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. Delhi, India, routinely experiences some of the world's highest urbanparticulate matter concentrations. We established the Delhi Aerosol Supersitestudy to provide long-termcharacterization of the ambient submicron aerosol composition in Delhi. Herewe report on 1.25 years of highly time-resolved speciated submicronparticulate matter (PM 1 ) data, including black carbon (BC) andnonrefractory PM 1 (NR-PM 1 ), which we combine to develop acomposition-based estimate of PM 1 (“C-PM 1 ”  =  BC  +  NR-PM 1 ) concentrations. We observed marked seasonal and diurnal variability in the concentration andcomposition of PM 1 owing to the interactions of sources and atmosphericprocesses. Winter was the most polluted period of the year, with averageC-PM 1 mass concentrations of ∼210 µ g m −3 . The monsoon was hot and rainy, consequentlymaking it the least polluted (C-PM 1 ∼50 µ g m −3 ) period. Organics constituted more than halfof the C-PM 1 for all seasons and times of day. While ammonium, chloride,and nitrate each were ∼10  % of the C-PM 1 for the coolermonths, BC and sulfate contributed ∼5  % each. For the warmerperiods, the fractional contribution of BC and sulfate to C-PM 1 increased, and the chloride contribution decreased to less than 2 %. Theseasonal and diurnal variation in absolute mass loadings were generallyconsistent with changes in ventilation coefficients, with higherconcentrations for periods with unfavorable meteorology – lowplanetary boundary layer height and low wind speeds. However, the variationin C-PM 1 composition was influenced by temporally varying sources,photochemistry, and gas–particle partitioning. During cool periods when windwas from the northwest, episodic hourly averaged chloride concentrationsreached 50–100  µ g m −3 , rankingamong the highest chloride concentrations reported anywhere in the world. We estimated the contribution of primary emissions and secondary processes toDelhi's submicron aerosol. Secondary species contributedalmost 50 %–70 % of Delhi's C-PM 1 mass for thewinter and spring months and up to 60 %–80 % for the warmer summerand monsoon months. For the cooler months that had the highest C-PM 1 concentrations, the nighttime sources were skewed towards primary sources,while the daytime C-PM 1 was dominated by secondary species. Overall,these findings point to the important effects of both primary emissions andmore regional atmospheric chemistry on influencing the extreme particleconcentrations that impact the Delhi megacity region. Future air qualitystrategies considering Delhi's situation in both a regionaland local context will be more effective than policies targeting only local,primary air pollutants.

Submicron aerosol composition in the world's most polluted megacity: the Delhi Aerosol Supersite study