Aerosols of the atmosphere are one of the most important factors influencing the level of radiation in the upper and lower layers of the atmosphere, and hence - the whole climate. However, the optical properties of the aerosols all over the world are not broadly studied yet.

The data on its optical is majorly collected by satellites. Ground-based systems, though, provide us with more accurate information. AERONET is a widespread automatized system, provided with sun and sky photometers that allow continuing and precise measurement of a number of aerosol parameters. The international program operates in MSU since 2001.

The Lomonosov Moscow State University meteorologists Natalia Chubarova, Alexey Polukhov and Ilona Gorlova studied season and long-term variations of aerosol percentage in the atmosphere over Eastern Europe. Simultaneously they tried to explain a tendency of aerosol optical depth of the atmosphere (AOD) and a role of human factor in its forming. For an extra-correction of data on aerosols, the clouds were also observed.

A significant aspect of the research was a correction of radiative properties of aerosols by taking nitrogen dioxide in consideration. NO2 has remarkable absorption edges in uv- and visible range of a specter, especially in its industrial and urban areas, and Moscow is a large city with a high concentration of nitrogen oxides in its atmosphere.

To analyze the influence of NO2 on the aerosol optical depth of the atmosphere, researchers measured its percentage in several spots within the city (Ostankino tower and MSU main building) in summer and winter.

As the result, two regimes of vertical distribution of nitrogen dioxide were detected, typical for a troposphere less than 2 km. General percentage of nitrogen in higher atmosphere layers is far lower. According to the results of the observations, a maximal level of nitrogen dioxide appeared in February when due to the heating season the amounts of gas emissions to the atmosphere peaked. Also, a percentage of nitrogen dioxide grows a little in summer because of peat bog fires.

Since the procedures of aerosol measurements are automatized, an extra observation of cloudage was included. While common weather stations measure parameters once in three hours, MSU weather station traced the data every hour.

Points of general cloudage varying from zero to ten (from clear sky to overcast correspondently) were used as an index. The results for an overcast sky were later excluded from the dataset, since a direct radiation disappears. Depending on a season this filter varies - in summer it makes 10 points, in winter - around 6. The applied filter reduces the amount of days, for which the calculations are absolutely correct, to 7 - 20% in warm periods, and to 25 - 45% in cold periods.

The AERONET filters show nearly no reaction on the homogeneous upper clouds - Cirrus and Cirrostratus clouds. An instrument is directed towards sun basing on astronomical formulae and the outcome measurements can be filtered according to the accepted standardized system. The first filter is based on a signal variation within one minute.

The instrument registers the lower layer clouds: Cumulus and Stratus clouds, hence, they are easy to exclude for AOD estimation. "Automatic filters, used by AERONET, are not always sensitive to the presence of upper-layer clouds, what can lead to the wrong estimations of aerosol characteristics," comments Alexey Polukhov, one of the researching team.

Natalia Chubarova, PhD in Geography, explains: "Studying of the aerosols is particularly important at the moment because of the global changes in climate.

Basing on the unique dataset collected by the MSU Meteorological Observatory and covering changes in aerosol optic depth throughout several years, we managed to detect a significant trend of lowering aerosol percentage in the XXI century. A detailed analysis allowed distinguishing that this tendency is connected with a reduction in anthropogenic emissions of proto-aerosol gases that is currently observed over Europe and Western part of Russia."

The scientists explain a trend towards lowering of polluting emissions to the atmosphere with a higher level of petrol quality and a reduction in anthropogenic emissions of sulfur oxides.

However, emissions are still largely responsible for aerosol pollution of Moscow. Considering season changes, the most significant AOD lowering falls on spring and autumn periods. Remarkably, natural processes seem not to influence the percentage of aerosols significantly.

According to Natalia Chubarova, the research is directly connected to the estimation of the climate effects of aerosols and plays an important role in tackling the climate change problem.

The research was published in Atmospheric Measurement Techniques (current IF - 2,929).