Background

Ozone is a good thing, right? Most people associate ozone with its ability to protect the Earth from the sun’s radiation and the well-publicized ozone “hole” that occurs in the upper atmosphere. However, ozone also exists near the ground, where it is a common air pollutant. Ozone is an extremely reactive chemical that has been shown to reduce visibility and have harmful effects on human health, commercial crops, and natural areas. Particulate matter (PM) is another common pollutant. It is not a single compound, but a collection of small particles composed of hundreds of chemical species. High levels of PM are significantly associated with adverse health effects, ecosystem damage, and degraded visibility. The recently revised and strengthened federal standards for ozone and PM emphasize the growing acknowledgement of their detrimental effects and compel cities to look for improved management strategies. The detection of meteorological variables controlling ozone and PM and the identification of emissions-controlled pollutant trends are needed to provide greater insight into the effects of past air quality management decisions and allow for more effective and proactive decisions to ameliorate future air pollution. Developing information on the risks associated with high ozone and PM concentrations has led to a proliferation of studies that examine these pollutants, particularly in the eastern United States.

Weather and Climate Connections

Research in other regions has indicated that ozone is increased by sunny skies and high temperatures, which enhance the conditions needed for the photochemical process that creates ozone. The influence of other meteorological variables is uncertain. Some research has suggested that large concentrations of water vapor and stagnant conditions also lead to increased ozone. These previous studies were conducted primarily in the eastern United States.

Wind speed, atmospheric mixing, and atmospheric moisture are the meteorological variables believed to exert the most influence on PM concentrations. Stagnant conditions usually associated with atmospheric inversions (condition in which the air temperature rises with increasing altitude, holding surface air down and preventing dispersion of pollutants) are thought to be associated with high PM concentrations. While high wind speeds can increase ventilation, they are normally correlated with high PM concentrations because they allow the resuspension of particles from the ground, as well as long-range transport of particulates between regions. High PM concentrations are normally associated with dry conditions due to increased suspension of dust, soil, and other particles. There have not been many studies conducted on PM and climate, particularly in the Southwest.

Air Quality in the Southwest

Air quality research carried out elsewhere, however, does not necessarily apply to the Southwest due to its unique climatic and demographic conditions. The Southwest experiences a highly variable climate due to the region’s position between the mid-latitude and subtropical atmospheric circulation systems, its complex topography, and its proximity to the Pacific Ocean, the Gulf of Mexico, and the Gulf of California (Sheppard et al. 2002). The complex terrain of the Southwest also causes thermally and topographically driven winds that affect pollutant transport and dilution.

The Southwest is the fastest growing region in the United States. It is also a highly urbanized region. The high rates of population growth and urbanization in the Southwest are troubling in light of air quality. More people lead to more houses, factories, shops, cars, and travel, all of which contribute to emissions of air pollutants. Cities in the Southwest are typically sprawling, with little public transportation available. There has been a rapid increase in vehicle miles traveled throughout most of the Southwest. This increase has offset any gains that may have come about through improved emissions standards in newer vehicles (Keyes et al. 2001).

Air quality in the Southwest is influenced by a wide variety of factors, both anthropogenic and biogenic. Anthropogenic sources of pollutants include emissions from automobiles, power plants, industry, and wood-burning stoves and fireplaces. Wildfires, airborne dust and soil particles, and hydrocarbons emitted by vegetation are examples of biogenic influences. Meteorological conditions, however, appear to have the greatest impact on daily variations in air quality. The strong linkage between weather conditions and pollutant levels can obscure the effects of changing emission levels over time. Air quality planners and managers must understand the link between climate and pollutants in order to select optimal pollutant reduction strategies and avoid exceeding the federal air quality standards.