Understanding the origin of carbonaceous aerosols during periods of extensive biomass burning in northern India.

Post-harvest crop residue burning is extensively practiced in North India, which results in enhanced particulate matter (PM) concentrations. This study explores the PM (particulate matter with aerodynamic diameter ≤ 2.5 μm) emissions during various time periods (pre-monsoon, monsoon, and post-monsoon) over the biomass burning source region in Beas, Punjab. The PM concentrations during the pre-monsoon period (106-458 μg m) and the post-monsoon period (184-342 μg m) were similar but much higher than concentrations during the monsoon season (23-95 μg m) due to enhanced wet deposition. However, the carbonaceous aerosol fraction in PM was nearly double in the post-monsoon season (∼27%) than the pre-monsoon period (∼15%). A higher contribution of secondary organic carbon (SOC) observed during the pre-monsoon season can be attributed to enhanced photochemical activity in dry conditions. Stable carbon isotope ratio (δC value) of ambient PM allowed elucidation of contributing sources. δC correlation with SOC during post-monsoon and pre-monsoon periods suggests significant influence of secondary formation processes during both time periods. The concentrations of carbon fractions in sampled sources and aerosols suggests contribution of biofuels, resulting in enhanced PM concentration at this location. δC values of pre- and post-monsoon samples show dominance of freshly emitted aerosols from local sources. Impact of biomass and biofuel combustion was also confirmed by biomass burning K tracer, indicating that major agriculture residue burning occurred primarily during nighttime. C plant derived aerosols dominated at the sampling location during the entire sampling duration and contributed significantly during the pre-monsoon season. Whereas, both fossil fuel and C plant combustion contributed to the total mass of carbonaceous aerosols during the post-monsoon and monsoon seasons.