Aerosols are fine particles of liquid or solid material with sizes in the nanometer to micrometer range. Their concentration varies strongly in space and time. Aerosols tend to decrease the climate warming by reflecting solar radiation back to space. They are also necessary to form clouds. Therefore, they play an important role in climate monitoring and research.
Aerosol and climate
Aerosols are solid or liquid air-borne particles that come from natural processes (soil erosion, sea salt crystals, forest fires) or anthropogenic processes (oil, coal, gas or wood combustion, agriculture, traffic and others). Although they can have a negative impact on human health, aerosols play an important role for the climate. First, they are the unique atmospheric compound that cools the climate. Depending on their chemical composition, aerosols scatter solar radiation back to space and therefore reduce the surface temperature.
Second, aerosols are needed for clouds to form. They act as so-called cloud condensation nuclei: if the air is supersaturated with respect to water, water condenses on the surface of the aerosols and forms droplets. As more droplets form and collide, the droplets grow, the cloud thickens, until the droplets are large enough to fall to the ground as rain. If the aerosol number increases in the area of an existing cloud, the number of cloud droplets will also increase, but the droplets will become smaller. As a result, the clouds appear brighter from above, they reflect more sunlight, an indirect effect that also tends to cool the climate. In addition, the formation of precipitation is potentially slowed down and decreased.
The cooling effects of the aerosols are accepted in the scientific community but are subject to large uncertainties despite advances in science over the last decades.
The purpose of the Global Atmosphere Watch (GAW) is to determine the chronological distribution of aerosols and their properties with respect to their influence on climate and air quality in time scales of up to several decades. The aerosol monitoring programme operated by the Laboratory for Atmospheric Chemistry at the Paul Scherrer Institute (PSI) at the global GAW Station on top of Jungfraujoch (3580 m above sea level) is among the most comprehensive ones in the world. Due to the high altitude of the station, the Jungfraujoch is partly located in the free troposphere. This zone of the atmosphere is detached from lower part of the atmosphere that is regularly fed with aerosols from natural sources and human activities. The lowest layer of the atmosphere is called boundary layer. Aerosols tend to accumulate in the boundary layer. For the aerosols to reach altitudes such as the Jungfraujoch, vertical transport is needed. Therefore, concentrations at Jungfraujoch are generally lower than in civilized areas in the boundary layer. At the Jungfraujoch, a seasonal cycle is observed for all measured aerosol parameters. This is due to vertical transport of aerosol-ladden air from the boundary layer toward the Jungfraujoch in the summertime. The lifting occurs because the air in the lower parts of Switzerland warms up in summer – and warm air rises because it’s lighter.
A 10 to 15 year trend analysis of the scattering and absorption coefficients as well as of the number concentration has revealed a global decrease in the aerosol load for most stations in North America and relative stability in Europe. The difference between both continents seems to be related to a massive reduction of pollutant emission in Europe since 1980's whereas the greatest emission abatement policies occurred a decade later in US. The European stations (mountainous and marine ones) are however not completely representative of the continental plateau.
Collaud Coen, M.; Andrews, E.; Asmi, A.; Baltensperger, U.; Bukowiecki, N.; Day, D.; Fiebig, M.; Fjaeraa, A. M.; Flentje, H.; Hyvärinen, A.; Jefferson, A.; Jennings, S. G.and Kouvarakis, G.; Lihavainen, H.; Lund Myhre, C.; Malm, W. C.; Mihapopoulos, N.; Molenar, J. V.; O'Dowd, C.; Ogren, J. A.; Schichtel, B. A.; Sheridan, P.; Virkkula, A.; Weingartner, E.; Weller, R. and Laj, P. Aerosol decadal trends : Part 1: In-situ optical measurements at GAW and IMPROVE stations Atmos. Chem. Phys., 2013, 13
Asmi, A.; Collaud Coen, M.; Ogren, J. A.; Andrews, E.; Sheridan, P.; Jefferson, A.; Weingartner, E.; Baltensperger, U.; Bukowiecki, N.; Lihavainen, H.; Kivekäs, N.; Asmi, E.; Aalto, P. P.; Kulmala, M.; Wiedensohler, A.; Birmili, W.; Hamed, A.; O'Dowd, C.; G Jennings, S.; Weller, R.; Flentje, H.; Fjaeraa, A. M.; Fiebig, M.; Myhre, C. L.; Hallar, A. G.; Swietlicki, E.; Kristensson, A. and Laj, P. Aerosol decadal trends: Part 2: In-situ aerosol particle number concentrations at GAW and ACTRIS stations Atmos. Chem. Phys., 2013, 13, 895-916
Collaud Coen, M., Weingartner, E., Furger, M., Nyeki, S., Prévôt, A.S.H., Steinbacher, M., and Baltensperger, U.: Aerosol climatology and planetary boundary influence at the Jungfraujoch analyzed by synoptic weather types, Atmos. Chem. Phys., 11, 5931-5944, 2011.
Collaud Coen, M., Weingartner, E., Apituley, A., Ceburnis, D., Fierz-Schmidhauser, R., Flentje, H., Henzing, J. S., Jennings, S. G., Moerman, M., Petzold, A. and others: Minimizing light absorption measurement artifacts of the Aethalometer: evaluation of five correction algorithms, Atmos. Meas. Tech., 3, 457-474, 2010.