2014 Progress Report: Response of Regional Air Quality to Severe Drought

EPA Grant Number: R835191
Title: Response of Regional Air Quality to Severe Drought
Investigators: Allen, David T. , Huang, Ling , Kimura, Yosuke , McDonald-Buller, Elena , McGaughey, Gary
Current Investigators: Allen, David T. , Huang, Ling , Kimura, Yosuke , McDonald-Buller, Elena , McGaughey, Gary , Zheng, Jeff
Institution: The University of Texas at Austin
EPA Project Officer: Chung, Serena
Project Period: June 1, 2012 through May 31, 2015 (Extended to May 31, 2016)
Project Period Covered by this Report: June 1, 2014 through August 17,2015
Project Amount: $750,000
RFA: Extreme Event Impacts on Air Quality and Water Quality with a Changing Global Climate (2011) RFA Text |  Recipients Lists
Research Category: Air Quality and Air Toxics , Global Climate Change , Water and Watersheds , Climate Change , Air , Water


Drought is a natural disaster that has profound and complex social, economic, and environmental impacts. In recent years, the severity of drought has varied spatially and temporally in Texas, which has highly diverse climatic conditions and land use and land cover profiles over its ten climate regions. The National Climate Assessment (Melillo et al., 2014) indicates an increase in the state over the next several decades in the annual numbers of consecutive dry days and numbers of days with temperatures exceeding 100°F. Changes in vegetation associated with more frequent droughts have the potential to affect regional air quality, which will be critical to understand as the state endeavors to achieve and maintain attainment with National Ambient Air Quality Standards (NAAQS) for ozone. This work characterizes land cover for eastern Texas climate regions (North Central Texas, South Central Texas, East Texas, and Upper Coast) where most metropolitan areas are located, explores variability in meteorological conditions, emissions of biogenic hydrocarbons, and the physical removal of pollutants via dry deposition during years with severe to exceptional drought conditions (2006 and 2011) as well as years with average to above average precipitation patterns (2007), and assesses effects on predictions of Texas air quality. Specific objectives have included: (1) investigating annual and seasonal variations in leaf area index (LAI), soil moisture, and meteorological variables on emissions estimates of biogenic hydrocarbons using the Model of Emissions of Gases and Aerosols from Nature (MEGAN) (Guenther et al., 2012); (2) characterizing differences between a regional land cover product with high spatial resolution and a widely used global land cover product on biogenic emissions estimates and ozone predictions; (3) examining seasonal and interannual changes in estimated ozone dry deposition velocities and component resistances over eastern Texas using an offline version of a dry deposition algorithm from the Comprehensive Air Quality Model with Extensions (CAMx) (ENVIRON, 2014); and (4) examining the relative contributions of different physical and chemical processes on predicted ground-level ozone concentrations using the process analysis diagnostic tool in CAMx during representative drought and wet periods.

Progress Summary:

Biogenic Emissions

Isoprene and monoterpenes are quantitatively among the most important biogenic volatile organic compounds (BVOCs) emitted globally from vegetation and have crucial roles in atmospheric chemistry and climate at regional and global scales. Annual biogenic emissions in Texas ranked first within the continental United States in the 2008 National Emission Inventory. Recognition of the roles of BVOCs in tropospheric ozone and organic aerosol formation has been critical for air quality planning efforts in the state. In 2008, for example, biogenic emissions accounted for 29% and 40% of the total VOC inventories in the Dallas/Fort Worth and Houston/Galveston/Brazoria ozone nonattainment areas, respectively.

The Texas Commission on Environmental Quality (TCEQ) has recently transitioned to the use of the MEGAN model to provide biogenic emissions estimates for their air quality modeling initiatives. Key input parameters to the model include land cover and land use classification, leaf area index (LAI; representing the ratio of total upper leaf surface of vegetation to land surface area), soil moisture, and meteorological fields for air temperature, solar radiation, relative humidity, and wind speed. Emissions estimates for isoprene and monoterpenes are calculated in MEGAN according to where Fi is the emission rate in units of flux (i.e., µg m-2 ground area h-1), ε is the basal emission factor for vegetation type j with fractional coverage χj, and γ is the overall emissions activity factor that accounts for variations in environmental conditions from a standardized set of environmental conditions (i.e., air temperature of 303 K, solar angle of 60°, photosynthetic photon flux density (PPFD) transmission of 0.6, LAI of 5 m2/m2 consisting of 80% mature, 10% growing and 10% old foliage, and volumetric soil moisture of 0.3 m3/m3). The overall activity factor is comprised of individual environmental activity factors that are applied multiplicatively to adjust emissions and is constructed differently for isoprene and monoterpenes due to differences in their light dependencies. These individual activity factors include those for leaf age, soil moisture (isoprene only), and the canopy environment, which is based on temperature, light, and LAI. Previous studies have utilized MEGAN in support of sensitivity analyses of emissions estimates via perturbations of environmental inputs or climate or vegetation scenarios. An objective of our work was to quantify and interpret seasonal and interannual differences in these environmental activity factors between years that had above average rainfall and average temperatures (2007) to years with substantially warmer and drier conditions (2006 and 2011) in eastern Texas by tracking their relative changes internal to the MEGAN estimation of isoprene and monoterpene emissions. This methodology maintained an environmentally consistent (i.e., “real-world”) set of model inputs. Model source codes were modified to enable the examination of individual activity factors for temperature, light, LAI, and soil moisture.

