Spatiotemporal variability of peroxy acyl nitrates (PANs) over megacities from satellite observations
December 05, 2022
Madison Shogrin
Committee: Emily Fischer (Adviser); Steven Miller; Jeffrey Pierce; Vivienne Payne (Jet Propulsion Laboratory); Sheryl Magzamen (Environmental and Radiological Health Sciences)
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Abstract
Peroxy acyl nitrates (PANs) are photochemical pollutants with implications for health and atmospheric oxidation capacity. PANs are formed via the oxidation of non-methane volatile organic compounds (NMVOCs) in the presence of nitrogen oxide radicals (NOx = NO + NO2). PANs serve as reservoir species and sources for NOx in outflow regions of megacities, facilitating O3 production downwind. While urban environments are large sources of PANs, in-situ observations in urban areas are generally limited. Here we use satellite measurements of PANs from the Tropospheric Emission Spectrometer (TES) and the S-NPP Cross-Track Infrared Sounder (CrIS) to evaluate the spatiotemporal variability of PANs over and surrounding 9 megacities: Mexico City, Beijing, Los Angeles, Tokyo, São Paulo, Delhi, Mumbai, Lagos, and Karachi. We use monthly mean values of PANs to determine the seasonal cycle within the urban center of megacities. We find pronounced seasonal cycles of PANs in megacities and seasonal maxima in PANs correspond to seasonal peaks in local photochemical activity. Local fire activity can explain some of the observed interannual variability in PANs over and around megacities. We use S-NPP CrIS data to probe the spatial outflow pattern of PANs produced within urban Mexico City during the month with the largest mixing ratios of PANs (April). Peak outflow in April occurs to the northeast of the city and over the mountains south of the city. Outflow to the northwest appears infrequent. CrIS is used to further explore changes in PANs associated with substantial declines in megacity NOx in response to the COVID-19 pandemic. We only identify two cities over which PANs changed significantly in response to NOx perturbations: Beijing and Los Angeles. This work demonstrates that the space-based observations provided by CrIS and TES can increase understanding of the spatiotemporal variability and sensitivity to precursor emissions of PANs over and around global megacities.
