As I will show in a minute, there is no obvious evidence of better air quality this week, and, in fact, during the past days the air quality has declined to moderate in some areas.
This morning, the U.S. EPA AirNow website showed a region of moderate air quality (yellow colors) over western B.C., Washington, and Oregon.
During the day, with a warming sun and better mixing, things improved substantially
Taking a look of a plot of air quality during the past week at Seattle (blue), Tacoma (red), Spokane (tan), and Marysville (purple), there are good levels (below 50) for Monday through Friday, but moving into the moderate zone last night. The substantial daily (diurnal) variations are obvious.
Air quality got better each day as solar heating causes the atmosphere to become well mixed.
Perhaps we can get some insights by plotting the air quality for the past years in Seattle (see below).
Mama Mia! The air quality today was the worst in a year! I don’t see how this could be due to coronavirus.
The moderate air quality this week was associated with relatively dry conditions over the region and high pressure aloft (see upper level map last night) which resulted in sinking in the middle atmosphere. Such sinking produces warming aloft and a lack of clouds, which
in turn allows good cooling at the surface (infrared radiation is able to escape the atmosphere more easily without clouds). With warming aloft and cooling at the surface, we tend to develop an inversion (temperature increasing with height), which helps trap pollutants near the surface. Take a look at the plot of temperature and dew point last night at Salem, Oregon, last night. The inversion is very clear.
It appears that meteorology (light winds, inversion, clear skies) is overwhelming the loss of emissions from less industrial and transportation activity. Yes, less folks are driving (I-5 was a ghost town yesterday evening), but all of us still have to heat our homes and apartments. In fact, folks are staying home more, so home heating is probably way up. Bus routes are still going and people still have to drive to get food and supplies. Might air quality have been a bit worse without the restriction? Perhaps. But the effect is not obvious.
And this week we will experience something that was rare during the past several days: some rain.