AEROSOL PROPERTIES IN MOSCOW ACCORDING TO 10 YEARS OF AERONET MEASUREMENTS AT THE METEOROLOGICAL OBSERVATORY OF MOSCOW STATE UNIVERSITY

Different microphysical, optical and radiative properties of aerosol were analyzed according to the 10 years of measurements (2001– 2010) at the Meteorological Observatory of Moscow State University within the framework of international AERONET program. Volume aerosol size distribution was shown to have a bimodal character with dominating the fine mode aerosol particles at effective radius of reff-fine = 0.15 μm. In smoke conditions reff-fine was shown to increase to 0.25 μm at extremely large aerosol optical thickness (AOT). Real and imaginary parts of refractive index are characterized by REFR = 1.45, REFI = 0.01 respectively, changing to REFR = = 1.49, REFI = 0.006 for smoke aerosol. AOT seasonal changes are characterized by the increase towards warm period with a local minimum in June. The joint analysis of aerosol characteristics with the NOAA_NCEP_CPC_ CAMS_OPI climatology shows the nature of these changes. For typical conditions aerosol single scattering albedo (SSA) is about 0.9 at 675nm and is characterized by a distinct decrease with wavelength while in forest fires conditions it is significantly higher (SSA = 0.95). The interaction between volume aerosol concentration of different aerosol fractions was obtained with a distinct decrease of variation towards large aerosol content.


Meteorological Observatory of Moscow State
University (MSU MO) joined the AERONET program in 2001, and since that time regular high quality spectral measurements of aerosol characteristics have been in operation with the instruments calibrated according to the international standards.The 10-year period of continuous measurements provides an excellent dataset for the analysis of different aerosol properties in Moscow.In this paper we analyze typical features of main aerosol parameters, its climatology and some aspects of aerosol characteristics during the severe fire events.

METHOD AND THE DESCRIPTION OF THE DATA
Fig. 1 presents a picture of the CIMEL instrument at the roof of the Meteorological Observatory of Moscow State University.Direct Sun measurements are made with 1.2° full field of view collimator at 340, 380, 440, 500, 675, 870, 940 and 1020 nm every 15 minutes during daytime [Holben et al., 1998].Measurements in the solar almucantar and at the solar principal plane are made with the help of the second 1.2° full field of view collimator in four channels: 440, 670,870 and 1020 nm every hour during daytime.The direct Sun measurements are used to compute aerosol optical thickness (AOT) except that for 940 nm channel, which is used to estimate the total water content W, and Angstrom exponent.The Angstrom wavelength exponent (α) is computed as the slope of the linear regression of lnAOT λ versus lnλ using the 440, 500, 675, and 870 nm wavelengths.This is an important characteristic for qualifying the main features of aerosol size distribution and the relative dominance of fine or coarse mode particles.The uncertainty of AOT measurements does not exceed 0.01 in visible range and 0.02 in UV spectral range (Eck

Microphysical and optical aerosol characteristics in Moscow
The AERONET inversion algorithm [Dubovik and King, 2000] provides the retrievals of many important microphysical and optical characteristics of aerosol.Fig. 2 shows the mean volume aerosol size distribution dV(r)/dlnr (μm 2. We should note that due to the increase in cloudy overcast situations and relatively small AOT it is almost impossible to retrieve refractive index and single scattering albedo during winter period.As a result, the obtained aerosol characteristics described above mainly characterize spring-summer-fall conditions (more than 92% of cases).
3. The statistics on SSA and refractive index is less due to the necessity of additional restriction on the AOT440 >0.4 in the retrieval algorithm.

