Current World Environment

Introduction

The physicochemical properties of the atmosphere get affected by particulate matter by altering its composition and thus may alter ambient air quality, visibility, cloud formation, and consecutively energy entering, absorbed, reflected, and emitted by earth system^1-3. Moreover, it can also affect human health adversely and may also alter the ecosystem^4-6. Therefore, the chemical characterization of PM is necessary for the development of an air quality improvement programme to control the implications^7-8. Delhi is considered a city where PM load is usually higher than the permissible limits, and hence its chemical description is needed at regular intervals of time. The primary sources of pollutants impacting the air of Delhi are the burning of fossil and biomass fuels, rapid urbanization, industrialization, and transportation^9-11. Due to the potential effect of water-soluble components on rain chemistry, human health, and air quality, water-soluble inorganic components are studied extensively^12-16. Several anthropogenic activities such as agricultural, vehicular, industrial, and burning of biomass consecutively transform secondary inorganic particles such as sulphate and nitrate^9,17-19.

Delhi is a typical site for studying the chemistry of primary pollutants and the role of the transformation of secondary pollutants due to the high loading of particulate matter. In India, most studies on chemical characteristics are reported on a 24 hours basis sampling, whereas data on day and nighttime during different seasons forPM is limited. This study presents the temporal variability (day and nighttime) of the chemical composition of ambient aerosols for the years 2012 and 2013.The importance of day and nighttime variability, and the role of seasonal and transport patterns of long-range major water-soluble ionic species (WSIS), including reactive nitrogen species inPM_2.5 and PM₁₀ aerosols at a site in central Delhi, have been discussed.

Methodology

Site Description

The samples (PM₁₀ and PM_2.5) were collected at the terrace of CSIR-National Physical Laboratory (28°38?N, 77°10?E; 218 m amsl) (Fig.1), New Delhi. The site is located in central Delhi, surrounded by IARI farmland, commercial and residential areas. The site also experiences local, regional, and long-range transport of pollutants depending on meteorological conditions. Therefore, the study site is under the influence of both local as well as transported particulate matter.

Among the local sources,~7.4 million automotive and registered factories (~8000), including rubber/plastic, chemical, metal, and, leather affect the level of pollutants in the city. The study area also experiences the seasonality in the transport of pollutants from regional to distant sources. During summer,the site is under the influence of local to regional transport from North-west while in winter regional to long-range transport. In addition, calm wind conditions, lower mixing height with haze and, foggy conditions prevail in winter, whereas in summer, frequent dust storms and higher mixing height have been observed at the study site.

Figure 1: Location of Sampling Site. Click here to view Figure

Sampling of PM₁₀and PM_2.5

High and low volume samplers were used for collecting PM₁₀(n= 160), and PM_2.5 (n = 188) samples respectively. The pre-combusted (at 550ºC for 5h)Whatman Quartz Microfibre filters (QM-A) were used to collectsamples. The samples were stored in desiccators for 24 h beforeand after sample collection. Respirable particle sampler (PM₁₀, Envirotech, APM 460 BL) and fine particulate sampler (PM_2.5, Envirotech, APM 550) were run for 12 hrs daytime (07:00–19:00h) and 12 hrs nighttime(19:00–07:00h next day). After collecting samples, filters were stored at low temperature in a refrigerator till further analysis.PM₁₀ sampler was operated in the flow range of 0.9-1.4 m³/minwhile PM_2.5 sampler at a 1 m³/hr flow rate.

Analysis

For the analysis of WSIS, filter cuts (0.536 cm² area) of collected samples were extracted in de-ionized water (Millipore, specific resistance: 18.2 M?-cm)using a sonicatorfor 90 minutes. A microporous membrane filter (PALL, Ultipor N Nylon 6, 6-Membrane, pore size 0.45-µm diameter 25 mm) was used to filter the extracted solution. After the extraction, water-soluble cations (Na⁺, K⁺, NH₄⁺,Ca²⁺ and Mg²⁺) and anions (Cl^-,SO₄^2- and NO₃^-) in PM_2.5 and PM₁₀ aerosols were determined using Ion Chromatograph (Metrohm 883). Similarly,the concentrations of these water-soluble ionic species werealso estimated in blank filters. Every time, before the analysis, a three-point calibration curve was also achieved for each species.

