New paper says a warmer Arctic will be a cleaner Arctic, finds new negative-feedback

from AGU Journal Highlights September 20, 2011

Arctic air may become cleaner as temperatures rise

The air in the Arctic is cleaner during summer than during winter. Previous studies have shown that for light-scattering pollutants, this seasonal cycle is due mainly to summer precipitation removing pollutants from the air during atmospheric transport from midlatitude industrial and agricultural sources.

With new measurements from Barrow, Alaska, and Alert, Nunavut, Canada, Garrett et al. extended previous research to show that light-absorbing aerosols such as black carbon are also efficiently removed by seasonal precipitation.

Precipitation removes these particles from the air most efficiently at high humidities and relatively warm temperatures, suggesting that as the Arctic gets warmer and wetter in the future, the air and snow might also become cleaner. If Arctic aerosols have a net warming effect, as is believed to be the case, precipitation removing these particles from the air would represent a negative climate feedback, mitigating anticipated Arctic warming.

GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L16805, 6 PP., 2011
doi:10.1029/2011GL048221

The role of scavenging in the seasonal transport of black carbon and sulfate to the Arctic

Key Points

• Arctic aerosol have a strong seasonal cycle that is dominated by wet scavenging
• Both soot and sulfate are affected nearly equally by wet scavenging processes
• We can anticipate from this study that a warmer Arctic will be a cleaner Arctic

Timothy J. Garrett et al

In a prior study, a decade-long dataset of ground-based aerosol and carbon monoxide measurements from Barrow, Alaska (71°N, 157°W) was used to show that surface air in the Arctic is clean during the summer, less due to inhibited transport of pollutants from mid-latitudes, and more because of efficient wet scavenging at temperatures near freezing. Here, the analysis is extended to light-absorbing aerosols, such as black carbon, and to measurements from Alert, Canada (82°N, 62°W). The data imply that both light scattering and light absorbing aerosols have similar seasonal cycles, independent of location, and they are controlled nearly equally by wet scavenging. Removal is particularly efficient at high relative humidities and warm temperatures, which suggests 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.