Seasonal distribution and deposition patterns of size-segregated particulate matter in human respiratory system in Central Delhi, India

Abstract

Seasonal patterns and potential exposure of size-segregated particulate matter (PM) were studied in central Delhi from January 2021 to December 2022. A total of 79 samples were collected using an eight-stage Andersen cascade impactor. The samples were categorized as submicron (PM_{< 0.43–1.1}), fine (PM_1.1–2.1), and coarse (PM_2.1- > 9) fractions of PM. During 2021, average mass concentrations of submicron, fine, and coarse PM were 67.2 ± 10.7, 33.6 ± 5.7 and 124.1 ± 9.1 µg/m³ respectively. During 2022, the corresponding average mass concentrations were 55.1 ± 7.5, 25.8 ± 3.6 and 117.2 ± 8.9 µg/m³ respectively. The submicron and fine particles were more prevalent during the post-monsoon and winter seasons, while coarse particles were more pronounced during summer. The lognormal mass-size distribution displayed a bimodal pattern during the winter and the post-monsoon seasons of 2021 and 2022. Conversely, summer and monsoon seasons exhibited unimodal distributions. The inhalation dose was calculated for all seasons, found in the order post-monsoon > winter > summer > monsoon. Total, regional, and lobar depositions of size-segregated PM in respiratory airways of various age groups were also quantified using the multiple-path particle dosimetry model. Among different age groups, the deposited concentrations ranged from 35–51% for inhaled submicron particles, 57–68% for fine particles, and 89–96% for coarse particles, respectively. The deposited mass in respiratory airways was maximum for coarse particles in summer, while maximum in winter for submicron and fine particles in all age groups. Concentration weighted trajectory analysis for various size ranges in different seasons highlighted the influence of local, regional, and long-range transport of pollutants at the receptor site.

Introduction

Rapid industrialization, urbanization, and population growth have deteriorated the air quality of urban areas, particularly in developing countries like India (Kunkel et al. 2013; Kumar and Yadav 2016; Pant et al. 2019). Particulate matter (PM) has been recognized as a significant factor responsible for air pollution. Numerous studies on coarse (PM₁₀) and fine (PM_2.5) particulate matter are available globally and in India with a focus on their mass concentrations (Saliba et al. 2010; Hu et al. 2013; Huang et al. 2015; Ahirwar and Bajpai 2017; Kumar et al. 2020), chemical composition (Simoneit et al. 1991; Park et al. 2007; Feng et al. 2012; Chandra et al. 2014; Kulshrestha et al. 2016; Singh et al. 2016; Kang et al. 2020), optical properties (Porch et al. 2007; Soni et al. 2011; Menon et al. 2014; Ningombam et al. 2019), source apportionment (Almeida et al. 2006; Chowdhury et al. 2007; Hu et al. 2013; Chandra et al. 2017; Jung et al. 2019; Kulshrestha et al. 2019; Singh et al. 2023a) and health effects Li et al. 2015a; Liu et al. 2019; Lv et al. 2021; Cipoli et al. 2023). Exposure to PM₁₀ and PM_2.5 has been linked to lung cancer, respiratory morbidity, cardiopulmonary mortality, asthma attacks, and other health issues (Sarath and Ramani 2014; Singh et al. 2019; Gautam et al. 2020; Murari et al. 2020). The regulatory bodies worldwide have regulated two size fractions, namely PM₁₀ and PM_2.5. Hence, the majority of PM investigations primarily revolve around these size categories. Different aerodynamic diameters are associated with distinct emission sources, transformation mechanisms, and lifetime in the atmosphere (Theodosi et al. 2011). Their size distribution, number concentrations, and chemical constitution differ temporally and spatially due to local emission sources like vehicles, waste incineration, industries, etc. The toxicity of size-segregated PM mainly depends on its ability to penetrate the human respiratory tract. In addition, the toxicity of PM is also influenced by its physical parameters such as morphology, surface area, porosity, hydrophobicity, hydrophilicity, and their chemical composition (Sharma and Maloo 2005; Moreno-Ríos et al. 2022). So, it becomes imperative to study various size fractions of PM.

Delhi, the capital city of India, has been facing severe air pollution issues for several years. It is known for its high population density, rapid urbanization, extensive vehicular traffic, industrial activities, and biomass burning practices in nearby areas. These factors contribute to high levels of ambient air pollutants, including PM, in the city. The regular surpassing of National Ambient Air Quality Standards (NAAQS) in Delhi poses a significant health risk to its residents (Jha et al. 2024). The complex interplay between meteorological factors, anthropogenic emissions, and long-range transport collectively defines the region's air quality (Buch and Pedersen 1967; Trindade et al. 1967; El-Shobokshy et al. 1990; Bhaskar and Mehta 2010; Jayamurugan et al. 2013; Mukherjee and Agrawal 2018). There is an urgent need to gain a comprehensive understanding of size distribution, seasonal variation, and health implications of size-segregated PM in Delhi to develop effective plans for improving air quality in the region.

This study examines the seasonal variation in mass concentrations of size-segregated PM in central Delhi from January 2021 to December 2022. By studying the variability of size-segregated PM vs meteorological parameters, a deeper understanding of the air pollution problem in central Delhi, along with its local and regional impacts, can be established. The average inhalation dose for size-segregated PM in different seasons has been estimated to comprehend their potential health impacts. In addition, the multiple-path particle dosimetry (MPPD) model has been applied to quantify total, regional, and lobar depositions, deposited mass, deposited mass per unit area, deposited mass rate, mass flux, and clearance of deposited size-segregated PM in respiratory airways of various age groups. The concentration weighted trajectory model, a receptor-based approach, has also been used to identify the possible source regions for observed mass concentrations.

A two-year comprehensive study was conducted to understand seasonal patterns clearly. In size-segregated PM studies, long-term investigations are relatively scarce. Studies are often either episodic or focussed on specific seasons. To the best of the authors’ knowledge, no prior studies in Delhi have provided with two yearly comparative and comprehensive analyses of size-segregated PM. This study aims to fill the gap by providing an in-depth data analysis over two years. The present study is the first of its kind in Delhi, and it is comprehensive, and intercompares yearly levels, distribution patterns, average inhalation doses, deposition, and clearance of size-segregated PM in human respiratory airways.