Meteorological Parameters and Trajectory Analysis

For the years 2012 and 2013, meteorological parameters such as temperature (T), relative humidity (RH), and mixing height (MH) were downloaded from http://ready.arl.noaa.gov/READYamet.php link. A clear seasonality was observed for T, which was lowestin winter and highest in summer months. The highest monthly mean value of Twas observed in May, while minima were recorded in January. The monthly mean of T varied from approximately 10^oC to 40^oC. A higher difference between day and nighttime mean temperature was observed during winter and summer compared to the monsoon season. The monthly mean of RH varied from approx. 15 to 75%. The diurnal variation of RH revealed higher nighttime levels than daytime. The mixing height (MH) at the site represented the potential of convective current in the lower atmosphere. MH for winter months was observed a few hundred meters, while for summer months it went up to few thousand meters. Day and nighttime mean mixing height explain, much higher daytime value compared to nighttime during summer season, while this difference gradually decreases during monsoon and winter seasons (Fig.2).

Figure 2: Meteorological Conditions During Sampling Period. Click here to view Figure

Results and Discussion

Annual Mean Day and Nighttime WSIS in PM_2.5 and PM₁₀

Fig.3 shows two years’average of mass concentrations of major ionic species in fine and coarse particles. Almost similar levels of Ca²⁺ and Mg²⁺were observed during the day and nighttime in fine mode,while in coarse mode,significant variation with higher daytime valueswere observed. Daytime and nighttime levels of Ca²⁺ in PM₁₀ were observed as 7.32 µg m^-3 and 4.25 µg m^-3, respectively. The higher daytime levels might be due to more resuspension of crustal Ca²⁺ due to agricultural and commercial activities, as thesoil is theprime source of Ca²⁺.  On the contrary, higher levels of NH₄⁺ and K⁺ were observed in nighttime compared to daytime in both PM_2.5 and PM₁₀ among cations. NH₄⁺was observed as 1.49 µg m^-3 during the day and 2.50 µg m^-3 during the nighttime in fine modewhile 2.39 and 5.54 µg m^-3, respectively in the coarse mode. A similar trend was also reported from other study²⁰, which reported higher nighttime levels of NH₄⁺ owing to the variation in emission sources and temperature difference during nighttime. Day and nighttime levels of K⁺ were observed as 1.11 and 1.65 µg m^-3 infine and 2.69 and 3.74 µg m^-3 in coarse particles, respectively. Both the anionic species showed the same trend with higher levels in nighttime than daytime. During the day and nighttime,NO₃^- was observed as 3.18 and 4.75 µg m^-3 in PM_2.5 while 7.37 and 9.47 µg m^-3 in PM₁₀ respectively. The high concentrations of NO₃^-during nighttime might be due to the formation of NO₃^-through hydrolysis of N₂O₅under high humidity conditions and stability of NH₄NO₃ at a lower temperature during nighttime^21-22. Similarly, low concentrations of NO₃^- during daytime might be due to evaporative loss of NH₄NO₃ at higher temperature^23-24. A similar trend was also reported in an earlier study at Kanpur²⁵, with a higherNO₃^- in nighttime (12.9 µg m^-3) than daytime (5.4 µg m^-3) in PM₁₀. Day and nighttime levels of SO₄^2- were observed as 8.66 and 9.25 µg m^-3 in PM_2.5 while12.80 and 12.95 µg m^-3 in PM_10, respectively.

Figure 3: Annual Mean Day and Nighttime Concentrations of WSIS in PM2.5 and PM10.  Click here to view Figure

The two years mean (daytime + nighttime) concentration difference in PM_2.5 and PM₁₀ was observed the highest for Ca²⁺while lowest for Mg²⁺. Around 5 times higher concentration of Ca²⁺ was observed in PM₁₀(5.8 µg/m³) compared to PM_2.5 (0.8 µg/m³). These observations are consistent with the other study, which reported higher Ca²⁺in coarse than fine particulate matter²⁰. The mean concentration of Mg²⁺ was 0.1 and 0.6 µg/m³ in PM_2.5 and PM_10, respectively. Similarly, NO₃^- and SO₄^2- were observed with ~4.5 and 3.9 times higher concentrations in PM₁₀thanPM_2.5, respectively. The levels of NO₃^-were observed as 4.0 and 8.4 µg/m³ in PM_2.5 and PM₁₀. SO₄^2- was observed as 9.0 and 12.9 µg/m³ in PM_2.5 and PM₁₀, respectively. K⁺ showed ~ 1.8 times mean concentration in PM₁₀ (3.2 µg/m³) compared to PM_2.5 (1.4 µg/m³).

