In the San Joaquin Valley there is dust. It is worst during the months of August and September. These are the months when almonds are harvested.
When PM10 is measured it includes PM2.5. This makes total PM10 readings highest during the winter months of November through February. But, during those months dust is not a problem. It is ammonium nitrate and ammonium sulphate.
Almond acreage has been growing. In 2000 there were slightly over 500,000 acres of bearing almond orchards. By 2016 this number had grown to nearly 950,000 acres.
PM10 is measured in micrograms per cubic meter. On the AQMIS California Air Resources Board web pages the current and historical levels of PM10 can be found. Historical data is found by changing the year in the small box at the bottom.
What has happened is PM10 average levels for the SJV have gone up and down through the years starting in 2000 and overall there has been a very slight increase in the annual average. An average below 50 micrograms for the year used to be considered healthy by the Environmental Protection Agency. EPA has dropped this annual standard in favor of an annual PM2.5 standard. California considers 25 micrograms as a healthy annual standard.
The graph below shows the annual average PM10 levels since 2000 taking the worst reading in the valley for each day.
Separating out the almond harvest season, a different situation arises. PM10 levels have risen overall, along with almond acreage, since the year 2000 during the months of August and September.
The other 10 months of the year, when almonds are not being harvested, PM10 has actually decreased slightly since 2000.
Notice that the annual average and the average for non-almond harvest months varies between 40 and 60 micrograms per cubic meter. But, the almond harvest season has an average between 50 and 90 micrograms.
If you live in the valley and have experienced the choking dust during the almond harvest season, these numbers should come as no surprise.