Unraveling the influences of pollution and climate trends on radiation fog frequency: A comparison of California's Central Valley and Italy's Po Valley

In California's Central Valley, episodes of wintertime radiation fog increased as much as 90% from 1930-1970, followed by a 76% reduction since 1980. As unregulated emissions increased air pollution from 1930-1970, it directly contributed to the formation of cloud condensation nuclei (CCN) necessary for the development of clouds and fog. With regulation increasingly mitigating emissions from 1970 to the present, pollutant concentration declined, thus reducing the frequency of fog events. Similar radiation fog trends have been observed in Italy's Po Valley - a location of comparable climatology and anthropogenic function - where a 50% fog reduction has also been observed over the past 40 years, concurrent with dramatic improvements in air pollution.

To identify the most significant variables affecting visibility, an indicator of dense fog, and to compare the fog trends in these two analogous regions, we used a generalized additive model to test the explanatory power of climate and pollution predictor variables related to fog formation. We ran the model for a range of sites in California and Italy with differing timescales (daily, monthly, annual) to elucidate the nonlinear relationships in the system. Over 60% of the variance was consistently explained by dew point depression, wind speed, NO_x (oxides of nitrogen) concentration (precursor for ammonium nitrate which acts as CCN), and precipitation. Trends in dew point depression, which incorporate both water availability and temperature, have greater influence on daily time scales, suggesting that short-term fog variability is primarily influenced by local weather patterns. The explanatory value of pollution concentration becomes more pronounced when analyzing on longer timescales where NO_x influence on fog formation is most clearly expressed independent of year-to-year meteorological variance. Fog frequency is most sensitive to changes in air pollution when dew point depression is low.