Extreme Air Pollution Events Spiking Ionic Levels at Urban and Rural Sites of Indo-Gangetic Plain

The present study is a modest attempt to quantify the water-soluble inorganic ions (WSIIs) in the early morning (fogforming) ambient air on the central Indo-Gangetic Plain (IGP) during winter via dissolution using a low-cost refluxing mist chamber. The chemical composition of the bulk samples showed significant differences in the WSIIs between the two monitored sites, one urban and the other rural, with NH4 (47% and 37%, respectively) and Ca (18% and 8%, respectively) being the major cations and NO3 (10% and 16%, respectively) and SO4 (9% and 14%, respectively) being the major anions. The WSII concentration spiked during November at both locations (∑WSII = 159.6 and 141.9 μg m) due to two extreme air pollution events, viz., i) the burning of crop residue and ii) the Diwali festival. These changes, which were corroborated by observations of the meteorological conditions, played an essential role in the wintertime atmospheric chemistry. On foggy days, significant scavenging of ions associated with crustal dust (Ca, Na, Mg and Cl) occurred, although the ambient concentrations of other species (K, NH4, NO2, NO3 and SO4) remained relatively unaffected. Furthermore, the relationship between the cations and anions demonstrated that NH3 and HONO, as the primary gaseous species, were involved in heterogeneous aqueous-phase reactions and the formation of secondary aerosols. Source apportionment based on principal component analysis and the mass ratios also indicated that local anthropogenic sources, in addition to natural ones such as soil/road dust and biogenic emissions, influenced the WSII composition. Vehicular exhaust, solid waste and biofuel burning were identified as the major contributors of WSIIs at the urban site, whereas biomass burning, agricultural activity and coal combustion in brick kilns were the predominant sources at the rural site.


INTRODUCTION
The onset of winter season in the Indo-Gangetic Plain (IGP) region has been found to be associated with the formation of a thick blanket of brown clouds within two to three kilometers above the surface (Ramanathan et al., 2005;Chung and Ramanathan, 2006;Nakajima et al., 2007). This seasonal phenomenon is an outcome of the complex interplay between various meteorological and anthropogenic factors. The meteorological factors mainly include calm winds, lower mixing height, temperature inversion, etc. They restrict the dispersion of pollutants and leads to their accumulation in the lower atmosphere (Nair et al., 2007;Gautam, 2014;Pani and Verma, 2014;David et al., 2011;Sen et al., 2017). As to the anthropogenic factors, the increased emissions of harmful pollutants from residential, industrial and transport sectors to meet the energy demands of this densely populated region undermines air quality Pan et al., 2015;Liu et al., 2018). Moving further, another vital connection between the deteriorating air quality and certain extraneous extreme pollution episodes such as stubble burning in the states falling upwind of the IGP and the use of firecrackers during the celebration of Diwali festival needs to be factored in while accounting for the overall atmospheric profile of the region during the winter season (Kulshrestha et al., 2004;Singh et al., 2010;Perrino et al., 2011;Jain et al., 2014;Singh and Kaskaoutis, 2014;Izhar et al., 2018). These cumulatively result in the reduction of visibility, poor air quality and jeopardizing of health indicators (Seaton et al., 1995;Gautam et al., 2007;Balakrishnan et al., 2019).
Furthermore, the winter season is characterized by higher moisture content in the form of fog which provides a potent medium for the chemical reactions of different atmospheric organic and inorganic species (Jain et al., 2000;Rajput et al., 2016;Tie et al., 2017;Bharali et al., 2019). As for the latter, significant concentrations of major inorganic ions have been reported in the non-precipitating clouds and fog (Jacob and Hoffmann, 1983;Jacob et al., 1984;Ram et al., 2012;Straub et al., 2012). Longer residence time and higher scavenging efficiency through aqueous-phase reactions explain the reason behind high concentrations of inorganic ions in the cloud and fog water (Jacob and Hoffmann, 1983;Fridlind and Jacobson, 2000). The ambient air chemical composition during winters is assumed to be determined by the following factors: (1) the composition of the activated cloud condensation nuclei (CCN) on which the water droplet condenses, (2) the absorption of atmospheric gases at the droplet's surface, and (3) the subsequent aqueous-phase reactions of homogeneous and heterogeneous species. Such physical as well as chemical interactions among water droplets, gases and aerosols in the ambient air invariably alter the regional-scale atmospheric chemistry (Seinfeld and Pandis, 1998;Raes et al., 2000).
Previous studies conducted in IGP have extensively reported the concentration of ions either in fog water or in the aerosols and gases separately in the atmosphere, which provide vital information regarding the different kinds of phase chemistry during winter period (Kulshrestha et al., 2005;Kaul et al., 2011;Gupta and Mandariya, 2013;Agarwal et al., 2017;Ghude et al., 2017;Nath and Yadav, 2018;Rajput et al., 2018;Izhar et al., 2020). But there are certain interactions going on in the ambient air, particularly during winters owing to aqueous-phase chemistry, which simultaneously includes interactions of gases and aerosol with fog droplets (Waldman et al., 1982;Jacob et al., 1984). Furthermore, the presence of fog, on the one hand, scavenge the aerosols through their dissolution in large droplets and on the other hand also enhance the formation of secondary aerosols, which accentuates the need to look into the overall chemical composition of dominant atmospheric species (Lopez-Hilfiker et al., 2014;Rao and Vejerano, 2018). Ionic composition regulates atmospheric acidity, which helps us understand the partitioning of semi-volatile water-soluble compounds between particulates and gas phase. In such a scenario, the study of bulk air mass samples will be helpful in characterizing the water-soluble inorganic ionic (WSII) species and major reaction chemistry occurring during fog-forming hours in ambient air. The present study has following research objectives: (1) to quantify overall (bulk) concentrations of WSIIs and monthly variation at different land use sites during fog-forming hours in winters, (2) to assess the impact of fog scavenging and ionic interactions of different WSIIs in ambient air and (3) to identify major sources of WSIIs using principal component analysis (PCA) model at different sites of the IGP.

