We have investigated the influence of clouds on ozone on a regional sc
ale (Europe) with a regional scale photochemical dispersion model (LOT
OS). The LOTOS-model calculates ozone and other photo-oxidant concentr
ations in the lowest three km of the troposphere, using actual meteoro
logic data and emissions. We accounted for the altered radiative trans
fer in clouds and aqueous-phase chemistry (with and without iron and c
opper reactions in the aqueous phase). These cloud effects have been t
ested separately to distinguish their individual contribution to the o
zone formation/degradation. Furthermore we estimated the effect of wet
deposition parameterizing a upper-limit scavenging rate. Model result
s are shown for a cloudy period in August 1990. We find for this perio
d that the radiative effect of clouds lead, locally, to ozone reductio
ns in the planetary boundary layer by as much as 22%; overall to a red
uction of ozone of 4%. This coincides with a reduction of 14% of the n
et ozone formation. When aqueous-phase chemistry was added the net ozo
ne formation and concentration were further reduced, up to 20% and 5%,
respectively. Although aqueous-phase chemistry can lead to a strong r
eduction of ozone formation in the cloud (up to 110%), the overall eff
ect is limited by the cloud volume fraction, cloud liquid water conten
t and pH. The iron and copper reactions change the aqueous-phase react
ion pathways substantially and render the droplet roughly a four times
smaller sink for ozone. (C) 1997 Elsevier Science Ltd.