The Effect of Recent Aerosol Trends on Solar Radiation in the Central and Southeastern United States: Implications for Regional Hydrology.

Abstract:

Observations show large increases in summertime (JJA) surface solar radiation (SWdn) over the central and southeastern United States during the last 20 years, as much as a +40 Wm^-2 at midday. At the same time, in response to environmental regulations in the early 1990s, emissions of U.S. aerosol precursors have decreased by as much as 60%. Detecting a possible connection between these two trends has been difficult due to the secondary effects of aerosols on cloud concentration and lifetime, and previous efforts have failed to find a direct link. Here we investigate the clear-sky direct effect of decreasing U.S. aerosols on climate, using a radiative transfer model (RRTMG) driven by 1997-2014 measurements of aerosol optical depth at Surface Radiation Budget Network (SURFRAD) sites in the central and southeastern United States. We impose aerosol asymmetry parameters and single scattering albedos from nearby Aerosol Robotic Network (AERONET) sites. Preliminary results indicate that declining aerosols drive a summer noontime change in clear-sky SWdn of +25 Wm^-2 since 1997 at Goodwin Creek, MS, accounting for 56% of the observed increase in SWdn at that site. Similarly, we find that aerosols increase clear-sky SWdn by +6.5 Wm^-2 in Bondville, IL, which accounts for 21% of the observed SWdn trend there. These results suggest that the climate in these regions of the U.S. may be sensitive to recent reductions in aerosol concentrations, especially during summer months. We also analyze in situ soil measurements from the Illinois Climate Network from 1990-present, and find that a significant decrease in soil moisture (-0.6 m³ m^-3 a^-1) accompanies the increase in SWdn, implying a link between aerosol trends and regional hydrology. Aerosol reductions are expected to continue in the United States and may further influence regional climate including hydrological factors. Our work has implications for polluted regions outside the U.S., where future reductions in the aerosol burden could increase vulnerability to drought.