Aerosols, also known as fine solid particles or PM (abbreviation for particulate matter), affect cloud formation and are relevant for climate research. The actual effect of aerosols on the atmosphere has yet to be fully researched. Analyses show that the aerosol exposure has decreased in North America over the past 10-15 years, while it remained stable in Europe.
Aerosols are solid or liquid air-borne particles, such as soot, mineral dust, salt crystals or ammonium sulphate, with a size ranging between several nanometres and several hundred micrometres. They can affect the atmosphere in two significant ways, by direct and indirect aerosol effect:
- The direct effect describes the mechanism by which aerosols scatter and absorb radiation, thus altering the radiation balance of the earth-atmosphere system.
- The indirect effect describes how aerosols alter the microphysical (and hence the radiation-relevant) properties, the quantity as well as lifespan of clouds.
In spite of the uncertainties, it is generally accepted that in areas where there is a high concentration of anthropogenic aerosols, the radiative forcing due to aerosols is in the same order of magnitude as radiative forcing due to greenhouse gases, albeit with reversed signs. Aerosols would thus contribute to reducing climate warming caused by greenhouse gases.
The radiative forcing of different influencing variables is analysed in the report of the Intergovernmental Panel on Climate Change (IPCC) of 2013. Based on it, the positive effect of greenhouse gases, ozone and water vapour is well explained. The direct and indirect effects of aerosols are considered to be negative, although there is still a great deal of uncertainty around this subject.
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 and is therefore well equipped to measure the aerosol background levels. A seasonal cycle is observed for all measured aerosol parameters. This is due to the convective transport of the planetary boundary layer toward the Jungfraujoch in the summertime.
A 10 to 18-year trend analysis of the scattering and absorption coefficients as well as the incidence density (Collaud Coen et al., 2013, Asmi et al., 2013) has revealed a global decrease in aerosol exposure in North America and relative stability in Europe. The difference between the two continents is probably due to the massive reductions in pollutant emissions in in Europe during the 1980s, while in North America, such measures were not implemented until later on. However, the European stations (in mountainous regions or close to the sea) are perhaps not fully representative of the continental plate.
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.