Abstract. Volatile organic compounds (VOCs) play a key role in atmospheric chemistry.
Emission and deposition on soil
have been suggested as important sources and sinks of atmospheric trace
gases. The exchange characteristics and heterogeneous chemistry of VOCs on
soil, however, are not well understood. We used a newly designed differential
coated-wall flow tube system to investigate the long-term variability of
bidirectional air–soil exchange of 13 VOCs under ambient air conditions of
an urban background site in Beijing. Sterilized soil was investigated to
address physicochemical processes and heterogeneous/multiphase reactions
independently from biological activity. Most VOCs revealed net deposition
with average uptake coefficients (γ) in the range of
10−7–10−6 (referring to the geometric soil surface area),
corresponding to deposition velocities (Vd) of
0.0013–0.01 cm s−1 and soil surface resistances (Rc) of
98–745 s cm−1, respectively. Formic acid, however, was emitted at a
long-term average rate of ∼6×10-3 nmol m−2 s−1,
suggesting that it was formed and released upon heterogeneous oxidation of
other VOCs. The soil–atmosphere exchange of one individual VOC species can
be affected by both its surface degradation/depletion caused by surface
reactions and by competitive uptake or heterogeneous formation/accommodation
of other VOC species. Overall, the results show that physicochemical
processing and heterogeneous oxidation on soil and soil-derived dust can act
as a sink or as a source of atmospheric VOCs, depending on molecular
properties and environmental conditions.
Physicochemical uptake and release of volatile organic compounds by soil in coated-wall flow tube experiments with ambient air