Methodology

Sampling site

Delhi is located in the upper Indo-Gangetic plain and geographically positioned between 28°24´17´´ to 28°53´00´´ N and 76°50´24´´ to 77°20´37´´ E with the Thar desert situated to its west. The Himalayas act as a barrier to the north of Delhi, preventing the dispersal of pollutants and leading to their accumulation within the region. Delhi’s climate exhibits characteristics of a semi-arid region featuring four different seasons. Winter season spans from December to February, summer occurs from March to June, monsoon extends from July to September, and post-monsoon covers October and November. The temperature usually reaches 45–48 °C in summer, while it drops to 1–4 °C in winter. The monsoon season, marked by high relative humidity and rainfall, is followed by the post-monsoon, characterized by reduced wind speeds and significant variation in diurnal temperatures (Singh and Kulshrestha 2024). In recent years, Delhi has consistently ranked among the most polluted cities worldwide (World Air Quality Report 2018, 2019, 2020, 2021). The air quality in Delhi worsens due to calm weather conditions that lead to the accumulation of pollutants in the atmosphere during winter and post-monsoon. The size-segregated PM samples were collected on the terrace of the CSIR-National Physical Laboratory (CSIR-NPL) located (28°37′52″ N and 77°10′01″ E) in the central region of Delhi, India. There are no direct emission sources near the sampling site making it a suitable site for collecting ambient air samples. The site is encompassed by abundant forest, agricultural, residential, institutional, and commercial areas (Fig. 1). Additionally, a busy traffic route is close to the sampling site.

Fig.1

Location of study site in New Delhi (India)

Sample collection

The samples were collected using an 8-stage Andersen Cascade Impactor, ACI (TISCH, 28.3 lpm flow rate) installed at the roof of CSIR-NPL, around 15 m above the ground (Sengupta 2003). The impactor stages had aerodynamic cut-off diameters of < 0.43, 0.65, 1.1, 2.1, 3.3, 4.7, 5.8 and > 9.0 µm. A dry gas meter (ITRON) ensured the average flow rate. Size-segregated samples were collected weekly from January 2021- December 2022. Sampling was done every eighth day so that all weekdays and weekends were covered uniformly throughout the year to get a uniform estimation of PM mass concentrations. The sampling duration was 72 h except from January- March 2021, where sampling was done for 24 h. The sampling duration was optimized from 24 to 72 h after observing less mass deposition on filters in 24 h. A total of 79 samples were collected during the sampling period. As the sampling duration of each sample was 72 h, start date of each sampling has been considered to explain the results. The samples during the summer of 2021 could not be collected due to the COVID-19 lockdown. Quartz microfibre filters (Dia 82.6 mm, Whatman) were used for sample collection. Additionally, a backup filter with a diameter of 81 mm was placed downstream of the sampler. Before sample collection, the filters were baked at 400ºC for 4 h to remove any contamination. An analytical microbalance (Mettler Toledo, XP205) was used to weigh the filters before and after sampling. The filters were kept in a desiccator for 24 h pre- and post-sampling to remove any moisture content. Meteorological data was obtained from the Automatic Weather Station (AWS) installed on the terrace of CSIR-National Physical Laboratory, which provides data every 15 min. This data was averaged according to the sampling period. AWS measures temperature from -40 °C to 125 °C, wind speed from 0–322 km/h, and relative humidity from 0–100% with an accuracy of ± 0.4 °C, ± 3 km/h, and ± 2%, respectively. AWS has a wind direction resolution of 1° with an accuracy of ± 3°.

Inhalation dose estimation

The inhalation dose of a pollutant refers to the amount that crosses the boundary of the lungs and enters the exposed individual's target tissue. The relationship between exposure and inhalation dose can be described as follows (Li et al. 2015b; Liu et al. 2019):

$$D= \int {C}_{P}\times {IR}_{(\Delta t)}dt$$

where D signifies the average inhalation dose (μg), Cp denotes the concentration of pollutants in a specific environment at a given time (μg/m³), IR_(∆t) stands for the inhalation rate (m³/h), and t denotes the time duration of individuals exposed to air pollutants during outdoor activities (h). The value of the inhalation rate, as per Risk Information provided by the US Environmental Protection Agency, is 1.62 m³/h. This value is the average short-term exposure rate for inhalation and corresponds to individuals (both male and female) aged 21–51 years during moderate-intensity physical activity levels (USEPA 2011). In this study, the exposure time for each sample was taken 24 h, as the sampling time spanned 72 h (3 days). It was assumed that individuals typically spend a minimum of 8 h per day outdoors. Continuous values of Cp were utilized in this estimation. Initially, the inhalation dose was calculated individually for each sample at each stage. Subsequently, the seasonal average of inhalation dose values was determined for each stage.

Multiple-path particle dosimetry (MPPD) model

The MPPD model (v3.04, https://www.ara.com/products/multiple-pathparticle-dosimetry-model-mppd-v-304) was used to estimate and visualize the depositions of size-segregated PM in different areas of the human respiratory system. The deposition fraction quantifies the proportion of inhaled particles of a specific size deposited within a specific respiratory tract region (Cheng and Swift 1995). The input parameter values used in this model for each age group are given in Table 1. For the airway morphometry, human species were selected. In this study, the age-specific 5-lobe model, which resembled the actual human airway morphology, was chosen. Six age groups were considered in the present study, i.e., 3 months, 3 years, 9 years, 14 years, 18 years, and 21 years. Model default values of functional residual capacity (FRC) and upper respiratory tract (URT) volumes for each age group were considered (Table 1). The distribution of particles was assumed to be single. Count median diameter (CMD) values of 1.1, 2.1, and 10, were used to calculate the deposition fraction of submicron, fine, and coarse particles. Default values given in the model for particle density, aspect ratio (AR), and geometric standard deviation (GSD) were 1 g/cm³, 1, and 1, respectively. The exposure condition was chosen as constant, and seasonal average mass concentrations of submicron, fine, and coarse fractions were used. Body orientation was chosen as upright from nine given orientations. For each age group, breathing frequency (BF) and tidal volume (TV) values were considered as per the International Commission on Radiological Protection (ICRP) and model recommendations (ICRP 1994). The inspiratory fraction was set to 0.5, considering the breathing scenario as nasal (Manojkumar et al. 2019). Deposition and clearance calculations were done after inputting the data.

Table 1 Input parameter values specified for the MPPD model [Source: Applied Research Associates (ARA), Inc. MPPD (v3.04) model]

Concentration weighted trajectory (CWT)

The CWT model is employed to ascertain the spatial distribution and relative contributions of potential source regions to the elevated concentrations detected at receptor sites. The whole geographical area is segmented into a grid of cells for estimating the trajectory-weighted concentration in each grid cell to perform CWT calculations. The calculation is done using the following equation (Hsu et al. 2003; Wang et al. 2009; Cheng et al. 2013):

$${C}_{ij}= \frac{1}{\sum_{l=1}^{M}{\tau }_{ijl}}{\sum }_{l=1}^{M}{C}_{l}{\tau }_{ijl}$$

where: C_ij is the average weighted concentration in the ij^th grid cell.

