Reduction in NOX emissions during the COVID-19 lockdown did not result in a comparable reduction in secondary PM levels
- 1Technical University of Munich, Environmental sensing and modelling, Germany
- 2School of Engineering and Applied Science, Harvard University, Cambridge, MA, USA
- 3Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA
The COVID-19 lockdown is viewed as a natural experiment that can put our current understanding of the contribution of secondary PM2.5 sources to the test. In ten German metropolitan areas, mean meteorology-accounted for PM2.5 concentrations dropped by 5 % during the 2020 lockdown period (spring) compared to 2019, but meteorology-accounted for NO2 concentrations decreased by 23 % during the same time. Furthermore, meteorology-accounted for SO2 and CO concentrations show no significant differences between the 2020 lockdown period and 2019. The GEOS-Chem model simulation with COVID-19 emission reduction scenario (23 % reduction in NOX emission with unchanged VOC and SO2) supports our findings of only a marginal decrease in PM2.5 and a significant decrease in NO2 levels and reveals that the atmosphere's oxidative capacity is increased in all three important oxidants, OH, O3, and night-time NO3. The night-time increase in O3 is the main cause of increase in night-time NO3 radical. The increase in OH does not compensate for the strong reductions in NO2, whereas the increase in NO3 radical at night roughly balances the effects of the NO2 reduction. As a result, compared to the Business As Usual condition, i.e., no lockdown, day-time PM nitrates are reduced while night-time PM nitrate formation is relatively unaffected. In addition to the above, slightly enhanced sulfate formation and decreased ammonium explain the small reduction in the total PM2.5 during the lockdown period. We also investigated the annual spring high PM2.5 episodes in German metropolitan areas. Satellite measurements show high ammonia (NH3) concentrations in the early spring and summer months, when high PM2.5 episodes are associated with high NH3 concentrations in the spring. We find that high atmospheric ammonia concentrations, combined with low temperature and low boundary layer height, are the most favorable conditions for PM2.5 formation. Based on our findings, we suggest that emission control policies should be more focused on limiting ozone that should also reduce PM2.5. Furthermore, ammonia emissions should be limited in order to control the high PM2.5 episodes in winter and spring.
How to cite: Balamurugan, V., Chen, J., Qu, Z., Bi, X., and Keutsch, F. N.: Reduction in NOX emissions during the COVID-19 lockdown did not result in a comparable reduction in secondary PM levels, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9565, https://doi.org/10.5194/egusphere-egu22-9565, 2022.
