Sensitivity of ozone to summertime climate in the eastern USA: A modeling case study

Abstract

The goal of this modeling study is to determine how concentrations of ozone respond to changes in climate over the eastern USA. The sensitivities of average ozone concentrations to temperature, wind speed, absolute humidity, mixing height, cloud liquid water content and optical depth, cloudy area, precipitation rate, and precipitating area extent are investigated individually. The simulation period consists of July 12–21, 2001, during which an ozone episode occurred over the Southeast. The ozone metrics used include daily maximum 8 h average O₃ concentration and number of grid cells exceeding the US EPA ambient air-quality standard. The meteorological factor that had the largest impact on both ozone metrics was temperature, which increased daily maximum 8 h average O₃ by 0.34 ppb K⁻¹ on average over the simulation domain. Absolute humidity had a smaller but appreciable effect on daily maximum 8 h average O₃ (−0.025 ppb for each percent increase in absolute humidity). While domain-average responses to changes in wind speed, mixing height, cloud liquid water content, and optical depth were rather small, these factors did have appreciable local effects in many areas. Temperature also had the largest effect on air-quality standard exceedances; a 2.5 K temperature increase led to a 30% increase in the area exceeding the EPA standard. Wind speed and mixing height also had appreciable effects on ozone air-quality standard exceedances.

Introduction

High concentrations of ozone (O₃), a major constituent of air pollution, have detrimental effects on human health (Godish, 2004). Reactions of ozone with tissue in the airways are believed to cause weakened immune response, decreased lung function, and increased morbidity from asthma (Bernard et al., 2001; Levy et al., 2001). Also, ozone has been shown to cause significant damage to crops (Heck et al., 1982).

The conditions necessary for high ozone concentrations in the lower troposphere generally include warm weather (Sillman and Samson, 1995), sunlight, and stagnating high pressure systems, making episodes of high tropospheric ozone concentrations generally a summer phenomenon. Because of the size and duration of these warm high-pressure systems, ozone episodes tend to occur at a regional scale and last several days. Ozone is formed through complex interactions among nitrogen oxides (NO_x) and volatile organic compounds (VOCs) in the presence of sunlight (Seinfeld and Pandis, 1998). Both NO_x and VOCs have natural and anthropogenic sources.

Ozone concentrations are influenced by meteorology in many ways. Ozone production is expected to be influenced by temperature because of the temperature dependences of the hundreds of reactions involved. A large contributor to this behavior is the temperature-dependent decomposition rate of peroxyacetylnitrate (PAN) and its homologs, which act as reservoir species for NO₂ (Sillman and Samson, 1995). Water vapor has competing effects on ozone levels. It begins with the photolysis of ozone, which can produce excited oxygen atom, O(¹D), and an oxygen molecule. The O(¹D) can then react with water vapor to produce a hydroxyl radical. The hydroxyl radical undergoes further reactions, some of which eventually lead to ozone production but many of which do not. The amount of ozone that subsequently forms depends on the NO_x/VOC mixture in a location. These reactions can collectively constitute a sink for ozone, due to the consumption of an O₃ molecule and an O(¹D) atom, or they can produce more ozone molecules due to the subsequent chemistry of the hydroxyl radical. High wind speed is generally correlated with low pollutant concentrations due to enhanced advection and deposition; the processes involved, however, are complex, and in some places wind speed is positively correlated with ozone concentration (Tecer et al., 2003). Changes in cloud cover can affect the photochemistry of ozone production and loss, though the extent to which cloud cover affects ozone concentrations is thought to be small (Korsog and Wolff, 1991). Precipitation changes are expected to affect the rates of wet deposition of ozone, aerosols, and precursors. Additionally, changes in mixing height could affect reaction rates and the dilution of pollutants.