Biogenic emissions peak during the summer in the four Texas climate regions that were the focus of this study. East Texas and the Upper Coast are dominated by broadleaf and needleleaf forests versus lowgrowing vegetation, such as grasses, in the central regions. East Texas exhibited the highest emissions among the four regions, primarily due to its densely forested areas. Summer emissions estimates can be more than three times greater than during the spring/fall in each region. Isoprene interannual variations ranged from 13.9% during summer in South Central Texas to 24.6% during spring in North Central Texas; monoterpenes exhibited weaker interannual variations by season.

Comparisons of results between drought and non-drought years reinforced the importance of temperature on predicted biogenic emissions. Decreases in emissions associated with reduced LAI were overwhelmed by emission increases caused by much warmer temperatures during periods of drought. However, soil moisture and associated wilting point may have substantial effects depending on the data product employed.

MEGAN simulates the drought response of isoprene emissions through a soil moisture activity factor based on a single observational study (Pegoraro et al., 2004). The soil moisture activity factor scales between 0 and 1 depending on the soil moisture and wilting point (the soil moisture content below which plants cannot extract water from soil), representing a negative influence on isoprene emissions under drought conditions. Previous studies with MEGAN have typically employed a single soil moisture database; predicted impacts on isoprene emissions have ranged from minimal to substantial. This work considered two soil moisture datasets: North American Land Data Assimilation System Phase 2 (NLDAS-2) with the Mosaic land surface model or the multi-parameterization options version of the Noah model (Noah-MP). Wilting point values were provided with each model. Comparisons were made with the limited in situ observations available for eastern Texas. Depending on the soil moisture database employed (i.e., Mosaic or Noah-MP), predicted reductions in isoprene emissions ranged from nearly negligible to almost -70% during the summer of 2011, a time period with all-time record drought in Texas.

For MEGAN and most biogenic emission models, land cover characterization, i.e., plant functional type, is an essential driving variable as it determines the phenological emission potential of a region. Another objective of this study was to investigate the influences of different land cover representations on MEGAN estimates of isoprene and monoterpene emissions using the Moderate Resolution Imaging Spectroradiometer (MODIS) global land cover product and a regional product with high spatial resolution and detailed land cover categories developed for the TCEQ (Popescu et al., 2011). In general, forest coverage was significantly lower in the global MODIS land cover product compared to the regional TCEQ product in Central Texas, which resulted in lower estimated monthly isoprene and monoterpene emissions by as much as 90%. Photochemical modeling was conducted to investigate these differences on predicted ozone concentrations. Mean differences in predicted maximum daily 8-hour average (MDA8) ozone concentrations were 2 to 6 ppb with maximum differences exceeding 20 ppb in eastern Texas. Misclassification between trees and grasses/crops has the potential to lead to large differences in biogenic emission estimates. This could also be of particular importance in other regions of the world where rapid land cover change, such as deforestation, is occurring.

Dry Deposition

Dry deposition is broadly defined as the transport of gaseous and particulate species from the atmosphere by turbulent transfer to surfaces in the absence of precipitation. On a regional level in Texas, dry deposition represents an important physical removal mechanism for ozone during the warm spring through early fall seasons; therefore, estimates of ozone dry deposition are required for air quality modeling and management. Ozone dry deposition is controlled by the combined effects of all removal pathways, which include stomatal and non-stomatal uptake (e.g. deposition to soils, cuticles, or any other external surface). The relative importance of stomatal and non-stomatal removal can vary with vegetation type as well as diurnally and seasonally. This work investigated the impacts of drought on ozone dry deposition during the daytime by exploring interannual variations in predicted dry deposition velocities and associated component resistances in eastern Texas. An offline version of the Zhang et al. (2003) dry deposition sub-module within CAMx was created and used to simulate ozone dry deposition velocities.

