A perfect smog storm: review of the particle pollution episode in the Midwest from January 31 to February 6, 2005

Particle pollution is a continuing air quality problem for many areas of the United States. Unlike summertime ozone, particle pollution episodes can occur year-round in most areas of the United States and produce both respiratory and cardiovascular health problems. Particle pollution (also called fine particles) consists of particulate matter with diameters less than 2.5 micrometers (PM_2.5). Typically these episodes occur when certain weather conditions allow emissions from various sources (vehicles, industry, power generation, and consumer products) to build up and subsequently cause particle concentrations to increase.

In early February 2005, a major wintertime PM_2.5 episode occurred in the Midwest. Particle pollution reached unhealthy levels in many cities in the Midwest, from Minneapolis, Minnesota, through Michigan and into northern Ohio. Detroit, Michigan, experienced seven days when air quality reached the Unhealthy category on the Air Quality Index. Weather conditions during this particle pollution episode were characterized by light-to-calm winds that allowed particle concentrations to increase; increased stability in the boundary layer that trapped particles near the ground and prevented dispersion; snow cover that increased low-level cooling and enhanced stability; and increased humidity from snow evaporation that aided the chemical formation of particulate matter.

This paper describes the key weather factors that produced the episode, discusses air quality conditions day by day, shows how the episode was covered by the media, and reviews the health effects associated with particle pollution.