• - C_l is the concentration observed at the sampling site on the arrival of trajectory l.

• - M is the total number of trajectories.

• - τ_ijl is the residence time of trajectory l in the ij^th grid cell associated with the C_l sample.

High C_ij values suggest that air parcels passing over the ij^th grid cell typically carry high concentrations at the receptor site. For this study, the CWT concentrations were computed using the TrajStat (Version- 1.2.2.6) software, with a grid size of 0.75° × 0.75°. The air mass back trajectories were simulated using the National Oceanic and Atmospheric Administration's (NOAA) Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model. 120-h back trajectories were calculated 500 m above ground level and 05 UTC for all sampling days. The mass concentrations of size-segregated PM were correlated with respective trajectories to elucidate potential source regions.

Results and discussion

Mass concentration of size-segregated particulate matter

Annual variation of submicron, fine and coarse PM concentrations

The annual average mass concentrations of size-segregated PM for 2021 and 2022 are shown in Fig. 2. The nine size ranges were grouped into three classes, i.e., submicron, fine, and coarse. Submicron ranged from < 0.43 to 1.1 µm, fine from 1.1 to 2.1 µm, and coarse > 2.1 µm (Singh and Banerjee 2021). During 2021, mass concentrations of submicron, fine, and coarse PM ranged from 14.8–254.2, 5.5–141.3, and 39.1–229.6 µg/m³ with average concentrations 67.2 ± 10.7, 33.6 ± 5.7, and 124.1 ± 9.1 µg/m³ respectively. During 2022, the corresponding ranges of mass concentration were 12.4–233.8, 3.4–102.1, and 13.9–284.1 µg/m³ with average concentrations 55.1 ± 7.5, 25.8 ± 3.6, and 117.2 ± 8.9 µg/m³ respectively. The variation in different size ranges was observed to be similar in both years, with slightly lower mean concentrations in 2022 (Fig. 2). The annual mean concentrations of submicron, fine, and coarse PM at an urban site (Durg) were reported as 65.7 ± 36.9, 135.0 ± 76.2 and 118.5 ± 32.8 µg/m³ respectively (Deshmukh et al. 2013); Nirmalkar et al. (2016) also reported annual average mass concentrations of 58.0 ± 30.9, 101.0 ± 55.7 and 79.9 ± 40.6 µg/m³ at a rural site (Rajim), corresponding to submicron, fine, and coarse fractions respectively. However, their classification of size ranges was different.

Fig.2

Annual average mass concentration of size-segregated particulate matter

Seasonal variation of submicron, fine, and coarse concentrations

The variation in submicron, fine, and coarse fractions during 2021–22 corresponding to meteorological variables viz., T, RH, WS, and RF is shown in Fig. 3. The lowest temperature was recorded in winter, whereas the highest in summer. Monsoon season was the most humid, while summer season was recognized as the driest. Summer season exhibited the highest wind speed, while winter and post-monsoon had the lowest. The highest rainfall was recorded in monsoon season. The seasonal polar plots of wind direction and mass concentration of size-segregated PM are shown in Fig. 4. In winter, monsoon and post-monsoon, the prevailing wind directions were primarily from the north and northwest. Winds were more dispersed in the summer season.

Fig.3

Seasonal variation in mass concentration of size-segregated particulate matter and meteorological variables in 2021-22 (W- Winter, S- Summer, M- Monsoon, PM- Post-monsoon, RH- Relative Humidity, T- Temperature, RF- Rainfall, WS- Wind Speed)

Fig.4

Seasonal polar plots of wind direction and mass concentration of size-segregated particulate matter in 2021-22

Submicron range

The mass concentration of submicron fraction varied in the range of 12.4–254.2 µg/m³ (Average: 60.3 ± 6.3 µg/m³) during the sampling period. Winter and post-monsoon seasons showed relatively higher mass concentrations of submicron fraction. Low T, low WS, and high RH characterize the winter season. These meteorological conditions result in a shallow planetary boundary layer (PBL), resulting in a buildup of air pollutants in the atmosphere. The high RH facilitates gas-to-particle partitioning increasing PM concentrations (Chen et al. 2020). In winter, burning wood and coal for heating purposes significantly increases submicron concentrations (Rajput et al. 2016). The post-monsoon period coincides with the crop residue burning period in nearby states. So, the crop residue burning (CRB) emissions were transported to the sampling site through wind from north and northwest directions (Fig. 4). During the summer season, RH is low, T remains high, and WS is usually high, resulting in increased convection activities, increased PBL height facilitating more dispersion of particles, contributing to lower mass concentrations of PM in the atmosphere. PM levels decreased during monsoon season due to their removal through rain. A sharp peak was observed on 14th January 2021 due to local wood-burning emissions during the Lohri festival (13th January), which increased concentration levels of particles in the atmosphere. Lohri festival, primarily celebrated in northern parts of India, signifies the commencement of the harvest season, which is marked by lighting bonfires, enjoying festive cuisine, and engaging in traditional dances. The peak observed on 7th February 2021 was attributed to elevated RH, which promoted the hygroscopic growth of particles, leading to higher mass concentrations (Wang and Ogawa 2015; Cheng et al. 2017). The peak observed on 6th November 2021 was due to increased local emissions during the Diwali festival (4th November). During this festival, candles/diyas burning, increased frequency of vehicles, and firecrackers burning were mainly responsible for increased mass concentrations in submicron ranges. Similarly, peaks on 24th December 2021 and 1st January 2022 were observed due to increased local emissions in celebrating the Christmas festival and New Year. Peaks other than these were observed due to local meteorological conditions. Two haze events were reported in the Delhi-NCR region between 2–5 and 8–11 November 2022 due to crop residue burning in Punjab and Haryana (Singh et al. 2023b). This resulted in higher mass concentrations on 1st November 2022 (1–4 November, 2022) and 9th November 2022 (9–12 November, 2022). A dip in mass concentration was observed on 17th November 2022 due to increased WS, which resulted in the dispersal of particles. A decline in mass concentration was also observed on 12th December 2022, caused by a sudden drop in RH and increased WS, which dispersed the particles and lowered their concentrations.

Fine range

Mass concentration of fine particles varied from 3.4–141.3 µg/m³ (Average: 29.1 ± 3.2 µg/m³). Fine particles followed a pattern similar to submicron particles, with higher concentrations in winter and post-monsoon while lower in summer and monsoon seasons. Higher concentrations were also observed during festivals like Lohri, Diwali, Christmas, and New Year.

