Emerging Major Role of Organic Aerosols in Explaining the Occurrence, Frequency, and Magnitude of Haze and Fog Episodes during Wintertime in the Indo Gangetic Plain.

Aerosols are an important part of Earth's atmosphere. They can absorb, scatter, or reflect the incoming solar radiation, which results in heating or cooling of Earth, thus impacting its climate. It affects the health of exposed human population adversely, reduces visibility, disturbs environmental systems, and causes material damage. This study summarizes the research carried out to understand the role of aerosol load and its physicochemical characteristics on occurrence, frequency, and magnitude of haze and fog events during wintertime within the Indo Gangetic Plain (IGP) in the past decade. For most species, the highest concentration was measured during foggy events at night-time over the winter season. A few species such as water-soluble organic and inorganic carbon (WSOC and WSIC), K+, SO4 2-, and NO3 -, owing to their hygroscopic nature, were efficiently scavenged, resulting in their lower concentration within the interstitial aerosol during fog episodes. Oligomerization with hydroxy and carbonyl functional groups during AFP (activating fog period) and DFP (dissipating fog period), respectively, accompanied by acidic aerosol (having catalytic ability) and high aerosol liquid water content conditions was found to be significant. Whereas the fragmentation process was dominant along with functionalization of -RCOOH or carbonyl (aldehyde/ketone) and -RCOOH moieties during FP (fog period) and PoFP (post-fog period), respectively. Transition metals play an important role in aqueous production of secondary organic aerosol (SOA) especially during the night-time. Crustal sources had the highest scavenging efficiency along with WSOC playing an important role in nucleation scavenging. Fine droplets had a higher concentration of species with a larger fraction of highly oxidized organic matter (OM) as compared to coarse or medium size droplets. Also, a new approach to calculate absorption by black carbon (BC) and brown carbon (BrC) was proposed, which found the water-soluble brown carbon (WSBrC) absorption value in aerosol to be up to 1.8 times higher than that measured in their corresponding aqueous extracts. Organic aerosol plays a vital role in facilitating fog formation and is responsible for the longer residence time in the ambient atmosphere. Ammonia plays an important role in stabilizing organic aerosol and aids to this recurring haze-fog-haze cycle that is dominant during wintertime in the IGP. Therefore, controlling the major anthropogenic sources of organic aerosol and ammonia should be our top priority in this part of the world.

Introduction

Fine aerosols play a crucial role in affecting the earth’s radiative budget and impact human health and visibility. The carbonaceous fractions of airborne particulates (carbonaceous aerosols, CA) comprise elemental carbon (EC) and organic carbon (OC). Carbonaceous aerosols generally contribute as a major fraction of PM loading. The light-absorbing component of aerosol is EC, which is also a chemically inert compound and is characterized by its insolubility and refractory nature.[1,2] EC emissions are mostly from primary sources like vehicular emissions, biomass burning, and industrial processes. On the other hand, OC is considered to be a representative of organic matter (OM), which is a complex mixture of several organic compounds with various molecular forms and volatilities.

Previously, OC was considered as a light-scattering component of aerosols and EC as a completely light-absorbing component of aerosols. However, in the past decade, some portion of organic aerosols has been observed to be light absorbing, which is termed as brown carbon (BrC). Organic aerosol predominates the aerosol mass concentration in the troposphere with a high spatiotemporal variability.[3,4]

Organic aerosols (OA) make up a large fraction of ambient fine aerosols (20–80%).[5−7] OA can be classified as primary and secondary OA with primary OA (POA) being emitted directly into the atmosphere via biomass burning (BB), fossil fuel combustion, industrial emission, volcanic eruptions, and sea spray. Secondary OA (SOA) is generally formed via a gas to particle partitioning mechanism.[8,9] An alternate aqueous phase mechanism[5,10−12] also leads to the SOA formation.