Variation of WSIS in PM_2.5 and PM₁₀

Several studies on water-soluble ionic species in particulate matter over Delhi and other cities of India have been carried out earlier^9,25-26. For example, Chandra et al.⁹ reported that the secondary inorganic aerosols (NO₃^-, SO₄^2- and NH₄⁺) contributed up to 85% of the annual average concentration.

In this study, the highest difference was observed forCa²⁺, which showed ~9 times higher mean concentration in PM₁₀ than PM_2.5 in the daytime(Fig.4). The levels for NH₄⁺, K⁺, and Mg²⁺ were observed as 1.6, 2.4, and 5.4 times higher in PM₁₀ than PM_2.5. For NO₃^- and SO₄^2- _, ~2.3 and 1.5 times higher mean concentrations were observed in PM₁₀ than PM_2.5. This suggested the dominance of Ca²⁺ and Mg²⁺among all species in coarse mode, while the dominance of NO₃^- and SO₄^2-in fine mode. In daytime, the trend of WSIS in PM_2.5 and PM₁₀was observed as SO₄^2->NO₃^-> NH₄⁺> K⁺> Ca²⁺> Mg²⁺and SO₄^2->NO₃^->Ca²⁺>K⁺ > NH₄⁺>Mg²⁺ respectively. Previous studies^27-28 also showed the dominance of SO₄^2-, NO₃^-and NH₄⁺ in PM_2.5 and PM₁₀.

During nighttime, the trends of WSIS in PM_2.5 and PM₁₀ were observed as SO₄^2->NO₃^-> NH₄⁺> K⁺> Ca²⁺> Mg²⁺ and SO₄^2->NO₃^->NH₄⁺> Ca²⁺>K⁺>Mg²⁺ respectively. Among cations,~ 5.3 times higher Ca²⁺ was observed in PM₁₀ compared to PM_2.5. Similarly, 2.2, 2.3 and, 3.2 times higher levels for NH₄⁺, K⁺, and Mg²⁺ were observed in PM₁₀ than PM_2.5.  NO₃^- and SO₄^2- also showed ~2 and 1.4 times higher levels in PM₁₀ than PM_2.5, respectively.

Figure 4: Mean Concentrations of WSIS During Day and Nighttime in PM2.5 and PM10. Click here to view Figure

Comparison of WSIS for the Year 2012 vs. 2013

A comparative study of WSIS in PM_2.5 and PM₁₀ was done for 2012 vs. 2013 (Fig. 5). In PM_2.5,daytime concentration difference between 2012 and 2013 was observed as the highest for NO₃^- (5.01 and 1.36 µg/m³) and the lowest for Mg²⁺ (0.16 and 0.07µg/m³). On the other hand, inPM_10, the day time difference between 2012 and 2013 was the highest forCa²⁺ (10.12 and 4.51 µg/m³) and the lowest for Mg²⁺ (0.93 and 0.47 µg/m³).

In the day time, all WSIS in PM_2.5 and PM₁₀ were found higher in 2012 compared to 2013 except K⁺. K⁺ in PM₁₀ showed comparable levels in both 2012 (2.64 µg/m³) and 2013 (2.73 µg/m³). In 2012, NH₄⁺, and Ca²⁺ in PM₁₀ were 3.53 and 10.12 µg/m³ respectively. Significantly high daytime Ca²⁺ in PM₁₀ suggested their dominance in coarse mode and favourable meteorological conditions due to solar, wind, and anthropogenic activities that induce mineral resuspension from the earth’scrust. The daytime NO₃^- and SO₄^2- in PM₁₀ were observed as 8.97 and 13.41 µg/m³  for 2012 and 5.78 and 12.18 µg/m³ for 2013,respectively. Moreover, daytime NO₃^- and SO₄^2- in PM_2.5  were observed as 5.01 and 10.31 µg/m³ for 2012 and 1.36 and 7.01 µg/m³ for 2013,respectively.