Sampling Sites
IGP region is known for its very high population density and stationary sources of pollutant emissions in the environment. Keeping in view the different land use patterns, two sites have been selected: Prayagraj as an urban site and Madhupur, a village in Pratapgarh district of UP, as a rural site as shown in Fig. 1.

Prayagraj (an Urban Site)
Prayagraj (25°54′N, 81°85′E), formerly known as Allahabad, is the administrative headquarters and among one of the most populous cities of Uttar Pradesh state. Located in the southern Uttar Pradesh, at the confluence of the Ganga and Yamuna, the city's area covers 70.5 km 2 area. As per the census 2011, the district has a population density of 1,086 per km 2 . National Highway-130 passes through the city and attracts very high traffic density. Allahabad Development Authority (ADA) continuously focuses on the planning of the suburban areas of the city. As a result, in 2011, it was ranked the world's 130 th fastestgrowing city. Within the city, sampling was conducted in the Phaphamau area, located in the northern part of the city. The sampler was mounted at 1.5 m above the surface on the terrace of a residential building in a colony situated nearby the highway.

Madhupur (a Rural Site)
Madhupur village (25°98′N, 82°04′E) were selected as rural site, which is in Pratapgarh, a district adjacent to the Prayagraj. Madhupur site is located 70 km away in the northeastern side of Prayagraj. Sai and the Ganges are the main rivers flowing through Pratapgarh district. As per the census 2011, the district has a population density of 854 per km 2 . The site comes under one of the most backward districts of India. Primarily an agrarian district, Pratapgarh has risen in ranks as the top producer of Aonla or Amla fruit, which is used as herbal medicine. The village is poorly developed as hardly any roads pass through the area with two brick kiln units within 5 km 2 of the site. It has very low vehicle density and high vegetation cover. Biomass burning, soil resuspension, and agricultural activities are the main source of pollution in the area. Sampling was conducted on the terrace of a twofloored house. Similar to the urban site, the fog sampler was mounted approximately 1.5 m above the surface of the terrace.

