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

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Final Report — September 2004 Photoacoustic light absorption measurements are plotted against aethalometer black carbon measurements in the scatter diagram in Figure 5-11. The slopes of the linear models are 8.4 m 2 /g and 9.9 m 2 /g, depending on whether the entire data set is considered (solid line), or just the data corresponding to BC > 0.15 µg/m 3 are used. (The corresponding R 2 values are 0.45 and 0.21, respectively.) The first slope represents the value that can occur when measurements are near the noise floors of the instruments. A prior study comparing versions of these two instruments at a mostly urban-aerosol-dominated site in Brighton, CO (Moosmüller et al., 1998) indicated an efficiency factor of 10.0 m 2 /g, which agrees with the value obtained at Big Bend when the black carbon concentration was greater than 0.15 µg/m 3 . It also agrees with the conventional value that is used, for example, in the IMPROVE formula for estimating extinction from particulate matter component concentrations. Babs (Mm -1 ) DRI PHOTOACOUSTIC INSTRUMENT 3 ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ 2 ¥ ¥ ¥ ¥ ¥¥ ¥ ¥¥ ¥ ¥¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥¥¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥¥ ¥¥ ¥ ¥ ¥¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥¥ ¥ ¥ ¥ ¥¥ ¥¥ ¥ ¥ ¥ ¥ ¥ ¥ 1 ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥¥ ¥¥ ¥ ¥ ¥ ¥ ¥¥ ¥ ¥¥ ¥ ¥ ¥ ¥ ¥¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ 0 ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ ¥ -1 ¥ -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 LOCAL BC (µg m -3 ) CSU AETHALOMETER Figure 5-11. Comparison of hourly averaged black carbon and light absorption measurements during the BRAVO study time period of Figure 5- 10. Linear regression lines are for the entire data set (solid) and just the data where BC > 0.15 µg/m 3 (dashed). One can interpret these slopes as absorption efficiency factors to use for estimating light absorption from aethalometer measurements of black carbon. (Note that aethalometer black carbon concentrations are themselves derived from apparent filter absorption by using an empirical efficiency factor of 19 m 2 /g, as discussed by Bodhaine (1995). The multiplescattering filter substrate tends to amplify light absorption beyond the in-situ value, which is a well-known characteristic of the aethalometer.) The coefficient of determination (R 2 ) of the data in Figure 5-11 is relatively small (0.45), which appears to be a consequence of the low concentrations of light absorbing species at Big Bend. The aethalometer BC values never go to zero because values below the minimum detection limit of the instrument were not included in the analysis. 5-18
Final Report — September 2004 This experiment showed that fast time-response light absorption measurements were feasible and that there was reasonable consistency between absorption determined from IMPROVE elemental carbon (or LAC) measurements on filters using the conventional efficiency of 10 m 2 /g, absorption derived from aethalometer measurements, and the photoacoustic measurements. (Comparison with the integrating sphere absorption measurements made by IMPROVE for many years was not addressed.) 5.3.2 Optical Measurements of the Big Bend Atmosphere The optical properties of the atmosphere at Big Bend National Park during the full period of the BRAVO Study were measured routinely in multiple ways. Light extinction was measured by transmissometers. Open-air nephelometry was used to measure ambient light scattering. (The difference between extinction and scattering is one measure of light absorption.) In addition, light scattering by particles smaller than 2.5 µm was measured, both under ambient relative humidity and for a dried aerosol. Fine and coarse particle absorption were estimated from light-absorbing carbon (LAC) measurements on PM 2.5 and PM 10 filter samples; absorption was also measured with an aethalometer. Optec LPV-2 transmissometers were used to determine ambient light extinction. Their use in remote locations such as national parks is discussed by Molenar et al. (1989), while Dietrich et al. (1989) presents their use in urban settings. Careful operation of the transmissometer (daily cleaning of optics and pre- and post-calibrations) should result in extinction measurements with an accuracy of about 10% (Molenar et al., 1989). Five Optec NGN-2 integrating nephelometers, in various configurations, were operated at Big Bend during the study. Details of ambient nephelometer measurements were covered in Malm et al. (1994) and Day et al. (1997). Two nephelometers utilized the openair configuration and were operated using IMPROVE protocols (Air Resource Specialists, 1995). The remaining instruments were modified for specialized measurements. One nephelometer was fitted with an Anderson PM 10 inlet and the other two operated at reduced flow rate (113 l/min) and each was fitted with a Bendix-240 cyclone with a 2.5 µm cutpoint. In addition to the Optec instruments, a Radiance Research M903 integrating nephelometer was configured to measure scattering by dried PM 2.5 . Sample air was drawn through a temperature controlled humidity conditioner and passed into the nephelometer. The humidity conditioner consisted of Permapure Nafion dryers; placing the dryers in a constant temperature water bath controlled their temperature. Three Rotronics mp 100f combination relative humidity and temperature sensors were housed in PVC holders and aspirated by a fan. The manufacturer reports the accuracy of the relative humidity sensor to be ±2%. The results of these measurements are presented in detail in the CIRA/NPS report (Schichtel et al., 2004), which is included in the Appendix. The discussion below summarizes the key findings. 5-19
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