Abstract
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.
Highlights
The main factors affecting atmospheric radiative transfer at wavelengths important for tropospheric chemistry are the solar zenith angle, absorption by stratospheric ozone, reflection from the surface, and scattering and absorption by cloud and aerosol particles
Using the MOZART-2 model coupled with the FTUV photolysis scheme Tie et al (2003b) found global ozone burden increases of 8–12% with clouds compared to clear-sky conditions, whereas Liu et al (2006) found increases of 3–5%
These previous studies provide valuable insight into global cloud-chemistry interactions. They either examine the effect of clouds on tracer concentrations without examining chemical budgets (Tie et al, 2003b; Liu et al, 2006), or they explore the ozone budget thoroughly without evaluating the effects of clouds in detail (Wild, 2007; Wu et al, 2007). We extend this earlier work by focusing on how clouds contribute to the large uncertainties in the global ozone budget in current models through their effects on photolysis rates
Summary
The main factors affecting atmospheric radiative transfer at wavelengths important for tropospheric chemistry are the solar zenith angle, absorption by stratospheric ozone, reflection from the surface, and scattering and absorption by cloud and aerosol particles. There was only a small change (2.5%) in the global ozone budget, and more recent studies have found similar results (Liu et al, 2006) The magnitude of these cloud effects varies significantly between models using different photolysis codes. Using the MOZART-2 model coupled with the FTUV photolysis scheme Tie et al (2003b) found global ozone burden increases of 8–12% with clouds compared to clear-sky conditions, whereas Liu et al (2006) found increases of 3–5% In their assessment of ozone budget uncertainties, Wu et al (2007) showed that global tropospheric P(O3) in GEOS-CHEM varied significantly when using different meteorological fields and that clouds were the major cause of these variations. By clouds have a greater effect on methane than larger OH increases found at higher levels
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