Emissions from agricultural fires in India: field measurements of climate relevant aerosol chemical and optical properties†

Abstract

Carbonaceous aerosol particles are associated with large uncertainties in their climate impacts because of incomplete knowledge of their optical properties and emission magnitudes. Biomass-burning sources significantly contribute to carbonaceous aerosol emissions in India, with crop residue burning being crucial during post-harvest months. Here, for the first time, we study the chemical and optical properties of emission aerosols using in situ real-time and filter-based measurements from significantly contributing crop residue straws, stalks, and stems in India. Emitted particles exhibited optical behaviour characteristic of the brown-black carbon absorption continuum, with large mass absorption cross-sections (MAC₅₂₀: 8.2 ± 9.6 m² g⁻¹) and small absorption Angström exponents (AAE_370/660: 1.97 ± 0.81). They contain significant amounts of lower volatility organic (OC_LV) and elemental carbon fractions. The relative abundances of OC_LV correlate positively with MAC₅₂₀ and negatively with AAE_370/660, implying significant absorption exerted by OC_LV, with likely atmospheric persistence. Additionally, we measured emission factors for a complete list of particulate chemical constituents. Emission factors of elemental carbon are larger than those in earlier studies, indicating a 1.6–3.8 times increase in the climate warming potential of the emitted particles from crop residue burning. The intrinsic property measurements and the emissions estimates made here can aid climate modelling efforts that underestimate aerosol absorption in the region.