Investigating Day and Night Time Variability of Major Water- Soluble Inorganic Species and Role of Reactive Nitrogen Species In PM2.5 and PM10-A Two Year Study

© 2021 The Author(s). Published by Enviro Research Publishers. This is an Open Access article licensed under a Creative Commons license: Attribution 4.0 International (CC-BY). Doi: http://dx.doi.org/10.12944/CWE.16.Special-Issue1.03 Abstract The study reports day and nighttime temporal variation of prominent water-soluble inorganic species (WSIS) of fine and coarse mode particulate matter at a site in central Delhi for 2012 and 2013. The two years’ concentrations of major ionic species (NH4, K+, Ca2+ and Mg2+ and NO3, SO4) showed that mean levels of both ∑cations and ∑anions were higher in nighttime than daytime in both PM2.5 and PM10. Moreover, the difference of mean levels in daytime and nighttime was noticed higher in PM2.5 compared to PM10. Among anions, more variation was observed for NO3, i.e., ~49% higher values in nighttime compared to daytime, while~9% higher values in the nighttime for SO4. The Higher variation of NO3 level might be due to active participation of NOx in daytime photo-oxidation process. Among cations,Ca2+ and Mg2+ showed higher concentration in daytime compared to nighttime. Higher day and nighttime variation of Ca2+ and Mg2+ in coarse particulate matter suggested their dominance in PM10 compared to PM2.5. 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 concentration in monsoon compared to summer showed its significant formation as secondary aerosols in fine mode. The reactive nitrogen species NO3 and NH4 were correlated during the day and nighttime in PM2.5 and PM10. Moreover, NH4 was highly correlated to SO4 and NO3, mainly in the form of (NH4)2SO4 and NH4NO3. Investigating Day and Night Time Variability of Major WaterSoluble Inorganic Species and Role of Reactive Nitrogen Species In PM2.5 and PM10-A Two Year Study

. The Higher variation of NO 3 level might be due to active participation of NOx in daytime photo-oxidation process. Among cations,Ca 2+ and Mg 2+ showed higher concentration in daytime compared to nighttime. Higher day and nighttime variation of Ca 2+ and Mg 2+ in coarse particulate matter suggested their dominance in PM 10 compared to PM 2.5 . In PM 10 , the highest Ca 2+ concentration was observed in daytime summer and least in nighttime monsoon, indicating the role of loose soil and meteorological conditions. The higher daytime SO 4 2concentration in monsoon compared to summer showed its significant formation as secondary aerosols in fine mode. The reactive nitrogen species NO 3 and NH 4 + were correlated during the day and nighttime in PM 2.5 and PM 10 . Moreover, NH 4 + was highly correlated to SO 4 2-and NO 3 -, mainly in the form of (NH 4 ) 2 SO 4 and NH 4 NO 3 .

Investigating Day and Night Time Variability of Major Water-Soluble Inorganic Species and Role of Reactive Nitrogen Species In PM 2.5 and PM 10 -A Two Year Study
SUBHASH CHANDRA 1 , MONIKA J KULSHRESTHA 2 * and RUCHI SINGH 3

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][2][3] Moreover, it can also affect human health adversely and may also alter the ecosystem. [4][5][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][10][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][13][14][15][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][18][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 for PM 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 in PM 2.5 and PM 10 aerosols at a site in central Delhi, have been discussed.

Methodology Site Description
The samples (PM 10 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.

Sampling of PM 10 and PM 2.5
High and low volume samplers were used for collecting PM 10 (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 collect samples. The samples were stored in desiccators for 24 h before and after sample collection. Respirable particle sampler (PM 10 , 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. PM 10 sampler was operated in the flow range of 0.9-1.4 m 3 /min while PM 2.5 sampler at a 1 m 3 /hr flow rate.

Analysis
For the analysis of WSIS, filter cuts (0.536 cm 2 area) of collected samples were extracted in de-ionized water (Millipore, specific resistance: 18.2 MΩ-cm) using a sonicator for 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 + , NH 4 + ,Ca 2+ and Mg 2+ ) and anions (Cl -,SO 4 2-and NO 3 -) in PM 2.5 and PM 10 aerosols were determined using Ion Chromatograph (Metrohm 883). Similarly, the concentrations of these water-soluble ionic species were also estimated in blank filters. Every time, before the analysis, a three-point calibration curve was also achieved for each species (R 2 =1).

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 lowest in winter and highest in summer months. The highest monthly mean value of T was observed in May, while minima were recorded in January. The monthly mean of T varied from approximately 10 o C to 40 o C. 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).

