Decadal Changes in Seasonal Variation of Atmospheric Haze over the Eastern United States: Connections with Anthropogenic Emissions and Implications for Aerosol Composition

The current seasonal summer maximum in surface fine particulate matter (PM_2.5) over the eastern United States has been well established. We find that this seasonality has historically changed substantially, based on long-term quality assured inverse visibility (1/Vis) data over 1946-1998. The median summer/winter 1/Vis ratio increased from about 0.8 over both the southeastern and northeastern United States in the late 1940s to 1.24 over the southeastern United States and to 1.04 over the northeastern United States in the mid-1970s. This ratio exhibits weaker changes in both regions afterward. The observed PM_2.5 seasonality after the year 2000 has similar spatial distribution as that in 1/Vis over the mid-1990s, with systematically higher summer/winter ratios which rapidly weaken after the mid-2000s. From 1956 to 1975, stronger increases in 1/Vis occurred in summer than in winter in both regions, associated with increases in sulfur dioxide emissions and reductions in anthropogenic carbonaceous emissions. Over the southeastern United States, the changes in aerosol seasonality, i.e., both the strengthened summer maxima over 1946-1975 and dampened summer maxima after 2007, suggest historical changes in anthropogenic influence on secondary organic aerosol (SOA) formation and suggest the prospect of reducing summer SOA through controls on anthropogenic emissions.