The effect of the interannual variability of clouds on photolysis rates and composition in highly polluted regions

Abstract:

Solar radiation (in the form of UV and visible radiation) determines the photochemical environment for the photolysis of several chemical species in the troposphere and stratosphere. These processes are a crucial part of the Earth system affecting atmospheric composition, air quality and climate and the reactants and products of photolysis reactions are highly variable temporally and spatially and are influenced by anthropogenic and natural emissions.

Clouds influence the composition and chemistry of the atmosphere in several ways but of particular importance is the way they modify solar radiation leading to changes in photolysis rates of several species. This in turn affects the oxidizing capacity of the atmosphere, concentration of greenhouse gases and pollutants from the surface and well into the stratosphere. However, this influence on the modification of photolysis rates and global atmospheric chemistry is a relatively unexplored area of study, particularly relating to the effect of cloud interannual variability (IAV). The analysis of a potentially critical connection between clouds and composition via the effects on photolysis is therefore essential.

This work examines the impact of clouds on the IAV of photolysis rates and oxidants in recent decades through hindcast experiments using the UK Met Office HadGEM3-GA7 model incorporating the UKCA chemistry model, and in comparison with various observational datasets. We begin with a direct assessment of the relative IAV (demonstrated by the coefficient of variation) of 2D cloud fraction, CO, ozone and NO2 calculated from the HadGEM3-UKCA simulations with the relative IAV of the same quantities calculated from MODIS, TES, MLS and OMI (for the periods for which there are observations). We then present the analysis of the impact of the IAV of clouds from 1990–2010 by calculating the percentage of variability that can be attributed to the IAV of clouds globally and over representative anthropogenic regions.

Key findings include an influence of cloud IAV on photolysis rates and OH IAV, and to a lesser extent, on ozone, CO and NO2 IAV over the regions examined.