Changes to Peroxyacyl Nitrates (PANs) over Megacities in Response to
COVID-19 Tropospheric NO2 Reductions Observed by the Cross-track
Infrared Sounder (CrIS)
Abstract
The COVID-19 pandemic perturbed air pollutant emissions as cities
shutdown worldwide. Peroxyacyl nitrates (PANs) are important tracers of
photochemistry that are formed through the oxidation of non-methane
volatile organic compounds (NMVOCs) in the presence of nitrogen oxide
radicals (NOx = NO + NO2). We use satellite measurements of free
tropospheric PANs from the S-NPP Cross-Track Infrared Sounder (CrIS)
over eight of the world’s megacities: Mexico City, Beijing, Los Angeles,
Tokyo, São Paulo, Delhi, Lagos, and Karachi. We quantify the seasonal
cycle of PANs over these megacities and find seasonal maxima in PANs
correspond to seasonal peaks in local photochemistry. CrIS is used to
explore changes in PANs in response to the COVID-19 lockdowns.
Statistically significant changes to PANs occurred over two megacities:
Los Angeles (PAN decreased) and Beijing (PAN increased). Our analysis
suggests that large perturbations in NOx may not result in significant
declines in NOx export potential of megacities.