The role of wet scavenging in controlling the impact of biogenic and anthropogenic pollution on Arctic cloud properties

Regional aerosol concentrations are governed by an evolving balance between aerosol sources and sinks. Here, a simple technique is described for making estimates of the extent to which seasonal aerosol variability is controlled by wet scavenging rather than the efficiency of transport from pollution source regions. Carbon monoxide (CO) is employed as an assumed passive tracer of pollution transport efficiency that can be compared to remotely sensed cloud properties or surface-based measurements of the magnitude of aerosol light scattering is compared. Because aerosols, unlike CO, are affected by wet scavenging as well as transport efficiency, the ratio of short-term perturbations in these two quantities provides a measure of the relative roles of these two processes. This technique is applied to surface measurements in the Arctic at Barrow, Alaska (71◦ N) for the decade between 2000 and 2009, and to MODIS/POLDER data during the IPY IOP in spring 2008. What is found is that a well-known seasonal cycle in “Arctic Haze” is dominated by variability in wet scavenging. Crossing the freezing threshold for warm rain production appears particularly critical for efficiently cleaning the air. Insofar as indirect effects of aerosols are concerned, we find that they are almost entirely mitigated by warm rain production along aerosol transport pathways.