AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
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Particle Size Distribution in a Polluted Megacity: The Delhi Aerosol Supersite Study
SHAHZAD GANI, Sahil Bhandari, Kanan Patel, Sarah Seraj, Prashant Soni, Zainab Arub, Gazala Habib, Lea Hildebrandt Ruiz, Joshua Apte, University of Texas at Austin
Abstract Number: 44 Working Group: Urban Aerosols
Abstract Delhi, India, routinely experiences some of the world's highest urban particulate matter concentrations. While fine particulate matter (PM2.5) mass concentration in Delhi are an order of magnitude or more higher than in many western cities, the particle number (PN) concentrations are not similarly elevated. Here we report on 1.25 years of highly time resolved particle size distribution (PSD) data from the Delhi Aerosol Supersite study. We also supplement some of our findings using aerosol chemical composition data from our site. Details of aerosol mass and composition have already been presented in Gani et al. (2018).
While the mass concentrations between the winter and summer dropped by more than 50%, the corresponding drop for PN concentration was less than 20%. Furthermore, while it is generally assumed that ultrafine particles (UFP, Dp < 100 nm) make up most of the PN concentration, we observed that large number of accumulation particles—that make up most of the PM2.5 mass—contributed to almost half of the PN concentrations for some of the extremely polluted periods. UFP concentrations were found to be lower during periods with some of the highest mass concentrations. Calculations based on measured PSDs and coagulation theory suggest wintertime UFP concentrations suppression by a rapid coagulation sink. High wintertime particle surface area, mostly from particles in the accumulation mode, provides a plausible sink for UFP. Implications of a strong accumulation mode coagulation sink for future air quality control efforts in Delhi are that a reduction in mass concentration, especially in winter, may not produce proportional reduction in PN concentration. Long term continuous observations of PSDs from Delhi provide important insights into the role of sources and atmospheric processes that drive aerosol number concentrations.
Gani et al. (2018), Atmospheric Chemistry and Physics Discussions, https://doi.org/10.5194/acp-2018-1066.