Introduction

The United States Environmental Protection Agency (USEPA) has prescribed the usage of certain types of air quality models to analyze characteristic ozone episodes and conceptualize ozone abatement strategies for ozone non-attainment regions in the corresponding State Implementation Plan (SIP).1 Each SIP must demonstrate through computer modeling analyses and estimates that any suggested regulatory proposals will enable the pollution levels in the region to meet federal air quality standards in the future. The modeled future case helps determine a hypothetical future scenario using contemporary meteorology and potential emission reductions and enables the planners to determine whether the decision-making processes to reduce emissions are relevant in the context of future growth and development. However, the current decision-making process does not account for possible variations in ozone concentrations in the future due to potential changes in climate. The interaction between air quality and climate is an interactive process since resultant climate changes impact global atmospheric chemistry and background levels of air pollutant concentrations. The Intergovernmental Panel on Climate Change report by Meehl et at.,2 based on results from most recent climatic models, predicts an average rise of global temperature between 1.4°C and 5.8°C by the year 2100. This significant rise in ambient temperature can impact global tropospheric chemistry as suggested by Fiore et at.,3 and can therefore alter the chemical composition of the troposphere and affect both the surface ozone concentrations and ozone exceedances on regional and urban scales. Figure 1 highlights the key temperature-dependent photochemistry involved in the ozone formation.

Meteorological parameters which influence advection, dispersion, dilution, and rates of atmospheric chemical mechanisms affect air quality variabilities in most regions. Therefore, it is imperative to account for climate changes in numerical modeling experiments while developing emission control policies for the future. Several past studies, such as those by Seaman et at.,4 and Sillman et at.,5 have made detailed impact assessment of meteorological conditions on the surface ozone concentrations and ozone exceedance events. The study of ozone sensitivity to different modifications in temperatures employing process modeling and chemical transport models as described by Baertsch-Ritter et at.,6 has revealed that increases in peak ozone concentrations are directly related to temperature increases and higher temperatures are usually associated with elevated

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Fig. 1. Atmospheric photochemical pathways for the formation of ozone.

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Fig. 1. Atmospheric photochemical pathways for the formation of ozone.

ozone concentrations. Temperature changes affect chemical dynamics and emission rates of anthropogenic and biogenic ozone precursors. It has also been established that temperature variations have the largest impact on peak ozone concentrations and ozone exceedances amongst all meteorological variables as per Dawson et al7 Therefore, it becomes important to assess the impact of future global temperature changes and consequently climate change on air quality decision-making processes.

The south Texas region has experienced several exceedances of the 8-h National Ambient Air Quality Standards (NAAQS) within different urban and semi-urban regions during the recent years. The region is characterized by a unique climatology of semi-arid coastal and inland areas and consists of a number of urban areas classified by the Texas Commission on Environmental Quality (TCEQ) to be in near non-attainment status of the 8-h ozone standards. These near non-attainment areas (NNA) have voluntarily opted to develop appropriate planning processes to continue to remain in attainment of the federal 8-h ozone standards. The future modeling scenario considered here was developed as a part of an attainment demonstration process. The emission estimations from anthropogenic sources were adjusted to account for such factors as anticipated growth or decline in population and economy as well as the impact of federal, state, and regional emission reduction measures.8 The biogenic emissions inventory for the 2007 future case was the same as that used in the 1999 base case, and this was developed in accordance with the USEPA guidelines.1

There has been little focus so far on the effect of temperature perturbations on regional and urban ozone quality in the South Texas region. Emission control policies in these parts are currently implemented assuming that the climate conditions remain constant. This study illustrates the significant impact climate changes can have on potential ozone concentrations in the future, despite the assimilation of substantial emission reduction procedures. It examines the impact of an array of temperature perturbations on the model-simulated 8-h averaged ozone concentrations for the 2007 future case where emissions and other meteorological parameters were held constant. This study provides an in-depth analysis of the importance of the role of possible climate variability on surface ozone concentrations in South Texas by investigating spatial and temporal responses in modeled peak surface ozone concentrations and ozone exceedances to various temperature perturbations. The promulgation of a longer-term 8-h ozone standards since 1997 based on more rigorous health assessments by the USEPA resulted in an overall increase of non-attainment areas. Spatial and temporal scales are intrinsically linked in the ozone process as mentioned by Rao et al.9 Therefore, the possible lowering of federal ozone standards to 70-75 ppb in the future10 will considerably enhance this problem. This issue of climate change then becomes even more critical if the 8-h standards become more stringent in the near future, and this is highlighted in the study described here.

The primary objective of this ozone sensitivity study is to enhance the awareness of decision-makers regarding climate change impact on surface ozone concentrations so that future potential emission abatement strategies may be developed, factoring climate change in the decision-making process for the South Texas region.

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