LOS ANGELES — There’s been a lot of talk lately about how clean the air is around Southern California. But it turns out it’s not just because millions of cars are off the road due to the Stay at Home order.

There’s another factor that has helped to improve our air quality: weather. With or without people staying home, the weather plays a big role in affecting our daily air quality.

For example, on a typical smoggy day, offshore winds can clear out the smog over downtown Los Angeles. It becomes so clear that people along the coast can see all the way to the mountains. Unsettled or stormy weather can also improve air quality by mixing up the atmosphere and preventing pollutants from building up. 


Southern California has experienced a long stretch of rainy days for much of March and the first part of April. The South Coast Air Quality Management District’s “Air Quality Map Index” showed that many cities were in the green or good quality range since about March 7 – one week before the Stay at Home order went into effect. 

The AQMD analyzed CalTrans traffic sensors on freeways throughout the basin and found notable traffic reductions in March: 33% in light-duty vehicles and 20% in heavy-duty vehicles. 

The AQMD believes that the weather is the most significant factor driving day-to-day changes in air quality. Meteorological factors must be removed or one must look at a long timeframe before quantifying effects of reduced emissions. 



“I do want to caution in terms of our air quality in Southern California. The cars have gotten so much cleaner over the years that they’re not as big a part of the problem as they used to be,” said Philip Fine, AQMD’s Deputy Executive Officer. “What we’re really looking at as our biggest source is the heavy-duty equipment – the trucks, the trains, ships, the cargo handling equipment.”

Southern California is home to two of the biggest ports in the United States: the Port of Long Beach and the Port of Los Angeles. About 40% of the imported container cargo comes through those two ports. 

“This is really our No. 1 challenge, our No. 1 source of emissions,” said Fine. “Even if there are a lot fewer cars, we would also have to seriously address those heavy-duty vehicles as well.”

All across the world, people are noticing environmental differences in their cities that appear to be due to a lack of normal daily human activity. 

Tourists and residents in Venice, Italy have been posting pictures on social media of the Venice Canals which they’re surprised to see is noticeably cleaner. The water is so clear that people, for the first time, have noticed all the fish in the canals. In China, NASA images from space shows a dramatic drop in nitrogen dioxide, pollution from emissions.


“My hope is that people will begin to take seriously what we’ve known for a very long time. We can actually prove the effects of mankind on changing climate,” said Ray Weiss, professor of Atmospheric Chemistry at Scripps Institution of Oceanography at UC San Diego. “It’s important for us to be objective about what we’re doing. Maybe this example will show people that they should take these things seriously.” 

Weiss is also involved in the Los Angeles Megacities project, a collaboration with the Jet Propulsion Laboratory and other organizations. Their mission is to try to understand what the greenhouse gas emissions of the greater L.A. area are and how they’re changing over time. 

All these agencies have a skeleton crew of researchers on campus keeping instruments running while others, working from home, continue to analyze the data. 

Researchers at Scripps Institution of Oceanography are looking at the big picture effects of COVID-19 and global carbon dioxide levels. They believe it may take up to a year of prolonged downturn for the drop in Co2 emissions to show up clearly at remote stations around the world.