Figure 5: Mean Concentration of WSIS During Day and Nighttime in PM2.5 and PM10. Click here to view Figure

In the nighttime, levels of all WSIS were observed higher in 2012 compared to 2013 in PM_2.5 and PM₁₀. In PM_2.5, NH₄⁺, K⁺, and Ca²⁺ were found 3.67, 1.94 and 1.17 µg/m³ in 2012, and 1.33, 1.37 and 0.45 µg/m³ respectively in 2013. Moreover,in PM₁₀, levels of NH₄⁺, K⁺, and Ca²⁺ were found as 6.80, 3.80, and 5.60 µg/m³ in 2012, and 4.29, 3.67, and 2.89 µg/m³ respectively in 2013. The nighttime levels of NO₃^- and SO₄^2- in PM_2.5 were observed as 7.06 and 10.80 µg/m³  for 2012 and 2.45 and 7.70 µg/m³ respectively in 2013. Moreover, nighttime levels of NO₃^- and SO₄^2- in PM₁₀ were observed as 9.57 and 13.63 µg/m³ for 2012 and 9.36, and 12.27 µg/m³ respectively in 2013. These observations are analogous to another study that also reported a similar trend²⁰.

Seasonal Variation of WSIS During 2012 and 2013

Time series of WSIS fromJanuary 2012 to December 2013 are presented in Fig.6. The frequency of high peaks of WSIS was recorded during the winter months and low in monsoon months. This seasonal pattern might be due to change in meteorological parameters. K⁺ showed a clear peak in both fine and coarse mode particles during the post-monsoon time, which suggested their relation with biomass/crop residue burning in nearby states. Earlier studies also suggested high K⁺ concentrations during the biomass burning period. Ahigher concentration of other ionic species such as NH₄⁺, Ca²⁺, Mg²⁺, NO₃^- and SO₄^2- was observed in the winter. Compared to other seasons,the high concentration trend in winter might be due to prevailing calm wind conditions and lower mixing height, restricting the dilution of WSIS in the atmosphere. High Ca²⁺ was noticed during the summer season, with higher levels observed in PM₁₀ than PM_2.5. Ca²⁺peaks in summer might be attributed to higher wind speed and temperature, which facilitate soil resuspension in the atmosphere. The only SO₄^2- showed some peaks in the monsoon season, which might be due to their formation in fine mode.

Figure 6: Temporal Variability in Concentrations of WSIS in PM2.5 and PM10. Click here to view Figure

The concentration of Mg²⁺ has followed almost the same pattern of variation as Ca²⁺ in both PM_2.5 and PM₁₀ during all seasons. However, in post-monsoon seasons,Mg²⁺ showed some extra peaks due to Fire crackers burning in the Diwali festival. Moreover, higher levels of NH₄⁺, NO₃^- and SO₄^2- were observed during the post-monsoon and winter months.

The concentration of each season during both daytime and nighttime was estimated to observe the seasonal mean variation for both years(Fig. 7). In the winterdaytime,?cations and anions were observed as 5.69 and 17.40 µg/m³ in PM_2.5 and 15.08 and 32. 29 µg/m³ respectively in PM₁₀.Whereas in monsoon season, the concentration was as lowas 1.88 and 10.62 µg/m³ in PM_2.5 and 6.07 and 7.55 µg/m³, respectively in PM₁₀. Results indicated that during winter, cations concentration in PM_2.5 rises 3 times than that of monsoon, while in PM₁₀, the rise was onlyup to 2.5 times. Significantly higher concentrations of cations during daytime in winters than monsoon indicated accumulation of particles transported from the long-range area and local sources like biomass burning, transport, brick kiln, etc. The highest concentration of pollutants during the winter months was also reported in previous studies owing to the lowering of boundary layer height and effect of source strength^9,25. The concentration of Ca²⁺ during daytime summer (10.33 µg/m³) was noticed much higher compared to monsoon (4.39 µg/m³) in PM_10, while in PM_2.5,the concentrations in both seasons observed in a similar range (0.89 and 0.86 µg/m³ respectively). The higher Ca²⁺ was also reported in summer months (3.5 µg/m³) compared to winter (2.0 µg/m³) in PM₁₀ aerosols at Kanpur²⁹. During monsoon, a significant reduction of Ca²⁺ concentration in PM₁₀ might be due to efficient wash out due to coarse mode existence and efficiently settling down these particles during rain. Moreover, the higher average value of Ca²⁺observed in daytime summer compared to winter might be due to high wind flow that carries a large amount of soil dust whereas, during winter and monsoon, meteorological conditions suppressed dust resuspension (Fig. 7). In the present study,NO₃^- and SO₄^2-also showed significant seasonal variations in PM_2.5 and PM₁₀. A sharp reduction in the concentration of NO₃^- was observed in daytime summer just after winter in both PM_2.5 and PM₁₀ aerosols. For the winter and summer seasons, NO₃^- concentrations were observed as 7.51 and 1.16µg/m³ in PM_2.5 and 13.49 and 2.08 µg/m³ in PM₁₀,respectively. The sharp reduction in daytime summer concentration NO₃^-might be due to the photochemical oxidation process of NOx consumed to form secondary atmospheric pollutants. A different seasonal variation was noticed in SO₄^2- in different size particulate matter, where daytime maximum concentration was observed in the rainy season (10.62 µg/m³) in PM_2.5 and in winter (18.79 µg/m³) in PM₁₀. Higher SO₄^2- in monsoon might be due to their preferable formation in fine mode. An earlier study has also reported a higher percentage of SO₄^2- in monsoon than pre-monsoon during both day and nighttime²⁶. In the daytime, K⁺ showed the highest level during post-monsoon in PM_2.5 and PM₁₀, i.e., 2.14 and 4.40 µg/m³, respectively. The rise of K⁺ concertation in post-monsoon was attributed to large-scale crop residue burning by farmers in nearby states.