Due to complex chemical transformation reactions, involvement of numerous start and end products, and variable physicochemical characteristics of existing compounds in the atmosphere, direct estimation of SOA is not possible. An indirect method, such as the EC tracer method, has been used by many researchers[13,14] to estimate SOA. Secondary organic carbon fraction can be calculated using different methods, and the most common one is a minimum OC/EC ratio or EC tracer method given by Castro et al.[15] For the EC tracer method, the regression between EC and OC values is observed from the subset of measurements where the production of SOA is least likely. These represent primary EC and OC emissions whose ratio OC/ECmin is assumed constant contrary to periods of high photochemical activity, in which the OC/EC ratio increases. The following equations were used to calculate secondary OC in PM1:In a previous study carried out simultaneously at Kanpur and Agra within the IGP, Villalobos et al.[16] reported an increasing contribution of SOA from October to December.

Water-soluble OA (WSOA) forms a large portion of OA and plays a significant role in influencing overall OA properties. This is due to the presence of a significant amount of LWC (liquid water content) in the form of water vapor. The particulate-bound water is at times more than the available dry aerosol mass in Earth’s atmosphere,[5] and the soluble part, i.e., WSOA, can easily interact with it. WSOC (water-soluble organic carbon) has the potential to alter the hygroscopicity of fine aerosols by reducing the surface tension of aqueous solutions, leading to more droplet formation and enhanced albedo.[17] The role of black carbon (BC ∼ EC) in warming the atmosphere has been recognized since a long time.[18] Brown carbon is often associated with direct absorption of incoming shortwave solar radiation, thus positively influencing radiative forcing.[19,20] Furthermore, a significant fraction of BrC is water-soluble in nature. Since SOA is broadly comprised of polar organics, thus WSOA is often considered to be a marker of SOA. However, a few studies have also reported that some fraction of POAs, such as BBOA (biomass burning OA) are also water-soluble and can contribute significantly (∼7–50%) to WSOA.[17,21]

Organic fractions of aerosols are composed of a mixture of numerous organic compounds, which are difficult to identify.[16] Some organic compounds, such as polycyclic aromatic hydrocarbons (PAHs), hopanes, or levoglucosan, serve as molecular markers to identify various sources.[3,4,16] Levoglucosan is identified as a biomarker for biomass burning emissions, whereas hopanes are used as tracers of vehicular exhaust emissions, coal combustion, and other liquid fuel combustion. However, PAHs are generated from incomplete combustion of fuels, biomass, or coal. The different sources can be identified using isomeric ratios of PAHs.[22] Organic compounds are present in the atmosphere in various forms. The quantitative identification of various organic compounds assists in accurate source apportionment. Several studies have been carried out for the source apportionment of organic aerosols and to better understand their absorption characteristics.[16,23−25] However, there is still a lot of uncertainty in quantifying the impact of organic aerosols on climate forcing, atmospheric processes, and in terms of their true health impacts on the exposed population.

In this review, trends in particulate matter concentration, secondary organic aerosol production (SOA), impact of transition metals and acidity on SOA production, techniques to help analyze the trends, scavenging efficiency of various species, effect of droplet size in aqueous phase processing, and a new approach to calculate brown carbon absorption efficiency are described in detail.

Sampling Location

The Indo Gangetic Plain (IGP) stretches from northwestern to northeastern region in India and covers ∼15% of the total geographical area in south Asia. IGP has a narrowing topography with the Himalayas to the north and the Deccan Plateau to the south. This region experiences extreme weather conditions with cold and foggy winters (December–February; temperature <5 °C) and hot and humid summers (April–May; temperature as high as 48 °C) and receives an annual rainfall of ∼900 m during June–September (monsoon). The entire IGP holds 40% of the population in south Asia. It is one of the world’s most densely populated regions and home to half of India’s population. High aerosol loading over this region has been observed throughout the year. Greater spatial variability in the changes of particulate matter concentration has been observed. Also, many hazy and foggy days have been observed in the northern part of the country, in particular, in Delhi, which affects the visibility and causes severe health problems. Due to a lack of visibility during these days, many flight delays and road accidents were reported from the Delhi region.