Emissions control policy is currently made assuming that climate will remain constant. However, climate changes over the next decades are expected to be significant and may impact O₃ concentrations; for example, global average temperatures are expected to rise 1.5–4.5 K over the next century (IPCC, 2001). Predictions of how wind speed will change in the USA vary depending on the area in question and on the model used. Predictions from one study differ in different areas in the same state (Bogardi and Matyasovszky, 1996). Breslow and Sailor (2002) predict decreases in wind speeds over the USA in the next 50 years. Water vapor concentration (absolute humidity) is generally expected to increase due to the higher saturation vapor pressure of water at higher temperatures (IPCC, 2001); to first order, it is expected that relative humidity remain constant with climate change (Held and Soden, 2000). Norris (2005) has observed decreases in cloud cover in recent decades over most of the planet. Simulations using general circulation models (GCMs) indicate that cloud cover will decrease if temperature is increased (Cess et al., 1990). GCM studies also predict small changes in summer and annual mean precipitation over the eastern USA (Räisänen, 2005); Leung and Gustafson (2005), however, predict changes in the number of summer days with precipitation in the eastern USA. Mickley et al. (2004) and Hogrefe et al. (2004) report increases in mixing heights for simulated future climates, though Murazaki and Hess (2006) see no significant changes in mixing heights in a future climate.

Determining how air quality changes as climate changes is an important step toward estimating future air quality. This may allow policy planners to relax the assumption of constant climate and meteorology, or it may indicate that the assumption of constant climate will have little effect on predicted air quality. It will also help show if climate changes can be accounted for with simple corrections to models run with constant climate or if a sophisticated modeling framework is necessary. In any case, the effects of climate changes on air quality must first be characterized.

The response of ozone to changes in temperature has been examined in the past with both process modeling and statistical studies. Higher temperatures have generally been associated with higher ozone concentrations (Tecer et al., 2003; Bloomfield et al., 1996; Guicherit and van Dop, 1977; Menut, 2003; Neftel et al., 2002), with some exceptions (McMillan et al., 2005). In an observational study of ozone in Chicago, ozone concentrations increase with temperature on days with high temperatures over approximately 50°F (Bloomfield et al., 1996). A chemical transport modeling study simulating an ozone episode over Milan in May 1998 finds a linear positive correlation between peak ozone concentration and temperature (Baertsch-Ritter et al., 2004). In a modeling study of an ozone episode over southern California in September 1996, Aw and Kleeman (2003) calculate an increase in peak ozone with temperature. High water vapor concentrations have been shown to have inconsistent effects on ozone concentrations from location to location or even at a fixed location due to these canceling influences on chemistry (Jacob et al., 1993), depending on whether an area is VOC- or NO_x-limited (Baertsch-Ritter et al., 2004). Baertsch-Ritter et al. (2004) also notice a weak connection between water vapor and ozone concentrations. Korsog and Wolff (1991) see a negative correlation between cloud cover and ozone, though the correlation between temperature and ozone was stronger. These studies have focused on small areas (e.g., one city) during ozone episodes; the response of ozone over both episode and non-episode areas or over large regions has been the focus of little research. Additionally few studies (Baertsch-Ritter et al., 2004) have calculated sensitivities of ozone concentrations to a comprehensive suite of specific meteorological parameters.

Several studies have used GCM-predicted future climates in models to predict future ozone concentrations. Hogrefe et al. (2004) examine ozone changes over the eastern USA under a changed climate and determine that average daily maximum 8 h ozone concentrations increase by 2.7 ppb by the 2020s and 5 ppb by the 2080s. Murazaki and Hess (2006) predict decreases in background ozone but increases in ozone in areas with high NO_x emissions. These studies do not separate the effects of specific meteorological changes on ozone; instead, they estimate the response to a combined set of changes in climate parameters.

The goal of this study is to determine how concentrations of ozone over the eastern USA respond to changes in climate parameters, specifically temperature, wind speed, absolute humidity, mixing height, cloud cover, and precipitation. This work investigates each of these parameters separately so that the effects of each can be determined. This will help identify the major factors that could have an effect on air quality as climate changes by determining the meteorological factors to which ozone concentrations have the largest sensitivities.