Anthropogenic perturbations to the Earth system are a major concern to anyone who is paying attention. Air pollution has many serious effects on our planet, not all of which are immediately obvious. Recent studies have indicated that increased aerosol pollution is leading to a decrease in precipitation. This could have serious consequences to local and regional weather patterns, including an increased chance of drought for some locations.
An aerosol is a very small droplet or particle that is suspended in the air. Aerosols come from both natural and anthropogenic sources. Natural sources include salt spray from the ocean, plant respiration and dust storms. The main concern regarding aerosols is in the anthropogenic sources, which are increasing rapidly worldwide. The main anthropogenic sources of aerosols are pollution, from vehicles and industry, and biomass burning, which is used in some agricultural practices. While dust is primarily considered a natural source, human land use practices can exacerbate the problem.
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When an air parcel rises its temperature drops and the relative humidity increases. When the temperature drops to the point of complete saturation a cloud will form. This level is called the lifting condensation level. In order for clouds to form, even if the air is saturated or supersaturated, there need to be particles in the air for the water vapor to condense onto. Thus cloud condensation nuclei (CCN) are essential to the cloud formation process. For more information on cloud formation and types of clouds try the CloudMan website.
Cloud droplets then combine through coagulation and coalescence to form larger drops. When the drops are large enough they fall out as precipitation.
With a small number of CCN in the air the water vapor has few options of where to condense. The water vapor is divided between a small number of CCN. This leads to large drops forming relatively quickly. Conversely, if there are many CCN the water vapor that condenses is divided between more particles. This leads to a greater number of smaller cloud droplets. Small droplets are much less efficient at colliding with one another, and this causes the water to stay longer in the atmosphere as a cloud droplet.
One way in which quantitative evidence was looked for to back up the qualitative observations was to examine the sizes of cloud droplets. Using satellite and aircraft measurements the size of cloud particles was determined in clouds formed in pollution plumes. This was then compared to clouds in pristine, unpolluted areas. The droplets found in the pollution plume were found to be smaller than those formed in clean air, considerably less than 14 µm in radius, compared to 25 µm (Rosenfeld, 2000). The threshold for cloud droplet radii that will form precipitation is at least 14 µm. This significantly shows that the droplets in the unpolluted cloud were able to form precipitation, while the droplets in the polluted cloud were not large enough.
Another study was done to quantitatively compare the amount of precipitation due to orographic lifting (see the Cloud Formation figure). The amount of precipitation during the study was compared to historical records of precipitation amounts for the same season and the same amount of time. Again areas of heavy pollution were compared to relatively pristine areas. The upslope precipitation in the polluted areas was found to have decreased by 15-25% (Givati and Rosenfeld, 2004). In the unpolluted areas no such decrease was found. One complicating factor in this study was the measured increase in precipitation on the downslope side of the mountain. This is due to the fact that increasing the amount of CCN, and thus creating smaller drops, delays precipitation. Since the water is not precipitated out on the upslope, as it normally would, there is more water available for precipitation on the downslope. The amount of precipitation increase on the downslope, however, is much smaller than the decrease on the upslope. Thus, all in all, there is shown a significant decrease in precipitation in the polluted areas.
An analogous study was performed on the effects of desert dust on precipitation (Rosenfeld et al., 2001). Again the cloud droplet size was measured and found to be smaller than the minimum size required for precipitation to form. This also reveals a possible feedback mechanism. The dust in the air reduces precipitation, which in turn increases the aridity of the land and more dust can be blown into the air, further reducing precipitation. While the dust studied in this process was natural desert dust, the results are applicable to human activities. Land use practices, such as building of roads, deforestation and some agricultural practices can also increase the amount of dust in the air.
The trend of precipitation decrease downwind of polluted areas is not, however, 100% consistent. There have been observations downwind of cities where precipitation has increased. One explanation for this occurrence is the heat island effect. Cities are generally warmer than their surroundings, leading to the term heat island. When a parcel of air is over the city it warms and can pick up moisture. After being blown out of the city the parcel will quickly cool, if the parcel cools enough or has a high enough water content it may reach saturation, and precipitation will occur. Another instance of precipitation increasing downwind of cities has been seen in situations where the relative humidity is very high (Fan et al., 2007). In this case the water content might be high enough to overcome the small droplet sizes due to increased CCN.
The reduction of rainfall could have drastic consequences on the hydrological system, upon which we rely completely. Droughts could occur in greater number, most significantly affecting those areas that are already vulnerable. If the problem were to become severe enough the lack of precipitation could lead to shortages of drinking water or water for irrigating crops.
The possible indirect effects of aerosols are less well understood. It is possible that by delaying rainfall increasing aerosols will increase the cloud cover. This in turn will raise the albedo of the Earth, which can have a cooling effect on the planet. It is thought that this might, to some extent, lessen the effects of global warming. Whether or not aerosols will lead to significant amounts of cooling, or any cooling at all, is still unknown. Further research is required to better understand this complicated problem.