Ozone damage to crops in southern Africa: An initial modeling study

Abstract

The Cross Border Impact Assessment Project (CAPIA) was designed to develop an understanding of regional surface ozone concentrations and their potential risk to agriculture in southern Africa. Surface ozone concentrations were estimated using the Comprehensive Air Quality Model with extensions (CAMx). The initial assessment of ozone risk to maize was characterised using the Accumulated Exposure over a Threshold of 40 ppb (AOT40). Modelled ozone concentrations exceed 40 ppb over much of southern Africa suggesting that the potential for ozone damage to maize exists across the region. The AOT40 approach has limitations, the most notable being its inability to account for modifying factors that limit the amount of pollutant taken up by the plant. The aim of this research is to investigate the feasibility of including the stomatal flux algorithms in the CAMx model, and so improve the estimates of ozone uptake in plants and the subsequent risk of ozone damage posed to crops. The initial model results indicate that the areas with elevated ozone concentrations are not the same as those with the highest ozone fluxes, suggesting that the application of the more biological relevant flux-based risk assessment methods would identify different regions within the modelling domain where damage to maize is more likely to occur. In addition, the algorithms in CAMx tend to underestimate both the deposition velocity and ozone flux in comparison to the flux method. The maximum modelled total ozone fluxes are above the critical stomatal flux values of 6 nmol m-2 s-1 currently defined and applied within Europe to assess risk and economic impacts of ozone to agricultural crops.