Regional radiative impact of volcanic aerosol from the 2009 eruption of Mt. Redoubt

High northern latitude eruptions have the potential to release volcanic aerosol into the Arctic environment, perturbing the Arctic's climate system. We present assessments of shortwave (SW), longwave (LW) and net direct aerosol radiative forcing efficiencies and atmospheric heating/cooling rates caused by volcanic aerosol from the 2009 eruption of Mt. Redoubt by performing radiative transfer modeling constrained by NASA A-Train satellite data. The optical properties of volcanic aerosol were calculated by introducing a compositionally resolved microphysical model developed for both ash and sulfates. Two compositions of volcanic aerosol were considered in order to examine a fresh, ash rich plume and an older, ash poor plume. Optical models were incorporated into a modified version of the SBDART radiative transfer model. Our results indicate that environmental conditions, such as surface albedo and solar zenith angle (SZA), can influence the sign and the magnitude of the radiative forcing at the top of the atmosphere (TOA) and at the surface and the magnitude of the forcing in the aerosol layer. We find that a fresh, thin plume (similar to 2.5-7 km) at an AOD (550 nm) range of 0.18-0.58 and SZA = 55 degrees over snow cools the surface and warms the TOA, but the opposite effect is seen for TOA by the same layer over ocean. The layer over snow also warms by 64 W m(-2)AOD(-1) more than the same plume over seawater. The layer over snow at SZA = 75 degrees warms the TOA 96 W m(-2)AOD(-1) less than it would at SZA = 55 degrees over snow, and there is instead warming at the surface. We also find that plume aging can alter the magnitude of the radiative forcing. An aged plume over snow at SZA = 55 degrees would warm the TOA and layer by 146 and 143 W m(-2)AOD(-1) less than the fresh plume, while the aging plume cools the surface 3 W m(-2)AOD(-1) more. Comparing results for the thin plume to those for a thick plume (similar to 3-20 km), we find that the fresh, thick plume with AOD(550 nm) = 3, over seawater, and SZA = 55 degrees heats the upper part of the plume in the SW similar to 28 K day(-1) more and cools in the LW by similar to 6.3 K day(-1) more than a fresh, thin plume under the same environmental conditions. We compare our assessments with those reported for other aerosols typical to the Arctic environment (smoke from wildfires, Arctic haze, and dust) to demonstrate the importance of volcanic aerosols.