This project was designed to provide air quality managers and decision makers with better information on the climatic influences on air quality by involving them in the research process. In December 2002, an Air Quality and Climate Variability taskforce was assembled, consisting primarily of members from local, state, and federal environmental agencies. Their feedback on the research project was solicited and used to shape and focus the research goals in a direction that will provide them with the most useful information possible. Research goals included: 1) Determining which meteorological variables most influence ground-level ozone and particulate matter (PM) air quality concentrations in the southwestern United States; 2) Examining the underlying air quality trends that emerged once the effects of these variables had been removed from the time-series; and 3) Determining whether the meteorological variables that control ozone and PM in the Southwest are similar to the meteorological controls found in other parts of the United States.
An important component of the study was to determine how the discovery of these trends could be made useful for air quality planners and managers. For this reason, a number of air quality forums and workshops were held in order to better understand which issues needed to be addressed. Stakeholder needs included more information on PM–weather relationships, straight-forward graphics and interpretation of research results, and information that would help determine the probability of the Southwest experiencing a climate year that is conducive to high pollutant concentrations.
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.
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 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.
Daily meteorological and air quality data were collected and analyzed for the time period 1990–2003, where available. Meteorological variables of interest that were analyzed in this study are maximum daily temperature, average daily wind speed, average daily dew point temperature, mixing height, average daily relative humidity, incoming solar radiation, and precipitation.
This study utilized a technique called the KZ filter to remove meteorological signals from the air quality time-series. This was done to reveal underlying air quality trends and contribute to an understanding of the mechanisms controlling the trends. The KZ filter is capable of separating a time-series into different temporal components by varying the length of the filter window and the number of iterations.
Although the KZ filter has been widely used for this type of trend separation in ozone studies in the eastern United States, this project aimed to extend the method in three key ways. First, the study was developed through a partnership between academic researchers and air quality planners and managers, and the output was tailored to be more applicable to decision makers’ needs. Second, the KZ filter was applied to PM in this study in order to determine the method’s effectiveness on this pollutant. Third, the method was applied to the Southwest in order to evaluate the effectiveness of the method in a region with weaker weather controls than the eastern United States.
Statistical techniques were used to evaluate the relationship between pollutants and meteorological variables, producing a set of recommended models for ozone and PM. The KZ filter was then used to separate meteorological and air quality data into short-term, seasonal, and long-term trend components. Statistical techniques and the KZ filter method were further applied to produce long-term ozone and PM trends that have had the influence of weather removed. These trends represent changes in pollutant concentrations attributable to sources other than the removed meteorological variables, such as emissions changes.
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Milanchus, M., S. Rao, and I. Zurbenko. 1998. Evaluating the effectiveness of ozone management efforts in the presence of meteorological variability. Journal of the Air & Waste Management Association, 48:201-215.
Rao, S. and I. Zurbenko. 1994. Detecting and tracking changes in ozone air quality. Journal of the Air & Waste Management Association 44:1089-1092.
Sheppard, P., A. Comrie, G. Packin, K. Angersbach, and M. Hughes. 2002. The climate of the U.S. Southwest. Climate Research 21:219-238.
Wise, E. K. 2005. Air quality effects from southeast Arizona wildfires. Bulletin of the American Meteorological Society 86(12):1719-1721.
Wise, E. K. 2005. Urban air quality impacts of wildfires in the U. S. Southwest. Pacifica, Fall 2005: 1, 5-6.
Wise, E. K. and A. C. Comrie. 2005. Extending the Kolmogorov-Zurbenko Filter: application to ozone, particulate matter, and meteorological trends. Journal of the Air and Waste Management Association, 55: 1208-1216.
Wise, E. K. and A. C. Comrie. 2005. Meteorologically-adjusted urban air quality trends in the southwestern United States. Atmospheric Environment, 39: 2969-2980.