Impact of Air Pollution Controls on Radiation Fog Frequency in the Central Valley of California

Abstract In California's Central Valley, tule fog frequency increased 85% from 1930 to 1970, then declined 76% in the last 36 winters. Throughout these changes, fog frequency exhibited a consistent north‐south trend, with maxima in southern latitudes. We analyzed seven decades of meteorological data and five decades of air pollution data to determine the most likely drivers changing fog, including temperature, dew point depression, precipitation, wind speed, and NO x (oxides of nitrogen) concentration. Climate variables, most critically dew point depression, strongly influence the short‐term (annual) variability in fog frequency; however, the frequency of optimal conditions for fog formation show no observable trend from 1980 to 2016. NO x concentration, which has a decreasing north‐south concentration gradient, declined continuously over this period, consistent with the long‐term temporal and spatial trends in fog. As development in the Central Valley increased direct particle and other pollutant emissions from 1930 to 1970, fog frequency increased. Following the Clean Air Act, particle emissions quickly declined, and NO x emissions declined steadily, reducing the cloud condensation nuclei (CCN) available for fog formation. As a precursor of ammonium nitrate aerosols, which are efficient CCN, we used NO x measurements and emission trends as a proxy for the CCN trend. We conclude that while the short‐term fog variability is dominantly driven by climate fluctuations, the longer‐term temporal and spatial changes in fog have been driven by changes in air pollution. For conditions close to the dew point, a decrease in fog of 5 days/year per 10 parts per billion NO x decrease occurred across the Central Valley.