10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Submicron Aerosol Composition in the World’s Most Polluted Megacity: The Delhi Aerosol Supersite Campaign
SHAHZAD GANI, Sahil Bhandari, Sarah Seraj, Dongyu S. Wang, Kanan Patel, Prashant Soni, Zainab Arub, Gazala Habib, Lea Hildebrandt Ruiz, Joshua Apte, University of Texas at Austin
Abstract Number: 1270 Working Group: Air Quality in Megacities: from Sources to Control
Abstract Delhi is one of the most polluted megacities in the world, where annual average PM2.5 levels exceed 130 µg m-3, and wintertime episodic concentrations commonly exceed 500 µg m-3. However, policies to improve air quality and public health are impeded by a limited understanding of the sources and atmospheric dynamics of air pollutants. Here we report on 1.5 years of data from the Delhi Aerosol Supersite. This site, located at Indian Institute of Technology Delhi, is the first long-term chemical characterization of ambient submicron aerosol in India, with near-continuous online measurements of aerosol composition and size distribution since January 2017. Our measurements include non-refractory PM1 (NR-PM1, via Aerodyne’s aerosol chemical speciation monitor, ACSM), black carbon (BC, via aethalometer) and particle size distributions (scanning mobility particle sizer).
We observe marked seasonal and diurnal variability in the concentration, composition and size distribution of PM1 owing to the interactions of sources and atmospheric mixing. Winter is the most polluted period of the year with average PM1 (NRPM1 + BC) mass concentrations of 130 µg m-3. Monsoon was the least polluted with 40 µg m-3 and summer, spring and autumn between 50 – 70 µg m-3. Organics are the single-largest PM1 mass component for all seasons and times of day (~50%). While the average BC concentration was 17 µg m-3 for winter and 7 – 13 µg m-3 for all other seasons, BC only contributed to 13% of the PM1 mass concentration in the winter compared to 16 – 22% for the other seasons. We observe average wintertime chloride concentrations of 11 µg m-3 (~10% of observed PM1 mass concentrations). Diurnal cycles of chloride have a pronounced early morning peak, with episodic concentrations exceeding 100 µg m-3, among the highest levels observed anywhere in the world. Sulfate contributes ~ 16 – 19% of PM1 mass in the warmer months; 9 – 11% for winter and spring, perhaps reflecting more efficient photochemical formation of sulfate during warmer months. Boundary layer height exerts strong influence on diurnal cycles of all primary pollutants, with nocturnal concentrations of primary species routinely exceeding daytime levels by 3-5×.
While the wintertime mass concentrations were 4-5× times higher than during the cleaner monsoon months, the average number concentrations were only 1.9× higher during wintertime compared to monsoon (37,000 # cm-3 vs. 20,000 # cm-3). We attribute this relatively low enhancement in wintertime particle number concentration to coagulative scavenging. Concurrently, coagulation leads to a strongly pronounced accumulation mode in particle size distributions, with unusually large count median diameters during polluted conditions (~90-130 nm).
Positive matrix factorization (PMF) conducted on the ACSM mass spectra provides further information on the sources and atmospheric processes that affect NR-PM1 levels in Delhi. Hydrocarbon-like organic aerosol (HOA) exhibits strong diurnal variability, reflecting the impact of primary combustion emissions modulated by diurnal cycles in mixing height. As observed in other megacities, oxidized organic aerosol (OOA) is the largest constituent of the organic aerosol throughout the year, demonstrating the profound influence of secondary formation on particle concentrations in Delhi. Biomass burning organic aerosol (BBOA) constituted majority of the organic aerosol during extreme wintertime biomass burning events, but was almost absent during the less polluted monsoon months.
Overall, these findings point to the important effects of both primary emissions and regional atmospheric chemistry on influencing the extreme particle concentrations that impact the Delhi megacity region. Future air quality strategies considering Delhi’s situation in a regional context will be more effective than policies targeting only local, primary air pollutants.