Measuring Changes in Air Quality from Reduced Travel in Response to COVID-19

Research Team: Michael Kleeman (lead) and Shenglun Wu

UC Campus(es): UC Davis

Problem Statement: The major source of oxides of nitrogen (NOx) that produce ground-level ozone (O3) come from mobile sources. Model calculations and ambient measurements both suggest that major California cities are currently in a “NOx-limited” regime where decreasing NOx concentrations lead to higher O3 concentrations, making current emissions control programs counter-productive in the short term. Shifting traffic patterns associated with COVID-19 may have reduced NOx emissions from mobile sources by more than ~50% in densely populated urban areas in California. This “natural experiment” provides an opportunity to (i) test the ability of models to simulate O3 response to deep cuts to ambient NOx concentrations, (ii) more accurately predict the amount of NOx reduction needed to achieve O3 benefits, and (iii) improve confidence in the long-term benefits of emissions control plans.

Project Description: This project developed new methods to directly measure O3 chemistry in the atmosphere and applied them over long-term campaigns in multiple California cities to quantify traffic contributions to O3 formation. A seasonal-cycle was observed of NOx-rich O3 chemistry during cooler months trending toward NOx-limited chemistry in warmer months. Superimposed on this seasonal cycle was a spatial pattern of NOx-rich chemistry in dense urban cores and NOx-limited chemistry in areas downwind of urban cores. Chemistry-based models with source tagging were also developed to better understand these trends. Seasonal changes to biogenic VOC and gasoline evaporative VOC emissions likely explain the seasonal changes in O3 formation chemistry. Reduced traffic emissions in March 2020 did not reduce O3 concentrations because the chemistry was heavily NOx-rich during the spring season. Extended model predictions suggest that similar traffic reductions could have reduced ambient O3 concentrations in small and intermediate cities if they would have occurred in summer months. Traffic reductions alone would not be sufficient to reduce O3 concentrations in the urban cores of larger cities. Reduced emissions from transportation sources can improve air quality in California, but transportation sources no longer exclusively dominate O3 formation. Future emissions controls should be coordinated across multiple sectors (including transportation) to achieve their objectives.

Status: Completed

Budget: $110,000