Results and Discussion
Annual Mean Day and Nighttime WSIS in PM 2.5 and PM 10 Fig.3 shows two years'average of mass concentrations of major ionic species in fine and coarse particles. Almost similar levels of Ca 2+ and Mg 2+ were observed during the day and nighttime in fine mode, while in coarse mode, significant variation with higher daytime values were observed. Daytime and nighttime levels of Ca 2+ in PM 10 were observed as 7.32 and 4.25 µg/m 3 , respectively. The higher daytime levels might be due to more resuspension of crustal Ca 2+ due to agricultural and commercial activities, as the soil is the prime source of Ca 2+ . On the contrary, higher levels of NH 4 + and K + were observed in nighttime compared to daytime in both PM 2.5 and PM 10 among cations. NH 4 + was observed as 1.49 µg/m 3 during the day and 2.50 µg/m 3 during the nighttime in fine mode while 2.39 and 5.54 µg/m 3 , respectively in the coarse mode. A similar trend was also reported from other study, 20 which reported higher nighttime levels of NH 4 + 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 in fine 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,NO 3 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 PM 10 , respectively. The high concentrations of NO 3 during nighttime might be due to the formation of NO 3 through hydrolysis of N 2 O 5 under high humidity conditions and stability of NH 4 NO 3 at a lower temperature during nighttime. [21][22] Similarly, low concentrations of NO 3 during daytime might be due to evaporative loss of NH 4 NO 3 at higher temperature. [23][24] A similar trend was also reported in an earlier study at Kanpur, 25 with a higher NO 3 in nighttime (12.9 µg/m 3 ) than daytime (5.4 µg/m 3 ) in PM 10 . Day and nighttime levels of SO 4 2were 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.
The two years mean (daytime + nighttime) concentration difference in PM 2.5 and PM 10 was observed the highest for Ca 2+ while lowest for Mg 2+ . Around 5 times higher concentration of Ca 2+ was observed in PM 10 (5.8 µg/m 3 ) compared to PM 2.5 (0.8 µg/m 3 ). These observations are consistent with the other study, which reported higher Ca 2+ in coarse than fine particulate matter. 20 The mean concentration of Mg 2+ was 0.1 and 0.6 µg/m 3 in PM 2.5 and PM 10 , respectively. Similarly, NO 3 and SO 4 2were observed with ~4.5 and 3.9 times higher concentrations in PM 10 than PM 2.5 , respectively. The levels of NO 3 were observed as 4.0 and 8.4 µg/m 3 in PM 2.5 and PM 10 . SO 4 2was observed as 9.0 and 12.9 µg/m 3 in PM 2.5 and PM 10 , respectively. K + showed ~ 1.8 times mean concentration in PM 10 (3.2 µg/m 3 ) compared to PM 2.5 (1.4 µg/m 3 ).

Variation of WSIS in PM 2.5 and PM 10
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 (NO 3 -, SO 4 2and NH 4 + ) contributed up to 85% of the annual average concentration.
In this study, the highest difference was observed for Ca 2+ , which showed ~9 times higher mean concentration in PM 10 than PM 2.5 in the daytime (Fig.4) 4 2also showed ~2 and 1.4 times higher levels in PM 10 than PM 2.5 , respectively. These observations are analogous to another study that also reported a similar trend. 20

