Space based evaluation of the aerosol indirect effect in the Arctic

During the Arctic winter and spring, enhanced levels of aerosol particles and trace gases form a pronounced haze originating primarily from industrial pollutants transported into the region. The haze rapidly dissipates during the late spring as pollution transport is inhibited and meteorological conditions favor pollutant removal. Prior ground based studies have found that aerosols associated with the "Arctic haze" have the potential to indirectly alter Arctic cloud surface radiative forcing in both the solar and thermal IR bands. While satellites have been used extensively to study the indirect effects of aerosols on clouds in lower latitude regions, they rarely are employed in Arctic studies. One limitation of using satellites to study aerosols and clouds is that they do not provide retrievals of aerosol concentrations under cloudy conditions nor do they resolve aerosol vertical profiles; co-location of aerosol and cloud fields is therefore impossible. The ubiquitous nature of Arctic clouds makes the common practice of comparing cloud properties to aerosol in nearby cloud free regions a difficult task in the Arctic, providing little information about aerosol-cloud interactions. Here, in order to circumvent these concerns, passive satellite cloud property retrievals are co-located horizontally, vertically and temporally with pollution tracers from a Lagrangian particle dispersion transport model. The advantage of this analysis approach is that clouds and pollution are compared where they are affected by the same meteorological conditions. This means that pollution can be treated as an independent variable affecting cloud properties. Cloud properties from low level liquid clouds north of 65 °N are colocated with fields of pollution tracer during the period March 20 to July 20, 2008. The analysis shows a high sensitivity of cloud optical depth and droplet effective radius to the anthropogenic and biomass burning pollution tracers. Furthermore, the cloud sensitivity to pollution is evaluated under different thermodynamic and physical constraints. Results of the analysis show a strong indication of wet-scavenging reducing the effects of pollution on clouds at warmer temperatures. Additionally, the sensitivity to pollution is higher for cloud optical depth than for droplet effective radius, suggesting that some sort of feedback process amplifies the radiative response through changes in liquid water path.