In 1981 President Ronald Reagan made quite a stir when he said, "Trees cause more air pollution than automobiles do." The remark was laughed at by many, however, the fact is that many trees do emit volatile organic compounds (VOCs), which, in the presence of sunlight and nitrogen oxides (NOx) may react to form photochemical smog, otherwise referred to as ozone.
There has been a lot of research done to estimate the emission rates of VOCs from trees and other plants. Cal Poly SLO even has a tree selection tool called SelecTree that can be used to pick trees based on their VOC or biogenic emission strength. With ratings from "low" to "high," the tool can be, and has been, used by environmental planners to recommend the types of trees that should be planted when certain projects are being reviewed for permits.
Here is a list of 6 trees with different biogenic emission rates commonly found in Bakersfield and the San Joaquin Valley, which has many, many smoggy days.