Gary McGaughey

Effects of temperature and land use on predictions of biogenic emissions in Eastern Texas, USA

Accurate estimates of biogenic volatile organic compound emissions are critical for air quality planning in areas such as Eastern Texas where biogenic emissions comprise a significant fraction of the total volatile organic compound inventory. Uncertainties in biogenic volatile organic chemical emission estimates associated with different land use databases, surface temperature databases, and temperature interpolation methods were quantified and compared. The sensitivity of isoprene emissions to land use classification was investigated by comparing predictions based on land use data recently compiled for Eastern Texas to those based on the Biogenic Emissions Landcover Database version 3.1 (BELD3). Previous studies have only made these comparisons with the previous BELD version 2 database. Isoprene emission increased throughout much of Eastern Texas because areas classified as agricultural or savannah in BELD3 were more accurately classified as Post Oak, Live Oak, mesquite, and juniper in the new database. These results indicate the need for land use studies in areas poorly characterized in the BELD3. The sensitivity of isoprene emission estimates to uncertainties in surface temperatures were investigated by comparing predictions based on two different temperature databases and three different interpolation techniques. Spatial interpolations of surface temperatures collected at available Automated Surface Observing System (ASOS) stations in Houston, Austin, and Dallas were similar to the spatial interpolations of surface temperatures obtained from the ETA Data Assimilation System (EDAS). As a result, substantial variations in isoprene emissions were not observed over the majority of the modeling domain; however, differences of 4 F over localized regions produced a 35% difference in isoprene emissions. Comparisons between the isoprene emissions of the three interpolation methods sometimes revealed large variations, with maximum temperature differences of 4 F resulting in 60% differences in isoprene emissions in areas with the highest isoprene emissions. It was noted that the ASOS stations were clustered in urban areas and not in areas with the highest biogenic emissions. More ambient temperature monitors need to be located in rural locations to provide robust estimates of biogenic emissions and facilitate validation of interpolated temperature fields.

Regional ozone impacts of increased natural gas use in the Texas power sector and development in the Eagle Ford shale.

The combined emissions and air quality impacts of electricity generation in the Texas grid and natural gas production in the Eagle Ford shale were estimated at various natural gas price points for the power sector. The increased use of natural gas in the power sector, in place of coal-fired power generation, drove reductions in average daily maximum 8 h ozone concentration of 0.6-1.3 ppb in northeastern Texas for a high ozone episode used in air quality planning. The associated increase in Eagle Ford upstream oil and gas production nitrogen oxide (NOx) emissions caused an estimated local increase, in south Texas, of 0.3-0.7 ppb in the same ozone metric. In addition, the potential ozone impacts of Eagle Ford emissions on nearby urban areas were estimated. On the basis of evidence from this work and a previous study on the Barnett shale, the combined ozone impact of increased natural gas development and use in the power sector is likely to vary regionally and must be analyzed on a case by case basis.

Flexible NOx abatement from power plants in the eastern United States

Emission controls that provide incentives for maximizing reductions in emissions of ozone precursors on days when ozone concentrations are highest have the potential to be cost-effective ozone management strategies. Conventional prescriptive emissions controls or cap-and-trade programs consider all emissions similarly regardless of when they occur, despite the fact that contributions to ozone formation may vary. In contrast, a time-differentiated approach targets emissions reductions on forecasted high ozone days without imposition of additional costs on lower ozone days. This work examines simulations of such dynamic air quality management strategies for NO(x) emissions from electric generating units. Results from a model of day-specific NO(x) pricing applied to the Pennsylvania-New Jersey-Maryland (PJM) portion of the northeastern U.S. electrical grid demonstrate (i) that sufficient flexibility in electricity generation is available to allow power production to be switched from high to low NO(x) emitting facilities, (ii) that the emission price required to induce EGUs to change their strategies for power generation are competitive with other control costs, (iii) that dispatching strategies, which can change the spatial and temporal distribution of emissions, lead to ozone concentration reductions comparable to other control technologies, and (iv) that air quality forecasting is sufficiently accurate to allow EGUs to adapt their power generation strategies.

Dynamic Management of NOx and SO2 Emissions in the Texas and Mid-Atlantic Electric Power Systems and Implications for Air Quality

Cap and trade programs have historically been designed to achieve annual or seasonal reductions in emissions of nitrogen oxides and sulfur dioxide from power plants. Emissions reductions may not be temporally coincident with meteorological conditions conducive to the formation of peak ozone and fine particulate matter concentrations. Integrated power system and air quality modeling methods were developed to evaluate time-differentiated emissions price signals on high ozone days in the Mid-Atlantic portion of the Pennsylvania-New Jersey-Maryland (PJM) Interconnection and Electric Reliability Council of Texas (ERCOT) grids. Sufficient flexibility exists in the two grids with marked differences in demand and fuel generation mix to accommodate time-differentiated emissions pricing alone or in combination with a season-wide program. System-wide emissions reductions and production costs from time-differentiated pricing are shown to be competitive with those of a season-wide program on high ozone days and would be more cost-effective if the primary policy goal was to target emissions reductions on these days. Time-differentiated pricing layered as a complement to the Cross-State Air Pollution Rule had particularly pronounced benefits for the Mid-Atlantic PJM system that relies heavily on coal-fired generation. Time-differentiated pricing aimed at reducing ozone concentrations had particulate matter reduction co-benefits, but if particulate matter reductions are the primary objective, other approaches to time-differentiated pricing may lead to greater benefits.

Comparison of regional and global land cover products and the implications for biogenic emission modeling

Accurate estimates of biogenic emissions are required for air quality models that support the development of air quality management plans and attainment demonstrations. Land cover characterization is an essential driving input for most biogenic emissions models. This work contrasted the global Moderate Resolution Imaging Spectroradiometer (MODIS) land cover product against a regional land cover product developed for the Texas Commissions on Environmental Quality (TCEQ) over four climate regions in eastern Texas, where biogenic emissions comprise a large fraction of the total inventory of volatile organic compounds (VOCs) and land cover is highly diverse. The Model of Emissions of Gases and Aerosols from Nature (MEGAN) was utilized to investigate the influences of land cover characterization on modeled isoprene and monoterpene emissions through changes in the standard emission potential and emission activity factor, both separately and simultaneously. In Central Texas, forest coverage was significantly lower in the MODIS land cover product relative to the TCEQ data, which resulted in substantially lower estimates of isoprene and monoterpene emissions by as much as 90%. Differences in predicted isoprene and monoterpene emissions associated with variability in land cover characterization were primarily caused by differences in the standard emission potential, which is dependent on plant functional type. Photochemical modeling was conducted to investigate the effects of differences in estimated biogenic emissions associated with land cover characterization on predicted ozone concentrations using the Comprehensive Air Quality Model with Extensions (CAMx). Mean differences in maximum daily average 8-hour (MDA8) ozone concentrations were 2 to 6 ppb with maximum differences exceeding 20 ppb. Continued focus should be on reducing uncertainties in the representation of land cover through field validation. Implications: Uncertainties in the estimation of biogenic emissions associated with the characterization of land cover in global and regional data products were examined in eastern Texas. Misclassification between trees and low-growing vegetation in central Texas resulted in substantial differences in isoprene and monoterpene emission estimates and predicted ground-level ozone concentrations. Results from this study indicate the importance of land cover validation at regional scales.