Aerosols—particles of hydrocarbons referred to as PM2.5 because they are smaller than 2.5 microns in diameter and easily lodge in the lungs—are proven to cause cardiovascular and respiratory problems.
As a result of strict vehicle emissions laws, organic aerosol levels have been significantly reduced throughout the United States, but the drop has been particularly dramatic in Los Angeles, which started out at a higher level.
Based on pollution measurements over the past 20 years, the UC Berkeley scientists found that concentrations of PM2.5 in the Los Angeles basin in 2012 were half what they were in 1999. As a result, from 2016 to 2018, there were almost no PM2.5 violations in the area when temperatures were low, below 68 degrees Fahrenheit. But at warmer temperatures, aerosol concentrations rose—over the same time period, 70% to 80% of days over 100 F exceeded the National Ambient Air Quality Standard (NAAQS) threshold.
Weird but fascinating article about how Los Angeles choice of palm trees may be effecting ozone levels on hot days. That said, color me skeptical as there are still a lot of tailpipes in a unique air basin that traps in pollution.