Sample Collection Using Mist Chamber
Collection of ambient air bulk samples during winter months was carried out using customized low-cost refluxing mist chambers. Very similar mist chamber has been used by Jain et al. (1999). A schematic design of the glass mist chamber used in the current study has been shown in Fig. 2. The air inlet sucks the air directly into the mist-forming spherical glass chamber, filled with collecting solution. The spherical glass chamber has a capacity of 250 mL and it consists of an air inlet, a nebulizing nozzle, and a port for the addition and removal of the solution. The collecting solution (Milli-Q water) is drawn from the reservoir into the nozzle, where impaction with the incoming air forms a fine aqueous mist. The fine aqueous drops provide a large surface area for efficient extraction of water-soluble compounds from the air. The mist is drawn upward through the mist chamber where aqueous droplets containing scrubbed atmospheric species collide and are refluxed into the reservoir. The upper side of the mist chamber was designed with a few bends, which help to reduce the sample loss and evaporation of the solution. It was connected to the well-calibrated rotameter to maintain the flow rate, which was further connected to the vacuum pump through a silicon tube. Though the flow rate was maintained at 15 L min -1 , the pressure drop at each stage was taken into the account (between 12 and 14 L min -1 ), which was corrected while data calculation and analysis.
The use of such type of mist chambers have been reported in various previous studies, which have shown a very high collection efficiency for gases (Stratton et al., 2001;Spaulding et al., 2002;François et al., 2005;Hennigan et al., 2018). More recently, as in the present study, the mist chambers have also been used for particle plus gas analysis (King and Weber, 2013). Here, the bulk samples which are collected through the incorporation of all the water-soluble components entering with the air stream into the mist drops and their subsequent dissolution in water represents the total concentration of a particular chemical species present in ambient air. The measurement of the collection efficiency of the sample collector was performed with the cations and anions analysis using ion chromatography. Two identical mist chambers were operated in a series similar to prior studies for characterizing their collection efficiency (Spaulding et al., 2002;Hennigan et al., 2018). Collection efficiency (CE) was calculated using the following formula: where C2 and C1 are the concentrations of the analyte in first and second sampler. For most of the analyzed species, the collection efficiency was found to be very high (> 95%), which strengthened the robust nature of the method. Replicate samples were also collected at each site. The result showed a difference of about 12-18% for all the cations and anions, which is within acceptable limits. However, a slight variation in replicate samples can be attributed to the changing dynamics of analytes in the atmosphere.
Since the winter period, particularly morning hours, has been evident of very high relative humidity (RH), most of the soluble gaseous species also get dissolved onto the water droplets or more precisely on fog droplets and alters atmospheric chemistry, which is the primary reason to use mist chamber during winter months. Along with efficient scrubbing of aerosols, mist chamber also allows the gas-and aqueous-phase diffusion of analytes onto the mist droplets inside the glass chamber. Most of the studies which focus on fog chemical characterization use condensation method, which ultimately gives the species concentrations in per volume of fog (in µeq L -1 ). However, the current study includes the overall concentration of measured chemical species in ambient air (µg m -3 ) during winters from November 2016 to February 2017. Considering the fog-forming hours in India, the sampling was conducted for 5 h from 03:00 to 08:00. About 60 mL of deionized Milli-Q water was used as collecting solution. Due to high RH, water loss through evaporation was very less and for most of the time it was less than 10 mL, which was corrected before analysis. After the collection of samples, glass chamber was rinsed (20 mL × 3) every time carefully to eliminate contamination in samples through the detachable lower part. Total 41 and 22 samples were collected at urban and rural sites, respectively. Due to limited power supply in villages, sample size was less at the latter site. Samples were stored using thymol as a preservative and kept in a refrigerator at -4°C to avoid any chemical alterations. Meteorological parameters were reported on a regular basis from online website wunderground.com. Wind speed, temperature and RH are primarily considered for the study.

Analysis on Ion Chromatography
Five major cations (Na + , NH 4 + , K + , Ca 2+ and Mg 2+ ) and four major anions (Cl -, NO 2 -, NO 3and SO 4 2-) in the bulk samples were analyzed using ion chromatography (883 Basic IC Plus; Metrohm). Cation column (Metrosep A Supp 4, 250/4.0; Metrohm) was used for the determination of cations using a mixture of 1.7 mmol L -1 NaHCO 3 and 1.8 mmol L -1 Na 2 CO 3 as eluent. Whereas, anion column (Metrosep C4, 100/4.0; Metrohm) was used to determine anions using a mixture of 0.7 mmol L -1 dipicolinic acid and 1.7 mmol L -1 nitric acid as eluent. MERCK certified standards for anions and cations were used for the calibration. Four standards of 1, 2, 5 and 10 ppm were used to draw the standard calibration curve for cations and anions. The correlation coefficient of standards was observed to be 0.99. Calibration curve was plotted every time the new eluent is prepared and quality was ensured by repeating the analysis of standards on regular interval. Replicate sample analysis has shown a relative deviation within a range of 12-18%. The detection limits (DLs) of Cl -, NO 2 -, SO 4 2-, NO 3 -, Na + , NH 4 + , K + , Ca 2+ and Mg 2+ for our instrument were calculated as 0.03, 0.75, 0.24, 1.13, 0.35, 0.29, 0.63, 0.85 and 0.33 mg mL -1 , respectively. For quality assurance and quality control (QA/QC), procedure field blanks were collected at the start of each month sampling. Blanks were handled and analyzed in the similar way as other samples. Most of the target ions were found below detection limits except Clwhich was subtracted/corrected at each step while calculations. The final concentration of ions was calculated using following formula: where X is the concentration of particular ion obtained from IC, Vs is the volume of sampling solution, R is the flow rate, t is the duration of sampling and C is the final concentration of X ion in the ambient air.