Kanpur is located on the southern bank of river Ganges, in the center of the Indo Gangetic Plain (26°47N latitude and 80°35E longitude). Sand, silt, and clay are the main constituents of the alluvium, which occur in variable proportions at different places. The climate is tropical; it has hot summers (March–June) followed by heavy rainfall in monsoon (July–September) and foggy winters (October–February). Kanpur is the most populous city in the state of Uttar Pradesh. Here, aerosol sampling was carried out at the Indian Institute of Technology Kanpur (IIT-K), which is located away from the city center but within the city boundaries. It is an educational institute with a lush green residential campus and having no commercial or industrial activities in its vicinity.

Here, so far, integrated sampling was carried out, which provided offline measurement of chemical species. For this, a single-stage round nozzle, grease impaction substrate-based impactor-type PM1 sampler was used which was previously developed at our IIT-K lab.[26,27] The sampler was validated using polydisperse artificial aerosol, which was generated in the lab and measured using an aerodynamic particle sizer (model 3021, TSI Inc., USA) following the well-established method for impactor characterization.[28,29]

An aerosol mass spectrometer (AMS) is available which can measure aerosol in real-time, and it can give nonrefractory chemical composition as a size segregated value. Offline fogwater analysis was also carried out in some studies via AMS + TD (thermodenuder), where the fogwater samples were atomized using a commercial TSI atomizer (model 3079), using particle-free compressed zero air. Similarly, offline AMS analyses were carried out to obtain the nonrefractory aerosol characteristics. The real-time measurement of aerosols during fog events using AMS can also provide ionic species (NH4+, Cl–, SO42–, NO3–) concentration of interstitial aerosols.[30,31]

During fog episodes, the interstitial aerosols can be collected onto the precombusted quartz filter using a PM1 impactor sampler. The collected filter samples were then extracted with Milli-Q water. The aqueous filter extracts were used to measure the ionic species concentration utilizing an ion chromatograph.[32,33]

Particulate Matter

Particulate matter refers to the mixture of particulate solids and liquid droplets present in the atmosphere. It can be produced directly or be formed because of complex chemical reactions. Particles far less than 1 μm can easily get into our lungs and even our bloodstream, and from there, it can reach various organs and damage them, thus negatively impacting human health. They also impart material and environmental damage and poor visibility in the urban air sheds. PM1, PM2.5, and PM10 can be defined as particles having aerodynamic diameters ≤1, 2.5, and 10 μm, respectively. A lot of studies have focused on PM2.5 and PM10. Here, we have focused on PM1 (Table ), which is comparatively much more harmful than PM2.5 and PM10.

Table 1

Comparison of Fine Aerosol Concentration of Various Studiesa

references	monitoring site	month/year of monitoring	aerosol concentration (μg/m3)	remarks
Chakraborty and Gupta[23]	IIT-Kanpur	December 2008 to January 2009	199.0 ± 66.0	PM1
Gupta and Mandariya[34]	IIT-Kanpur	November 2009 to February 2010	day: 114 ± 80	PM1
			night: 143 ± 86
Kaul et al.[35]	IIT-Kanpur	January 2010 to February 2010	foggy: 427.3 ± 209	PM1
			nonfoggy: 525.6 ± 261
Chakraborty et al.[30]	IIT-Kanpur	November 2011 to January 2012	foggy: 139 ± 44	PM1
			nonfoggy: 100 ± 37
Singh and Gupta[37]	IIT-Kanpur	August to November 2013	132.87 ± 27.97	PM1
Singh et al.[38]	IIT-Kanpur	November 2014 to February 2015	foggy: 107 ± 58	PM1
			nonfoggy: 247 ± 113
Mandariya et al.[36]	IIT-Kanpur	December 2015 to January 2016	PrFP: 207.8 ± 59.6	PM1
			AFP: 249.8 ± 47.8
			FP: 153.1 ± 37.8
			DFP: 205.0 ± 69.7
			PoFP: 190.8 ± 61.6

PrFP, pre-fog period; AFP, activating fog period; FP, fog period; DFP, dissipating fog period; PoFP, post-fog period.

