Co-authored with Bill Brick (San Diego Air Pollution Control District)
Over the past several weeks there has been a reduction in daily vehicle miles traveled (VMT) in the San Diego region due to Shelter-in-Place orders issued by the Governor of California. As discussed in a previous blog post, this decline in VMT has led to a reduction in greenhouse gases, the key gases in the changing climate. But there is also a relationship between VMT and daily and short-term air pollution.[1] We know that over two-thirds of smog-forming emissions in San Diego county are generated from mobile sources.[2] Air pollutants emitted from cars, diesel-powered trucks, buses, and other heavy‑duty equipment include oxides of nitrogen (NOx) as well as diesel particulate matter (PM).[3] It is reasonable, therefore, to expect that the reduction in VMT also would result in a reduction in such mobile source air pollutants.
The media has reported on how lock-downs have affected air pollution in many parts of the world and how clean and clear the air has become in metropolitan areas. People in the San Diego region have been asking the same question of how this reduction in VMT has affected our air quality. Due to this heightened interest in recent air monitoring data, the San Diego Air Pollution Control District (District) has provided a preliminary assessment of air quality measurements (even before the March 2020 data have been fully processed and validated). This post summarizes these preliminary findings.