Radiative characteristics of aerosol
Aerosol optical thickness is the most widely used aerosol parameter for characterizing the aerosol loading within the atmospheric column.It is also used for estimation of the radiative budget of the atmosphere.Fig. 3 presents the seasonal changes of aerosol optical thickness at different wavelengths according to the whole 2001-2010 dataset.
There is a distinct spectral dependence in AOT characterized by an AOT increase towards smaller wavelengths due to dominating the fine mode aerosol observed in Moscow conditions (see Fig. 2 and discussion in the previous section).The prevalence of the relatively small aerosol particles according to the Mie theory [Liou, 1980] provides a decrease in an extinction aerosol coefficient with an increase of wavelength.Since AOT is   accompanied usually by the prescribed fires, also creates additional source of fine-mode aerosol (difference in fine mode volume concentration (VolCon-fine) comprises +0.01 μm 3 /μm 2 ) with some decrease in effective radius r eff-fine from 0.18 to 0.14 μm.In addition, there is an increase in volume concentration of coarse mode particle (difference in VolCon-coarse = +0.026μm 3 /μm 2 ), which is also accompanied by the decrease in r eff-coarse from 2.5 to 2.2 μm.
In May there are pronounced changes in the circulation processes over Europe with amplifying the Azore anticyclone and spreading its wedges over the Mediterranean area.At the same time, the aerosol loading over Central and Eastern Europe reduces due to the increase in precipitation, the domination of northern air advection from Scandinavian regions and more intensive uptake of aerosol by grass and leaves.In June further increase of precipitation over the northern Eastern Europe and ceasing the air advection from the south are responsible for distinct local minimum in aerosol optical thickness over the vast territory to the north of 45°N and to the east of 15°E [Chubarova, 2009].In addition, this local AOT minimum can be attributed to comparatively high water store in soil and vegetation, which can also prevent active mineral dust aerosol formation.In July aerosol optical thickness as well as Angstrom exponent increase due to the additional generation of aerosol fine mode particles from anthropogenic emissions in conditions of high photolysis rates and elevated temperatures in the absence of wet removal of aerosol from the atmosphere.The AOT maximum in August is explained by the effects of forest fires in 2002 and 2010, which have led to extremely high aerosol loading.Excluding these two years from the sample has changed the August AOT to approximately the AOT in July (AOT∼0.22 in visible region of spectrum).So the forest fires can be responsible for 1.5 times increase in monthly mean AOT values.
In September lower temperatures and photolysis rates as well as the intensification of westerlies should decrease the aerosol loading.However, relatively high AOT level is also explained by the effects of fires in 2002 and its removing significantly decrease the AOT level by more than 30%.In October-November further cleaning of the atmosphere is observed due to prevailing westerlies, which is consistent with the attenuation of the Angstrom exponent values over the vast area with a distinct tendency to decreasing towards the Atlantic Ocean [Chubarova, 2009].
Fig. 5 demonstrates 3D picture with seasonal and year-to year variations of AOT at 500 nm.
One can see high variability in year-to-year AOT changes, some common features of decreasing AOT towards winter in different years can be also observed.There are pronounced maxima in August-September 2002 and in August 2010 during severe fire events.We had an extremely high monthly mean AOT maximum in 2010, which exceed 1.1 (more than 3 times higher than the mean value AOT500 = 0.348).However, in 2002 the period of elevated AOT values was much longer and lasted from the very end of July to the middle of September.The absolute AOT maximum was observed on August, 7th, when AOT500 reached approximately 5! This was one and a half time larger than the absolute maximum observed during the previous mega-fire event in 2002.However, due to the change in atmospheric circulation at the end of August and advection of very clean air from the western regions, the AOT dropped to 0.06 on August 20, 2010.
Another important radiative characteristics of aerosol -an asymmetry factor of the aerosol phase function (g) and aerosol single scattering albedo (SSA) -can be obtained from AERONET inversion algorithm using a combination of direct and diffuse radiance measurements.They are defined according to the following equations: where θ is the scattering angle, Р(θ) is the aerosol phase function; where α λ -extinction coefficient (1/c) σ λscattering coefficient (1/cm).
According to the equations the g and SSA parameters are dimensionless and can change from zero to 1.The asymmetry factor characterizes the shape of phase function and is close to 1 when the size of particles is much higher than wavelength and aerosol phase function has a distinct forward peak of scattering.For example, for visible spectral range this can be observed for large cloud particles (r eff = 7-8 μm).SSA values are close to 1 when the absorption is close to zero and the attenuation is observed only due to scattering processes.The statistics of SSA and factor of asymmetry at 675nm over the whole period of measurements in Moscow are shown in Table 1.Fig. 6 presents the spectral dependence of aerosol asymmetry factor and single scattering albedo.The aerosol asymmetry factor decreases within

