Abstract
Long-term Aqua and Terra MODIS (MODerate resolution Imaging Spectroradiometer) Collections 5.1 and 6.1 (c051 and c061, respectively) aerosol data have been combined with AERONET (AERosol RObotic NETwork) ground-based sun photometer observations to examine trends in aerosol optical thickness (AOT, at 550 nm) over Northern Europe for the months April to September. For the 1927 and 1559 daily coincident measurements that were obtained for c051 and c061, respectively, MODIS AOT varied by 86 and 90%, respectively, within the predicted uncertainty of one standard deviation of the retrieval over land (ΔAOT = ±0.05 ± 0.15·AOT). For the coastal AERONET site Gustav Dalen Tower (GDT), Sweden, larger deviations were found for MODIS c051 and c061 (79% and 75%, respectively, within predicted uncertainty). The Baltic Sea provides substantially better statistical representation of AOT than the surrounding land areas and therefore favours the investigations of trends in AOT over the region. Negative trends of 1.5% and 1.2% per year in AOT, based on daily averaging, were found for the southwestern Baltic Sea from MODIS c051 and c061, respectively. This is in line with a decrease of 1.2% per year in AOT at the AERONET station Hamburg. For the western Gotland Basin area, Sweden, negative trends of 1.5%, 1.1% and 1.6% per year in AOT have been found for MODIS c051, MODIS c061 and AERONET GDT, respectively. The strongest trend of –1.8% per year in AOT was found for AERONET Belsk, Poland, which can be compared to –1.5% per day obtained from MODIS c051 over central Poland. The trends in MODIS and AERONET AOT are nearly all statistically significant at the 95% confidence level. The strongest aerosol sources are suggested to be located southwest, south and southeast of the investigation area, although the highest prevalence of pollution events is associated with air mass transport from southwest.
Highlights
Global warming is primarily a problem of too much carbon dioxide (CO2) in the atmosphere
The strongest trend of –1.8% per year in aerosol optical thickness (AOT) was found for AERONET Belsk, Poland, which can be compared to –1.5% per day obtained from MODIS c051 over central Poland
Comparison of MODIS AOT against AERONET measurements
Summary
Global warming is primarily a problem of too much carbon dioxide (CO2) in the atmosphere. Anthropogenic emissions of aerosols impact the Earth’s radiation balance and climate as well (IPCC, 2013). CO2 is relatively stable in the atmosphere (with a residence time of about 100 years), and evenly distributed over remote areas of the Earth. Anthropogenic aerosols feature substantially shorter residence times in the lower atmosphere ($1 week) and cause mainly local or regional effects. The full impact of increased greenhouse gas concentrations on global temperature is not known as man-made aerosols mask the heating of the planet to an uncertain degree. The direct and indirect aerosol effects (in the cloud-free and cloudy atmosphere, respectively) have been known for decades, the estimates of the impacts of aerosols on the radiation balance are still associated with large uncertainties (IPCC, 2013).
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