Figure 7: Day Time and Nighttime Variation of WSIS in PM2.5 and PM10 During Different Seasons. Click here to view Figure

At nighttime, the seasonal variation of all WSIS followed an almost similar trend with a slight difference in magnitude. The ?cations and ?anions levels in nighttime winter were observed 8.09 and 24.73 µg/m³ in PM_2.5and 15.95 and 37.18 µg/m³in PM₁₀, respectively. NH₄⁺ was observed as the highest in nighttime winter while the lowest in monsoon in PM_2.5 (5.81 and 0.17 µg/m³ respectively) and PM₁₀ (11.42 and 0.03 µg/m³) respectively. During summer and post-monsoon,NH₄⁺levels were observed as 1.72 and 3.33 µg/m³ in PM_2.5 and 2.87 and 8.92 µg/m³in PM₁₀, respectively. The highernighttime concentration of NH₄⁺ in winter, summer and post-monsoon might be due to lower mixing height compared to daytime (Fig.2). In nighttime, K⁺ concentrations were observed the highest in post-monsoon and least in monsoon in PM_2.5 (3.79 and 0.67 µg/m³ )and PM₁₀ (9.39 and 1.58 µg/m³)respectively.On the contrary, Ca²⁺ levels were noticed as the highest in summer season (0.95 and 7.24 µg/m³) in both PM_2.5 and PM₁₀respectively.  The highest level of K⁺ in post-monsoon was attributed to biomass burning while the highest level of Ca²⁺ in summer attributed to more active crustal sources. In this study, summer nighttimeCa²⁺was noticed slightly lower than daytime despite lower mixing height, which might be due to more soil resuspension in daytime. Similar to daytime, NO₃^- concentration during nighttime of summer observed with sharp reduction in its level compared to winter. Summer and winter night time levels of NO₃^- were observed as 1.71 and 12.64 µg/m³in PM_2.5 and 3.04 and 17.64 µg/m³in PM₁₀, respectively. Moreover, during night time of winter and summer seasons,SO₄^2- concentrations were observed as 12.09 and 6.89 µg/m³in PM_2.5 and 19.54 and 8.86 µg/m³in PM₁₀, respectively. SO₄^2-concertation in nighttime of monsoon were observed as 10.13 and 5.01 µg/m³ in PM_2.5 and PM₁₀respectively, suggesting the preferable formation of fine mode SO₄^2- compared to coarse mode.

Percent Distribution of WSIS in PM_2.5 and PM_2.5-10

An analysis to observe the dominance of WSIS in PM_2.5 and PM_2.5-10 was done. In this analysis PM_2.5 has been subtracted from PM₁₀, which gives PM_2.5-10. The percentage of Ca²⁺ in PM_2.5-10  was 89% and 81% during day and nighttime, respectively (Fig.8). Whereas, percentage distribution study of SO₄^2- reveals their dominance in PM_2.5. The fine mode SO₄^2- was found 68% and 71% during day and nighttime,respectively. A higher percentage in fine mode suggested their origin as secondary aerosol (via gas-to-particle conversion).