Average PM1 concentrations of different elements and anions are the highest in winter because of enhanced biomass combustion, low temperature, and wind speed, which leads to lower mixing height and resulting in poor dispersion of aerosol. These are lowest in the monsoon season (average PM1 concentration: 30.1 ± 13.7 μg/m3) because of washout by rainfall and high relative humidity.[23] PM1 concentration increases significantly from daytime (114 ± 80 μg/m3) to night-time (143 ± 86 μg/m3), usually due to frequent inversion conditions which trap the aerosol near the ground surface.[34] Most species in aerosol have higher concentration during foggy episode as compared to nonfoggy episodes even after wet scavenging. This is true for most species except for a few such as K+, WSOC, WSIC, SO42–, and NO3–. These were found to have lower concentrations possibly due their highly hygroscopic nature owing to their secondary or biomass burning origin.[35] Mandariya et al.[36] characterized the entire fog episode into five phases, namely, pre-fog period (PrFP), activating fog period (AFP), fog period (FP), dissipating fog period (DFP), and post-fog period (PoFP), and found that PM1 concentrations were highest in AFP (249.8 ± 47.8 μg/m3), followed by PrFP (207.8 ± 59.6 μg/m3), DFP (205.0 ± 69.7 μg/m3), PoFP (190.8 ± 61.6 μg/m3), and lowest during FP (153.1 ± 37.8 μg/m3); more details are provided in the Supporting Information.

According to Chakraborty and Gupta,[23] nitrate and sulfate constitute a large portion of total anionic concentrations (approximately 80–90%; nitrate: 36.61 ± 11.51 μg/m3, sulfate: 33.02 ± 14.72 μg/m3). Gupta and Mandariya[34] suggested that secondary ionic species (NH4+, NO3–, and SO42–) exist in the form of NH4HSO4, (NH4)2SO4, and NH4NO3. Mandariya et al.[36] observed that, during foggy nights, NO3– was higher during PrFP to FP, whereas SO42– was higher during DFP to PoFP. It was suggested that aqueous-phase production of SO42– was faster than wet removal during FP period, whereas the decrease of nitrate and ammonium concentrations after FP was attributed to wet deposition as well as evaporation at higher temperature during the DFP and PoFP periods (Supporting Information).

The study conducted by Chakraborty et al.[31,39] detected organosulfates in water-soluble aerosols in Allahabad and Kanpur, indicating aqueous processing of OA during sampling periods. Another study witnessed significant secondary formation of WSON during foggy conditions (nonfoggy condition WSON: 10.0% of total N, foggy conditions WSON: 29.7% of total N), suggesting enhanced organonitrate formation.[14]

A compilation of total OC and EC concentration at various sites in India is shown in Figure . In this plot, it is visible that OC concentration for sites within the IGP is quite high compared to that for other sites in India.

Figure 1

Overview of carbonaceous aerosols at different locations in India: Patiala,[41] Hisar,[42] New Delhi, Agra, Manora Peak,[42] Allahabad,[22] Jaduguda,[42] Hyderabad, Chennai, Bangalore, Pune, Mumbai, Ahmedabad,[43] Mt. Abu,[44] Jorhat,[45] Tezpur (PM10, Bhuyan et al.[46]).

The organic aerosols and its precursors continuously evolve in the atmosphere to form more oxidized and less volatile organic compounds.[5,7] Elevated organic aerosol loading can be observed within the IGP during wintertime, which is a major cause of concern due to its negative impact on human health and climate. The OC/EC ratio is opted for basic identification of various sources. The OC/EC (measured through NIOSH protocol) ratio of <4 represents the dominance of vehicular exhaust emission, whereas a ratio >6 implies biomass burning emission loading.[32,40] The water-soluble fraction of OC (WSOC) generally originates from biomass burning emission and secondary organic aerosol formation.[21] It can be observed that the OC/EC ratios of ambient aerosols over the central IGP are higher than those of the upper and lower IGP region.

Secondary Organic Aerosol (SOA)

In the gas to particle partitioning mechanism, atmospheric oxidation transformations of volatile organic compounds (VOCs) directly emitted from primary sources takes place. These are oxidized by radicals and oxidants like OH, NO2, and O3, leading to the formation of semivolatile organic compounds that condense over the pre-existing aerosols or may form new particles.

