Chemical characteristics of aerosol and rain water during an El Niño and PDO influenced Indian summer monsoon

- Chemical characteristics of aerosols and rain water in IGP during SW-monsoon.
- Predominant neutralization of acidic species by NH4+ (rain water pH: 6.4-7.6).
- NO3− formation processes during SW-monsoon.
- Below-cloud scavenging is predominant wash-out mechanism.
- Dry and wet deposition fluxes have been estimated.

According to the meteorological long-term variability pattern, year 2015 was influenced by El Niño and PDO (Pacific Decadal Oscillation; causes weakening of Indian Summer Monsoon). These conditions facilitate the assessment of chemical characteristics of fine-mode ambient aerosols (PM2.5; n = 48) and individual rain waters (pH: 6.4-7.6; n = 15) during the South-west monsoon (July-September 2015) in the central Indo-Gangetic Plain (IGP; Kanpur). Water-soluble ionic species (WSIS) have been measured to assess the undergoing processes (neutralization, formation and below-cloud scavenging) and estimate their dry and wet deposition fluxes. The ∑WSIS varies from 4 to 32 μg/m3 in PM2.5, whereas it ranges from 32 to 102 mg/L in rain waters. The NH4+ and SO42− are found to be predominant in PM2.5 (16-120 μg/m3), whereas HCO3− and Ca2+ are predominant in rain water samples. The difference in chemical composition of PM2.5 and rain water is largely attributed to additional contribution of coarse-mode mineral dust in rain water. The Ca2+ and Mg2+ in both aerosols and rain water samples are associated with HCO3−. The NO3− and SO42− are neutralized predominantly by NH4+ and ∑−/∑+ ratio is ≈ 1 in both aerosols and rain waters. Furthermore, co-variability of NO3− with nss-Ca2+ in PM2.5 indicates role of fine-mode mineral dust surface in the formation of ammonium nitrate. Characteristic mass ratios (HCO3−/Ca2+ and SO42−/NH4+) in rain water look quite similar to those in aerosols (PM2.5). This suggests that below-cloud scavenging is predominant mechanism of aerosols wash-out. Dry deposition fluxes of Mg2+, NH4+ and SO42− are ∼13% of their wet deposition fluxes, whereas for K+, Ca2+ and NO3− it is <6%.

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