Interaction between microphysical, optical and radiative aerosol properties
One can see some interesting features if compare the relationship of some microphysical, optical, and radiative aerosol properties in real atmospheric conditions.Fig. 7, a, b shows the dependence of effective radii of fine and coarse mode as a function of aerosol optical thickness at 500nm.We should mention that high AOT500 (AOT500 > 0.8) were observed in situations with forest fires.One can see a significant drop in r eff variation for AOT500 > 0.8 with a tendency to increasing the fine mode effective radii from 0.18 to 0.25 μm and decreasing coarse mode radii towards higher AOT500 from 2.8 to 2.5 μm.
At smaller AOT we have an extremely high r eff variation from 0.1 to r eff -fine = 0.26 μm and from 1 to r eff -fine = 3.7 μm at small AOT < 0.15.This happen possibly due to different types and properties of aerosol that can depend on specific type of aerosol.The explanation of this phenomenon should be studied further.
There is also an interesting dependence of different contribution of fine and coarse modes volume concentration to the total volume concentration.Fig. 8 shows the approximately linear dependence of fine mode concentration with AOT while for the coarse mode there is a maximum at AOT less 0.4.The volume concentration of coarse mode has a maximum of about 20% for forest fire smoke aerosol while for other situations it can vary within large range from 8 to 90%.
In addition, there is a distinct decrease in deviation both in real and in imaginary part of refractive index with AOT increase (Fig. 9).There is some REFR increase in fire smoke conditions (REFR = 1.49) compared with typical aerosol conditions (REFR = 1.49Some interactions between volume concentrations of different aerosol fractions were obtained with a distinct decrease in deviation towards larger AOT values.

Fig. 1 .
Fig. 1.The Cimel sun/sky photometer at the roof of the Meteorological Observatory of Moscow State University

Fig. 3 .
Fig. 3. Seasonal change of mean (a) and median (b) values of spectral aerosol optical thickness AOT in Moscow for the 2001-2010 period

Table 1 . Diff erent microphysical and optical aerosol characteristics from AERONET data Statistics Eff ective radius, μm r eff -fi ne Eff ective radius, μm r eff -coarse Volume Concentration (μm 3 /μm 2 ) VolCon-total Volume Concentration (μm 3 /μm 2 ) VolCon-fi ne Volume Concentration (μm 3 /μm 2 ) VolCon-coarse Real part of refractive index, REFR (675 nm) Imaginary part of refractive index, REFI (675 nm) Single scattering albedo, SSA (675 nm) Factor of asymmetry, g (675 nm)
[Chubarova, Sviridenkov, Smirnov, and Holben, 2011]μm 2[Chubarova, Sviridenkov, Smirnov, and Holben, 2011].Table1presents the statistics of main microphysical and optical aerosol parameters during 10 years of continuous measurements in Moscow, which include effective radii for fine and coarse modes, volume concentration, real and imaginary part of refractive index.The obtained mean volume aerosol distribution for Moscow conditions corresponds to the mean effective radius of fine mode particle with r eff-fine = 0.15 μm and to the coarse mode particle with r eff-coarse = 2.34 μm.However, gi111.indd 22 gi111.indd22 03.08.2011 14:38:22 03.08.2011 14:38:22 of the coarse standard deviation is very high, especially for the coarse mode particles.The frequency distribution of effective radii has a lognormal type with a clear positive asymmetry.Volume aerosol concentration also has a distinct positive asymmetry and varies within a large range from 0.009 to 0.47 μm 3 /μm 2 with the mean value of VolCon = 0.07μm 3 /μm 2 .It is characterized by the prevalence of fine mode particles (about 57% of the total volume concentration).Mean values of real and imaginary parts of refractive index (REFR and REFI, see Table 1) comprise respectively REFR = 1.45 ± 0.01 and REFI = 0.01 ± 0.01.The frequency distribution of the REFR belongs to a normal law distribution, while that of the REFI is better described by a lognormal type with a positive asymmetry.The typical values of REFR correspond to the non-hydroscopic type of aerosol particles [Dubovik et al., 2002].

Table 2 . Several statistics of AOT500, water vapor content and Angstrom exponent for the 2001-2010 period. Monthly mean value, mean variation coeffi cient, mean minimum and maximum and mean day number
[Abakumova and Gorbarenko, 2008]1 ± 0.01) for Moscow conditions.Smoke aerosol was shown to have higher REFR (REFR = 1.49) and lower REFI = 0.006 than those values in typical conditions.Seasonal changes in AOT are characterized by the AOT increase towards warm period with local minimum in June which has been confirmed by our previous results obtained from satellite data for a comparatively large territory and is in agreement with standard actinometrical observations of the transparency characteristics described in[Abakumova and Gorbarenko, 2008].The joint analysis of aerosol parameters with NOAA_NCEP climatology shows the nature of this effect, which is determined by the change in air advection.The single Fig. 7. Aerosol effective radius of fine (a) and coarse (b) mode versus aerosol optical thickness at 500 nm (AOT500)