Seasonal Variation of WSIS during 2012 and 2013
Time series of WSIS from January 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 near by states. Earlier studies also suggested high K + concentrations during the biomass burning period. A higher concentration of other ionic species such as NH 4 + , Ca 2+ , Mg 2+ , NO 3 and SO 4 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 Ca 2+ was noticed during the summer season, with higher levels observed in PM 10 than PM 2.5 . Ca 2+ peaks in summer might be attributed to higher wind speed and temperature, which facilitate soil resuspension in the atmosphere. The only SO 4 2showed some peaks in the monsoon season, which might be due to their formation in fine mode. The concentration of Mg 2+ has followed almost the same pattern of variation as Ca 2+ in both PM 2.5 and PM 10 during all seasons. However, in post-monsoon seasons, Mg 2+ showed some extra peaks due to Firecrackers burning in the Diwali festival. Moreover, higher levels of NH 4 + , NO 3 and SO 4 2were 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 winter day time, ∑cations and ∑anions were observed as 5.69 and 17.40 µg/m 3 in PM 2.5 and 15.08 and 32. 29 µg/m 3 in PM 10 respectively. Whereas in monsoon season, the concentration was as low as 1.88 and 10.62 µg/m 3 in PM 2.5 and 6.07 and 7.55 µg/m 3 , in PM 10 respectively. Results indicated that during winter, cations concentration in PM 2.5 rises 3 times than that of monsoon, while in PM 10 , the rise was only up 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 Ca 2+ during daytime summer (10.33 µg/m 3 ) was noticed much higher compared to monsoon (4.39 µg/m 3 ) in PM 10 , while in PM 2.5 , the concentrations in both seasons observed in a similar range (0.89 and 0.86 µg/m 3 respectively). The higher Ca 2+ was also reported in summer months (3.5 µg/m 3 ) compared to winter (2.0 µg/m 3 ) in PM 10 aerosols at Kanpur. 29 During monsoon, a significant reduction of Ca 2+ concentration in PM 10 might be due to efficient washout due to coarse mode existence and efficiently settling down of these particles during rain. Moreover, the higher average value of Ca 2+ 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, NO 3 and SO 4 2also showed significant seasonal variations in PM 2.5 and PM 10 . A sharp reduction in the concentration of NO 3 was observed in daytime summer just after winter in both PM 2.5 and PM 10 aerosols. For the winter and summer seasons, NO 3 concentrations were observed as 7.51 and 1.16 µg/m 3 in PM 2.5 and 13.49 and 2.08 µg/m 3 in PM 10 , respectively. The sharp reduction in daytime summer concentration NO 3 might be due to the photochemical oxidation process of NOx consumed to form secondary atmospheric pollutants. A different seasonal variation was noticed in SO 4 2in different size particulate matter, where daytime maximum concentration was observed in the rainy season (10.62 µg/m 3 ) in PM 2.5 and in winter (18.79 µg/m 3 ) in PM 10 . Higher SO 4 2in monsoon might be due to their preferable formation in fine mode. An earlier study has also reported a higher percentage of SO 4 2in monsoon than pre-monsoon during both day and nighttime. 26 In the daytime, K + showed the highest level during post-monsoon in PM 2.5 and PM 10 , i.e., 2.14 and 4.40 µg/m 3 , respectively. The rise of K + concertation in post-monsoon was attributed to large-scale crop residue burning by farmers in nearby states.
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/m 3 in PM 2.5 and 15.95 and 37.18 µg/m 3 in PM 10 , respectively. NH 4 + was observed as the highest in nighttime winter while the lowest in monsoon in PM 2.5 (5.81 and 0.17 µg/m 3 respectively) and PM 10 (11.42 and 0.03 µg/m 3 ) respectively. During summer and post-monsoon, NH 4 + levels were observed as 1.72 and 3.33 µg/m 3 in PM 2.5 and 2.87 and 8.92 µg/m 3 in PM 10 , respectively. The higher nighttime concentration of NH 4 + 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/m 3 )and PM 10 (9.39 and 1.58 µg/ m 3 ) respectively. On the contrary, Ca 2+ levels were noticed as the highest in summer season (0.95 and 7.24 µg/m 3 ) in both PM 2.5 and PM 10 respectively. The highest level of K + in post-monsoon was attributed to biomass burning while the highest level of Ca 2+ in summer attributed to more active crustal sources. In this study, summer nighttime Ca 2+ was noticed slightly lower than daytime despite lower mixing height, which might be due to more soil resuspension in daytime. Similar to daytime, NO 3 concentration during nighttime of summer observed with sharp reduction in its level compared to winter. Summer and winter nighttime levels of NO 3 were observed as 1.71 and 12.64 µg/m 3 in PM 2.5 and 3.04 and 17.64 µg/ m 3 in PM 10 , respectively. Moreover, during nighttime of winter and summer seasons, SO 4 2concentrations were observed as 12.09 and 6.89 µg/m 3 in PM 2.5 and 19.54 and 8.86 µg/m 3 in PM 10 , respectively. SO 4 2concertation in nighttime of monsoon were observed as 10.13 and 5.01 µg/m 3 in PM 2.5 and PM 10 respectively, suggesting the preferable formation of fine mode SO 4 2compared 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 PM 10 , which gives PM 2.5-10 . The percentage of Ca 2+ in PM 2.5-10 was 89% and 81% during day and nighttime, respectively (Fig.8). Whereas, percentage distribution study of SO 4 2reveals their dominance in PM 2.5 . The fine mode SO 4 2was 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).