The aqueous phase mechanism takes place in fogs and clouds in two steps:As these fog droplets evaporate, the oxygenated compounds are formed, which condense over the interstitial particles, making them ideal FCN (fog condensation nuclei).[35]

VOCs are scavenged and dissolved into the droplets.

Dissolved VOCs are oxidized by the dissolved oxidants present inside the fog droplets to produce more oxygenated organic compounds than those produced from gas to particle mechanisms.

SOA Production and Role of Transition Metals

Transition metals play an essential role in the oxidation of WSOC of POA as they react with WSOC, leading to the formation of an organic complex with them. In terms of enhancing the WSOC and assisting SOA formation, the major and significant transition metals are Fe and Cu, whereas the minor ones are Cr and Mn.[37,47] The catalytic ability of transition metals present in aerosols is enhanced by complex formation with transition metal ions.[48] The complex formation leads to other oxidation reaction pathways which help in producing highly oxidized OM driving night-time oxidation reactions among other pathways.[31] Also, the neutralizing capability of ammonium ion and its excess availability leads to enhanced stability and residence time of WSOC in the ambient aerosol, allowing higher time for its processing.[47]

In a box model study conducted by Chakraborty et al.[31] to predict the trends in aqueous SOA formation, only glyoxal was used as a SOA precursor. Two simulations were carried out: first, considering all OH in the aqueous phase comes from the gas phase, and second, the consideration of Fenton reactions as an additional OH source and subsequent recycling of oxidized metal ions through the following reactions:Model results also indicate the importance of metal-assisted Fenton reactions in producing highly oxidized OM and suggest that variation in metal concentrations along with processing times estimates are required to fully represent variability in oxygenation states within a fog droplet population. A previous study by Lin et al.[49] also reported an aqueous phase SOA formation mechanism like photolysis of the Fe-oxalate complex.

Role of Aerosol Acidity in SOA Formation

Aerosol neutralization ratio (ANR) has been used by many researchers[30,36,50] to assess the acidity of the aerosol indirectly. It is the ratio of measured NH4+ to predicted NH4+ concentration, which is defined as follows:where NH4+(m), SO42–(m), NO3–(m), and Cl–(m) are the measured concentration of the samples. Also, it is assumed that NH4+ is the only cation balancing the anions. This is a reasonable assumption as previous studies have reported that most of the metallic cations which can contribute to aerosol neutralization are associated with coarse mode (>2.5 μm) particles. The ANR value averaged at ∼1 and 0.8 during foggy and nonfoggy periods, respectively, at Lumbini, Nepal.[50] Another study from Kanpur stated that interstitial aerosols were significantly neutralized (ANR ∼ 1) accompanied by highly oxidized aerosol during foggy events.[30] These were slightly less neutralized; that is, NH4+ is insufficient to completely neutralize anions and accompanying less oxidized aerosol during nonfoggy events. Contrary to interstitial aerosols, the ANR value for fogwater averaged around 0.51, suggesting insufficient ammonia to neutralize anions.[50] It is observed that calcium concentration was substantially high in fogwater such that calcium and ammonium together neutralized the anions.

Also, in a study conducted by Mandariya et al.,[36] acidic conditions were observed during AFP (pH = 1.4 ± 0.7) and DFP (pH = 2.0 ± 2.1), where oligomerization was considered a possible source of OOA formation through aqueous phase processing. The production of SOA is reflected by changes in the microphysical properties of the aerosols, such as (a) changes in the organic carbon of the aerosols because of condensation of the semi-VOCs over pre-existing aerosols; (b) increase in the modal diameter of the size distribution during foggy episodes and growth in the diameter of the particles upon the fog evaporation;[35] (c) aerosol number—new particle formation due to oxidation of VOCs may lead to an increase in number; (d) surface and volume concentrations—condensation of semi-VOCs on the existing aerosols and aqueous chemistry followed by the growth of the aerosols may lead to increased volume concentration. The growth of the particle and change in the number size distribution due to aqueous SOA formation have also been previously documented.[51]

