Comparisons of modeled and observed isoprene concentrations in southeast Texas

Biogenic emissions of hydrocarbons, primarily isoprene, dominate the VOC emission inventory in eastern Texas. Air quality model predictions of isoprene in southeast Texas were evaluated using ground and aircraft measurements collected during the Texas Air Quality Study 2000. The effects of two different vertical mixing schemes on model predictions of isoprene concentrations were also evaluated. The photochemical and biogenic emission estimation models used were the Comprehensive Air Quality Model with Extensions and the Global Biosphere Emissions and Interactions System. Ground level isoprene concentrations predicted by the models showed markedly good agreement with measured diurnal isoprene patterns. The vertical mixing schemes were most influential on surface concentrations, resulting in differences of as much as 270% in modeled isoprene concentrations. The model over predicted observations from airborne canister samples by as much as a factor of two over rural areas, but under predicted observations over urban areas. Modeled isoprene concentrations were also compared with measurements from an airborne Proton Transfer Reaction Mass Spectrometer, and the results indicated under prediction of isoprene by the model over urban areas, but better agreement in rural areas. The impacts of the vertical mixing schemes on isoprene concentrations were less direct aloft than at the surface. The resulting vertical redistribution of isoprene affected transport rates, chemistry, and the accumulation of mass. As a result, differences in concentrations aloft ranged from none to as much as 30%. This study reinforces the challenges of air quality model validation for highly reactive species such as isoprene, and the need for carefully controlled studies of biogenic emissions and chemical processing during different meteorological conditions in regions with spatially heterogeneous land use.