SEEC-2017; Feb 26-28 2017, Mohali India

International Conference on Sustainable Energy and Environmental Challenges (SEEC-2017; Feb 26-28 2017, Mohali India)

HETEROGENEOUS REACTIVITY ONTO SURFACE OF FINE-FRACTION MINERAL DUST Pradhi Rajeev Department of Civil Engineering, Indian Institute of Technology Kanpur. Email: [email protected] Vikram Chaudhary Department of Civil Engineering, Indian Institute of Technology Kanpur.

Prashant Rajput Department of Civil Engineering, Indian Institute of Technology Kanpur. Fena Sorathia Department of Civil Engineering, Indian Institute of Technology Kanpur.

Tarun Gupta Professor Department of Civil Engineering, Indian Institute of Technology Kanpur. Email: [email protected]

ABSTRACT Fine-fraction ambient aerosols (PM2.5; n= 32) have been studied during the South-west monsoon (July‒ September, 2015) at Kanpur (central Indo-Gangetic Plain). Water-soluble ionic species (WSIS) have been measured to assess the undergoing processes. ∑WSIS varied from 4‒32 μg/m3 in PM2.5. NH4+ and SO42- were found to be predominant in PM2.5 (16‒120 μg/m3). Ca2+ and Mg2+ were associated with HCO3-, while NO3- and SO42- are neutralized predominantly by the NH4+ and ∑-/∑+ ratio is ≈ 1 in PM2.5. Furthermore, heterogeneous formation of SO42- onto the surface of fine-fraction mineral dust has been inferred from co-variability of nss-SO42- with nssCa2+. Keywords : Heterogeneous reactivity, fine-fraction mineral dust, South-west monsoon, IGP, Ionic composition. INTRODUCTION Complexity in fine particulate matter composition arising due to secondary aerosol formation has been a matter of concern from the view point of atmospheric chemistry, human health and other environmental issues. Massive anthropogenic emissions and their reactivity due to photochemical reactions, high RH condition, finefraction mineral dust and transition metals are potential factors enhancing secondary aerosol species contribution [1-6]. Formation mechanism of secondary species through homogeneous pathway has been assessed and well

documented in literature for most of the identified species in atmosphere [7-9]. However, heterogeneous phase reactivity occuring in ambient atmosphere demands to cater many studies [10-13]. Recently, many studies have observed enhanced role of heterogenous phase chemistry in influencing atmospheric budget of secondary species (SO42-, NO3-) [12-17]. In year 2015, weak SW-monsoon due to El-Niño has resulted in less rainfall (~ 375 mm) [26]. Relatively high wind-speed during the study period (~ 8 m/s) compared to other seasons of the year, has led to higher upliftment of mineral dust [14]. Manifestation of high abundance of finefraction mineral dust and low precipitation (inefficient wetscavenging) could result into enhanced collisions of pollutants. METHODOLOGY Aerosol sampling was done from July−September, 2015 at Kanpur (26.30 ○N, 80.14 ○E, 142 m above mean sea level). During South-west monsoon in 2015, low precipitation events were recorded (~ 40% of the normal precipitation) which has led us to assess aerosol chemical characteristics during this period. Ambient temperature varied from 22−37 ○C whereas RH from 53−97% during the sampling period. Using a high-volume air-sampler, PM2.5 samples (n = 32) were collected onto the pre-baked quartz filters (WhatmanTM; 20.3 x 25.4 cm2) (Figure 1) [18-19]. Mass concentrations of PM2.5 have been ascertained gravimetrically after equilibrating quartz filters 1


at 25 ± 2 ºC temperature and 40 ± 5% RH. The cations (Na+, NH4+, K+,Ca2+ and Mg2+) and anions (F-, Cl-, NO3-, SO42- and PO43-) have been measured in aerosol extracts using a dual-channel ion-chromatograph (Metrohm, Swiss) [14, 20-23]. HCO3‒ concentration has been measured in all samples by titration using a 5 mM HCl solution and Methyl orange indicator (Thomas Baker). Analyses of samples in duplicate provided analytical uncertainty of the measurement (± 7%), while quality control of the data has been assured from check standards and procedural blanks.

FIGURE 2. PM2.5 concentration and percentage ionic composition in PM2.5. A strong correlation of Ca2+ with HCO3- (R2 = 0.96) represents a characteristic feature of mineral dust in fine fraction over the IGP (Figure 3). As reported earlier in different studies, that major mineral in the mineral dust over IGP is calcite (CaCO3) [14, 24].

