Space-Based Evaluation of the Aerosol Indirect Effect in the Arctic

Clouds play a complex and important role in Earth's radiative balance, which is particularly evident in the Arctic, where the dry atmosphere and high albedo amplify cloud's radiative influences. Arctic clouds contribute to the surface radiation budget by emitting longwave radiation and by reflecting incident shortwave radiation, overall they exert a net warming effect on the Arctic climate. Anthropogenic aerosol pollution is capable of altering cloud micro-physical properties, such as cloud droplet radii, which perturbs cloud radiative characteristics in what is known as the aerosol indirect effect. By this mechanism, pollution aerosols transported into the Arctic, may enhance the emissivity of low level water clouds, which in turn causes an increase in down-welling radiation that ultimately would result in sea ice thinning and warmer surface temperatures. Satellites can provide fairly accurate retrievals of aerosol concentrations, however they cannot do so under cloudy conditions nor accurately resolve the vertical profile. One common approach is to pair satellite derived cloud properties with satellite derived aerosol concentrations from an adjacent cloud free area. This adds a degree of uncertainty because the cloud free region is experiencing different meteorological conditions than the cloudy region. This study uses a high resolution Lagrangian chemical tracer model to serve as a proxy for aerosol concentrations, eliminating the need for in situ data and satellite aerosol retrievals. Anthropogenic carbon monoxide (CO) is used as a pollution tracer because its concentrations are dependent on pollutant source strength and atmospheric mixing and is effectively independent of oxidation and removal processes, CO's passive nature also allows for the examination of wet- scavenging. As pollution is transported from its source to the Arctic, the active cloud condensation nuclei (CCN) may be removed by wet-scavenging while CO concentrations remain constant. While this is occurring, the potential for CO concentrations to reflect the aerosol indirect effect will diminish with distance from the source region. Cloud property data from the CERES and MODIS instruments,for the region north of the Arctic Circle and spanning the month of April 2006, are collocated temporally, vertically and horizontally with FLEXPART CO concentration fields. Cloud properties depend first and foremost on the meteorological conditions; by using this analysis approach, both cloud properties and pollution fields are affected by the same meteorological conditions and a less ambiguous evaluation can be achieved. Further components of this study are addressing the influence of different pollutant source regions, age classes of the CO tracer, wet scavenging, and meteorological conditions. This research demonstrates a novel technique that avoids the ambiguities of comparing aerosol and cloud properties from differing locations and heights and can be used to better constrain the magnitude and extent of the aerosol indirect effect.