Insights from dendrochronology have provided a new seasonal predictor for air pollution meteorology. In the San Francisco Bay Area summer ozone excesses over the federal ozone standard are correlated (correlation coefficient r = .87) with precipitation for the two preceding winters, a factor related to tree-ring width in a precipitation-stressed climate. The hypothesis that reactive hydrocarbon emissions from vegetative biomass affects these ozone excesses was supported by a similar correlation between summer hydrocarbon average maximums and the two-winter precipitation factor, reaching r = .88 at suburban stations. A weak tendency for hot summers to follow wet winters (in 16 years of California data) explains only a minor part of the ozone-rain relationship in multiple correlations.

closed atmosphere to propose what may be happening in a natural environment (3). Furthermore, there is more evidence than they acknowledged that nonmethane hydrocarbons (NMHC) from natural sources may never reach significantly high concentrations. For example, two recent studies (4,5) showed that the total carbon from natural hydrocarbons did not exceed 0.1 part per million (ppm) in rural and remote areas.'These studies were carefully designed to minimize calibration errors that would make it difficult to distinguish between natural and anthropogenic hydrocarbons. In other studies, whenever rural hydrocarbons were higher, there was evidence of contributions from anthropogenic sources (6,7 Publ. 450/3-75-036 (1975 PubI. 600/3-77-001 (1977