Ion Balance Check
An ionic charge balance is usually performed in aerosols to check the data quality, where the sum of the equivalent ratios of cations and anions are assessed to fulfil the principal of electro-neutrality and characterizing the aerosol acidity (Pani et al., 2017). In the present study, the method does not include exclusively aerosols. The bulk concentration of WSIIs in fog-forming air mass simultaneously include fog droplets and diffused gases (mainly NH 3 and HONO) along with the aerosols and their collection in the sampling solution (Milli-Q water), not filters. The ion balance check revealed an excess cation case as reported by earlier studies in the Indian region (Kulshrestha et al., 2003). According to Kulshrestha et al. (2005) bicarbonate measurements substantially fill up the gap in the cation and anion difference in the Indian region. This is because much of the calcium which is a dominant cation is associated with HCO 3in the samples which is corroborated with the ion balance studies reported by Satyanarayana et al. (2010) and Kumar et al. (2016). In the present study we found the ratio of sum of equivalents of anion and cations (∑ -/∑ + ) in the range of 0.08-0.33 at the urban site and 0.12-0.47 at the rural site ( Table 1). The deficiency of unmeasured water-soluble organic anions (such as acetate and formate) might also have led to the lower ratio values, which have been reported to be present in abundance in the wintertime studies of IGP (Verma et al., 2017;Nath and Yadav, 2018). The importance of organic anions in charge balance has been also reported in previous studies (Kerminen et al., 2001;Lee et al., 2003). Furthermore, the lower ratios were identified due to NH 4 + -rich conditions owing to its efficient collection and higher solubility. In this context, Trebs et al. (2005) have also reported an excess in NH 4 + species due to greater diffusion of gaseous NH 3 , which might be neutralizing the other unmeasured anions in the atmosphere. The ratio of sum of cations other than NH 4 + and sum of anions was still observed to be in lower range (0.51 and 0.86 for urban and rural sites, respectively), which is probably due to excess in Ca 2+ concentrations. However, the major limitation of such neutralization process is the unmeasured highly soluble bicarbonate ions formed through the dissolution of CO 2 whose solubility depends on available cation species in the atmospheric samples. In this context higher concentration of unmeasured bicarbonate ions might be compensated with the excess of NH 4 + and Ca 2+ ions. Additionally, such interactions with measured ions have been explained using scatter plots in the results.

Variations in pH Values of Samples over the Sites
The pH values are one of the major indicators of dominant  (Wang et al., 2005;Morgan, 1982). The pH was analyzed to get the relative difference in the acidity of ambient air bulk samples (Fig. 3). The average pH of the collected urban samples was found to be 5.7 ± 0.1, which ranged from 5.1 to 6.2. Whereas, rural samples have shown a relatively lower average pH of 5.1 ± 0.1 ranging from 4.5 to 5.6. In the absence of effective neutralization, pH of rural samples indicates the higher concentrations of H + ions resulting in greater atmospheric acidity. However, higher pH of urban samples shows the efficient neutralization of acidic species in urban atmosphere which can be attributed to the presence of alkaline species from dust particles emitted from construction activities, deforestation, and road dust and soil resuspension (Kulshrestha and Sharma, 2015). The presence of gases like CO 2 , SO 2 and NO 2 also greatly affect the pH values, depending upon their concentration and dissolution properties in water (Lekouch et al., 2010).

Relative Contribution of WSIIs
The relative contribution of different analyzed WSIIs has been shown in Fig. 4. NH 4 + and Ca 2+ were found as major cations and NO 3and SO 4 2as major anions at both the sites. NH 4 + contributed around 47% of analyzed WSIIs at the urban site and 37% at the rural site. NH 4 + has been reported to be dominating atmospheric species in fog water and wintertime aerosol studies (Ali et al., 2004;Nath and Yadav, 2018;Ge et al., 2019). Since animal/human excreta are one of the most important sources of NH 3 , densely populated IGP has been observed as one of the major hotspots of ammonia emission in the world (Galloway et al., 2004). Owing to higher solubility, NH 4 + readily gets scavenged by water droplets present in the air during humid conditions in winter mornings and reacts with the atmospheric acidic components such as chlorides, nitrates and sulfates (Waldman et al., 1982;Jacob and Hoffmann, 1983;Bongartz et al., 1995;Ge et al., 2019). Urban sources of NH 3 include vehicular exhaust, open waste burning and human excreta/sludge degradation, whereas biomass burning, emissions from agriculture and livestock farming remain as major sources in the rural areas . At urban site NH 4 + was followed by Ca 2+ > NO 3 -> SO 4 2-> Cl -> Na + > K + > NO 2 -> Mg 2+ . However, at the rural site NH 4 + was followed by NO 3 -> SO 4 2-> Ca 2+ > K + > Na + > Cl -> Mg 2+ > NO 2 -. Ever-growing urbanization, migration and unplanned construction activities are continuously lowering the urban green cover which leads to very high erosion of surface dust and the main sources of Ca 2+ in urban areas . However, in rural areas, high green cover limits the emission of Ca 2+ in the atmosphere (Tiwari et al., 2016;Mishra and Kulshrestha, 2017). Both NO 3and SO 4 2were found to be higher at the rural site as compared to urban site, which primarily can be attributed to the solid fuel burning in the form of wood, crop residues, dung cakes, domestic wastes or low-grade coal burning in the villages .

