Boulder County sits at the confluence of the high alpine wilderness of the Rocky Mountains, the urbanized city of Denver, Colorado, and the agricultural and intensive gas development activities in Northeast Colorado. This urban/rural interface creates a complex air quality environment characterized by shifting upslope and downslope air flow conditions that may elevate air toxics concentrations in areas with few local sources. The Denver area, including Boulder County, was designated as a non-attainment area on November 20, 2007 for failing to meet the Environmental Protection Agency’s (EPA) 8-hour primary ozone standard. Therefore, the Boulder County Public Health Department (BCPH) and its partners recently completed one year of monitoring low molecular weight carbonyl compounds, volatile organic compounds (VOCs), and ground-level ozone at five locations around Boulder County to aid in air toxics source apportionment and model evaluation. Carbonyls and VOCs were collected on a 1-in-6 day schedule at each location from March 1, 2007 to February 24, 2008. In addition, meteorological data including wind speed, wind direction, temperature, and barometric pressure were collected at each location. Principal Component Analysis was used to help identify sources and other factors influencing hydrocarbon concentrations.

Each monitoring site showed different results for ozone, especially during the ozone season when values approached and occasionally exceeded the EPA primary 8-hour ozone standard. Concentrations of formaldehyde and acetaldehyde measured in the Boulder study were generally lower than those observed in earlier monitoring studies, with especially pronounced differences for acetaldehyde. Prior to the Boulder study, acetaldehyde/formaldehyde ratios measured in the Denver metropolitan area were much higher than values measured in other studies performed in the United States and abroad. Ratios from the Boulder study are more in line with those measured in other locations. Concentrations of VOCs, such as benzene and toluene, measured in the Boulder Study were similar to other studies previously conducted in Denver. Duplicate 24-hour samples collected during the Boulder study show strong agreement, as did the sum of the eight 3-hour samples collected along with the 24-hour samples. Clean field blanks collected during the Boulder Study also support the validity of the current measurements.