Coarse range

Mass loading of coarse particles ranged from 13.9–284.1 µg/m³ (Average: 120.2 ± 6.4 µg/m³). The levels of coarse particles were comparatively higher during the summer season. During the summer, T remains high, RH remains low, and WS is generally high, which leads to the resuspension of dust and soil particles. High WS also carries winds laden with dust, resulting in higher mass concentrations of coarse particles. During the summer of 2022, a sharp peak with high concentration was observed on 1st May 2022. This could be attributed to a landfill fire held at the Bhalswa landfill site (~ 12 km north of the sampling site) from 26 April to 5 May, 2022 (Sharma et al. 2024). Dust storm events were observed on 17, 23, 24, and 30 May, 2022 (IMD 2022; The Economic Times 2022; The Hindu 2022). A peak of coarse particles was observed on 17th May due to a dust storm. Lower mass concentrations observed on 23rd April, 9th May, 25th May, and 18th June 2022 were due to the removal of coarse particles through rainfall. During monsoon season, coarse particle mass concentrations were low due to precipitation except for a few peaks. A few higher concentrations of the coarse particles were mainly due to south-westerly winds, which carried coarse particles besides local emissions (Fig. 4). Peaks observed during the post-monsoon season were due to the Diwali festival and crop residue burning.

Figure 5 shows the boxplots of submicron, fine, and coarse mass concentrations in different seasons during 2021–22. The results suggested that the seasonal concentrations of submicron and fine particles varied significantly during post-monsoon and winter. In contrast, these concentrations remained relatively stable during the summer and monsoon seasons. Winter recorded the maximum concentrations of submicron and fine particles, while monsoon recorded the minimum. The summer exhibited maximum concentrations of coarse particles, while the monsoon recorded the lowest concentrations. In the years 2021 and 2022, it was observed that the average concentrations remained similar across all seasons for all size fractions, which suggested the presence of similar sources. However, coarse particle concentrations showed variation during the post-monsoon season between the two years. The average mass concentration in 2022 (118.6 ± 20.0 µg/m³) was lower than 2021 (167.4 ± 12.8 µg/m³). Continuous rainfall events were observed from 7–9 October 2022, which coincided with the first sampling of post-monsoon 2022 (8–11 October, 2022). This resulted in washout of coarse particles, hence, reduced their average concentrations. Coarse particles are more effectively washed out during continuous rain events compared to submicron and fine particles (Guo et al. 2016).

Fig.5

Boxplots of (a) submicron, (b) fine and (c) coarse particles concentrations in different seasons

Mass-size distribution of size-segregated particulate matter

The lognormal mass-size distribution of size-segregated PM in different seasons during 2021–22 is shown in Fig. 6. Figure 6(a) shows the combined seasonal distribution and 6(b) shows the intercomparison of distribution observed during 2021 and 2022. The pattern of size-segregated PM in 2021 and 2022 and combined plots of corresponding seasons were similar.

Fig.6

Lognormal mass-size distribution of size-segregated particulate matter in different seasons

Winter season exhibited bimodal distribution in both years. The peaks were observed in the 0.65–1.1 μm submicron range and 4.7–5.8 μm coarse range, respectively. Atmospheric PM levels are notably affected by wood burning and fossil-fuel combustion in the winter season (Rajput et al. 2019). Peaks in 2022 were observed to be lower than in 2021. The average wind speed in winter 2022 was higher than in 2021, which dispersed the particles and diluted the concentrations. Researchers at Raipur (urban) and Rajim (rural) also observed bimodal distribution during winter with peaks in fine (0.4–0.7 μm) and coarse (4.4–5.8 μm) fractions (Nirmalkar et al. 2016; Mahilang and Deb 2020). Anthropogenic activities like domestic heating, cooking, and biomass burning were the reasons for observed high-intensity peaks of fine range. Trimodal distribution during winter, with peaks at 0.7–1.1, 1.1–2.1, and 2.1–2.5 μm, was reported at Durg City (urban) (Deshmukh et al. 2012a).

During the post-monsoon season in 2021, the particles exhibited bimodal distribution, and peaks were observed in 0.43–1.1 μm (submicron) and 4.7–5.8 μm (coarse) size ranges. Post-monsoon in 2022 also showed bimodal distribution, but the peaks were observed in 0.65–1.1 μm (submicron) and 4.7–5.8 μm (coarse) size fractions due to CRB emissions during the post-monsoon season (Rajput et al. 2019). The coarse particles' peak was higher in 2021 than in 2022. Relatively higher rainfall in 2022 than in 2021 washed out the particles and reduced the concentration levels. A study at an urban site reported bimodal distribution during post-monsoon, and peaks were observed in 0.4–0.7 and 4.4–5.8 μm ranges (Mahilang and Deb 2020).

Unimodal distribution was observed during summer and monsoon seasons, with a peak in the 4.7–5.8 μm (coarse) range, with a higher value in 2022 than in 2021. In 2021, fewer samples were collected in the summer due to the COVID-19 lockdown, showing lower concentrations. Nirmalkar et al. (2016) reported a similar distribution in summer with an intense peak in the coarse range, i.e., 4.4–5.8 μm. In a study at Durg City, bimodal distribution during summer and unimodal during monsoon season was reported, with peaks in coarse ranges. In summer, the peaks were in the 2.5–4.4 and 5.8–9.0 μm ranges, while in monsoon, the peak was in the 2.5–4.4 μm range. The prevalence of dust and soil particles due to dry atmospheric conditions was attributed to the high-intensity peaks in the coarse range (Deshmukh et al. 2012a). In the present study, among all the seasons, monsoon had the lowest levels/peaks in all size ranges due to scavenging through rainfall. These observations indicated that variation existed in the natural and anthropogenic contributions throughout different seasons at the receptor site.

Statistical analysis of size-segregated particulate matter

A one-way ANOVA (Analysis of variance) was conducted to assess the variability of submicron, fine and coarse mass concentrations within different seasons in 2021 and 2022. The F-values were computed at a 5% significance level (α = 0.05). Table 2 shows the ANOVA results for the seasonal variability of submicron, fine and coarse mass concentrations. Significant variation in mass concentrations of all size fractions were observed during 2021 and 2022 as p values ranging from 1.3 × 10^–6 to 1.4 × 10^–3 in 2021, and 4.4 × 10^–7 to 7.0 × 10^–6 in 2022 were less than α in all size fractions. Similarly, F-values ranging from 6.64 to 16.95 in 2021 and 13.78 to 16.51 in 2022 were greater than corresponding critical values (F_C) in all size fractions. The results suggested the emission of particles by different sources during different seasons.