Temporal Variation of WSIIs
Since the study includes an overall characterization of WSIIs, i.e., without phase differentiation, here samples were not classified based on fog occurrence. The concentration of major WSIIs in ambient air bulk samples at urban and rural sites is given in Table 2.

Urban Site
Temporal variation of measured cations and anions at the urban site is shown in Fig. 5. Being dominant species during winters, mean concentration of NH 4 + at urban site was 58.2 ± 17.5 µg m -3 ranging from a minimum 24.7 µg m -3 to a maximum 123.5 µg m -3 . The IGP has been majorly reported as the hotspot of NH 3 emission (Behera et al., 2013). Apart from natural sources, higher traffic density and vehicular exhausts are the potential sources of NH 3 emission in urban areas. However, being one of the most soluble atmospheric gases, NH 3 readily gets absorbed by fog droplets and get converted into NH 4 + (Jacob et al., 1984;Shi et al., 1999). It was followed by Ca 2+ with an average concentration of 21.8 ± 7.1 µg m -3 ranging from a minimum 10.6 µg m -3 to a maximum 35.8 µg m -3 . Construction activities and resuspended road dust are most important urban sources of Ca 2+ . Since Ca 2+ concentration occurs higher in coarse mode and primarily in the form CaCO 3 , these large particulates provide a surface for the fog water accumulation. Due to the lower mixing height in winters, these particles remain suspended in the lower atmosphere and enhance the fog formation rate (Finlayson-Pitts and Pitts, 2000). Both Ca 2+ and NH 4 + have been reported to be crucial for the neutralization of acidic species in the atmosphere (Khemani et al., 1987). Na + , K + and Mg 2+ mean concentrations were 7.2 ± 7.7 µg m -3 , 5.1 ± 3.6 µg m -3 and 1.8 ± 0.7 µg m -3 , respectively. Na + and Mg 2+ mostly come from crustal sources, sea salts or longrange transport (LRT); however, K + could have natural (crustal) as well as anthropogenic (biomass burning) sources .
Average NO 3and SO 4 2concentrations at the urban site were observed to be 14.4 ± 6.8 µg m -3 and 12.6 ± 6.3 µg m -3 , respectively. NO 3and SO 4 2generally occur as secondary pollutants formed from their precursor gases NO, NO 2 , volatile HNO 3 and SO 2 . Prayagraj has a very high traffic density due to its large urban population where unregulated engines of local public transports such as Vikram tempo, buses and heavy-duty trucks, pass from nearby highways and emit enhanced amounts of these pollutants. During winters mixing height goes as low as 200 m and these exhaust emissions get accumulated in the lower atmosphere. Fog has been reported to intensify in the presence of precursor gases like NO 2 and SO 2 (Klemm and Lin, 2016). In aqueous phase, Such reactions in the presence of high moisture content and polluted atmosphere promote smog formation and cause thick-haze-like conditions and reduce visibility. Clconcentration was 10.4 ± 7.4 µg m -3 , which can be attributed to alkaline saline soil dust transported from upwind regions, HCl evaporation and municipal waste burning, which is very common for locals (Zhang et al., 2008' Perrino et al., 2011. Mean NO 2concentration was found to be 2.5 ± 1.4 µg m -3 , which could be formed as a result of Eqs. (2) and (3) during morning hours and play as crucial interstitial oxidizing species (Finlayson-Pitts and Pitts, 2000).

Rural Site
Temporal variation of measured cations and anions at the rural site is shown in Fig. 6. NH 4 + was observed as the dominant species also at the rural site, having an average value of 38.3 ± 13.1 µg m -3 with a minimum of 18.9 µg m -3 and maximum 62.6 µg m -3 . Agricultural activities and livestock are the major sources of NH 3 emission in rural areas . Madhupur village has agriculture practice, as their primary source of income. NH 4 + was followed by NO 3and SO 4 2with an average concentration of 16.4 ± 7.6 and 15.1 ± 11.1 µg m -3 , respectively. Despite having very few vehicles around the village, NO 3and SO 4 2concentrations were higher in rural samples. Biomass burning for cooking and heating purposes during winters in the villages of the IGP is a very common practice, which releases harmful primary pollutants NO 2 and SO 2 Singh et al., 2016). In the absence of coal burning by local people, two operational units of brick kiln in the nearby area (in the range of 5 km 2 ) were also observed as a significant source of SO 2 , which use low-grade coal as a fuel for the production. Since these precursor gases are highly hygroscopic, they quickly get incorporated into fog droplets. Also, they lead to the acidity in fog water samples in the absence of effective neutralization by alkaline species such as NH 4 + and Ca 2+ (Reactions (1)- (6)). Furthermore, the Ca 2+ concentration was found to be 8.1 ± 0.5 µg m -3 which is about three times lower as compared to urban site and could be associated with the soil dust resuspension. Following these, the mean K + concentration was 6.8 ± 6.3 µg m -3 and found to be significantly higher as compared to the urban site due to the prevalence of biomass burning in rural areas during winters (Kaneyasu et al., 1999). Average Na + concentration was found to be 6.6 ± 6.7 µg m -3 and followed by Clconcentration with an average of 6.4 ± 5.4 µg m -3 . Alkaline saline soil dust have been reported to be the primary source of Na + and Clover IGP owing to the prevalence of continental air masses from northwestern regions (Ghosh et al., 2014). Mg 2+ and NO 2were also found in a significant amount with average values of 1.2 ± 0.5 and 1.1 ± 0.5 µg m -3 , respectively.

