Annual and Monsoon Distribution of Biogenic NMVOCs and SOAs over India using High-resolution WRF-Chem Model

Low or semi-volatile organic gases undergo gas-particle conversion, condensation, nucleation and various multiphase reactions forming secondary organic aerosols (SOAs). In the Indian context, the evergreen forests and monsoon's meteorological parameters regulate dispersion, atmospheric chemistry and formation of SOA. This study analyses annual and monsoon (June, July and August) spatial distribution of major biogenic NMVOCs (b-NMVOCs) (isoprene (ISO), limonene (LIM) and -pinene (API)), which are a major cause of biogenic secondary organic aerosols (b-SOAs). Model of Emission of Gases and Aerosol (MEGAN) was used for extracting b-NMVOC emissions and Weather Research and Forecasting with Chemistry (WRF-Chem) model was used for estimating SOA levels. The model performance was validated against available observed concentrations (Fu et al., 2010, 2016; Mahilang & Deb, 2020). Evergreen broadleaf forested regions in the north of north India (NI), northeast India (NEI) and eastern central India (CI) show high levels of ISO intensity (>1.70 ton/km2/year). The total Indian ISO emissions are 4.09 ton/km2/year (2.74 % of global emissions). Aydin et al. (2014) state that broadleaf trees are major contributors towards isoprene emissions while monoterpenes are contributed by coniferous trees. In monoterpenes, the emission intensity of LIM 0.22 ton/km2/year and that of API 0.48 ton/km2/year is mostly from coniferous forested regions. Annual emissions of b-NMVOCs were 19.47 Tg including isoprene as 70.41% and monoterpenes as 29.59%. Monsoon showed higher emission intensity than other seasons. The regions (Indo-Gangetic Plain and NEI) with moderate sunlight (~1150kWh/m2) and daytime temperature (<300C) show high isoprene emissions. Annually, isoprene SOA (i-SOA) contributes 26.00% and monoterpene SOA(m-SOA) contributes 22.75% to total SOA. There was more contribution of i-SOA than m-SOA in broadleaf forested areas. Although, b-SOAs levels are high but their formation has significant contribution from anthropogenic activities. Thus, control over anthropogenic emissions can reduce b-SOA levels to a large extent.