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Aerosols: Detergent for the Atmosphere

The world population is constantly changing its habitat due to its growing need for energy, food, and goods. The associated anthropogenic emissions are also increasingly influencing the material composition of the atmosphere.

This leads to changes in atmospheric self-purification, meaning atmospheric chemistry processes in which climate-forceful components of the air harmful to health are degraded by so-called OH-radicals or else secondary pollutants, such as ozone and particles, are formed as secondary products. For climate forecasts, understanding how particles form–also referred to as aerosol formation–currently represents the greatest challenge. Aerosols impact the climate directly through their interaction with solar radiation and indirectly as cloud condensation nuclei. At present, it is assumed that they have a predominantly cooling effect on climate. At the same time, aerosols pose a health hazard and thus negatively impact air quality.

Two recent studies have revealed considerable gaps in our understanding of the chemical degradation of hydrocarbons by OH radicals and the resulting formation of organic aerosols (Hofzumahaus et al., Science 2009; Kiendler-Scharr et al., Nature 2009).

In studies on the air quality in the densely populated Pearl River Delta in Southern China, concentrations of the OH radical were higher by a factor of three to five than predicted by current atmospheric chemistry models (Hofzumahaus et al., Science 2009). Accordingly, the degradation rate of air pollutants is accelerated by several orders of magnitude. The analysis of field measurements proves that a previously unknown formation process is responsible for the high concentration of radicals and is most likely related to the degradation of biogenic (emitted from plants) hydrocarbons.

In further simulation experiments, the formation of organic aerosols through atmospheric chemistry oxidation of biogenic hydrocarbons was measured in a plant chamber. In the experiments, it was observed that isoprene, the hydrocarbon most frequently emitted by plants, effectively suppresses the formation of aerosols (Kiendler-Scharr et al., Nature 2009). In boreal forests, climate models predict a future temperature-related increase in isoprene in the atmosphere. This increase in isoprene could reduce the atmospheric formation of aerosols in future and the associated cooling climate effect. The findings of both studies could be related to the same chemistry. For further research, experiments are currently being carried out in the SAPHIR atmosphere simulation chamber.