Seasonal Variation in Chemical Composition of Size-Segregated Aerosols Over the Northeastern Arabian Sea

Water-soluble species constitute a significant fraction (up to 60–70%) of the total aerosol loading in the marine atmospheric boundary layer (MABL). The “indirect” effects, that is, climate forcing due to modification of cloud properties depend on the water-soluble composition of aerosols. Thus, the characterization of aerosols over the MABL is of greater relevance. Here, we present 1-year long aerosol chemical composition data of PM10 and PM2.5 at a costal location in the northeastern Arabian Sea (Goa; 15.45°N, 73.20°E, 56 m above the sea level). Average water-soluble ionic concentration (sum of anion and cation) is highest (25.5 ± 6.9 and 19.6 ± 5.8 μg·m−3 for PM10 and PM2.5, respectively) during winter season and lowest during post-monsoon (17.3 ± 9.1 and 14.4 ± 8.1 μg·m−3 for PM10 and PM2.5, respectively). Among water-soluble ionic spices, SO42- ion was found to be dominant species in anions and NH4+ is dominant in cations, for both PM10 and PM2.5 during all the seasons. These observations clearly hint to the contribution from anthropogenic emission and significant secondary inorganic species formation. Sea-salt (calculated based on Na+ and Cl−) concentration shows significant temporal variability with highest contribution during summer seasons in both fractions. Sea-salt corrected Ca2+, an indicator of mineral dust is found mostly during summer months, particularly in PM10 samples, indicates contribution from mineral dust emissions from arid/semiarid regions located in the north/northwestern India and southwest Asia. These observations are corroborated with back-trajectory analyses, wherein air parcels were found to derive from the desert area in summer and Indo-Gangetic Plains (a hot spot for anthropogenic emissions) during winter. In addition, we also observe the presence of nss-K+ (sea-salt corrected), for PM2.5, particularly during winter months, indicating influence of biomass burning emissions. The impact on aerosol chemistry is further assessed based on chloride depletion. Chloride depletion is observed very significant during post-monsoon months (October and November), wherein more than 80 up to 100% depletion is found, mediated by excess sulfates highlighting the role of secondary species in atmospheric chemistry. Regional scale characterization of atmospheric aerosols is important for their better parameterization in chemical transport model and estimation of radiative forcing.

Impact of organic acids on chloride depletion of inland transported sea spray aerosols

Heterogeneous reactions on sea spray aerosols (SSA) are the main pathway to drive the circulation of chlorine, nitrogen, and sulfur in the atmosphere. The release of Cl will significantly affect the physicochemical properties of SSA. However, the impact of organic acids and mixing state on chloride depletion of SSA is still unclear. Hence, the size and chemical composition of individual SSA particles during the East Asian summer monsoon were investigated by a single particle aerosol mass spectrometer (SPAMS). According to the chemical composition, SSA particles were classified into SSA-Aged, SSA-Bio and SSA-Ca. In comparison to the aged Na-rich SSA particles (SSA-Aged), some additional organic species related to biological origin were observed in SSA-Bio, and each of two types accounts for approximately 50 % of total SSA particles. SSA-Ca may associated with organic shell of Na-rich SSA particles, which only accounts for ~ 3 %. Strongly positive correlations between Na and organic acids (including formate, acetate, propionate, pyruvate, oxalate, malonate, succinate, and glutarate) were observed for the SSA-Aged (r2 = 0.52, p

How Are Strong Acids or Strong Bases Substituted by Weak Acids or Weak Bases in Aerosols?

<p>Due to significant influence on global climate and human health, atmospheric aerosols have attracted numerous interests from the atmospheric science community. To provide insight into the aerosol effect, it is indispensable to investigate the aerosol properties comprehensively.</p><p>Since atmospheric aerosols are surrounded by substantial gas phase and have high specific surface area, the composition partitioning between particle phase and gas phase must be considered as a key aerosol property, which is termed as volatility for volatile organic/inorganic components. Recent studies show that the aerosol volatility can also be induced by the reaction of components in addition to the volatile compositions. Herein, we summarize four types of volatility induced by reaction, namely chloride depletion, nitrate depletion, ammonia depletion and volatility induced by salt hydrolysis. For chloride depletion and nitrate depletion, these processes can be regarded as reactions that strong acids are substituted by weak acids. The high volatility of the formed HCl or HNO<sub>3</sub> drives the reaction continuously moving forward.</p><p>For ammonium depletion, we observed the reaction occurs between (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> and organic acid salts during dehydration process by ATR-FTIR. For example, when molar ratio is 1:1, significant depletion of ammonium was observed in the disodium succinate/(NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> particles, indicating the evaporation of NH<sub>3</sub>. Besides, the hygroscopicity of the aerosol particles decreased after the dehydration, which should be attributed to the formation of less hygroscopic succinic acid and ammonium depletion. By regarding organic acid salts as weak bases, the ammonium depletion is a reaction that strong base substituted by weak base, driving by the continuous release of NH<sub>3</sub>. In addition to volatility induced by reactions within multi-component aerosols, we also found that the salt hydrolysis can also cause the formation of volatile product. For magnesium acetate (MgAc<sub>2</sub>) aerosols, we found significant water loss of the aerosol particles under constant relative humidity condition, while the amount of acetate was also decreased. We infer that the acetic acid (HAc) evaporation is caused by the hydrolysis of MgAc<sub>2</sub>, leading to the volatility and declined hygroscopicity. Two factors contribute to the volatility of MgAc<sub>2</sub> aerosols. One is the volatile acid donner (Ac<sup>2-</sup>), which can lead to the formation of volatile HAc. The other is the residual ion accepter (Mg<sup>2+</sup>), which can combine residual OH<sup>-</sup> after the proton is depleted by the evaporation of HAc. The formation of insoluble Mg(OH)<sub>2</sub> effectively maintains the aqueous pH in a suitable range, keeping the reaction moving forward. It should be noted that the co-exist of volatile acid donner and residual ion accepter is indispensable for the volatility induced by hydrolysis.</p><p>Generally, for the volatile species present in atmosphere, the aerosol volatility induced by the reaction of components can be an important pathway for their recycling processes. Due to the substantial composition modification, the hygroscopicity is also affected by such reaction. Therefore, this partitioning behavior of aerosols needs to be considered in the future atmospheric aerosol study, which may prevent the underestimate of particle volatilization or overestimate of hygroscopicity.</p>