Estimates of mean daytime ozone dry deposition velocities in the four climate regions ranged from 0.27 to 0.46 cm s-1 during the spring through fall seasons of 2011, which had historic drought conditions. Variations in deposition velocities between climate regions during a season were generally less than 10%. Reductions in area-averaged daytime ozone deposition velocity and mass during 2011 were substantial relative to 2007, which had above average rainfall and average temperatures. Changes in ozone deposition velocities ranged from -10.9% to -24.2%; ozone deposition mass was reduced by -13.6% to -23.5% across eastern Texas. Drought-induced changes in meteorological fields (including temperature, wind speed, vapor pressure deficit) and LAI resulted in opposing responses of stomatal and non-stomatal conductances: stomatal conductances generally decreased under drought conditions while non-stomatal conductances showed increases associated with higher wind speeds and smaller LAI values. Overall deposition velocities increased during the spring of the drought year but decreased during the summer (>0.1 cm/s) and fall seasons. Forests exhibited the most significant reductions in ozone dry deposition velocities.

Predicted Ozone Concentrations and Process Analysis

CAMx simulations were conducted representing meteorological conditions during August 2007 and August 2011 over a nested 36-km/12-km/4-km domain that included eastern Texas. Anthropogenic emissions inventories from point sources, mobile sources, and area sources for June 2012 were provided by the TCEQ, and episode means were generated by weekday (i.e., Monday-Thursday, Friday, Saturday, and Sunday). Daily specific biogenic emissions estimates were obtained from MEGAN. The Zhang algorithm was implemented for dry deposition. Meteorological fields were provided by simulation with the Weather Research and Forecasting (WRF) model. The soil moisture algorithm was only applied to the 4-km domain, using soil moisture data generated by the Noah land surface model. Process analysis (PA) has been widely used to investigate the formation of ozone. The integrated process rate (IPR) method is one of the three components of the process analysis diagnostic tool implemented in CAMx. The IPR analysis calculates the hourly contributions of each physical process (i.e., advection, diffusion, deposition, and chemistry) on ozone formation (ENVIRON, 2014) and was activated over the 4-km domain for this work. Five processes including chemistry, plume-in-grid change, horizontal/vertical transport, and dry deposition were considered for the Austin/San Antonio, Houston, and Dallas-Fort Worth areas.

Analysis of integrated process rates (IPR) indicated that for all three regions, vertical transport and dry deposition represented the dominant mechanisms for surface ozone production and removal during both wet and drought periods. Chemical processes contributed to ozone formation during the daytime and destruction during the night. Ozone formed by chemical reactions in higher layers and entrained into the surface layer contributes to the vertical transport component. Predicted ground level, 8-hour averaged ozone concentrations were higher by as much as 8 ppb in parts of eastern Texas for the 2011 drought scenario relative to 2007. Median values of maximum daily average 8-hour (MDA8) ozone concentrations during the episode were higher by 5.3, 4.2, and 2.6 ppb in the Austin/San Antonio, Dallas/Fort Worth, and Houston urban areas, respectively, during the drought year.

Future Activities:

A one-year no-cost extension was requested and granted. We expect to further investigate the soil moisture characterization in MEGAN and its effects on biogenic emission estimates during drought conditions, focusing on root layer depth, soil type, and plant functional type. We will leverage available observations for soil moisture and wilting point and continue our evaluation of the land surface models.



Journal Articles on this Report : 3 Displayed | Download in RIS Format

Other project views: All 11 publications 4 publications in selected types All 4 journal articles
Type Citation Project Document Sources
Journal Article Huang L, McDonald-Buller EC, McGaughey G, Kimura Y, Allen DT. Annual variability in leaf area index and isoprene and monoterpene emissions during drought years in Texas. Atmospheric Environment 2014;92:240-249. R835191 (2013)
R835191 (2014)
R835191 (Final)
  • Full-text: ScienceDirect-Full Text HTML
  • Abstract: ScienceDirect-Abstract
  • Other: ScienceDirect-Full Text PDF
  • Journal Article Huang L, McGaughey G, McDonald-Buller E, Kimura Y, Allen DT. Quantifying regional, seasonal and interannual contributions of environmental factors on isoprene and monoterpene emissions estimates over eastern Texas. Atmospheric Environment 2015;106:120-128. R835191 (2014)
    R835191 (Final)
  • Full-text: ScienceDirect-Full Text PDF
  • Abstract: ScienceDirect-Abstract
  • Other: ResearchGate-Full Text PDF
  • Journal Article Huang L, McDonald-Buller E, McGaughey G, Kimura Y, Allen DT. Comparison of regional and global land cover products and the implications for biogenic emission modeling. Journal of the Air & Waste Management Association 2015;65(10):1194-1205. R835191 (2014)
    R835191 (Final)
  • Abstract from PubMed
  • Full-text: ResearchGate-Prepublication Full Text PDF
  • Abstract: Taylor & Francis-Abstract
  • Other: Taylor & Francis-Full Text PDF
  • Supplemental Keywords:

    biogenic emissions, drought, dry deposition, ozone, particulate matter, MEGAN, CAMx

    Progress and Final Reports:

    Original Abstract
    2012 Progress Report
    2013 Progress Report
    Final Report