Atmospheric abundance of HULIS during wintertime in Indo-Gangetic Plain: impact of biomass burning emissions

This study reports for the first-time the ambient concentrations of HULIS mass (HULIS-OM, Humic-like substances) and HULIS-C (carbon) in PM₁₀ (particulate matter with aerodynamic diameter?≤?10?μm) from the Indo-Gangetic Plain (IGP at Kanpur, wintertime). HULIS extraction followed by purification and isolation protocol with methanol: acetonitrile (1:1?v/v) on HLB (Hydrophilic-Lipophilic Balanced) cartridge has been established. Quantification of HULIS-C was achieved on a total organic carbon (TOC) analyser whereas HULIS-OM was determined gravimetrically. Consistently high recovery (> 90%) of HULIS-C based on analysis of Humic standard (sodium salt of Humic acid) suggested suitability of our established analytical protocol involving solvent extraction, purification and accurate quantification of HULIS. HULIS-OM varied from 17.3–38?μg?m^?3 during daytime and from 19.8–40.6?μg?m^?3 during night in this study. During daytime the HULIS-OM constituted 20–30% mass fraction of OM_Total and 10–15% of PM₁₀ mass. However, a relatively low contribution of HULIS-OM has been observed during the night. This observation has been attributed to higher concentrations of OM and PM₁₀ in night owing to nighttime chemical reactivity and condensation of organics in conjunction with shallower planetary boundary layer height. Strong correlation of HULIS-C with K⁺_BB (R²?>?0.80) and significant day-night variability of HULIS-C/WSOC ratio in conjunction with air-mass back trajectories (showing transport of pollutants from upwind IGP) suggest biomass burning emission and secondary transformations as important sources of HULIS over IGP. High-loading of atmospheric PM₁₀ (as high as 440?μg?m^?3) with significant contribution of water-soluble organic aerosols (WSOC/OC: ~ 0.40–0.80) during wintertime highlights their plausible potential role in fog and haze formation and their impact on regional-scale atmospheric radiative forcing over the IGP.