Meteorological Impact and Monthly Variation
Changing meteorological conditions affect dilution and diffusion, as well as the accumulation and elimination of the air pollutants in the lower atmosphere (Wang et al., 2014;Xu et al., 2018). Pollutants get concentrated in humid and stagnant weather conditions, which are predominant in winters over IGP. Since major properties of atmospheric components are linked to their solubility in water and NH 3 is reported to be highly soluble in increased RH conditions, NH 4 + was found as the dominant species in the collected bulk samples which varied widely in each month at both the sites (Fig. 7). Despite higher RH at the rural site as compared to urban site in each winter month (Table 3), its concentration remained higher at urban site in all the four months which suggests additional biogenic emissions of NH 3 from the municipal sludge and sewage waste from the urban regions. However, its concentration is recorded to be highest in January at both the sites which can be corroborated with the high RH, relatively lower temperature and shallow boundary layer in the month of January. Most of the other ions varied widely in each month and did not show any common pattern. Additionally, the mass ratio of (NO 2 -+ NO 3 -)/SO 4 2have shown an average value of 1.5 for both the sites and observed to have similar monthly trend (Nov. > Dec. > Feb. > Jan.) as shown in Table 3. This has suggested towards higher emission from stationary sources during January month for both the sites. But on an average November month samples have shown higher concentration of ∑WSII with a spike in combustion-generated ions at both the sites (Table 3), which suggests that firecrackers pollution during Diwali festival and crop residue burning incidents in that period have a significant impact on the atmospheric composition of WSIIs. The five days' air mass backward trajectories (plotted   (Fig. 8). The northwestern region of IGP is known for the large-scale crop residue burning in November month which could also be contributing to the higher concentrations of WSII (Jain et al., 2014;Singh and Kaskaoutis, 2014). Monthly comparison of major inorganic ions (Fig. 7) clearly shows that most of the ions were higher at the urban site as compared to the rural site in all the four months except K + , NO 3and SO 4 2which is primarily due to plant-based combustion sources at the rural site.

Fog Scavenging Using Statistical Analysis
In order to understand the impact of fog occurrence on ambient concentrations of WSII suspended in the atmosphere, the sample data were classified into foggy days and nonfoggy days to perform a paired t-test. Due to unavailability of visibility data, foggy days were assumed as the days with more than 80% RH and a temperature less than 20°C combined with visible documentation fog occurrence. High RH values during foggy days promote the wet scavenging of aerosols and gases and subsequently, their deposition in the form of dewdrops (Nath and Yadav, 2018). Result of paired t-test at a 95% significance level at urban and rural site is shown in Table 4. The ambient concentrations of NH 4 + , NO 2 -, NO 3and SO 4 2were not affected by fog occurrence at both the sites, suggesting their presence in the form of aerosols or suspended as fog droplets. This can also be attributed to the semi-volatile nature of these ions. However, concentrations of Ca 2+ and Na + were found to be  significantly different on foggy and non-foggy days at both the sites, showing greater scavenging by fog droplets and deposition in the form of dewdrops. A significant difference of K + and Mg 2+ on foggy days at urban site suggests its association with Ca 2+ in the form of alkaline dust. Unlike urban site, the concentrations of Mg 2+ and K + were not altered based on fog occurrence at the rural site owing to its continuous emission during sampling period. Clwas found to be effectively scavenged during foggy days at the rural site but was not affected at the urban site, which can be attributed to its stable sources at the latter site.

Winter Chemistry of WSIIs
In order to investigate the major interactions of WSIIs occurring in the atmosphere, correlation coefficients (Table 5) along with the scatter plots (Figs. S1 and S2) were analyzed between each of the major cation with major anions.

