Chemical characteristics of aerosol and rain water during an El Niño and PDO influenced Indian summer monsoon: Fid-Bau Portal

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

Rajeev, Pradhi / Rajput, Prashant / Gupta, Tarun

Abstract 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 SO4 ²⁻ 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 SO4 ²⁻ 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 SO4 ²⁻/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 SO4 ²⁻ are ∼13% of their wet deposition fluxes, whereas for K⁺, Ca²⁺ and NO3 ⁻ it is <6%.

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Highlights • 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.