(BRAVO) Study: Final Report. - Desert Research Institute

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Final Report — September 2004 enabling comparison of their measurements. Concurrently, the routine BRAVO sampling included analysis of 24-hr filter samples for light-absorbing carbon. The basic design of the photoacoustic instrument operated by DRI was an update of the instrument described by Arnott et al. (1999). The sample inlet included a 2.5 µm cut point cyclone (URG) and an annular denuder (URG model 962) coated with MnO 2 to take out NO 2 that might otherwise produce a photoacoustic signal from light absorption in the green wavelength region. The minimum detection limit (MDL) of the instrument used in the BRAVO Study was 0.5 Mm -1 for an integration time of 8 minutes. A Magee Scientific aethalometer (Model AE-16) was operated by CSU continuously during the study. The aethalometer measures the concentration of absorbing aerosol by measuring the optical attenuation of light caused by a particulate matter sample deposited continuously on quartz fiber filter tape. It is assumed that aerosol absorption is due mainly to carbonaceous particles (referred to here as black carbon, BC). A beam of light is directed at the aerosol deposit spot on the tape and the attenuation of light due to aerosol absorption is measured by detection of transmitted light through the sample on the filter. After the BRAVO field study, the minimum detection limit of the aethalometer was determined at the Atmospheric Simulation Laboratory at Colorado State University. Particlefree air was delivered to the instrument and from these measurements the MDL was determined to be 71 ng/m 3 for an hourly average. Figure 5-10 compares time series of photoacoustic measurements, as 12-hour averages, with aethalometer BC measurements (top) and IMPROVE elemental carbon measurements. (The relative scaling of photoacoustic absorption and aethalometer BC concentration axes in the top plot was based on the linear regression described below. The same scaling was used in the lower plot.) The top panel of Figure 5-10 shows that the maximum light absorption was 2.1 Mm -1 , and the maximum black carbon concentration was 0.2 µg/m 3 , consistent with other measurements in the arid southwest U.S. (Pinnick et al., 1993). Between September 20-23, the temporal variations of the measurements are very similar. Around September 25 and October 2, both measurements indicate low values, and the aethalometer measurements are smoother than the photoacoustic measurements. During October 8-16, the photoacoustic measure of light absorption is proportionally lower than the aethalometer measure of black carbon. In summary, there are periods of agreement and disagreement in the relative trends of black carbon measurements by the two methods, perhaps due to compositional changes and to the difference in the detection limits of the methods. The lower panel of Figure 5-10 compares photoacoustic aerosol light absorption measurements with IMPROVE elemental carbon (light absorbing carbon) concentration measurements. Note that the EC signal is noisier than the BC signal. Also note that relatively good agreement in this panel is found around October 8, though relatively poor agreement is found near October 17. Owing to the relatively low signal levels at this site, firm statistically sound conclusions cannot be drawn from this data set. However, one could state that use of the conventional light absorption efficiency factor of 10 m 2 /g for the IMPROVE EC measurements places IMPROVE absorption values into the same ballpark as the photoacoustic measurements of aerosol light absorption. 5-16
Final Report — September 2004 3 PHOTOACOUSTIC AETHALOMETER 0.35 2.5 0.3 Babs (Mm -1 ) DRI PHOTOACOUSTIC INSTRUMENT 2 1.5 1 0.5 0.25 0.2 0.15 0.1 0.05 LOCAL BC (µg m -3 ) CSU AETHALOMETER 0 0 -0.5 17-Sep 24-Sep 1-Oct 8-Oct 15-Oct 12 Hour Average Data, (BRAVO, 1999) -0.0625 Figure 5-10. Comparisons of 12-hour averages of the photoacoustic light absorption measurements and the aethalometer black carbon measurements (top) and IMPROVE EC measurements (bottom) during the BRAVO Study. The black carbon and EC concentrations were scaled to the pressure and temperature of the site for direct comparison with photoacoustic data. 5-17
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(BRAVO) Study: Final Report. - Desert Research Institute

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