Over central IGP, the enhanced formation of SOA has been reported during the fog period compared to that in the nonfog period.[30,36,52] During fog episodes, a water-soluble fraction of aerosols gets dissolved in fog droplets, forming highly oxidized and less volatile OA.[5,52] In Figure , we can see the SOC/OC ratio during fog and nonfog events at various sites. A distinct difference in secondary organics percentage can be observed during fog periods. Thus, we now know that secondary formation of organic aerosols increases during a high relative humidity (fog) period. A recent study has also given deeper insights about the changes in the organic aerosols’ composition and secondary aerosols’ formation by aqueous processing.[36] This study reports the chemical characteristics of organic aerosols in prefog and during fog and post-fog periods, which has given us a much clearer idea about aqueous processing of aerosols. However, there is still numerous unanswered questions about the aerosol chemistry during the fog period.

Figure 2

SOC/OC ratio during fog and nonfog periods at different sites in IGP along with their OC/EC ratio: Delhi,[53,54] Agra[55] Sikandarpur (rural site in central IGP),[56] Kanpur,[52] Allahabad,[22] Lumbini.[58] All OC-EC concentrations used are analyzed using NIOSH protocol. The “*” symbol means OC-EC is measured using the IMPROVE TOR method. The “#” at Delhi is for OC/EC concentrations shown for the whole winter time.

A Van Krevelen (VK) diagram (H/C vs O/C plot) shows the effect of functionalization reaction of organic compounds on the H/C and O/C ratios of aerosol (see Supporting Information for details). In the VK diagram, slopes of 0, −1, and −2 represent addition of an alcohol/peroxide group, a carboxylic acid group, and a ketone/aldehyde group to the organic compound, respectively. Furthermore, it can be observed from the VK plot that H/C > 1 and O/C > 0.5, which indicates that there is less influence of aromatic compounds and dominance of aliphatic compounds in WSOA at both sites.[57] Mandariya et al.[36] observed a shift of the VK slope toward shallower from PrFP (−0.56 ± 0.11) to AFP (−0.47 ± 0.06), indicating the importance of −OH functionalization and fragmentation reactions. The −OH functionalization was supported by movement of the OA position toward higher f43 in a triangular plot. This was further verified by an increase in negative OSC and a decrease in OSC of OOA. Due to higher BB loading and acidic conditions (pH = 1.4 ± 0.7), oligomerization can be a possible reason for OOA formation through aqueous phase processing. This was backed by an overall shifting of OA toward higher Nc and lower OSC. Here, OSC and Nc refer to average carbon oxidation state and carbon number of the organic aerosol, respectively.

From AFP to FP, the VK slope shifts toward a steeper one, from −0.47 ± 0.06 to −0.59 ± 0.06, indicating dominance of fragmentation and addition of −OH along with carbonyl/aldehyde or the −RCOOH group. This was also supported by an overall shifting of OA toward higher Nc and higher OSC.

During DFP, the VK slope changes from −0.59 ± 0.06 to −0.62 ± 0.11, indicating possible addition of more carbonyl groups than with −RCOOH groups. Also, the decrease in Nc and OSC suggests oligomerization with addition of carbonyl groups, as increased acidity of aerosol (pH = 2.0 ± 2.1) can trigger oligomerization reaction.

The VK slope further steepens from DFP to PoFP. Reduction in Nc and increment in OSC of OOA likely indicate OOA formation through fragmentation and addition of −RCOOH groups.

Mass Scavenging Efficiency of Fog Droplets

Izhar et al.[50] calculated the mass scavenging efficiency (MSE, ratio of species concentration in fogwater to the total concentration (fogwater + aerosol)) for water-soluble components during the foggy night. It has been reported that crustal sources had the highest scavenging efficiency, followed by secondary components and biomass burning species (Ca2+ (0.92) followed by Mg2+ (0.85) > NO3– (0.73) > WSOC (0.69) > Na+ (0.66) > SO42– (0.57) > K+ (0.52) > NH4+ (0.49) > Cl– (0.34)).

