With the establishment of the U.S. Clean Air Act, air quality surveillance has relied on ambient air and stack emission monitoring to provide data for attainment determination, health studies, dispersion modeling, air quality trends and industrial emission inventories. Establishment of the U.S. National Ambient Air Quality Standards (NAAQS) has identified criteria pollutants which require monitoring due to their adverse health effects and other indirect impacts on human health and life style. With increased technical ability and further understanding of the pollutant behavior, ambient air monitoring has been expanded to include more focused groups which pin point the pollutant sources affecting the ambient air quality. Heavy emphasis of the specialized ambient air monitoring is currently geared toward roadway emission monitoring.
It is estimated that in the United States, over 35 million people live within 300 feet of a major transportation infrastructure and as per the U.S. Department of Transportation, Federal Highway Administration, there are approximately 250 million vehicles on the roads. Data from numerous studies and near-road monitoring research projects suggests that the automobile exhaust is complex in composition, containing up to 1000 different compounds, and poses a significant risk to human health with the variety of compounds emitted by cars, trucks, buses and other commercial vehicles. Accounting for the proximity of human habitats to the extensive transportation systems and the number of vehicles present, it is evident that specialized near road monitoring stations play an important role in determining how mobile sources affect human health, what dispersion patterns are present and what can be done to limit the exposure.
In order to achieve a comprehensive near-road monitoring study, several factors must be taken into consideration including roadway design, traffic patterns, meteorology, air quality and the population size affected by the road network. The network design includes careful planning on number and type of analyzers as well as site locations.
The aim of any roadway monitoring is to provide real-time data to develop correlation between traffic patterns, meteorology and air quality. The pollutants of interest at a near-road monitoring station include nitrogen oxides (NOx), carbon monoxide (CO), volatile organic compounds (VOCs), particulate matter (PM) and ground level ozone (O3) therefore making each station a multi-pollutant monitoring station. In addition, a full meteorological sensor suite is required for detailed weather pattern examination and the ability to quantify automobile volume, speed and type. Combining all of the factors together will yield in a complete monitoring effort allowing for mobile source air pollution characterization.
Whether it is mandated by the U.S. Environmental Protection Agency (U.S. EPA), taken on as a research study or is resulting from legal action, roadway monitoring is guided by specific regulations issued by the U.S. EPA. As of February 2010, the U.S. EPA has proposed amending Appendix D to 40 CFR Part 58, “Network Design Criteria for Ambient Air Quality Monitoring”, to require NO2 monitors at near-road monitoring locations. This change, published in the Federal Register, specifies that at minimum, one micro-scale near-road NO2 monitor is required in each area with a population greater than 500,000 persons. A secondary near-road NO2 monitoring station is required for any area with a population of 2,500,000 persons or more, or in any area with a population of 500,000 or more persons and a major road system with an average annual daily traffic of 250,000 or greater.
The amendment further details that the roadway NO2 monitoring stations must meet the following criteria; the location of the site must be adjacent to roadways where maximum hourly NO2 concentration is expected, and the monitoring location must meet all siting requirements as established by the U.S. EPA. In the event of multiple site identification meeting or exceeding the design criteria, a decision must be made based on the potential of population exposure. Further specifications mandate that in areas where a minimum of two roadway NO2 monitoring stations are to be implemented, such as in case of an urban area with population of 2.5 million or greater, the sites may be located along the same roadway if at least one site selection factor can be uncommon – traffic patterns, prevalent vehicle type, terrain or other. The secondary option for locating two monitoring sites within one populated area is to place the monitoring sites on unique roadways including routes, interstate highways etc.
The final specification of this amendment requires that NO2 measurements are taken utilizing Federal Reference Method chemiluminescence monitors and must include at a minimum: NO, NO2, and NOx.
Monitoring for the criteria pollutants including CO, NOx, PM and O3 has been well established and there are various manufacturers offering monitoring equipment to suit those needs. One of the recent advances in near road monitoring includes the use of field gas chromatographs (GC) to monitor for toxic compounds. Volatile organic compounds emitted by automobiles include 1,3 butadiene, benzene, toluene, ethylbenzene, M&P,O-xylenes. To date, the VOC compounds have been sampled using metal canisters with either passive or active collection techniques followed by TO-14, TO14a or TO-15 laboratory chromatography analysis. Sampling is normally conducted on a 1 in 12 day schedule, with the sample being integrated over 24 hour period. In roadway monitoring, sampling frequency must be increased to compensate for traffic variations and can result in up to 10 canister samples per day, integrated in advanced statistical models over 365 days to achieve the desired 1 hr averaging interval. Employment of a continuous gas chromatograph at a near road monitoring station shows several advantages over the current canister sampling method. One of the main benefits is the continuous data stream. Data is generated on a 15-30 minute analysis cycle which is crucial in determining the daily maximum and minimum concentrations of ambient VOCs. Diurnal traffic patterns (i.e. morning and evening rush hour) will have effect on the ambient VOC concentrations and an integrated canister sample will only show a single concentration for the 24 hour sampling period. Another benefit of using the GC is the reduced operation cost as compared to canister sampling methods. Conducting analysis on site eliminates the need for shipping and handling costs, laboratory analysis costs, canister cleaning, canister purchasing or renting and operator time. Continuous gas chromatography has been employed in various air monitoring efforts around the world and has been adapted to specifically suit field applications. The optimal GC unit is fully autonomous, self calibrating, requiring little to no human interaction and the entire system is to be fully integrated into the industry standard 19 inch instrument enclosure. Data is presented in terms of a chromatogram with individual compound concentrations that are provided to researchers for data comparison and atmospheric chemistry analysis. Capital cost of the investment in a real time gas chromatograph is quickly offset by the reduced operating cost and typical laboratory costs involved in canister sampling.
The objective of roadway emission monitoring is to characterize the mobile source emissions, define factors affecting the pollutant concentrations, improve modeling tools for exposure assessments and further the understanding of health effects attributed to roadway emission exposures. With the changing regulations affecting and/or establishing the near-road monitoring protocols, and the enhanced capabilities of monitoring equipment, the roadway monitoring projects around the country will provide scientists and researchers with data to produce detailed health impact assessment reports and develop human exposure models.