Secondary pollutant ozone (O 3) formation in a particular area is often influenced by various factors. Source of emissions is one of the factors. In south east Texas, Houston-Galveston-Brazoria (HGB) is a marginal non-attainment area for ozone (O 3). A summer episode of May 28 to July 2, 2006 is simulated using Comprehensive Air Quality Model with extensions (CAMx). During this period O 3 concentration in HGB often exceeds the National Ambient Air Quality Standards (NAAQS) 0.075 ppm of average 8 hour O 3 concentration. HGB area has numerous point sources. Various studies found that some specific volatile organic compounds are very reactive in atmosphere. The objective of this study is to analyze the influence of volatile organic compounds present in point source emissions on the air quality of HGB area. For this purpose ozone sensitivity for HGB area is analyzed by the ratio of hydrogen peroxides (H 2O 2) to nitric acid (HNO 3). HGB area is found NOx limited but reactive VOCs are found to be influential too. From (1-4 June, 2006) maximum O 3 concentration was found on weekend, June 3. VOCs such as Acetaldehyde (ALD 2), Formaldehyde (FORM) and Alkane (ETHA) showed good correlation with O 3 concentrations on that day. In addition, Peroxyacetyl nitrate (PAN) formation was found correlated to higher ozone production. Criteria pollutant Sulfur dioxide (SO 2) was found to influence the ALD 2 and ETHA concentrations, and thus indirectly influenced O 3 production.
Air pollution is a great challenge for Houston Galveston Brazoria (HGB) area. This area consists of eight counties: Brazoria, Chamber, Fort Bend, Galveston, Harris, Liberty, Montgomery and Waller. Rapid urbanization and industrialization with meteorological variability has taken alarming dimensions for air quality of this area. This area is classified as marginal nonattainment as per 2008 eight-hour ozone NAAQS of 75 ppb [
A study [
In between January 31, 2003 and January 30, 2004, total of 1887 O3 exceedance events were reported in TCEQ Region 12, Houston [
Comprehensive Air Quality Model with extensions (CAMx) is a 3-D photochemical grid model. In this studyCAMx 5.40 was used to estimate the air pollutants concentration in the troposphere. This version of CAMx uses chemical mechanism 7, known as CB6. This mechanism uses 218 reactions and up to 114 chemical species [
For this paper, three emissions scenarios based upon point source emissions in HGB were simulated by CAMx 5.40. Point source emissions file for case HGB was generated using EPS3. After the simulation, CAMx output was visualized using tools like Visualization Environment Rich Data Interpretation (VERDI) [
In this work, CAMx Lambert Conformal Conic (LCC) map projection was used. CAMx uses nested grid domain for simulations, i.e. master grid input data move into finer grid input data. CAMx modeling domain parameters are shown in
Domain (km) | Easting (E) | Northing (N) | Number of cells | |
---|---|---|---|---|
(E) | (N) | |||
rpo_36 | −2735, 2592 | −2088, 1944 | 148 | 112 |
tx_12 | −984, 804 | −1632, −312 | 149 | 110 |
tx_4 | −328, 436 | −1536, −644 | 191 | 218 |
km for each day.
This study is a continuation of a previous study [
TCEQ modifies Texas Ozone Season Day (OSD) data by State of Texas Air Reporting System (STARS) and produces AIRS Facility Subsystem (AFS) archived ASCII file format which is compatible for Emission Pro- cessing System (EPS3). “afs.osd_2006_with_ards_removed_CB06_RPOlcp_v3” under Rider 8 program was used for case HGB point source emissions. Emissions files for area source, on-road source, non-road source and biogenic source were collected from TCEQ Rider 8 program [
EPS3 processed emission data and TCEQ files were used for CAMx simulations. Pollutants concentration in HGB area [(95 - 160) East grid cells and (65 - 120) North grid cells] were interpreted from CAMx tx_4 km domain results using VERDI tile plots.
proposed that the ratio of hydrogen peroxides (H2O2) to nitric acid (HNO3) is the transition point and the indicator to find out to be whether Ozone formation in an area is NOx limited or VOC limited. Also, ENVIRON [
HGB area is found to be NOx limited for these days which means the ozone formation is dependent on NOx and should be relatively unaffected by changes in VOCs [
From EPS3 message file of GRDEM module, it was found that point sources in HGB area release considerable amount of VOCs in lower atmosphere. Among the VOCs-Paraffin carbon bond (PAR), Terminal olefin carbon bond (OLE), Toluene and other monoalkyl aromatics (TOL), Xylene (XYL), Ethene (ETH) were emitted in large amounts (more than 5 tons/day). Formaldehyde (FORM), Acetaldehyde (ALD2), Ethanol (ETOH), Me- thanol (MEOH), Ethane (ETHA), Propionaldehyde and higher Aldehydes (ALDX) were released in relatively smaller amounts (about 3 tons/day). The minimum emitted VOCs were Isoprene (ISOP) and Terpene (TERP),
(less than 2 tons). For further study VOCs concentrations from CAMx output for June 3, 2006 were analyzed and concentration graphs are shown in
In Figures 4(a)-(c), concentrations of PAR, OLE and ETH species were found similar for all three cases. As the concentrations of these species did not show any change when Ozone concentration was changing, so it was difficult to find any correlation of these species with O3. It seems for that particular day, these VOCs were not directly contributing to O3 exceedance. This may be explained by the fact that VOCs from biogenic sources also contribute to total Ozone concentration in the ambient air.
VOCs and O3 concentration for case ALL and HGB were used for the correlation graphs (
Ozone (O3). This value reflects that ALD2 was strongly related to higher O3 formation for that day.
Studies reported that emissions from point sources have many chemical compounds, particularly alkanes and aromatics; they are less frequent in the reporting database than alkene releases. Also, Ozone production was directly related to the amount of hydroxyl radicals produced from the photolysis of Formaldehyde and other aldehydes in every simulation [
A study [
is well correlated with ETHA with a R2 value of about 0.65 (65%). But SO2 showed less correlation with FORM and PAN as shown in Figures 6(c)-(d). This could be the effect of meteorological conditions because Rappengluck [
Point source is one of the major sources of air pollutants which initiates air quality degradation. In this paper the significance of point source emissions on Ozone (O3) production was studied using CAMx model output. EPS3 processed emissions files and TCEQ Rider 8 Base-case input files were used. Weekdays (June 1 and June 2) and weekend days (June 3 and June 4) in summer time 2006 for three different scenarios (ALL, TCEQ, WOP) were simulated. While analyzing the reasons behind high O3 formation in HGB area, O3 sensitivity for the precursors was evaluated. Ratio of hydrogen peroxide to nitric acid (H2O2/HNO3) showed that O3 formation in HGB area was NOx limited for the simulated episode. So, control of nitrogen oxides emissions is imperative to control O3 exceedance in HGB area. On the other hand, some VOCs showed good correlations with high Ozone concentrations. ALD2, FORM, ETHA and PAN showed correlation (R2) of 0.81, 0.53, 0.5 and 0.67 respectively with O3. Sulfur dioxide (SO2) was found to have good correlation with the VOCs, such as ALD2 and ETHA. So, reactivity of specific VOC species is significant for O3 exceedence. While considering the emission control strategies these VOCs emissions limit should also be considered along with the NOx. For this study only one episode of summer 2006 was analyzed. To acquire effective results more simulation and analysis is recommended.
This work is supported by the National Science Foundation (NSF) through the Center for Energy and Environmental Sustainability (CEES), a CREST Center, award no. 1036593