Clouds, photolysis and regional tropospheric ozone budgets 1Centre for Atmospheric Science, University of Cambridge, UK
2Lancaster Environment Centre, Lancaster University, UK
3Met Office, Exeter, UK
*now at: the NASA Goddard Inst. for Space Studies & Columbia Univ., Center for Climate Systems Res., New York, USA
Received: 09 April 2009 – Published in Atmos. Chem. Phys. Discuss.: 25 June 2009 Abstract. We use a three-dimensional chemical transport model to examine the shortwave
radiative effects of clouds on the tropospheric ozone budget.
In addition to looking at changes in global concentrations as previous studies
have done, we examine changes in ozone chemical production and loss caused
by clouds and how these vary in different parts of the troposphere. On a global
scale, we find that clouds have a modest effect on ozone chemistry,
but on a regional scale their role is much more significant, with the size of
the response dependent on the region. The largest averaged changes in chemical budgets
(±10–14%) are found in the marine troposphere, where cloud optical depths are high. We demonstrate that
cloud effects are small on average in the middle troposphere because this is a
transition region between reduction and enhancement in photolysis rates.
We show that increases in boundary layer ozone due to clouds are driven by
large-scale changes in downward ozone transport from higher in the troposphere
rather than by decreases in in-situ ozone chemical loss rates. Increases in
upper tropospheric ozone are caused by higher production rates due to backscattering of
radiation and consequent increases in photolysis rates, mainly J(NO2). The
global radiative effect of clouds on isoprene, through decreases of OH in the lower
troposphere, is stronger than on ozone. Tropospheric isoprene lifetime increases
by 7% when taking clouds into account. We compare the importance of clouds in
contributing to uncertainties in the global ozone budget with the role of other
radiatively-important factors. The budget is most sensitive to the overhead ozone
column, while surface albedo and clouds have smaller effects. However, uncertainty
in representing the spatial distribution of clouds may lead to a large
sensitivity of the ozone budget components on regional scales.
Revised: 29 September 2009 – Accepted: 19 October 2009 – Published: 03 November 2009
Citation: Voulgarakis, A., Wild, O., Savage, N. H., Carver, G. D., and Pyle, J. A.: Clouds, photolysis and regional tropospheric ozone budgets, Atmos. Chem. Phys., 9, 8235-8246, doi:10.5194/acp-9-8235-2009, 2009.