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.

We use new peroxyacetyl nitrate (PAN) observations from the Cross-Track Infrared Sounder (CrIS) on the Suomi National Polar-orbiting Partnership satellite to investigate PAN over the western U.S. during the summer 2018 North American wildfire season. This period coincides with the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN). When combined with favorable background conditions, the resolution and sensitivity of CrIS is sufficient to observe PAN production in plumes. CrIS PAN normalized excess mixing ratios (NEMRs) in the Pole Creek Fire increase from 0.2% to 0.4% within 3-4 hours of physical aging, consistent with NEMRs calculated from WE-CAN observations. CrIS is also able to detect PAN and CO enhancements in plumes that have been transported hours to days downwind. On average for the study period, 24-56% of PAN in the free troposphere during the afternoon over the western U.S. can be attributed to fires.