Impacts of Diwali fireworks on aerosol levels, composition, chemistry, and toxicity: Insights from a combined indoor-outdoor study

We investigated PM _2.5 levels and chemical composition during the Diwali period at the residential area of the Indian Institute of Technology Bombay (IITB) by sampling at the Hostel 14 terrace (outdoor) and an adjacent residential room (indoor). The average PM _2.5 in the indoor and outdoor locations increased by approximately two times during Diwali (Di) compared to Pre-Diwali (PrD) levels, and remained elevated (~15%) even in the Post-Diwali (PoD) phase, indicating prolonged pollution effects. Among OC–EC fractions, EC1 increased sharply during Di by ~447% indoors and 319% outdoors, highlighting the strong influence of firecracker combustion. Potassium (K⁺), a marker of pyrotechnic activity, also surged during Diwali by ~184% indoors and ~68% outdoors compared to PrD. During Di, sulfate dominated the water-soluble anions. The ammonium to sulfate+nitrate ratio dropped below 1 during Diwali, suggesting enhanced aerosol acidity due to incomplete neutralization of acidic species. HR-ToF-AMS analysis revealed the steeper slope between fC ₂ H ₃ O ⁺ and fCO ₂ ⁺ [ C ₂ H ₃ O ⁺ (m/z=43) , and CO ₂ ⁺ (m/z=44) to Organic Aerosol ratio] during Di time, indicating rapid decarboxylation or enhanced fragmentation. Volume-normalized oxidative potential (OP ^DTTv ) reached 8.46 (±2.25) and 8.98 (±2.52) nmol min ^-1 m ^-3 indoor, and outdoor, respectively, reflecting a ~270% ,and ~216% enhancement relative to PrD. These results demonstrate that short-term pyrotechnic activities during Diwali substantially altered aerosol chemistry, carbonaceous composition, and oxidative potential, even within a relatively low-traffic academic–residential environment. The average number of passively smoked cigarettes (PSC) reached more than 20 for cardiovascular mortality during the Di period.