may sometimes be hot spots.3 In the absence of data on pollutant concentrations in hot spots, characterization of pollutant exposures of persons who work in, reside in, or travel through them is problematic (see Box 7-5 in Chapter 7).
Standard operating procedures, measurement methods, and quality assurance and quality control (QA/QC) procedures are critical to ensure that the data sets from monitoring networks can be directly compared and integrated for use in trends analysis, for investigations of atmospheric processes, and for improvement of predictive models. In accordance with the Code of Federal Regulations (40 CFR Part 53 ), EPA established federal reference methods (FRM) and federal equivalent methods (FEM) to be used in measurements of criteria pollutants (EPA 2002q). Further development of monitoring techniques is needed, however, to address some of the following concerns:
The instrumentation used to measure NO and NO2 in the FRM does not have the sensitivity or the specificity to measure nitrogen oxides accurately over the full range of conditions typically encountered by such instrumentation (McClenny et al. 2002).
PM10 and PM2.5 are generally measured once every 3 or 6 days, a sampling rate that is too infrequent to capture the true variability of PM concentrations.
New analysis systems were developed in response to the PAMS initiative and its requirements for measurements of speciated hydrocarbons and carbonyl compounds. However, routine monitoring of hydrocarbons and carbonyls is a challenging task, even for research institutions, and thus far, delivery and analysis of quality-ensured data from PAMS have been limited (see NARSTO 2000).
PM2.5 measurements will continue to pose a technical challenge because of the complex multiphase mix of constituents in ambient aerosols. As noted earlier, current sampling methods are subject to various artifacts.
Hot spots are locales where pollutant concentrations are substantially higher than concentrations indicated by ambient outdoor monitors located in adjacent or surrounding areas. Hot spots can occur in indoor areas (for example, public buildings, schools, homes, and factories), inside vehicles (for example, cars, buses, and airplanes), and outdoor microenvironments (for example, a busy intersection, a tunnel, a depressed roadway canyon, toll plazas, truck terminals, airport aprons, or nearby one or many stationary sources). The pollutant concentrations within hot spots can vary over time depending on various factors including the emission rates, activity levels of contributing sources, and meteorological conditions.