Clouds: The innocent bystanders of long range pollution

So carrying on this long range pollution theme, and a particular focus on Aerosols, I wished to burrow deeper into the literature of the impacts of aerosols in the Arctic. Within this post I aim to open your eyes to another impact, in the Arctic, which you may not have heard about before; the impacts of aerosols on clouds.

Sulphate aerosols, in particular, can alter the micro-physical properties of clouds. "Aerosols in the lower atmosphere can modify the size of cloud particles, changing how the clouds reflect and absorb sunlight, thereby affecting the Earth's energy budget". Changes in the energy budget have played a role in causing amplified warming in the Arctic. Kay and Gettelmen (2009) argue that feedback systems, created by changes to cloud composition, can be held accountable for the great loss of sea ice during the 2007 event.

I bet you don’t think about the clouds when you think of the impacts of
 long range pollution in the poles, do you?

Impacts of aerosols on clouds

A series of complex interactions link clouds, aerosols and sea ice in the Arctic. Aerosols can act as both a warming and cooling agent in this case. Clouds, in the Arctic, can also have “an annual-mean net warming effects on the Arctic surface”. When surface temperatures are cooler than internal temperature of low clouds, infra-red radiation emitted from clouds, warms the surface. The presence of long range aerosols can alter the mirco-physical properties of the clouds and consequently alter the annual net energy budget (this factor is often ignored when modelling Arctic change). This influence on the natural energy budget is refereed to as the aerosol indirect effect.

Changes to the effective radius of water droplets can have varying implications of the surface energy budget. Optically thin spring time clouds typically contain low droplets concentrations. The presence of aerosols causes a greater number of smaller water droplets, consequently optimizing longwave emissivity of clouds and increasing surface warming. A 50% change in the radius of water droplets can alter the energy budget by 40 W m^-2.  

In many cases a mixture of aerosols black carbon can increase water droplet size, within the cloud, thus increasing the albedo of clouds and causing surface cooling. However, this is minimal in comparison to the warming influence of clouds, as they already sit above a highly reflective surface (ie snow) (Graversen et al., 2008). 

Sensitivity and Meteorological conditions

Tietze et al. (2011) illustrated the sensitivity of clouds, to long range aerosols, by analysing changes to properties including liquid cloud effective radius and optical depth. Pollutants were identified to have the biggest impact around the freezing point, sensitivity decreases above or below these temperatures. Results also indicated the optical depth was up to 4 times more sensitive (than effective radius) to the presence of aerosol pollutants. This difference was attributed to an unknown feedback system.  

However, simply looking at the properties of clouds does not create a true representation of the systems in the Arctic. To get a true picture, when modelling the influence of aerosols, meteorological conditions should also be incorporated. This is illustrated by the results of Coopman et al. (2015) who contradicted Tietze et al, by stating optical depth and liquid cloud effective radius have very similar sensitivities to aerosol pollution (under similar conditions). Furthermore, localised meteorological conditions influence the extent to which aerosols alter certain properties (increased water vapour within the air can increase the impact of aerosols on cloud sensitivity). Differences in meteorological conditions can also account for the differences between optical depth and effective radius found within the Tietze et al. study.

High levels of uncertainty are associated with modelling studies in the Arctic. I have noticed a there is a large gap between firstly the simulated and observed results and also little consistency between the results of different studies. This uncertainty can often be associated with changes in cloud properties. In particular, the influence of meteorological conditions are often not considered and the incorporation of the scientific knowledge (discussed above) will help reduce such uncertainty.