It was reasoned that since WSOC’s MSE is fairly high, it could have played an equally active role in nucleation scavenging as the other hygroscopic inorganic species. Also, for secondary species, MSE was found to be directly dependent on liquid water content and inversely on aerosol concentration. Furthermore, calculating the ratio of fogwater to aerosol enrichment factor revealed Cu, Mn, and Ba to be highly soluble, whereas Al and Pb tend to be more insoluble.

Many studies have revealed that fine fog droplets (diameter: 4–16 μm) are more concentrated in organic and inorganic solutes compared to medium (16–22 μm) and coarse (>22 μm) ones because of their large surface area and higher residence time.[31,35,52] In the study conducted by Chakraborty et al.,[31] WSOC concentrations were found to be 126, 24, and 19 mgC/L for fine, medium, and coarse fog droplets, respectively. Also, the concentration of Cu increased from 0.23 to 1 ppm and that of Fe increased from 0.01 to 0.18 ppm from coarse to fine fog droplets. Furthermore, NO3– and SO42+ concentrations showed a relative increase in their contribution to 21 and 15%, respectively.

Fogwater pH was slightly acidic to neutral (5.1 < pH < 7.3), with NH4+ being sufficient to neutralize SO42– and NO3–. Fine droplet samples were visibly yellow in color compared to the almost clear solution for other droplet sizes. Other studies of locations impacted by biomass burning have also reported light-absorbing material in fog and cloud droplets.[19,59,60]

Residues of fine fog droplets are more oxidized (O/C = 0.88) OM as compared to medium (O/C = 0.74) and coarse (O/C = 0.68) ones. Also, fine fog droplet residues were found to have a larger fraction of extremely low volatility OM than medium and coarse fog droplet residues, indicating a majority of residual OM to be semivolatile to moderately volatile in nature.[31] Polluted air with higher number concentration and mass loading tends to form fog with more numerous but smaller droplets. The smaller drops settle down gradually and thus enhance fog lifetime and limit natural cleansing of the atmosphere. Therefore, this shift toward reduced fog droplet sizes might lead to enhanced production of highly oxidized, less volatile SOA mass, which in turn might impact the resulting particle properties in terms of hygroscopicity and lifetime. Such trends might (partially) reverse the role of fog as a natural cleansing agent of the atmosphere due to efficient fine particle scavenging and settling of droplets.[31]

Emerging Important Role of Brown Carbon

Choudhary et al.[61] has reported the light absorption properties and direct radiative forcing (DRF) of WSBrC and EC during fog and nonfog periods. It has been reported that a significant enhancement has been observed during fog episodes in the light absorption properties, such as absorption coefficient (babs_365) and mass absorption efficiency (MAE) except AAE (angstrom absorption exponent), which remains the same during both periods. This study also highlighted the higher light-absorbing potential of ambient BrC at Kanpur compared to that of other sites in the IGP. Furthermore, enhancement in MAE of BrC and EC during fog episodes has been reported to be ∼15 and 20%, respectively. Moreover, DRF of BrC with respect to EC has been observed to be 2 times higher during fog episodes compared to that with nonfog conditions, which is attributable to the aqueous processing of organic compounds.[61]

Previous studies have used just the soluble part of BrC using the filter-based solvent extraction method to estimate its contributions to radiative forcing.[62,63] However, water-insoluble brown carbon (WIBrC) may have a significant contribution to total BrC absorption.[64,65] The existing approaches assume fixed AAE for BC (AAEBC = 1) to apportion light absorption into black carbon (BC) and BrC, which is not always true for ambient aerosols.[66] Izhar et al.[66] derived the BC and BrC absorptions using a pre-existing simplified two-component model but by obtaining BCAAE optimal by constraining the BCAAE value between 0.8 and 1.4 for every sample.

The optimal value was obtained by performing a least-squares minimization model between the observed and estimated carbonaceous absorption values[67] by applying following constraints: first, non-negative absorption values for BC and BrC at all wavelengths, and second, AAE for BC and Ae,BrC (aerosol brown carbon) within their possible ranges of 0.8 to 1.4[68] and 1.5 to 15,[69,70] respectively.