A comparative study on the sum of all cations and anions suggesteda higher percentage of cations in PM_2.5-10in daytime (73%)  andnighttime (63%) and a higher percentage of anions in fine mode in both daytime (59%) and nighttime (62%)respevtively.  Earlier studies also suggested the cations such as Ca²⁺ and Mg²⁺ of crustal origin and NO₃^- and SO₄^2- from anthropogenic sources. The fine mode existence of NH₄⁺ was noticed higher in the daytime (63%) compared to nighttime (45%). At the same time,K⁺ depicted a comparable percentage in fine mode during daytime (59%) and nighttime (56%).

Figure 8: The Percent Distribution of WSIS in PM2.5 and PM2.5-10. Click here to view Figure

NO₃^-/SO₄^2-mass ratios in PM_2.5 and PM₁₀

The mass ratio of NO₃^-/SO₄^2-has been widely used by several researchers as amarker for the relative contribution of mobile vs. stationary sources of nitrogen and sulphur species in the atmosphere^{17, 30}. The mass ratios (NO₃^-/SO₄^2?)at the study site suggestedthe dominance of source type. Ratio values>1 indicate the prevalence of mobile sources, whereas the mass ratios value<1 suggeststhe dominance of stationary sources³¹. In this study, the seasonal average mass ratios of NO₃^-/SO₄^2- in PM_2.5 and PM₁₀were observed in daytime and nighttime (Fig.9). The mass ratioswere found in the range of 0.1- 1.3 during different seasons of 2012 and 2013.In general,NO₃^-/SO₄^2- mass ratios showedcomparatively higher values in winter and post-monsoon whereas least values in summer and monsoonperiod. The results of this ratio analysis suggestedthedominance of stationary sources over mobilesources in the winter period, which might be facilitated byprevailing calm wind conditions.

Figure 9: Seasonal Variation of Mass Ratios of NO3-/SO42- in PM2.5  and PM10 During Day and Nighttime.  Click here to view Figure

Behaviour of Reactive Nitrogen During Day and Nighttimein PM_2.5 and PM₁₀

The acidic species such as H₂SO₄ and HNO₃ are considered secondary air pollutantsformed in the atmosphere viathe oxidation process of their primary gaseous precursors (SO₂ and NOx) in the atmosphere³².The neutralization of these acidic species in the atmosphere is done by alkaline ions such as NH₄⁺ and Ca²⁺. Therefore, the availability and correlation among these acidic and alkaline species decide the acidic/alkaline nature of dry/wet deposition.

Therefore, a correlation study between nitrogenous cation (NH₄⁺) and anion (NO₃^-) in both during the day and nighttime in PM_2.5 and PM₁₀, was done (Fig.10). In PM_2.5, the correlation coefficient (r) between NH₄⁺ and NO₃^- was observed higher in daytime (r= 0.84) compared to nighttime (r=0.67). On the contrary, in PM₁₀ higher correlation was observed between NH₄⁺ and NO₃^- in nighttime (r=0.73) compared to daytime (r= 0.67). The result suggested that fine mode NH₄⁺ has a higher affinity with NO₃^- in the daytime, whereas coarse mode NH₄⁺ showed a higher affinity with NO₃^- in the nighttime. These results suggested that these ions were secondary in nature, and NH₄⁺ mainly existed as (NH₄)₂SO₄ and NH₄NO₃ during the day and nighttime.

Figure 10: Correlation Between NH4+ and NO3- During Day and Nighttimein PM2.5 and PM10. Click here to view Figure

