Last summer saw similar trends — though they were at first unexpected. In the spring, when lockdowns shut down the city, an assumed silver lining was a reduction in air pollution from vehicle traffic. Ground-level ozone is a byproduct of other pollutants; it forms when nitrogen oxides react with volatile organic compounds, or VOCs. Though both chemicals have a variety of sources — including industrial emissions — much of the pollution is caused by vehicles. So when traffic dropped off dramatically in the spring of 2020 — reducing nitrogen dioxide pollution in the Valley — some were hopeful that ozone levels, too, would dip. But they didn’t; ozone levels exceeded standards on 40 days in 2020, about the same as the year prior.