Urban Site
Table 5 (Fig. S1) represents the relation of each anion with each of the cations at urban site. Despite being highest in concentration, NH 4 + showed least correlation with SO 4 2-, Cland NO 3 -(r = 0.04, 0.20 and 0.09, respectively), which are the most important component of secondary aerosols; however, it showed a good correlation (r = 0.71 at p = 0.01) with NO 2 -. This can be attributed to the possible heterogeneous gaseous reaction of HONO with NH 3 onto the fog droplets, which lead to the formation of NH 4 + and NO 2as an end product. HONO has a maximum concentration in morning hours before sunrise during winters which also affects the stoichiometry of atmospheric aqueous-phase reactions (Finlayson-Pitts and Pitts, 2000). However, a good correlation between them is suggestive of their common emission sources in urban areas such as vehicular emission or incorporation of diffused gases into mist during the sampling process. Furthermore, the anions were found to be poorly correlated with Ca 2+ and slightly positive with NO 2 -(r = 0.27), showing its limited presence as a secondary aerosol. Rather, it could be directly emitted from excessive road construction activities in the city along with other crustal elements. Ca 2+ might also act as a surface for fog formation because of its higher solubility in the humid environment. K + and Na + have shown a strong correlation with Cl -(r = 0.87 and 0.88, respectively at p = 0.01) and a good correlation with NO 3 -(r = 0.56 and 0.56, respectively at p = 0.05), which is indicating towards their presence in the form of secondary aerosols as KCl, KNO 3 , NaCl and NaNO 3 . SO 4 2was also positively correlated with Na + (r = 0.46) and K + (r = 0.44 at p = 0.05), which suggests that they might act as primary alkaline species in the atmosphere for acid neutralization in urban samples.

Rural Site
Table 5 (Fig. S2) represents the relation of major anions with each cation at the rural site. Na + has shown a strong positive correlation with Cl -(r = 0.60 at p = 0.01), which shows the influence of alkaline saline soil dust or in the form of secondary salts as NaCl in the ambient air during sampling hours at rural site. Na + has also shown a strong correlation with NO 2 -(r = 0.66 at p = 0.01), which implies to the formation of NaNO 2 . Being dominant inorganic ion in the rural samples, NH 4 + has shown a good correlation with the NO 2 -, NO 3and Cl -(r = 0.46, 0.48 and 0.51, respectively at p = 0.05) suggesting efficient neutralization of acidity and the formation of secondary aerosols, viz. NH 4 NO 2 and NH 4 NO 3 . However, it was least correlated with SO 4 2-(r = 0.17). SO 4 2has shown a strong correlation with K + (r = 0.63 at p = 0.01), which suggests their common sources and strongly indicating towards biomass burning in rural areas. Furthermore, the good correlation of K + with Cl - (r = 0.71 at p = 0.01) suggests that the waste burning could be taking place for heating or cooking purposes during winters. Ca 2+ and Mg 2+ have shown highest correlation with NO 2 -(r = 0.80 and 0.66 at p = 0.01, respectively) as compared to other anions which suggest that crustal components might be providing active sites for nitrogenous gases oxidation.

Source Apportionment
Factor analysis is a statistical method, based on a mathematical model of the reduced factor analytical solution, to find small number of factors from a dataset of many correlated variables which explains a major part of the result thereby extracting latent information. To analyze factors, the original data matrix gets decomposed into the number of factor loading and a matrix of factor scores along with variance of dataset, i.e., analytical uncertainties. SPSS version 20.0 for Windows was used to analyze the principle components for each site. PCA was performed in order to determine the factors underlying the inter-correlations between the measured species, having a factor loading more than 0.5. In accordance with this, the grouping of ions in each factor was attributed to physical, chemical, meteorological reasons as well as to the common sources.

Urban Site
PCA of WSIIs at urban site (n = 41; Table 6) identified four principle factors, which explains data loading for 91% (cumulative variance) where most of the components have significant loading, i.e., more than 0.5 values. The first factor explains about 45% of dataset, with the major loading of Na + , K + , Mg 2+ , Cl -, NO 3and SO 4 2-. This suggests the common emissions of K + , Cl -, NO 3and SO 4 2from municipal solid waste and biomass burning during winters as the dominant sources at urban site which are responsible for about half of the ambient levels of WSIIs. Such attributions can be further corroborated with the higher ratios of Cl -/K + (2.2) for municipal waste burning and SO 4 2-/K + (3.3) for biomass burning (Fourtziou et al., 2017). The presence of Na + and Mg 2+ in Factor-1 is indicating towards their existence in the form of NaCl and MgCl 2 salts. Second factor explains about 23% of dataset with a heavy loading of Ca 2+ and NH 4 + and light loading of Mg 2+ and NO 2 -. This suggests the oxidization of gases like NH 3 and NO 2 into NH 4 + and NO 2on the dust particles, which is providing a surface for heterogeneous aqueous-phase interactions. NH 3 sources in urban areas mainly include biogenic emission from sewage microbial degradation (Jain et al., 2000;Tiwari et al., 2012;Behera et al., 2013). Furthermore, the mean ratio of Mg 2+ /Ca 2+ (0.08) has shown to have a crustal origin following the previously reported winter season ratio by Mu et al. (2019). Third factor explains 15% of dataset and primarily loaded with NO 2and NH 4 + which indicates their common gaseous emission from vehicular exhausts. Fourth factor included only SO 4 2and explains 8% of the dataset which suggests the SO 2 sources around the sampling sites. Coal burning in local food dhabas (restaurants) and heavyduty (diesel) vehicles are the primary sources of SO 2 in urban areas. Vehicular emission of SO 4 2was further supported by the lower mean ratio (1.2) of NO 3 -/SO 4 2- (Rajput, 2018). PCA clearly indicates that primary sources at urban sites are solid waste burning, biomass burning, soil dust, vehicular exhausts and biogenic emissions.

