The effects of drought on predicted air quality in Texas

Drought is a natural disaster that has profound and complex social, economic, and environmental impacts. As drought is predicted to occur more frequently within Texas with changes in future climate, it is critical to understand its impacts on regional air quality as the State endeavors to achieve and maintain attainment with National Ambient Air Quality Standards for ozone and fine particulate matter. Drought-induced changes in various natural systems, including emissions of biogenic hydrocarbons from vegetation and the physical removal of pollutants by vegetation via dry deposition, have the potential to effect air quality. This work characterizes land cover for eastern Texas climate regions during years with severe to exceptional drought conditions as well as years with average to above average precipitation patterns. Variability in meteorological conditions, biogenic emissions, and dry deposition rates is explored with widely applied global and regional models that have been configured specifically for multi-year analysis of eastern Texas conditions. The Comprehensive Air Quality Model with Extensions (CAMx), which has been used for air quality planning and management efforts in Texas, is used to quantify the relative contributions of various physical and chemical processes to ground-level ozone formation and changes in ground-level ozone concentrations during representative drought and wet periods. The analyses indicate that drought influences air quality in complex ways. This work suggests that the two largest drought driven changes to the physical and chemical processes that influence air quality are increased biogenic emissions due to elevated temperatures and decreased air pollutant removal through dry deposition due to changes in leaf-level processes. Both of these changes degrade air quality and their combined effect can be as large as an increase of approximately 5 ppb in ground level, 8-hour averaged ozone concentrations in parts of eastern Texas. The effects of soil moisture on biogenic emissions estimates can be as significant as temperature, but current land surface model configurations and the adequacy of the Model of Emissions of Gases and Aerosols Nature (MEGAN) algorithm to fully represent short and long-term responses to soil moisture remain highly uncertain. The characterization of soil moisture through ground and satellite-based measurement programs and validation of global and regional-scale land cover distributions should continue to be high priorities to support air quality planning in Texas.