FIGURE 1. High- volume PM2.5 sampler equipped with the novel slit impactor assembly (HVIA) [18]. RESULTS AND DISCUSSION Mass concentration of PM2.5 varied from 15.9−119.6 (35.8 ± 20.5) µg/m3. Major ion chemistry in aerosols (PM2.5) collected during the SW-monsoon (July– September, 2015) have been assessed in this study. Watersoluble inorganic species (WSIS) including Na +, NH4+, K+, Ca2+, Mg2+, HCO3-, F-, Cl-, NO3-, PO43- and SO42- have been analyzed, which constitute 35% of the PM2.5. ΣWSIS concentration varied from 3.9−32.1 (11.6 ± 5.6) µg/m3 of which major ions (NH4+, K+, Ca2+, HCO3-, NO3-, PO43- and SO42-) accounts for 97% of the ΣWSIS and the rest was accounted by Na+, Mg2+, F-, Cl-. Figure 2 shows the mass concentration of PM2.5 and the percentage ionic composition of several ions constituting PM2.5 mass. In PM2.5, the dominant ionic species follows the following trend : SO42- (11%) > NH4+ (8%) > HCO3- (5.2%) > PO43(4.7%) > NO3- (3.3%) > Ca2+ (1.5%). Thus, high abundance of anthropogenic species was observed in this region as reported earlier [20]. HCO3- and Ca2+ concentration in PM2.5 indicates the presence of significant fraction of mineral dust in fine fraction aerosols. The method detection limit (MDL) for ionic composition varied from 0.02−0.07 µg/m3.

FIGURE 3. Scatter plot of Ca2+ with HCO3-. Temporal variability plot of equivalence ratio of NH4+/ (NO3- + SO42-) (Figure 4a) indicates complete neutralization of acidic species by NH4+. It means that NH4+ is present in excess (NH4+/ (NO3- + SO42- = 1.39 ± 0.05) during the study. The excess NH4+ was associated with PO43-, indicating agricultural activities in IGP as one of the major sources of atmospheric ammonia. Apart from this, all anions and cations are also charge balanced (Σ-/Σ+ = 0.96 ± 0.01) within the analytical uncertainity (Figure 4b). We have collected fine-fraction (PM2.5) aerosols owing to its larger surface-area as compared to coarser particles as our major interests in this study was to assess the heterogeneous formation of SO42- aerosols. Moreover, it is widely known that SO42- aerosol is formed predominantly during daytime and therefore we preferably collected the PM2.5 samples during daytime only. The formation of CaSO4 by heterogeneous reaction of SO42- with CaCO3 is 2


overruled in this study as we have shown above that all SO42- is neutralized by NH4+. Figure 5 shows the covariability of nss-Ca2+ with nss-SO42-.

aerosols in presence of high RH condition have been reported in several studies [13].

CONCLUSION We document here the chemical characteristics of ambient PM2.5 during the SW-monsoon (July‒September 2015) in the Indo-Gangetic Plain. In PM2.5, SO42- (33.6%) and NH4+ (23%) contributions to ∑WSIS are predominant. HCO3- is associated with Ca2+ and Mg2+, whereas NO3- and SO42- are completely neutralized by NH4+ in fine-aerosols. Heterogeneous formation of sulfate aerosols onto the surface of fine-fraction mineral dust has been inferred from this study. ACKNOWLEDGMENTS This study has been carried out utilizing internal funds from IIT Kanpur. REFERENCES FIGURE 4. Temporal variability of equivalence ratio of: (a) NH4+/ (NO3- + SO42-) and (b) Σ-/Σ+. The non-sea salt ionic concentration has been estimated by following formulae using the sea-water ratios well documented in literature [25, 27] : nss-Ca2+ = (Ca2+)aerosol – (Na+)aerosol X (Ca2+/Na+)sea-water nss-SO42- = (SO42-)aerosol – (Na+)aerosol X (SO42-/Na+)sea-water

FIGURE 5. Co-variability of nss-Ca2+ and nss-SO42-. Thus, it heterogeneous mineral dust formation of

is logical to infer the occurrence of formation of SO42- onto the surface of fine (a field-based evidence). Heterogeneous SO42- onto the surface of fine-fraction

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