Fig. 7: Day time and night time variation of WSIS in PM 2.5 and PM 10 during different seasons
A comparative study on the sum of all cations and anions suggested a higher percentage of cations in PM 2.5-10 in daytime (73%) and nighttime (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 Ca 2+ and Mg 2+ of crustal origin and NO 3 and SO 4 2from anthropogenic sources. The fine mode existence of NH 4 + 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%). CHANDRA etal., Curr. World Environ., Vol. 16 (Special Issue 1) 16-28 (2021)

NO 3 -/ SO 4 2-Mass Ratios in PM 2.5 and PM 10
The mass ratio of NO 3 -/SO 4 2has been widely used by several researchers as a marker for the relative contribution of mobile vs. stationary sources of nitrogen and sulphur species in the atmosphere, 17,30 The mass ratios (NO 3 -/SO 4 2− ) at the study site suggested the dominance of source type. Ratio values>1 indicate the prevalence of mobile sources, whereas the mass ratios value<1 suggests the dominance of stationary sources. 31 In this study, the seasonal average mass ratios of  Behaviour of Reactive Nitrogen during Day and Nighttime in PM 2.5 and PM 10 The acidic species such as H 2 SO 4 and HNO 3 are considered secondary air pollutants formed in the atmosphere via the oxidation process of their primary gaseous precursors (SO 2 and NOx) in the atmosphere. 32 The neutralization of these acidic species in the atmosphere is done by alkaline ions such as NH 4 + and Ca 2+ . 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 (NH 4 + ) and anion (NO 3 -) in both during the day and nighttime in PM 2.5 and PM 10 , was done (Fig.10).
In PM 2.5 , the correlation coefficient (r) between NH 4 + and NO 3 was observed higher in daytime (r= 0.84) compared to nighttime (r=0.67). On the contrary, in PM 10 higher correlation was observed between NH 4 + and NO 3 in nighttime (r=0.73) compared to daytime (r= 0.67). The result suggested that fine mode NH 4 + has a higher affinity with NO 3 in the daytime, whereas coarse mode NH 4 + showed a higher affinity with NO 3 in the nighttime. These results suggested that these ions were secondary in nature, and NH 4 + mainly existed as (NH 4 ) 2 SO 4 and NH 4 NO 3 during the day and nighttime.  . This suggested crustal sources and secondary aerosol formation at low temperatures in the fine mode particles. Similarly, PC3 showed a negative correlation between K + and SO 4 2with Ca 2+ during the daytime, whereas K + was negatively correlated with Ca 2+ and SO 4 2during nighttime in 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, SO 2 and NOx also participate in other chemical conversions than their sulphate and nitrate salts.

Conclusions
Two years of continuous study of water-soluble ionic species in PM 2.5 and PM 10 at a site in central Delhi provides comprehensive data on the behaviour of these species during day and nighttime. The results showed that the mean levels of both ∑cations and ∑anions were higher in nighttime than daytime in PM 2.5 and PM 10 . Furthermore, it revealed the higher average concentration of Ca 2+ and Mg 2+ in the daytime compared to nighttime, while higher values of K + , NH 4 + , NO 3 and SO 4 2-in nighttime compared to daytime in PM 2.5 and PM 10 . A higher daytime Ca 2+ indicated the role of active crustal sources. While the lower daytime value of NO 3 might be due to the photochemical oxidation process of NOx in the daytime. Moreover, the different levels in day and nighttime were noticed higher values in PM 2.5 compared to PM 10 . Among anions, significant variation was observed in NO 3 , showing ~49% higher values in the nighttime than daytime, while for SO 4 2only ~9% higher values were noticed in the nighttime. The higher variation of NO 3 level might be due to their active participation in the daytime photo-oxidation process. The higher day and nighttime difference of Ca 2+ and Mg 2+ were observed in PM 10 compared to PM 2.5 , suggesting their dominance in the coarse mode. In PM 10 , the highest Ca 2+ concentration was observed in daytime summer and least in nighttime monsoon, indicating the role of loose soil and meteorological conditions. The higher daytime SO 4 2concentration in monsoon compared to summer indicated the preferable formation of secondary aerosols in fine mode. A high correlation was observed between NO 3 and NH 4 + in fine and coarse mode particles in the day and nighttime. Also, NH 4 + levels were found to be in good correlation to SO 4 2-and NO 3 -, indicating the role of secondary aerosol formation. In both day and nighttime, NH 4 + mainly existed as(NH 4 ) 2 SO 4 and NH 4 NO 3 .