Issues with Ozone Attainment Methodology for Houston, TX

Abstract

To comply with the federal 8-hr ozone standard, the state of Texas is creating a plan for Houston that strictly follows the U.S. Environmental Protection Agency's (EPA) guidance for demonstrating attainment. EPA's attainment guidance methodology has several key assumptions that are demonstrated to not be completely appropriate for the unique observed ozone conditions found in Houston. Houston's ozone violations at monitoring sites are realized as gradual hour-to-hour increases in ozone concentrations, or by large hourly ozone increases that exceed up to 100 parts per billion/hr. Given the time profiles at the violating monitors and those of nearby monitors, these large increases appear to be associated with small parcels of spatially limited plumes of high ozone in a lower background of urban ozone. Some of these high ozone parcels and plumes have been linked to a combination of unique wind conditions and episodic hydrocarbon emission events from the Houston Ship Channel. However, the regulatory air quality model (AQM) does not predict these sharp ozone gradients. Instead, the AQM predicts gradual hourly increases with broad regions of high ozone covering the entire Houston urban core. The AQM model performance can be partly attributed to EPA attainment guidance that prescribes the removal in the baseline model simulation of any episodic hydrocarbon emissions, thereby potentially removing any nontypical causes of ozone exceedances. This paper shows that attainment of all monitors is achieved when days with observed large hourly variability in ozone concentrations are filtered from attainment metrics. Thus, the modeling and observational data support a second unique cause for how ozone is formed in Houston, and the current EPA methodology addresses only one of these two causes. INTRODUCTIONSeveral factors combine to make Houston's ozone (O 3 ) problem unique in comparison to other metropolitan cities in the United States. These include the (1) complex interactions between land-sea breeze circulations, (2) intense clustering of industrial emission sources in the Houston Ship Channel and coastal areas, (3) significant precursor emissions from the heavily urbanized eightcounty Houston Galveston Bay (HGB) area, and (4) potential pollution transport from domestic and international source regions. The culmination of these factors has resulted in a complex and difficult environment to understand. This difficulty has garnered the attention of researchers and regulators with the common goal of understanding how O 3 is formed. The resulting science and policies of Texas regulators describe two unique causes, or conceptual models, for how high O 3 is formed in Houston. One conceptual model is a gradual region-wide increase in O 3 concentrations "typical" of many large U.S. metropolitan cities. This conceptual model is well represented in the gradual evolution of O 3 seen in observations and predicted in photochemical models. The second conceptual model was first discovered in Houston and has been linked to episodic ...

Assessment of a regulatory model's performance relative to large spatial heterogeneity in observed ozone in Houston, Texas

“…A necessary next step is to perform a meteorological analysis similar to the studies described above (Byun et al, 2007;Vizuete et al, 2009). It is important to understand the regional meteorology that was present on NTOC days, because high O 3 in Houston is known to be dependent upon wind fields.…”

“…These policies are described and justified with modeling and observational data in a document called the State Implementation Plan (SIP) (Texas Commission on Environmental Quality [TCEQ], 2010a), which follows the U.S. Environmental Protection Agency (EPA) attainment methodology (EPA, 2007). The EPA attainment methodology describes the use of observational and modeling data to show attainment, and the assumptions underlying it have been the focus of several studies (Jones et al, 2005;Sistla et al, 2004;Vizuete et al, 2010, Vizuete et al, 2011. Specifically for Houston, researchers have found that the metrics used in the attainment methodology were influenced by large hourly changes in observed O 3 concentrations.…”

“…Specifically for Houston, researchers have found that the metrics used in the attainment methodology were influenced by large hourly changes in observed O 3 concentrations. In the observational data used by the 2010 SIP, it has been shown that some of the highest 8-hr O 3 levels included hourly changes in O 3 concentrations of at least 40 ppb, or 60 ppb in 2 hr (Vizuete et al, 2011). These large hourly changes usually appear at only a few monitors spanning a narrow geographic area.…”

“…These meteorological conditions have been described in detail and are characterized at surface monitors by low wind speeds and a rotational wind field (Banta et al, 2005;Cowling et al, 2007;Ngan and Byun, 2011). An analysis of winds and NAAQS exceedances in Houston, however, determined that O 3 conducive meteorological conditions are necessary but insufficient to produce high O 3 (Vizuete et al, 2009).…”

Near-source air quality impacts of large olefin flares

“…The observed transient high ozone events (THOEs; defined by regulators as monitored ozone increases >40 ppb in 1 hr) were at first thought to be the consequence of routine emissions of highly reactive volatile organic compounds (HRVOCs) from the Ship Channel facilities (e.g., leaks from process vents), together with combustion emissions of nitrogen oxides (NO x ¼ NO þ NO 2 ) and favorable meteorological conditions such as stalled sea breeze fronts and stagnant rotating winds (e.g., Banta et al, 2005). Later, it was hypothesized that flares and other stochastic emission events also contribute to the occurrence of THOEs (Nam et al, 2006;Olaguer et al, 2006), an opinion supported by Vizuete et al (2011), who showed that model simulations of ozone events in Houston based solely on routine point source emissions can only account for broad ozone plumes, as opposed to the narrow ones frequently observed emanating from the Ship Channel.…”