Teve Brown of NOAA said the valley suffers from cows + cards. At Harris Ranch a large industrial cattle farm trucks drive 6,000miles/day for 60 loads of feed producing nitrogen oxides (NOx). NOx combines with the ammonia from cow manure and urine to from ammonium nitrate which accounts for more that 1/2 of the areas most polluted days of PM2.5.
In addition, Interstate is a major thorough bringing more traffic pollution and farming practices including nitrogen fertilizer contributes 1/3 of NOx in California air. The SJV also holds 9000 oil wells and because all the light oil has been drilled, the current production is described as the "thickest, dirtiest petroleum" in the nation.
Intersecting factors: landscape (bowl shape of the Valley); economic (agriculture that contributes to PM2.5); transportation corridor that add more traffic pollution; and state-wide wildfires that bring more particulate pollution which is trapped; and political environment in which area elects representatives (ex: Devin Nunes) who deny global warming and reject environmental protection.