Table 2 Analysis of variance results for the seasonal variability of size-segregated particulate matter concentrations

A two-sample t-test was also conducted to assess the variability of particle concentrations between the same seasons in different years. i.e., winter 2021 vs winter 2022, summer 2021 vs summer 2022, monsoon 2021 vs monsoon 2022 and post-monsoon 2021 vs post-monsoon 2022. The t-values were computed at a 5% significance level (α = 0.05). Table 3 shows the two-sample t-test results for the variability of submicron, fine, and coarse mass concentrations between the corresponding seasons of 2021 and 2022. It was noted that there was no significant variation between the concentrations of submicron and fine particles in corresponding seasons as p values ranging from 0.25 to 0.48 for submicron and 0.18 to 0.41 for fine were found to be greater than α in all seasons (Table 3). Similarly, values of t ranging from -0.04 to 0.71 for submicron and -0.50 to 0.94 for fine were always less than corresponding critical values (t_C) (Table 3). So, it can be inferred that the emission sources remained nearly constant in corresponding seasons irrespective of the year. The same trend of insignificant variation was also found for coarse concentrations in all seasons except post-monsoon. In contrast, a significant difference was observed in the coarse concentrations in the post-monsoon seasons of 2021 and 2022, where the p-value (0.03) was less than α, and t (2.06) was greater than t_C (Table 3). Weekly concentrations of coarse particles were similar in all the samples in the post-monsoon seasons of 2021 and 2022, except one. Unusually low coarse concentration was observed in that sample due to rainfall, which scavenged coarse particles.

Table 3 Two-sample t-test results for the variability of size-segregated particulate matter concentrations between corresponding seasons of 2021-22

Correlation of size-segregated particulate matter and meteorological parameters

Pearson correlation analysis among meteorological parameters and mass concentrations of submicron, fine, and coarse fractions in different seasons during 2021–22 is shown in Fig. 7. The correlation from 0.1–0.3 was considered weak, 0.3–0.5 moderate, and > 0.5 strong.

Fig.7

Pearson correlation statistics among meteorological parameters and size-segregated particulate matter in different seasons in 2021-22 (T = Temperature in °C; RH = Relative Humidity in %; WS = Wind Speed in km/h; RF = Rainfall in mm; SM = Submicron mass concentration in μg/m³; F = Fine mass concentration in μg/m³; C = Coarse mass concentration in μg/m.³)

Winter

During the winter, temperature showed a strong negative correlation with submicron and fine while a weak negative correlation with coarse particles’ mass concentrations. RH showed a strong positive correlation with submicron and fine while a weak positive correlation (0.03) with coarse particles’ mass concentrations. The correlations of particles’ concentrations in all three fractions were moderate to strong negative with wind speed. A moderate negative correlation was observed between RF and coarse particles’ mass concentrations. In the winter, when the surface temperature is low, even a slight temperature rise may create the temperature inversion layer that restricts the movement of pollutants (Li et al. 2015b; Xu et al. 2019). Low WS further suppresses the convective activities and thus favours the accumulation of PM in the atmosphere. Moreover, high humidity condition during winter allows hygroscopic growth of particles which leads to high mass concentrations (Wang and Ogawa 2015; Cheng et al. 2017; Liao et al. 2017). Additionally, high RH facilitates gas-to-particle partitioning, resulting in increased PM concentrations. Tiwari et al. (2014) reported a similar negative correlation (-0.55) between WS and PM_2.5 concentration, while a positive correlation (0.32) between RH and PM_2.5 concentration during the winter season at a site in New Delhi. Local emissions were reported as the primary source for high concentrations of PM_2.5 during winter, and low WS could not flush the air pollutants. Submicron concentration was positively correlated to fine and coarse concentrations. Additionally, fine particles showed a strong positive correlation with coarse particles. These findings suggest that these fractions’ emission sources were either similar or simultaneously emitted from different sources.

Summer

A strong negative correlation of temperature with submicron and fine particles’ mass concentrations was observed in the summer season. Conversely, a moderate positive correlation was observed between temperature and coarse particles' mass concentrations. RH showed a weak and moderate positive correlation with fine and submicron particles' mass concentrations, respectively, while a strong negative correlation with coarse concentrations. WS exhibited a moderate to strong positive correlation, while RF showed a weak to moderate negative correlation with all three fractions. High temperature during summer allows more evaporative losses of submicron and fine particles, including loss of volatile/semi-volatile constituents, resulting in their low concentrations (Liu et al. 2015; Wang et al. 2006). On the contrary, high temperature and low RH conditions favour intense convective activities promoting local resuspension of dust and soil particles, increasing coarse particles' mass concentrations. Strong winds during summer carry pollutants from other regions and increase particles' concentrations in the atmosphere. Precipitation has a negative effect on PM concentrations due to the washout of atmospheric column (Jian et al. 2012; Li et al. 2015a). However, coarse particles are scavenged more effectively than submicron and fine particles due to rainfall events during summer. Fine particles, being smaller than coarse particles, were less effectively washed away by precipitation. Similar results were observed in New Delhi during the summer season, i.e., coarse particles were positively correlated (0.46) with temperature while negatively correlated with RH (-0.51) (Tiwari et al. 2014). Submicron and fine particles were positively correlated, while a weak correlation was observed with coarse particles. This indicated similar sources of submicron and fine particles, while different sources of coarse particles during the summer. Dust storms are common during summer, and carry coarse particles from distant regions like the Thar desert and local PM sources.

Monsoon

During the monsoon season, a weak positive correlation was observed for temperature with submicron and fine particles, while a moderately strong correlation with coarse particles. Submicron and fine particles showed a weak negative correlation, while coarse particles showed a strong negative correlation with RH. WS was moderately correlated with submicron (negative) and coarse (positive), while weakly correlated with fine (positive) particles. RF showed weak and moderate negative correlation with submicron and coarse particles, respectively, whereas weak positive correlation with fine particles. High RH during monsoon causes suspended particles to coalesce to form large particles, making them heavy enough to undergo dry deposition (falling to the ground) and wet deposition (being washed away by precipitation) (Li et al. 2015a; Li et al. 2015b). This process significantly lowers the concentration of PM in the atmosphere. A similar study in New Delhi reported a similar strong positive correlation (0.58) of temperature and a negative correlation of RH (-0.70) with coarse particles during monsoon season (Tiwari et al. 2014). A strong positive correlation between submicron and fine particles, and fine and coarse particles was also observed. These findings suggest that these fractions’ emission sources were either similar or simultaneously emitted from different sources.

