Air Quality Monitoring

Unlike emission inventories, air quality monitors determine actual pollutant loading in the atmosphere and are therefore the most direct measure of air quality on which air regulatory programs are based. Samples of particulate matter in the atmosphere (PM10 or PM2.5, or both) are also used for source apportionment purposes to identify the origin of the aerosols. Monitoring of smoke from fires, however, presents several unusual technical challenges that affect results. These challenges center on the fact that smoke from fires has several unique characteristics.

Current Monitoring Techniques

The three principal methods of measuring air pollution are samplers, optical instruments, and electrochemical devices (each described below). Samplers are most common for long-term monitoring. Data from optical meters and electrochemical devices can be stored in a computer or datalogger on site or transmitted from remote locations to provide real-time information.

Samplers

Samplers collect aerosols on a filter or chemical solution. A simple gravimetric measure of mass concentration may be obtained, or different types of filters or solutions can be used, to help define chemical species and particle sizes. For chemical speciation, filters must be sent to a laboratory for analysis. For this reason, sampling information usually is delayed by days to weeks after the sampling period. Active samplers are the most accurate as they use a pump to pass a known volume of air through the collector. Passive samplers are the least expensive, allowing air to reach the collector by some physical process such as diffusion. Tapered Element Oscillation Microscales (TEOMs) are a special class of samplers that provide a gravimetric measure of mass concentration at the studied site without having to transport filters to a laboratory.

All sampling devices lose some degree of semivolatile fine particulates (Eatough and Pang 1999). Positive and negative organic carbon artifacts are just two of several factors that contribute to variability between different colocated instruments. To minimize this variability, consistent sampling methods are used throughout a sampling network to help recognize such artifacts.

The analytical technique used to quantify carbon concentrations from filters also can cause discrepancies between measurements (Chow 2000). For example, the NIOSH 5040 method (Cassinelli and OConner 1994) is a thermal-optical transmittance method of speciating total, organic, elemental, and carbonate (inorganic) carbon being adopted by the EPAs PM2.5 program. This method is a departure from the thermal-optical reflectance method that has been used in the IMPROVE program. Recent comparisons between ambient samples have identified differences as great as 17.5 ± 15 percent (EPA 2000a), which can be significant when monitoring for National Ambient Air Quality Standards (NAAQS) violations.

Because filters can become overfull, they must be changed regularly and are not suitable for sites close to fires where particulate concentrations are heavy.

Optical Instruments

Optical instruments use a light source to measure the atmospheres ability to scatter and absorb light. Common devices are photometers, which measure the intensity of light, and transmissometers, which are photometers used to measure the intensity of distant light. Photometers and transmissometers have a direct relation to visual range. Nephelometers measure the scattering function of particles suspended in air. They can be used to determine the visual range, as well as the size of the suspended particles, by changing the wavelength of the light source. Wavelengths of 400 to 550 nm are common for monitoring smoke from biomass fires, while wavelengths of 880 nm are more common for road dust measurements. Because the instruments have increasing application for both long-term and real-time monitoring of smoke, Trent and others (2000) evaluated the accuracy of several different optical instruments by comparing their output to gravimetric samples.

Investigators have found some problems in field reliability and temperature drift among photometers and nephelometers (Trent and others 1999, 2000). While Davies (2002) recommends a general coefficient for relating scattering coefficient to drift smoke from a DataRAM nephelometer, a precise relation between a nephelometers measured scattering coefficient and particle concentration depends on the wavelength of the instrument and the particle distribution of the medium, which varies by combustion stage and fuel type.

Electrochemical Devices

Electrochemical devices have been used in industrial applications for many years. Their small size and ability to measure criteria pollutants, such as carbon monoxide, make them suitable for personal monitoring or monitoring in extremely remote locations. Thus, they are gaining value for monitoring wildland smoke impacts. For example, Reinhardt and Ottmar (2000) recommend the use of an electrochemical dosimeter for monitoring exposure levels experienced by wildland fire fighters (Reinhardt and Ottmar 2000).

States, Tribes, and local air agencies use a variety of instruments to monitor long-term and real-time smoke impacts for both NAAQS and visibility to suit their local interests and regulatory needs. The Interagency Monitoring of Protected Visual Environments (IMPROVE) program is one of few nationally coordinated monitoring projects.  IMPROVE was established in 1985 in response to the 1977 amendment of the Clean Air Act requiring monitoring of visibility-related parameters in Class I areas throughout the country. The IMPROVE network uses a combination of speciation filters on active samplers to measure physical properties of atmospheric particles (PM2.5 and PM10) that are related to visibility. Many sites also include transmissometers and nephelometers optical devices. Also, cameras are used document the appearance of scenic vistas. Because the samplers collect for 24 hours every 3 days, their information is used for determining long- term trends in visibility. The optical and camera devices can monitor more frequently and can help define short-term or near real-time changes in visibility impact.