2012 Progress Report: Response of Regional Air Quality to Severe Drought
EPA Grant Number:
Response of Regional Air Quality to Severe Drought
Allen, David T.
, Huang, Ling
, Kimura, Yosuke
, McDonald-Buller, Elena
, McGaughey, Gary
Allen, David T.
The University of Texas at Austin
EPA Project Officer:
June 1, 2012 through
May 31, 2015
(Extended to May 31, 2016)
Project Period Covered by this Report:
June 1, 2012 through May 31,2013
Extreme Event Impacts on Air Quality and Water Quality with a Changing Global Climate (2011)
Air Quality and Air Toxics
Global Climate Change
Water and Watersheds
Droughts can occur over most parts of the world and vary substantially, in intensity, severity, duration, spatial extent, and frequency. Throughout the southwestern United States, severe droughts are a recurring phenomenon. Most climate models suggest that these droughts will become more severe in the future as climate changes in response to increased concentrations of greenhouse gases and other radiative forcing species in the atmosphere. The 2011 Texas drought resulted in record agricultural losses and was associated with the worst year for wildfires in Texas history. As drought is predicted to occur more frequently within the state, which also faces requirements to achieve and maintain attainment with the National Ambient Air Quality Standard (NAAQS) for ozone in several large metropolitan areas, it is important to understand the effects of drought on regional air quality. The objective of this project is to advance the understanding of the effects of drought-induced changes in natural systems, including biogenic emissions and dry deposition from vegetation, and changes in agricultural practices on Texas air quality.
The project has focused on the following activities during the first year: (1) a literature review and attendance at technical meetings to develop an understanding of drought classification, drought indices, the effects of drought on regional scales throughout the world, and the particular impacts of drought on vegetation; (2) development of a climatology for Texas focusing on 2006 through 2011, a time period that included a year with greater than average precipitation (2007) as well as 2 years with droughts that were among the most severe in Texas history (2006 and 2011); (3) investigation of spatial and annual variations in leaf area index (LAI) derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) on-board the Terra and Aqua satellites and the relative effects of LAI versus other input parameters on predictions of biogenic emissions using the Model of Emissions of Gases and Aerosols from Nature (MEGAN); (4) training and ongoing preparation of meteorological simulations with the Weather Research and Forecast (WRF) Model.
The climate and land use/land cover of Texas vary widely across its 10 climate regions. Most large metropolitan areas in the state are located within one of four climate regions: North Central Texas that includes Dallas/Fort Worth; South Central Texas that includes Austin and San Antonio; East Texas; and the Upper Coast that includes Houston/Galveston. Both temperature and precipitation gradually decrease inland from the Gulf of Mexico. Moderate or worse drought was a recurring concern in 2006 through 2011; at its worst, in late September 2011, 85% of Texas was classified as under exceptional drought.
Vegetation has an important influence on tropospheric chemistry and global climate forcing and serves as a sink for atmospheric pollutants through dry deposition. The role of biogenic volatile organic compounds (BVOCs), in particular isoprene, in the formation of tropospheric ozone in Texas and other areas of the United States has been recognized as critical for air quality planning. A primary goal during the first year of the project has been to understand the driving parameters that influence predictions of biogenic emissions in drought and non-drought years. The inter-annual variability (IAV) in leaf area index (LAI), a key parameter in the modeling of biogenic emissions, was investigated using the MODIS 4-day LAI product for major land cover types in the four eastern Texas climate regions during 2006 through 2011.
LAI had a strong seasonal pattern with the lowest values in winter and highest in late spring and summer. LAI varied between climate regions even for the same land cover types and were generally highest in East Texas and lowest in North/South Central Texas; a result consistent with temperature and precipitation distributions. Maximum monthly IAV in LAI during 2006 through 2011 exceeded 20% for land cover types in North/South Central Texas, but less than 20% in East Texas and the Upper Coast. Reductions in the spatial extent of spring greening and LAI due to the onset and persistence of drought in North and South Central Texas were pronounced during 2011. Unlike North/South Central Texas where grasses and cereal crops with shallow root systems predominate, broadleaf and needleleaf forests cover ~ 60% of East Texas. Drought resistant tree species, such as post oak (Quercus stellata) and loblolly pines (Pinus taeda), may be associated with smaller IAV in LAI for East Texas forests.
Deviations in predicted isoprene and monoterpene emissions were well-correlated with deviations of LAI, suggesting that LAI has a significant contribution to year-to-year variations of isoprene and monoterpene emissions. In the two central regions, isoprene/monoterpene emissions were notably lower (by as much as -22%) from the mean during the summers of 2006 and 2011 (drought years), associated with the significant reduction of LAI. This result suggests that drought could affect isoprene/monoterpene emissions through changes in LAI. Annual changes in meteorological fields for temperature, wind speed, PAR, and humidity collectively contributed to larger IAV in isoprene and monoterpene emissions than LAI alone. LAI and meteorological factors may have complex and perhaps competing impacts on isoprene emissions. In the next year, meteorological simulations with the WRF model for 2006 through 2011 will be used to explore the impacts of drought on atmospheric parameters (e.g., temperatures, clouds, mixing heights). Output from the WRF simulations will be integral to studies of biogenic emissions and dry deposition and their effects on predictions of air quality.
The second year of the project will focus on completion of the WRF meteorological simulations, which will be used to analyze the impact of drought on atmospheric parameters (e.g., temperatures, clouds, mixing heights) through inter-comparisons of results for representative dry and wet years. Sensitivity studies will explore inter-annual variations in predicted biogenic emissions in eastern Texas using MEGAN, as well as the driving variables that influence emissions estimates in climatological average, wet, and drought years. Similar studies will explore inter-annual variations in predicted dry deposition velocities and component surface resistances in Texas using the Zhang and Wesely/Slinn algorithms for dry deposition. A review of emerging science in the literature on the impacts of drought on stomatal conductance, including factors such species-specific variability in growth rate and recovery, will be conducted. Results from the sensitivity studies with the biogenic and dry deposition models will be selected, and initial simulations that will examine the effects on air quality with the Comprehensive Air Quality Model with Extensions (CAMx) will be prepared.
No journal articles submitted with this report: View all 11 publications for this project
Biogenic emissions, drought, dry deposition, ozone, particulate matter
Progress and Final Reports:
2013 Progress Report
2014 Progress Report