Using the CAM-chem Model, we simulate the response of chemical species in the free troposphere to changes in emissions of primary pollutants during the COVID-19 pandemic. Zonally averaged ozone concentrations in the free troposphere during Northern Hemisphere spring and summer were 5 to 15 % lower than 19-year climatological values, in good quantitative agreement with ozone observations. About one third of this anomaly is attributed to the drastic reduction in air traffic during the pandemic, another third to reductions in surface emissions, the remainder to 2020 meteorological conditions, including the exceptional springtime Arctic stratospheric ozone depletion. The overall COVID-19 reduction in mean northern hemisphere tropospheric ozone in June is less than 5 ppb below 400 hPa, but reaches 8 ppb at 250 hPa. In the Southern Hemisphere, COVID-19 related ozone reductions by 4 to 6% were masked by comparable ozone increases due to other changes in 2020.

It was recently proposed (Franco et al., Nature 2021) that methanediol (MD, HOCH2OH ) formed by hydration of formaldehyde in liquid cloud droplets is outgassed to a larger extent than previously estimated, and reacts in the gas phase with the hydroxyl radical (OH), leading to formic acid (HCOOH). Whereas the resulting global production of formic acid is greatly dependent on poorly constrained parameters, such as the Henry’s law constant (HLC) of methanediol and the rate constant of its reaction with OH, Franco et al. suggest, based on global model calculations and on newly conducted chamber experiments (for the rate constant) and on statistical prediction methods (for the HLC), that this mechanism explains the large “missing source” of HCOOH in the atmosphere (e.g. Stavrakou et al. 2012). If true, this finding would be of tremendous importance for our understanding of the biogeochemical cycling of oxygenated organic compounds. For this reason, it is of utmost importance to double check the validity of thehypotheses and parameterizations behind this assessment. Here we examine two critical aspects of this determination: the HLC (taken equal to either 10^4 or 10^6 M atm^-1 in model simulations by Franco et al.) and the rate of the MD+OH reaction (taken equal to 7.5 × 10^12 cm^3 s^-1 ). The representation of chemical processing in liquid clouds in global models is also briefly discussed. Plausible ranges for those parameters are proposed , and causes of uncertainty are discussed . The potential consequences for the resulting production of formic acid are briefly explored.

This study compares recent CO, NO, NMVOC, SO, BC and OC anthropogenic emissions from several state-of-the-art top-down estimates to global and regional bottom-up inventories and projections from five SSPs in several regions. Results show that top-down emissions exhibit similar uncertainty as bottom-up inventories in most regions, and even less in some such as China. In general, for all species the largest discrepancies are found outside of regions such as the U.S., Europe and Japan where the most accurate and detailed information on emissions is available. In some regions such as China, which has undergone dynamical economic growth and changes in air quality regulations during the last several years, the top-down estimates better capture recent emission trends than global bottom-up inventories. These results show the potential of top-down estimates to complement bottom-up inventories and to aide in the development of emission scenarios, particularly in regions where global inventories lack the necessary up-to-date and accurate information regarding regional activity data and emission factors such as Africa and India. Areas of future work aimed at quantifying and reducing uncertainty are also highlighted. A regional comparison of recent CO and NO trends in the five SSPs indicate that SSP126, a strong-pollution control scenario, best represents the trends from the from top-down and regional bottom-up inventories in the U.S., Europe and China, while SSP460, a low-pollution control scenario, lies closest to actual trends in West Africa. This analysis can be a useful guide for air quality forecasting and near-future pollution control/mitigation policy studies.

We use the global Community Earth System Model to investigate the response of secondary pollutants (ozone O3, secondary organic aerosols SOA) in different parts of the world in response to modified emissions of primary pollutants during the COVID-19 pandemic. We quantify the respective effects of the reductions in NOx and in VOC emissions, which, in most cases, affect oxidants in opposite ways. Using model simulations, we show that the level of NOx has been reduced by typically 40 % in China during February 2020 and by similar amounts in many areas of Europe and North America in mid-March to mid-April 2020, in good agreement with space and surface observations. We show that, relative to a situation in which the emission reductions are ignored and despite the calculated increase in hydroxyl and peroxy radicals, the ozone concentration increased only in a few NOx-saturated regions (northern China, northern Europe and the US) during the winter months of the pandemic when the titration of this molecule by NOx was reduced. In other regions, where ozone is NOx-controlled, the concentration of ozone decreased. SOA concentrations decrease in response to the concurrent reduction in the NOx and VOC emissions. The model also shows that atmospheric meteorological anomalies produced substantial variations in the concentrations of chemical species during the pandemic. In Europe, for example, a large fraction of the ozone increase in February 2020 was associated with meteorological anomalies, while in the North China Plain, enhanced ozone concentrations resulted primarily from reduced emissions of primary pollutants.