Effects of organic aerosol loading and fog processing on organic aerosol volatility

A detailed time-resolved chemical characterization and volatility study of winter time ambient non refractory submicron aerosols (NR-PM1) was conducted at Kanpur, a polluted city of India. Two very distinct, high (HL, ~ 240 μg/m3) and low (LL, ~100 μg/m3) aerosol loading periods were observed during the campaign, impacted by frequent fog events (n=17). On average, organic aerosols (OA) contributed nearly 60% of the total NR-PM1 mass. Overall, OA volatility, (as measured by mass fraction of OA remaining after passing through a thermo denuder kept at 300 °C) decreased significantly (60%) from HL to LL period. OA volatility is anti-correlated to OA loading but much more strongly so in LL compared to HL period. Volatilities of different types of OAs, as identified by positive matrix factorization (PMF) method, showed significant variations (up to 300%) from HL to LL period. This indicates that nature (like oxidation state, molecular structure, functional groups) of the OAs might have changed with variations in loading conditions. The presence of fog had little or no impact on overall OA volatility, in spite of (15-30) % enhancement in the ambinet OA oxidation ratio (O/C ratio) during fog. This study examines combined effects of OA loading and fog aqueous processing on the ambient OA volatility for the first time.