Abstract
Siberia with its large area covered with boreal forests, wetlands and tundra is believed to be an important sink for ozone via dry deposition and reactions with biogenic volatile organic compounds (BVOCs) emitted by the forests. To study the importance of deposition of ozone in Siberia, we analyse measurements of ozone mixing ratios taken along the Trans-Siberian railway by train, air-borne measurements and point measurements at the Zotino station. For all data, we ran the Lagrangian particle dispersion model FLEXPART in backward mode for 20 d, which yields the so-called potential emission sensitivity (PES) fields. These fields give a quantitative measure of where and how strongly the sampled air masses have been in contact with the surface and hence possible influenced by surface fluxes. These fields are further statistically analysed to identify source and sink regions that are influencing the observed ozone. Results show that the source regions for the surface ozone in Siberia are located at lower latitudes: the regions around the Mediterranean Sea, the Middle East, Kazakhstan and China. Low ozone mixing ratios are associated to transport from North West Russia, the Arctic region, and the Pacific Ocean. By calculating PES values for both a passive tracer without consideration of removal processes and for an ozone-like tracer where dry deposition processes are included, we are able to quantify the ozone loss occurring en route to the receptor. Strong correlations between low ozone concentrations and the spatially integrated footprints from FLEXPART, especially during the period summer to autumn, indicate the importance of the Siberian forests as a sink for tropospheric ozone.
Highlights
Forests and wetlands act as biological sources and sinks for many atmospheric compounds and, play an important role for the chemical composition of the atmosphere (Gao et al, 1993)
The vertical gradient in ozone-mixing ratios observed above Siberia is much steeper in summer when the vegetation is active than in spring. We take these findings as a motivation for this study, in which we investigate measured ozone-mixing ratios over Siberia from three different measurement platforms and research programmes: (1) train measurements performed by the TRans-Siberian Observations Into the Chemistry of the Atmosphere (TROICA) campaigns since the year 1999, (2) aircraft measurements from YAK-Airborne Extensive Regional Observations in Siberia (AEROSIB) campaigns during the years 2006Á2008 and (3) measurements made at the Zotino research station
This means that air masses that are exposed to strong dry deposition in the region will eventually lose most of their O3, regardless of their initial ozone mixing ratio imported to Siberia
Summary
Forests and wetlands act as biological sources and sinks for many atmospheric compounds and, play an important role for the chemical composition of the atmosphere (Gao et al, 1993). Low surface ozone concentrations in large forest areas have been reported both for Siberia (Pochanart et al, 2003) and Amazonia (Kirchhoff, 1988) and have been explained both by the effective dry deposition on leaf surfaces and destruction by BVOCs. Given the globally rising ozone concentrations, these ozone sink areas are very important and deserve more attention. The vertical gradient in ozone-mixing ratios observed above Siberia is much steeper in summer when the vegetation is active than in spring We take these findings as a motivation for this study, in which we investigate measured ozone-mixing ratios over Siberia from three different measurement platforms and research programmes: (1) train measurements performed by the TRans-Siberian Observations Into the Chemistry of the Atmosphere (TROICA) campaigns since the year 1999, (2) aircraft measurements from YAK-AEROSIB campaigns during the years 2006Á2008 and (3) measurements made at the Zotino research station. This work is performed as part of a joint NorwegianÁRussian research program; Study of Russian Air Pollution Sources and their Impact on Atmospheric Composition in the Arctic using the TROICA railway carriage, data from Svalbard and the FLEXPART transport model (RAPSIFACT)
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