Post-monsoon

During post-monsoon season, temperature, WS, and RF showed moderate to strong negative correlation, while RH showed weak to strong negative correlation with all size fractions. Low temperature and low WS weaken the atmosphere’s convective activities and increase the particles’ buildup in the atmosphere (Li et al. 2014; Li et al. 2015b). RF negatively influences the particle concentrations, but the washing effect is more significant for coarse particles than for submicron and fine particles. A moderate negative correlation (-0.32) between WS and fine particles has been reported in an earlier study during post-monsoon in New Delhi (Tiwari et al. 2014). Like the winter season, all three size fractions were found to be strongly positively correlated with each other. This shows that emission sources of these fractions were similar during post-monsoon season.

Possible human health implications of size-segregated particulate matter

Inhalation dose

In recent years, the quality of the air and its implications on public health have been of global concern. UNICEF's report revealed that approximately 0.6 million children lose their lives annually in developing countries due to bad air quality (UNICEF 2016). Airborne particles enter the human body through breathing and deposit in the respiratory track according to their aerodynamic sizes. These particles can be categorized into three principal fractions, i.e., the inhalable fraction, the respirable fraction, and the alveolar fraction. The inhalable fraction, with particles > 4.7 µm, can enter the nose and/or mouth during inhalation. The respirable fraction ranging from 1.1 to 4.7 µm that can penetrate beyond the larynx and enter the lungs. The alveolar fraction comprises particles < 1.1 µm, capable of reaching the gas exchange region, known as the alveolar region, within the lungs (Nirmalkar and Deb 2016).

The estimated average inhalation dose for size-segregated PM during different seasons in 2021–22 is shown in Fig. 8. The winter season had higher inhalation doses of respirable and alveolar fractions. Parallelly, high concentrations of submicron and fine particles were observed during winter. This implied that inhalation of these particles resulted in high inhalation doses in these fractions. During post-monsoon season, inhalation doses were higher in respirable, alveolar, and inhalable fractions. Crop residue burning in nearby states and celebrations during the Diwali festival were responsible for the increased concentrations, leading to higher inhalation doses. During summer, doses of inhalable fraction elevated significantly due to higher levels of coarse particles. During monsoon season, the inhalation doses were found to be minimal as particles were scavenged due to rainfall. The concentrations of respirable and alveolar particles were reported to be higher during biomass burning, agricultural waste burning, fossil fuel combustion, etc. (Deshmukh et al. 2012a; Khaparde et al. 2012; Nirmalkar et al. 2013). Winter and post-monsoon seasons emerged as periods of higher health risk due to elevated concentrations of submicron and fine particles (Fig. 9). The findings suggest that there could be significant health problems, especially in children, elderly individuals, and those with breathing or other health issues.

Fig.8

Estimated average inhalation dose for size-segregated particulate matter during different seasons in 2021–22

Fig.9

Percent mass loading of submicron, fine, and coarse particles in different seasons in 2021-22

Particulate matter deposition in human airways

Deposition fractions (DF)

The total and regional deposition fractions of submicron, fine, and coarse particles in various age groups are shown in Table 4. Among different age groups, the deposited mass ranged from 35–51% for inhaled submicron particles, 57–68% for inhaled fine particles, and 89–96% for inhaled coarse particles. The total DF was observed to be maximum for coarse particles, followed by fine and submicron respectively. Coarse particles exhibited the highest deposition in 18 and 21 years old, while the lowest was in 3 years old. Fine particles had the highest deposition in 9 and 21 years old, while the lowest was in 3 years old. Submicron particles showed the highest deposition in the respiratory tracts of 9 years old, while the lowest was in 18 years old. Similar total DF results have also been observed in other studies (Manojkumar et al. 2019; Rajput et al. 2019; Madureira et al. 2020; Lv et al. 2021; Manojkumar and Srimuruganandam 2022). Moreover, deposition within a specific age group can vary depending on gender, lung morphology, breathing cycle, breathing scenario, and body position (Salma et al. 2002).

Table 4 Total and regional deposition fractions of submicron, fine and coarse particles in various age groups

Considerable variation was observed in the deposition of PM sizes across the head (H), tracheobronchial (TB), and pulmonary (P) regions. Coarse particles exhibited the highest deposition in the H region and the lowest deposition in the P region for all age groups. A combination of impaction and sedimentation mechanisms in the upper respiratory tract can elucidate the deposition of coarse particles (Behera et al. 2015). In contrast, the highest deposition of fine and submicron particles was observed either in the H or the P region and the lowest in the TB region. Fine and submicron deposition is influenced by Brownian diffusion, allowing them to deposit preferentially in the P region (Behera et al. 2015). Lower deposition in TB region can be attributed to mucociliary clearance mechanism which removes particles (Izhar et al. 2018); Manojkumar et al. (2019) reported similar regional deposition patterns of PM₁₀, PM_2.5, and PM₁ in Chennai, India. ICRP equations were used to calculate regional depositions during outdoor exercises at Dhanbad, India. This study found that the respective depositions of PM₁₀, PM_2.5, and PM₁ were in the order head (96, 80, and 69%) > alveolar (2, 13, and 24%) > tracheobronchial regions (2, 7, and 7%) (Gupta and Elumalai 2017). A study at Barcelona subway locations revealed similar pattern of PM_2.5 depositions in the human respiratory tract using the ExDoM model. The maximum deposition was in the H (68%) region, followed by alveolar (10%) and TB (4%) regions (Martins et al. 2015). Similar dominant deposition of coarse particles (PM₁₀) in the H region was observed in other studies in Hungary and Singapore (Salma et al. 2002; Behera et al. 2015). Earlier research has shown that coarse particles predominantly deposit in the head region during high breathing flow rates, while submicron particles are more likely to reach deeper airways irrespective of the flow rates (Islam et al. 2017). According to the model, the DF is influenced by the aerodynamic size of the particles rather than their mass concentrations. Therefore, regardless of mass concentration differences in different seasons, DF values were identical at the same aerodynamic diameter (Madhwal et al. 2020).