Principal Component Analysis of WSIS in PM_2.5 and PM₁₀

To study the origin and sources of major water-soluble inorganic species, Principal Component Analysis (PCA) was performed. PCA is a multivariate technique used to convert data intoasmall dataset of theindependent variable or principal components (PCs)³³. Factor loading >0.50 was included for the source apportionment. Data analysis for PM₁₀ and PM_2.5 was performed during theday and nighttime for 2012-2013 (Table 1). For coarse fraction analysis,a total of 3 PCs were extracted, explaining 87.4% and 91.81 % of data during the day and nighttime, respectively. In PM₁₀,no significant difference was observed during the day and nighttime PCA analysis. PC1 in both day and nighttime explained maximum emission,reflectingthesignificantcontribution from anthropogenic sources of air pollution, i.e., SO₂ and NO_x, which eventually converts into secondary aerosol formation. PC2 explained ~ 31 % data and showed high correlations of Ca²⁺and Mg²⁺. Therefore, this PC indicates emissions from crustal sources or windblown dust. PC3 explained ~ 10 % data during theday and nighttime with the correlation between NH₄⁺, K⁺, and Ca²⁺ indicating biomass burning and secondary aerosol formation.

For the fine mode particles,a total of3 & 4 PCs were extracted during the day and nighttime, respectively, explaining >80 % data. The first PC explained maximum data with high correlations of SO₄^2- and NO₃^- with NH₄⁺ and K⁺ indicating anthropogenic emissions including vehicular and biomass burning and consecutive secondary aerosol formation. During thedaytime, PC2 explained 30% data with high correlations of  Ca₂⁺ and Mg₂⁺ indicating crustal sources. But during nighttime, PC2 showed a positive correlation of Ca²⁺ and Mg²⁺and a negative correlation of NH₄⁺ and NO₃^-. This suggested crustal sources and secondary aerosol formation at low temperatures in the fine mode particles. Similarly, PC3 showed anegative correlation between K⁺ and SO₄^2- with Ca²⁺ during the daytime, whereas K⁺was negatively correlated with Ca²⁺ and SO₄^2-during nighttimein PC4. This suggested biomass burning and secondary aerosol formation. These results indicated that in the fine mode particles, secondary aerosol formations are more prominent than coarse mode, and at low temperature, SO₂ and NO_x also participate in other chemical conversions than their sulphate and nitrate salts.

Table 1: Principal Component Analysis of WSIS in PM_2.5& PM₁₀

Conclusions

Two years of continuous study ofwater-soluble ionic species in PM_2.5 andPM₁₀ at a site in central Delhi provides comprehensive data on thebehaviour of these species during day and night time.  The results showed that the mean levels of both ?cations and ?anions were higher in nighttimethan daytime in PM_2.5 and PM₁₀. Furthermore, it revealedthe higher average concentration of Ca²⁺ and Mg²⁺ in the daytime compared to nighttime, while higher values of K⁺, NH₄⁺, NO₃^- and SO₄^2- in nighttimecompared to daytime in PM_2.5 and PM_10. A higher daytime Ca²⁺ indicated the role of active crustal sources.  While thelower daytime value of NO₃^- might be due to thephotochemical oxidation process of NOx in thedaytime.  Moreover, the differentlevels in day and nighttimewere noticed higher in PM_2.5 compared to PM₁₀. Among anions,significantvariation was observed in NO₃^-, showing ~ 49% higher in the nighttime than daytime, while for SO₄^2- only ~9% higher values were noticed in thenighttime. The higher variation of NO₃^- level might be due to their active participation in thedaytime photo-oxidation process. The higher day and nighttime difference of Ca²⁺ and Mg²⁺wereobserved in PM₁₀ compared to PM_2.5, suggesting their dominance in thecoarse mode. In PM₁₀, the highest Ca²⁺ concentration was observed in daytime summer and least in nighttime monsoon,indicating the role of loose soil and meteorological conditions. The higher daytime SO₄^2- concentration in monsoon compared to summer indicatedthe preferable formation of secondary aerosols in fine mode.  A highcorrelation was observed between NO₃^- and NH₄⁺ in fine and coarse mode particlesin the day and nighttime.Also, NH₄⁺levels were found to be in good correlation to SO₄^2- and NO₃^-,indicatingthe role of secondary aerosolformation. In both day and nighttime, NH₄⁺ mainly existed as(NH₄)₂SO₄ and NH₄NO₃.

Acknowledgements

The authors are thankful to the Director, CSIR-NPL, New Delhi,for encouraging and support this study. Subhash Chandra is grateful for his CSIR Fellowship (JRF and SRF) for carrying out this work, and Ruchi Singh is grateful for her DST fellowship(SRF) during the study period(Grant No. GAP-113332). Partial funding from Grant No. PSC-0112 for consumables is gratefully acknowledged.

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