Formaldehyde integral content in troposphere of Moscow region: preliminary results of 6 years of measurements using DOAS technique

Formaldehyde (HCHO) in the atmosphere is directly emitted by anthropogenic and biogenic sources and, more significantly, produced during oxidation of methane and other volatile organic compounds (VOCs), and so its content is one of observable indicators of air pollution by VOCs. HCHO has a sufficiently large absorption cross-section in the UV spectral region to be detected by the technique of the differential optical absorption spectroscopy (DOAS). Spectral measurements of scattered solar radiation are performed at Zvenigorod Scientific Station (ZSS, 55°41'49'"N, 36°46'29'"E) located in 38 km west from Moscow Ring Road by MAXDOAS instrument since 2008. This location of the observational station allows evaluating the background levels of formaldehyde in the troposphere and the levels that are associated with pollution from Moscow. For analysis of the HCHO variability we selected spectra taken in cloud free conditions from October 2009 to April 2016. Version 1.3 of the retrieval algorithm is used. It uses information on the surface albedo and the height of the atmospheric boundary layer inferred from a model. It has optimized interpolation parameter of DOAS processing. Cloud screening algorithm using UV color index (the ratio of 370-nm radiance to 340-nm one) was implemented. The obtained data quantify the Moscow megapolis influence on air quality at Zvenigorod by comparison of HCHO VCD for east and west wind directions. HCHO VCD at East winds in average more than one at West winds for 0.4 ± 0.1×10 16 mol×cm -2 at air temperatures from +5 to +35°C, and for 0.8 ± 0.2×10 16 mol×cm -2 at temperatures from -20 to +5°C. It may be caused by Moscow emissions of HCHO precursors. Strong dependence of HCHO VCD on air temperature is noticeable in our data for air temperatures from +5 to +35°C. In different wind conditions the gradient of the temperature effect is about 0.86 ± 0.07 ×10 15 mol×cm -2 ×°C -1 in average. The increase of the formaldehyde content with the increase of the air temperature can be caused by the HCHO formation from non-methane biogenic volatile organic compounds for which more emission is expected at higher temperatures.