Lobar deposition

Lobar deposition fractions of size-segregated PM in different age groups are shown in Fig. 10. The right lung is divided into three lobes, i.e., the right lower lobe (RL), right middle lobe (RM), and right upper lobe (RU). In contrast, the left lung has two lobes namely the left lower lobe (LL) and left upper lobe (LU), to accommodate space for the heart (Islam et al. 2017). It was observed that lower lobes had higher depositions compared to upper/middle lobes, while the middle lobe had the lowest depositions in all three size fractions. The different path lengths and lobes’ volumes were responsible to the observed differences in particle depositions. Lower lobes with higher volumes exhibited higher depositions, whereas middle lobes with lower volumes had lower depositions (Manojkumar et al. 2019; Madureira et al. 2020). The findings of Islam et al. 2017 and Manojkumar et al. 2019 were consistent with the present study, i.e., higher deposition fraction in the lower lobes compared to the upper/middle lobes. Another study explored the impact of varying gravitational forces (0G, 1G, 2G) on particle deposition across different lung lobes. Across all three gravitational conditions, deposition was higher in the lower lobes than in the other lobes. Moreover, increasing gravitational force resulted in distinct deposition patterns among the lobes (Asgharian et al. 2006). Fine particles had the highest deposition fraction across all five lobes in all age groups. This deposition of fine particles in lobar regions may lead to reduced lung function and an increased risk of developing chronic obstructive pulmonary disease and respiratory morbidity (Zhao et al. 2017;Guo et al. 2018).

Fig.10

Lobar deposition fractions of size-segregated particulate matter in different age groups

Deposited mass (DM)

The seasonal DM of size-segregated PM in airways of various age groups was calculated in this study. Visualizations of DM, along with airway geometries of each age group, are shown in Fig. 11. The overall DM was higher for coarse particles than submicron and fine particles in corresponding age groups in all seasons. This can be attributed to their larger particle sizes and higher masses (Madureira et al. 2020). DM of submicron and fine particles was observed to be higher in adults (18y, 21y) followed by children (3y, 9y, 14y) and infants (3 m) in all seasons. The highest DM of submicron and fine particles was observed in 21 years old respiratory airways in all seasons, with a maximum in the winter season (1.025 × 10^–4 and 5.416 × 10^–5 µg, respectively). DM values increased with age, and similar results were reported by Manojkumar et al. (2019) and Cipoli et al. (2023). DM values of coarse particles were in the order children > infants > adults in all the seasons. The highest deposition of coarse particles was observed in 9-year-old children’s respiratory tract in all seasons, with a maximum in the summer (1.597 × 10^–3 µg). Considering all age groups, the DM of coarse particles was the highest in summer, followed by post-monsoon, winter, and monsoon seasons. In contrast, DMs of submicron and fine particles were in the order winter > post-monsoon > summer > monsoon, which aligns with the observed mass concentration patterns.

Fig.11

Seasonal deposited mass visualizations of submicron, fine and coarse fractions in respiratory airways of various age groups [(a) winter, (b) summer, (c) monsoon, (d) post-monsoon]

Deposited mass per unit area (DM/A)

The seasonal DM/A of size-segregated PM in airways of various age groups were also calculated, and visualizations are shown in Fig. 12. The airway geometries of children and infants showed higher DM/A values than adults in all size fractions during all seasons. The maximum DM/A was observed for 9y (27.9 µg/m² in summer), followed by 3 m (9.1 µg/m² in summer), and 18y (0.1028 µg/m² in winter) in children, infants, and adults categories respectively. A study in Chennai reported similar results with higher DM/A in infants and children than in adults (Manojkumar et al. 2019). While the mass deposited in infant airways was relatively low, the high DM/A could be attributed to the larger surface area-to-volume ratio than adults (Manojkumar et al. 2019; Madureira et al. 2020). Considering all age groups, the DM/A followed the same seasonal pattern as the DM of submicron, fine, and coarse particles. Vulnerability orders with respect to DM/A for submicron depositions were 9y > 18y > 3y > 3 m > 14y > 21y, for fine depositions 9y > 3 m > 3y > 18y > 14y > 21y, and for coarse depositions 9y > 3 m > 3y > 14y > 18y > 21y, respectively for all seasons. However, submicron and fine particles can penetrate deep inside the lungs and can be associated with higher health risks. Many studies have indicated that these are likely to carry carcinogens, mutagens, and bypass clearance mechanisms. This can impair lung function, cause pulmonary inflamation, an increased risk of asthma, and, in extreme cases, mortality (Cutrufello et al. 2012; HEI 2013; Kesavachandran et al. 2015; Singh and Gupta 2016).

Fig.12

Seasonal deposited mass per unit area visualizations of submicron, fine and coarse fractions in respiratory airways of various age groups [(a) winter, (b) summer, (c) monsoon, (d) post-monsoon]

Deposited mass rate (DMR)

The DMR (µg/min) represents the frequency at which particles are deposited in the human respiratory airways over a given period of time. Figure 13 shows the visualizations of seasonal DMRs of size-segregated PM in airways of various age groups. Considering all seasons, submicron and fine particles had the highest DMRs in winter, followed by post-monsoon, summer, and monsoon for all age groups. On the other hand, DMRs of coarse particles followed the order summer > post-monsoon > winter > monsoon. DMRs of submicron and fine particles were observed the highest in 21y and the lowest in 3 m old respiratory airways in all seasons. The highest submicron and fine DMRs for 21y old were 1.43 × 10^–3 and 7.58 × 10^–4 µg/min, respectively. The lowest corresponding submicron and fine DMRs for 3 m old were 3.57 × 10^–5 and 2.05 × 10^–5 µg/min. In all seasons, DMRs of coarse particles were the highest in 9y and the lowest in 21y old respiratory airways i.e., 2.72 × 10^–2 and 3.24 × 10^–4 µg/min respectively. Similar observations with higher deposition rates of fine particles in adults (2.76 × 10^–4 µg/min) and coarse particles in children (2.70 × 10^–4 µg/min) were reported by Cipoli et al. 2023; Manojkumar et al. 2019 also reported similar size-segregated PM deposition rates in Chennai where PM₁₀ had the highest deposition in the 9y (1.27 × 10^–3 µg/h) category in the TB region. PM_2.5 had the highest deposition in 21y (6 × 10^–5 µg/h) and the lowest in 3 m (4.7 × 10^–6 µg/h).

Fig.13

Seasonal deposited mass rate visualizations of submicron, fine and coarse fractions in respiratory airways of various age groups [(a) winter, (b) summer, (c) monsoon, (d) post-monsoon]

Mass flux

Mass flux (µg/min/m²) is the rate at which aerosol mass is deposited per unit area of the airway. Figure 14 shows the visualizations of seasonal mass flux of size-segregated PM in respiratory airways of various age groups. For all age groups, the mass flux of submicron, fine, and coarse particles followed similar seasonal trends as DM and DMR. Mass flux values for submicron fraction were in the order 9y > 3 m > 3y > 18y > 14y > 21y while for fine and coarse fractions were 9y > 3 m > 3y > 14y > 18y > 21y. Mass flux was observed to be the highest in children (9y) and infants (3 m) while the lowest in adults (21y) for all size ranges in all seasons. Higher mass flux of fine particles in infants and children than in adults was also reported at a similar site in Delhi during a smog episode (Fatima et al. 2022). Similar observations were also reported in Chennai during winter (Manojkumar et al. 2019).

