Effect of Biomass Burning on PM2.5 Composition and Secondary Aerosol Formation During Post‐Monsoon and Winter Haze Episodes in Delhi. Issue 1 (30th December 2021) - Lalchandani, V.
Abstract: Delhi metropolitan area suffers from extreme haze during the post‐monsoon and winter season, impacting climate, public health, and economy. We used a high‐resolution time‐of‐flight aerosol mass spectrometer (HR‐ToF‐AMS) and aethalometer at two urban locations in Delhi to capture non‐refractory PM2.5 (NR‐PM2.5 ) and black carbon (BC) during the post‐monsoon and winter season of 2019–2020. Four haze periods with high composition based‐PM2.5 (C‐PM2.5 = NR‐PM2.5 + BC) concentration and distinct chemical composition were identified, during all of which organics dominated but with varying contribution (∼[50%–70%] of C‐PM2.5 ). Biomass burning organic aerosol (BBOA) was dominant in all periods (∼[31%–45%] of OA), but the majority of it was highly aged ∼(45%–50%) with high O/C (0.71 and 0.46 at the two sites), formed most likely through rapid dark oxidation of freshly emitted and partially oxidized BBOA. High polycyclic aromatic hydrocarbons (PAH) signals in the fresh BBOA mass spectra suggest incomplete combustion activities such as open biomass burning emissions as major source. During an agricultural burning event in north‐western India, we estimated that ∼(44%–53%) of total C‐PM2.5 (combined contribution of aged BBOA and oxygenated OA) measured in Delhi was influenced by long‐range transported biomass burning emissions. During winter, secondary inorganics constituted a significant fraction apart from organics ∼(48%–55%), mainly in the form of ammonium nitrate (NH4 NO3 ; up to ∼[19%–25%] of C‐PM2.5 ) and ammonium sulfate (NH4 SO4 ; up to ∼[27%–38%]). Enhanced formation of NH4 NO3 and related‐secondary organic aerosol (SOA) were linked to nighttime oxidation of BBOA, while NH4 SO4 and related‐SOA were linked to heterogeneous aqueous phase oxidation under high RH conditions (>90%). Plain Language Summary: During post‐monsoon and winter seasons, Delhi experiences high concentration of fine particles leading to haze events. To design targeted mitigation policies, an understanding of key factors responsible for haze events is needed. We used an advanced suite of instrumentation to capture temporal trends of real‐time particle chemical composition at two urban locations in Delhi. Using advanced receptor modelling and molecular level source signatures, the open burning of biomass material was estimated to be the largest contributor to haze events during post‐monsoon and winter haze events. The post‐monsoon haze pollution was largely affected by the aged biomass burning particles likely from the long‐range transport of agricultural residue burning emissions in the upwind states of Delhi. Winter haze episodes were also highly affected by biomass burning emissions, likely from the local burning of wood, coal and/or roadside trash for heating and/or cooking purposes. However, the winter haze particles contained significant aged inorganic material including ammonium nitrate, ammonium sulfate and ammonium chloride aerosol apart from aged organics. This study also suggests that the nighttime aging of biomass burning emissions contributed in the rapid formation of aged organic and inorganic particles, causing haze events. Key Points: Four haze periods with distinct PM2.5 chemical composition were identified during post‐monsoon and winter season in Delhi Biomass burning organic aerosol was dominant in all haze periods with the majority of it present in highly aged form Open biomass burning emissions undergo rapid nighttime oxidation resulting in high yields of secondary organic and inorganics