Photochemical Assessment Monitoring

The Clean Air Act Amendments of 1990 required EPA to promulgate regulations for the "enhanced" monitoring of ozone and its precursors for ozone nonattainment areas classified as serious, severe or extreme. Both Congress and EPA recognized the need for an improved understanding of the ozone problem, and better feedback mechanisms for evaluating the effectiveness of ozone control strategies. In 1993, EPA published the final rule detailing the minimum requirements for Photochemical Assessment Monitoring Stations (PAMS), which includes measurements of nitrogen oxides (NOx), speciated volatile organic compounds (VOCs), and meteorological parameters. These monitoring regulations provide for the collection of an "enhanced" ambient air quality database which can be used to better characterize the nature and extent of the ozone problem, aid in tracking VOC and NOx emission inventory reductions, assess air quality trends, make attainment/nonattainment decisions, and evaluate photochemical grid-model performance.

The size and design of a PAMS network for a serious, severe or extreme ozone nonattainment area can vary from just 2 to as many as 4-5 sites, depending on the area's population size. There are four different types of PAMS sites, each serving a specific monitoring purpose: a Type 1 is upwind of the urban area and represents background concentrations coming into the area; a Type 2 is on the downwind fringe of the urban area and represents maximum precursor concentrations from the area; and a Type 4 is at extreme downwind location to represent long-range transport from the area. Therefore, PAMS sites required for an area will not necessarily all be located within the boundaries of the non-attainment area. This is how Maine, with no serious, severe or extreme non-attainment areas itself, can end up having three PAMS sites in operation during 1995.

A Type 4 PAMS site was established at Two Lights State Park in Cape Elizabeth in the spring of 1993 to represent extreme downwind impacts from the Greater Connecticut non-attainment area. This site operated in 2007 with an ozone (O3) monitor, a low-level nitrogen oxides (NO/NOy) monitor, a carbon monoxide monitor, a meteorological system and an automated gas chromatograph (GC) analytical system. A Type 2 PAMS site was established in the spring of 1995 at the Frisbee School in Kittery to represent maximum precursor emissions coming from the Portsmouth-Dover-Rochester NH non-attainment area. This site was operated by the State of New Hampshire but was discontinued after the 2005 season. A Type 4 PAMS site was established in the spring of 1997 on the top of Cadillac Mountain in Acadia National Park to represent downwind impacts from the Boston nonattainment area. This site operated in 2007 with an ozone monitor, a NOx monitor, a meteorological system and an automated gas chromatograph analytical system.

monitoring shelter cadilac mt, acadia

Monitoring shelter located on the peak of Cadillac Mountain in Acadia National Park

The distinguishing features that make PAMS sites truly “enhanced” ozone monitoring stations is that they collect data simultaneously on the essential ingredients (NOy and VOCs) and factors (meteorology) that produce ground-level ozone through photochemical reactions in the atmosphere. Gaining a better understanding of how these variables interplay with each other in actual real-world instances is absolutely necessary in order to effectively address Maine's and the nation's continuing ozone problem.

There are a number of hydrocarbons which are of particular interest to EPA and the DEP regarding ground-level ozone formation. A majority of these hydrocarbons are associated with gasoline, before and after combustion, and are dominant in urban atmospheres due to mobile source emissions. These hydrocarbon compounds contain two to eleven carbon atoms and are known to be photochemically reactive. Obtaining hourly speciated VOC data via a GC operated in the field compared to operating one in the laboratory, to be certain, is a technically demanding challenge. A 40-minute integrated ambient air sample is taken at the start of each hour, then during the remaining 20 minutes, it is analyzed for 55 separate VOCs. This is done 24 hours a day between June 1 st and September 30 th . Effectively handling and managing the tremendous volume of VOC data generated by these auto-GC systems is an equally challenging task. The list of hydrocarbons which are of interest to the EPA and DEP as well as a summary of some of the data collected at the PAMS sites is available in the latest annual air quality report.