Fig. 14

Seasonal mass flux visualizations of submicron, fine and coarse fractions in respiratory airways of various age groups [(a) winter, (b) summer, (c) monsoon, (d) post-monsoon]

Clearance of deposited PM in TB and alveolar regions

Clearance is a two-phase process through which particles are removed from the lungs. Initially, particles are removed rapidly via mucociliary activity in the TB region, followed by slow removal via alveolar macrophages in the P region (Cipoli et al. 2023). Clearance of the 9y age group was considered having the highest DM/A values the most vulnerable for all size fractions. The winter season mass concentrations of submicron, fine, and coarse, were taken for clearance modeling. TB and alveolar clearance of size-segregated PM in the 9y age group are shown in Fig. 15. For clearance calculations, the number of exposure hours per day was taken as 8, the number of days per week as 5, and the number of weeks as 1. It is evident that TB clearance surpasses alveolar clearance across all size fractions. Initially, TB clearance is very high, but it decreases significantly over time. This rapid clearance is primarily due to mucociliary activity. This activity involves the secretion of mucus by glands, which facilitates the elimination of deposited PM through coughing and ciliary movements (Walsh et al. 2011). In contrast, the alveolar region exhibits a consistently low clearance rate throughout the process. Unlike TB region, the alveolar region lacks the protective mucus layer, leading to slower particle clearance. This poses significant health risks since the alveolar region is responsible for gas exchange (Mbazima 2022). This observed low clearance rate can be attributed to the slower phagocytosis and lymphatic transport mechanisms, which are responsible for PM clearance (Lippmann et al. 1980; Manigrasso et al. 2017).

Fig.15

Clearance in TB and alveolar regions during post-exposure period

Concentration weighted trajectory (CWT)

The CWT model was utilized in this study to identify the potential source regions responsible for the observed mass concentrations of size-segregated PM at the sampling site (Chandra et al. 2017; Rani et al. 2023; Singh and Kulshrestha 2024). Figure 16 shows the CWT plots of submicron, fine, and coarse particles in different seasons during 2021–22. Air parcels carrying pollutants from Delhi were referred to as local sources, from different regions of India (other than Delhi) as regional sources, and from regions outside India as long-range sources respectively. During the winter, the air masses coming from Uttar Pradesh and some regions of Pakistan, in addition to local emission sources, carried higher concentrations of submicron and fine fractions. Local emissions and air parcels from neighboring states Haryana, Punjab, Uttar Pradesh, and regions of Pakistan were responsible for higher coarse fractions. IGP region, Bay of Bengal, eastern and remaining northern states of India, and some areas of Bangladesh, Myanmar, and China contributed to moderate to low concentrations of particles. Higher carbonaceous aerosol concentrations in PM₁₀ during winter were attributed to trajectories originating from Pakistan, Punjab, Haryana, and the upper IGP regions, which are in agreement with the present study (Singh et al. 2023b). Other researchers reported similar results for PM_2.5 and PM₁₀ (Saxena et al. 2017; Singh et al. 2020). During the summer, air parcels from Haryana, Punjab, Himachal Pradesh, and Pakistan contributed to the higher concentrations of all three fractions. Medium to lower concentrations were coming from Rajasthan, Gujrat, Madhya Pradesh, the IGP region, the Bay of Bengal, Persian Gulf, the United Arab Emirates (UAE), the Arabian Sea, Afghanistan, Iran, Saudi Arabia, Oman and the Gulf of Aden regions. Rajasthan (Thar desert) and Gujrat were reported as the main source regions for higher PM concentrations at Delhi during summer (Ghosh et al. 2015; Saxena et al. 2017). During monsoon season, high concentrations of submicron and fine particles were transported from parts of Punjab. Trajectories from almost all the states of India, Nepal, Bangladesh, Bhutan, the Bay of Bengal, the Arabian Sea, the Gulf of Oman, and the Gulf of Aden regions contributed to relatively low concentrations of particles. A similar study in central Delhi also reported that most air parcels originated from the Arabian Sea, Bay of Bengal, and the IGP region during monsoon season, were responsible for higher concentrations of n-alkanes (Singh and Kulshrestha 2024). During post-monsoon season, Haryana, Punjab, Himachal Pradesh, Uttar Pradesh, and some parts of Pakistan, as well as the local emissions sources, contributed to higher concentrations of submicron and coarse particles (Singh et al. 2016). Higher fine particle concentrations were mainly due to local sources and air parcels coming from Himachal Pradesh. Low to moderate coarse fractions came from Gujarat and Rajasthan states, as well as Afghanistan, Iran, and the Arabian Sea regions. Earlier studies also showed highly localized and northwest regions as significant contributors of PM during the post-monsoon season (Chandra et al. 2014; Saxena et al. 2017; Singh et al. 2020). Overall, it was observed that the site is under the influence of long-range and regional transport of air pollutants, in addition to local sources of emissions.

Fig.16

CWT plots of submicron, fine, and coarse particles during different seasons in 2021–22 (Concentration: µg/m³, Figures are software-based and do not represent political map)

Conclusion

This study investigated the influence of seasonality on mass concentration, inhalation dose, deposition fractions, and clearance of size-segregated PM in an urban environment. It was noted that the concentration patterns were similar in both years, with slightly lower average concentrations in 2022. Submicron and fine particles showed higher mass concentrations during the winter and post-monsoon seasons, while coarse particles during the summer. Winter and post-monsoon seasons emerged as periods of higher health risk due to elevated concentrations of submicron and fine particles. These smaller particles have a greater tendency to penetrate deep into the respiratory system, potentially reaching the gas exchange region in the lungs. Deposition fractions further revealed the influence of particle size, with coarse particles predominantly depositing in the head region, while submicron and fine particles reaching deeper into the lungs. These findings align with existing knowledge that links exposure to fine and submicron particles resulting in various health problems such as asthma, pulmonary inflammation, and even mortality. Children and infants were found to be more vulnerable due to a higher deposited mass per unit area. Lower lobes exhibited higher depositions compared to upper/middle lobes. During the post-exposure period, the clearance of deposited PM was rapid in the TB region, whereas it was much slower in the alveolar region. This study lays the groundwork for a more comprehensive understanding of the health risks associated with exposure to size-segregated PM. This knowledge can help to develop targeted air quality management strategies, particularly for vulnerable populations, to protect public health in urban environments.