Item

Abstract

Quantifying the relative importance of gas up-
take on the ground and aerosol surfaces helps to determine
which processes should be included in atmospheric chem-
istry models. Gas uptake by aerosols is often characterized
by an effective uptake coefficient (
γ
eff
), whereas gas uptake
on the ground is usually described by a deposition velocity
(
V
d
). For efficient comparison, we introduce an equivalent
uptake coefficient (
γ
eqv
) at which the uptake flux of aerosols
would equal that on the ground surface. If
γ
eff
is similar to
or larger than
γ
eqv
, aerosol uptake is important and should
be included in atmospheric models. In this study, we com-
pare uptake fluxes in the planetary boundary layer (PBL) for
different reactive trace gases (O
3
, NO
2
, SO
2
, N
2
O
5
, HNO
3
and H
2
O
2
), aerosol types (mineral dust, soot, organic aerosol
and sea salt aerosol), environments (urban areas, agricultural
land, the Amazon forest and water bodies), seasons and mix-
ing heights.
For all investigated gases,
γ
eqv
ranges from magnitudes
of 10
−
6
–10
−
4
in polluted urban environments to 10
−
4
–10
−
1
under pristine forest conditions. In urban areas, aerosol up-
take is relevant for all species (
γ
eff
≥
γ
eqv
) and should be con-
sidered in models. On the contrary, contributions of aerosol
uptakes in the Amazon forest are minor compared with the
dry deposition. The phase state of aerosols could be one of
the crucial factors influencing the uptake rates. Current mod-
els tend to underestimate the O
3
uptake on liquid organic
aerosols which can be important, especially over regions with
γ
eff
≥
γ
eqv
. H
2
O
2
uptakes on a variety of aerosols are yet to
be measured under laboratory conditions and evaluated.
Given the fact that most models have considered the up-
takes of these species on the ground surface, we suggest also
considering the following processes in atmospheric models:
N
2
O
5
uptake by all types of aerosols, HNO
3
and SO
2
uptake
by mineral dust and sea salt aerosols, H
2
O
2
uptake by min-
eral dust, NO
2
uptakes by sea salt aerosols and O
3
uptake by
liquid organic aerosols.