Rural Site
PCA of WSIIs at the rural site (n = 22; Table 7) identified three principal factors, which explains data loading for 80% cumulative variance where each factor has a variable loading more than 0.5 values. The first factor explains about more than half of the total variance, i.e., 51% with a high loading of Na + , Ca 2+ , Mg 2+ , Cland NO 2which suggests the influence of natural sources like crustal dust and salts including dominant oxidation processes induced by HONO as a primary factor during early hours in winters. However, a higher ratio of Mg 2+ /Ca 2+ (0.14) as compared to the urban site is indicating towards additional plant-based sources of Mg 2+ . The dominance of K + and SO 4 2in the second factor explains about 17% of the dataset, which is clearly indicating towards the biomass burning sources used for cooking and heating purposes in winters in rural areas. However, the brick kiln units have also been observed as an additional source of SO 2 , since none of the other coal-burning sources were observed in the area. Furthermore, the lower mean ratio of Cl -/K + (1.3) due to increased levels of K + and a higher ratio of SO 4 2-/K + (4.4) due to increased concentration  of SO 4 2ions cumulatively indicated towards biomass and coal burning as their primary sources (Mu et al., 2019). Third factor dominated with NH 4 + and NO 3explains 12% of the dataset. This can be attributed to their common sources such as agriculture activities, gasoline vehicles and secondary aerosol formation as NH 4 NO 3 . Summing up, the PCA at rural site have shown that biomass burning, soil dust, coal combustion in brick kilns, agricultural emission (livestock and fertilizers) and light-duty vehicles as primary sources.

Overview of Research Conducted and Implications
Atmospheric abundance of WSIIs has been assessed from an urban (Prayagraj) and a rural location (Madhupur) in Indo-Gangetic Plain. The study has identified the sources of contamination in fog water. The urban site was highly influenced by the emissions of automobiles, solid waste and biofuel combustion while the rural site was influenced by biomass burning, agricultural activities and brick kilns. The crustal sources also contributed significantly. The study suggests to develop the regulations for pollution control accordingly. The levels of sulfates and nitrates in fog water give an idea about their role in visibility reduction during foggy days. The knowledge of gas-to-particle transformation can partially be utilized in understanding the scavenging of cloud condensation nuclei in the region.

CONCLUSIONS
This study investigated the interactions of the major water-soluble inorganic ions (WSIIs) in the early morning ambient air during winter and their principal causative factors at an urban and a rural site on the IGP. We found significantly high concentrations of cations and anions at both of the sites, largely owing to the higher humidity during wintertime and consequent aqueous-phase chemistry, which played a crucial role in the scavenging of pollutants and resulted in the rapid condensation and evaporation of water droplets on preexisting particles during the morning/foggy period. Compared to the urban site, the rural site displayed higher ambient concentrations of acidic species, which were primarily responsible for the lower pH at this location. In general, NH 4 + , Ca 2+ , NO 3and SO 4 2+ , as the dominant ionic species, clearly governed the ambient chemistry of the WSIIs. Furthermore, the higher mean concentration of ∑WSII during November (based on the collected samples) indicated that extreme pollution episodes arising from festivities and crop residue burning in this month may have altered the atmospheric ionic composition. Fog scavenging significantly influenced the levels of the dust-associated components but did not affect the other species (e.g., K + , NH 4 + , NO 2 -, NO 3and SO 4 2-). Natural sources of WSIIs, such as crustal dust and biogenic emissions, were apparent at both sites. However, fossil fuel combustion, and solid waste and biofuel burning were the major anthropogenic contributors at the urban site, whereas biomass burning, brick kiln combustion and agricultural activity were the predominant ones at the rural site. The increased pollution, which is emitted by various sources, and the occasional spike in pollutant concentrations on the IGP is a serious concern in terms of both human health and regional ecosystems.

ACKNOWLEDGMENT
We sincerely thank the financial support received through CSIR (SRF) and DST-PURSE. We thank CIF, SES for providing analytical help during this research work. This work has been part of DRSNet-India network of Prof. U. C. Kulshrestha group.

SUPPLEMENTARY MATERIAL
Supplementary data associated with this article can be found in the online version at http://www.aaqr.org.