Chemical characteristics of rainwater and fine particulate matter (PM2.5) over an urban site in the middle Indo-Gangetic Plain

Environ Geochem Health. 2026 Apr 29;48(7):324. doi: 10.1007/s10653-026-03207-7.

ABSTRACT

Rainfall plays a key role in the removal of atmospheric pollutants (both particulate matter and gaseous pollutants) through wet scavenging and thus, the interaction between rain and atmospheric pollutants significantly influences the chemical composition of rainwater. Therefore, in this study, PM_2.5 and rainwater samples were collected from an urban site in the middle Indo-Gangetic Plain, Varanasi (25.28° N, 82.96° E), India during January-December 2022. The water-soluble inorganic ions (WSIIs), as well as carbonaceous and nitrogenous species in PM_2.5 and rainwater samples were analysed to understand their sources, seasonal variability, scavenging processes, and deposition fluxes. PM_2.5 mass concentrations ranged from 15.5 to 279.3 µg m^⁻3, with an annual mean of 91.6 ± 50.5 µg m^⁻3, significantly exceeding the Indian NAAQS annual limit of 40 µg m^⁻3. The total WSIIs (sum of concentrations of measured anions and cations) was 29.6 µg m^⁻3 accounted for ~ 33% of the total PM_2.5 mass and was dominated (66%) by secondary inorganic aerosols ( ${\text{SO}}_4^{2–}$ , ${\text{NO}}_3^{–}$ and ${\text{NH}}_4^{+}$ ), indicating strong anthropogenic influence. In rainwater, Ca²⁺, ${\text{NH}}_4^{+}$ , and ${\text{SO}}_4^{2–}$ were the major ions, contributing over 76% of total ionic content, with near-neutral pH (mean 6.6 ± 0.5) indicating effective neutralization by alkaline species. Seasonal variation showed higher ionic, dissolved organic carbon, and nitrogen concentrations during non-monsoon period, indicating pollutant accumulation under dry conditions whereas dilution during the monsoon. Wet deposition fluxes were consistently higher than dry deposition fluxes for all major ionic species. Relatively, high scavenging ratios, particularly for ${\text{NO}}_3^{–}$ and Ca²⁺, indicate efficient washout of both anthropogenic and crustal components, with below-cloud scavenging identified as the dominant mechanism. The relatively low ${\text{NO}}_3^{–}$ / ${\text{SO}}_4^{2–}$ ratios in both rainwater (0.54) and PM_2.5 (0.33) indicate predominant influence of stationary emission sources (e.g., coal/biomass burning and industrial emissions). Statistical analysis and diagnostic ratios further indicate contributions from biomass burning and anthropogenic activities (agriculture, vehicular emissions, and construction) for both rainwater and PM_2.5 samples. Moreover, back-trajectory analysis supports the combined influence of local emissions and regional transport. Overall, our study demonstrates a strong coupling between PM_2.5 composition and rainwater chemistry over the IGP and highlights the importance of wet deposition in controlling the overall deposition pattern over Varanasi.

PMID:42056635 | DOI:10.1007/s10653-026-03207-7