Item

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 at-
mospheric trace gases. The exchange characteristics and het-
erogeneous 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 variabil-
ity of bidirectional air–soil exchange of 13 VOCs under am-
bient air conditions of an urban background site in Bei-
jing. Sterilized soil was investigated to address physicochem-
ical processes and heterogeneous/multiphase reactions in-
dependently 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 sur-
face area), corresponding to deposition velocities (
V
d
) of
0.0013–0.01 cm s
−
1
and soil surface resistances (
R
c
) of 98–
745 s cm
−
1
, respectively. Formic acid, however, was emit-
ted at a long-term average rate of
∼
6
×
10
−
3
nmol m
−
2
s
−
1
,
suggesting that it was formed and released upon heteroge-
neous oxidation of other VOCs. The soil–atmosphere ex-
change of one individual VOC species can be affected by
both its surface degradation/depletion caused by surface re-
actions and by competitive uptake or heterogeneous forma-
tion/accommodation of other VOC species. Overall, the re-
sults show that physicochemical processing and heteroge-
neous 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.