Why is the Arctic summer clean? Will the Arctic get cleaner? How wet scavenging controls the impacts of mid-latitude pollution in the Arctic. (Invited)

Aerosols influence regional climates through their impacts on the radiative scattering and absorption, either directly or by changing cloud properties. The primary constraint on these effects is the removal of aerosols from the atmosphere by dry deposition and, most rapidly, by wet scavenging. Yet wet scavenging rates can be very difficult to quantify since they involve a range of processes that span nucleation of clouds from aerosols, condensational growth, and rain and snow formation, none of which are very well understood. And, because the processes are small-scale and rapid, they can be difficult to directly model within global transport models. Nonetheless, a simple technique can be used to at least make estimates of the efficiency of wet scavenging, without having to know any of the time history of atmospheric mixing and cloud microphysics. Within a tracer transport model or in situ measurements, and with some care taken with regards to atmospheric chemistry, carbon monoxide (CO) can be employed as an assumed passive tracer of pollution transport, to which aerosols and their effects on clouds can be compared. Because aerosols, unlike CO, are affected by wet scavenging as well as transport, the ratio of these two quantities provides a measure of the relative roles of wet scavenging and transport efficiency. This approach has been applied to study the impacts of mid-latitude pollution on the Arctic. Combined with a tracer transport model, space based measurements indicate that Arctic clouds are highly susceptible to modification by pollution plumes from mid-latitudes, but only weakly so where the clouds lie along warmer, moister isentropes, where wet scavenging removes aerosols while leaving inert CO behind. Additionally, in surface measurements from Barrow, Alaska (71 degrees N) and Alert, Canada (82 degrees N), a well-known seasonal cycle in 'Arctic Haze' is found to be dominated by wet scavenging rather than transport efficiency: the ratio of aerosol to CO is roughly four times lower in summer than in winter. Somewhat remarkably, wet scavenging appears to control the cycles of light absorbing and light scattering aerosols roughly equally. The summer is cleaned of soot and sulfate more because precipitation rates are higher in warm weather than because pollution transport from mid-latitudes is less direct. It seems possible that a future warmer and wetter Arctic may also be cleaner. Assuming aerosol pollutants generally have a warming effect in the Arctic, such an increase in wet scavenging would represent a negative Arctic climate feedback.