Recent findings have shown that BrC absorption in aqueous extracts (Aq,BrC) is not the same as originally present in aerosol or particulate forms (Ae,BrC).[65,71] Izhar et al.[65] found that the WSBrC absorption value in aerosol to be 1.8 times higher than that obtained in aqueous extracts, and mass absorption efficiency at 365 nm (MAE365) of WSBrC in aerosol was lower than that of WIBrC by 2.2 times. Also, despite scavenging, the MAE of BC and BrC for the fog droplet associated events was found to be increased, indicating that aqueous processing can significantly enhance their absorption capacity. Furthermore, a recent study reported the discrepancy in the OC and BC data obtained from reanalysis data, which was attributed to the variable emission sources, secondary formation, and aging of aerosols.[72]

Summary and Future Outlook

In summary, trends from various studies show that the highest concentration can be found during foggy winter nights for species other than K+, WSOC, WSIC, SO42–, and NO3–. Also, including AFP and DFP for fog episodes can prove beneficial in bridging the gap between modeled and measured SOA by helping us understand the role of OA in wintertime fog formation chemistry and dissipation processes observed over the central IGP.

The oligomerization mechanism could be significant for the formation of SOA along with functionalization of −OH and carbonyl (aldehyde/ketone) moieties during AFP and DFP, respectively, accompanied by acidic aerosol as well as high aerosol liquid water content conditions. Also, fragmentation process can be dominant along with functionalization of −RCOOH or carbonyl (aldehyde/ketone) and −RCOOH moieties during FP and PoFP periods, respectively.

Transition metals such as Fe, Cu, Mn, and Cr have been found to provide alternate pathways for aqueous production of SOA by forming a complex with organic ligands especially during the night-time. Crustal sources had highest MSE, followed by secondary and biomass burning sources. It is suggested that WSOC also plays an active role in nucleation scavenging.

A new approach of calculating BC and BrC absorptions revealed that the WSBrC absorption value in aerosol is 1.8 times higher than that obtained in aqueous extracts, and MAE365 of WSBrC in aerosol was lower than that of WIBrC by 2.2 times. Furthermore, light absorption capacity of WSBrC has been found to be enhanced during fog episodes. Hence, it is of prime importance to have a more detailed study to understand all of the components of OC which can be part of BrC in terms of their concentration, nature of its origin, atmospheric transformation, and processing.

Organic aerosol plays a vital role in facilitating fog formation as well as being responsible for their longer residence time in the ambient atmosphere. Due to their lower hygroscopicity, organic aerosol led to formation of finer fog droplets. Also, finer fog droplets have been found to contain a higher concentration of organic and inorganic solutes when compared with medium and coarser fog droplets. These were visibly yellow in color, containing more amounts of highly oxidized OM than coarse and medium ones. In addition, ammonia/ammonium ions play an important role in stabilizing organic aerosol and aid with this recurring haze–fog–haze cycle that is dominant during wintertime in the IGP. Therefore, controlling the major anthropogenic sources of organic aerosol and ammonia needs to be our top priority in this part of the world. To further understand the atmospheric processing during fog periods and production of SOA, the following future studies must be conducted: (a) A time-resolved study should be performed to better understand the correlation between transition metals and WSOC and if there exist alternate oxidation and aging mechanisms of organic aerosol. (b) Stable isotope study during different stages of fog period should be conducted to better understand the fractionation processes occurring in the atmosphere. Also, compound-specific stable isotope analysis of individual organic compounds like WSOC and hydrocarbon compounds will further help to identify the various factors responsible for secondary aerosol formation (reaction pathway, rate of formation). (c) Chemical speciation of WSOC and carbonaceous components like EC and OC, both in fogwater and interstitial aerosol, will help us to understand the organic classes of compounds and their interaction during the fog occurrence in the Indo Gangetic Plain area. Furthermore, determining relative scavenging efficiencies for different source markers can help us understand their residence time.