Aerosols, also known as fine particles or PM (abbreviation for particulate matter), affect the cloud formation and are relevant for climate research. The effect of aerosols on the radiative forcing 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 constant in Europe.
Aerosols are solid or liquid particles in suspension in the air, such as for example soot, mineral dust, salt crystals or ammonium sulfate, 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 effects:
- The direct effect describes the mechanism by which aerosols scatter and absorb solar 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.
The actual effect of aerosols on the atmosphere is still unknown. As of now, not all phenomena have yet been fully understood or identified. In spite of uncertainties, it is assumed that the effect of aerosols in regions with high anthropogenic aerosol levels might be in the same order of magnitude as the effect of all greenhouse gases – albeit with reversed signs. Aerosols slow therfore climate warming.
The radiative forcing of different influencing variables is summarized 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 usually rated negative, although these statements are tainted with uncertainty.
The purpose of the Global Atmosphere Watch (GAW) is to determine the spatio-temporal the spatio-temporal distribution of aerosol properties related to climate forcing and air quality up to multi-decadal time scales.. The aerosol monitoring program operated by the Laboratory for Atmospheric Chemistry of the Paul Scherrer Institute (PSI) at the global GAW Station of the 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 suited to the measurement of the background aerosol. A seasonal cycle is observed for all measured aerosol parameters. This is principally due to the convective transport of the planetary boundary layer toward the Jungfraujoch in the summertime.
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.