Air Quality Models and Unusually Large Ozone Increases: Identifying Model Failures, Understanding Environmental Causes, and Improving Modeled Chemistry

Several factors combine to make ozone (O3) pollution in Houston, Texas, unique when compared to other metropolitan areas. These include complex meteorology, intense clustering of industrial activity, and significant precursor emissions from the heavily urbanized eight-county area. Decades of air pollution research have borne out two different causes, or conceptual models, of O3 formation. One conceptual model describes a gradual region-wide increase in O3 concentrations "typical" of many large U.S. cities. The other conceptual model links episodic emissions of volatile organic compounds to spatially limited plumes of high O3, which lead to large hourly increases that have exceeded 100 parts per billion (ppb) per hour. These large hourly increases are known to lead to violations of the federal O3 standard and impact Houston's status as a non-attainment area. There is a need to further understand and characterize the causes of peak O3 levels in Houston and simulate them correctly so that environmental regulators can find the most cost-effective pollution controls. This work provides a detailed understanding of unusually large O3 increases in the natural and modeled environments. First, we probe regulatory model simulations and assess their ability to reproduce the observed phenomenon. As configured for the purpose of demonstrating future attainment of the O3 standard, the model fails to predict the spatially limited O3 plumes observed in Houston. Second, we combine ambient meteorological and pollutant measurement data to identify the most likely geographic origins and preconditions of the concentrated O3 plumes. We find evidence that the O3 plumes are the result of photochemical activity accelerated by industrial emissions. And, third, we implement changes to the modeled chemistry to add missing formation mechanisms of nitrous acid, which is an important radical precursor. Radicals control the chemical reactivity of atmospheric systems, and perturbations to radical budgets can shift chemical pathways. The mechanism additions increase the concentrations of nitrous acid, especially right after sunrise. The overall effect on O3 is small (up to three ppb), but we demonstrate the successful implementation of a surface sub-model that chemically processes adsorbed compounds. To our knowledge, this is the first time that chemical processing on surfaces has been used in a three-dimensional regulatory air quality model.Doctor of Philosoph

High ozone events and attainment demonstrations in Houston, Texas

The Houston-Galveston-Brazoria area has had multiple decades of persistent high ozone (O3) values. We have analyzed ten years of ground-level measurements at 25 monitors in Houston and found that peak 1-h O3 concentrations were often associated with large hourly O3 increases. A non-typical O3 change (NTOC) - defined here as an increase of at least 40 ppb/hr or 60 ppb/2hrs - was measured 25% of the time when concentrations recorded at a monitor exceeded the 8-h O3 standard. CAMx model simulations were found to be limited in their ability to simulate NTOCs, under predicted maximum observed rates of O3 increases by more than 50 ppb/hr, and had difficulty simulating spatially isolated, high O3 events measured at monitors that routinely violate the 8-h O3 standard. Our results suggest that this modeling system will be unable to guide the selection of effective control strategies required to meet a more stringent federal 8-h O3 standard

Houston's rapid ozone increases: preconditions and geographic origins

Many of Houston’s highest 8-h ozone (O3) peaks are characterised by increases in concentrations of at least 40 ppb in 1 h, or 60 ppb in 2 h. These rapid increases are called non-typical O3 changes (NTOCs). In 2004, the Texas Commission on Environmental Quality (TCEQ) developed a novel emissions control strategy aimed at eliminating NTOCs. The strategy limited routine and short-term emissions of ethene, propene, 1,3-butadiene and butene isomers, collectively called highly reactive volatile organic compounds (HRVOCs), which are released from petrochemical facilities. HRVOCs have been associated with NTOCs through field campaigns and modelling studies. This study analysed wind measurements and O3, formaldehyde (HCHO) and sulfur dioxide (SO2) concentrations from 2000 to 2011 at 25 ground monitors in Houston. NTOCs almost always occurred when monitors were downwind of petrochemical facilities. Rapid O3 increases were associated with low wind speeds; 75 % of NTOCs occurred when the 3-h average wind speed preceding the event was less than 6.5 km h−1. Statistically significant differences in HCHO concentrations were seen between days with and without NTOCs. Early afternoon HCHO concentrations were greater on NTOC days. In the morning before an observed NTOC event, however, there were no significant differences in HCHO concentrations between days with and without NTOCs. Hourly SO2 concentrations also increased rapidly, exhibiting behaviour similar to NTOCs. Oftentimes, the SO2 increases preceded a NTOC. These findings show that, despite the apparent success of targeted HRVOC emission controls, further restrictions may be needed to eliminate the remaining O3 events

Houston’s rapid ozone increases: preconditions and geographic origins

Many of Houston’s highest 8-h ozone (O(3)) peaks are characterised by increases in concentrations of at least 40 ppb in 1 h, or 60 ppb in 2 h. These rapid increases are called non-typical O(3) changes (NTOCs). In 2004, the Texas Commission on Environmental Quality (TCEQ) developed a novel emissions control strategy aimed at eliminating NTOCs. The strategy limited routine and short-term emissions of ethene, propene, 1,3-butadiene and butene isomers, collectively called highly reactive volatile organic compounds (HRVOCs), which are released from petrochemical facilities. HRVOCs have been associated with NTOCs through field campaigns and modelling studies. This study analysed wind measurements and O(3), formaldehyde (HCHO) and sulfur dioxide (SO(2)) concentrations from 2000 to 2011 at 25 ground monitors in Houston. NTOCs almost always occurred when monitors were downwind of petrochemical facilities. Rapid O(3) increases were associated with low wind speeds; 75 % of NTOCs occurred when the 3-h average wind speed preceding the event was less than 6.5 km h(−1). Statistically significant differences in HCHO concentrations were seen between days with and without NTOCs. Early afternoon HCHO concentrations were greater on NTOC days. In the morning before an observed NTOC event, however, there were no significant differences in HCHO concentrations between days with and without NTOCs. Hourly SO(2) concentrations also increased rapidly, exhibiting behaviour similar to NTOCs. Oftentimes, the SO(2) increases preceded a NTOC. These findings show that, despite the apparent success of targeted HRVOC emission controls, further restrictions may be needed to eliminate the remaining O(3) events

Toward a consistent modeling framework to assess multi-sectoral climate impacts

Efforts to estimate the physical and economic impacts of future climate change face substantial challenges. To enrich the currently popular approaches to impact analysis - which involve evaluation of a damage function or multi-model comparisons based on a limited number of standardized scenarios - we propose integrating a geospatially resolved physical representation of impacts into a coupled human-Earth system modeling framework. Large internationally coordinated exercises cannot easily respond to new policy targets and the implementation of standard scenarios across models, institutions and research communities can yield inconsistent estimates. Here, we argue for a shift toward the use of a self-consistent integrated modeling framework to assess climate impacts, and discuss ways the integrated assessment modeling community can move in this direction. We then demonstrate the capabilities of such a modeling framework by conducting a multi-sectoral assessment of climate impacts under a range of consistent and integrated economic and climate scenarios that are responsive to new policies and business expectations