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

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Final Report — September 2004 Wood smoke was not identified by these methods as a significant contributor to the carbonaceous aerosol during the BRAVO Study period. Using levoglucosan concentrations, typical estimated wood smoke contributions were lower than 1%. Detection of other wood smoke marker compounds, however, questions the validity of the use of levoglucosan as a conservative tracer. Source apportionment using vanillin as a tracer indicated up to approximately 6% of the particulate organic matter being derived from wood smoke. The difference in wood smoke contribution estimates may reflect degradation of levoglucosan during transport in the presence of acidic aerosols and/or secondary production of other markers like vanillin. A number of indicators, including low source contributions of primary compounds and absence of marker compounds, suggest that a large portion of the organic aerosol present during BRAVO was secondary. Comparisons of observed OC/EC ratios with wintertime OC/EC ratios in the park (see Brown et al. (2002) for details) suggest that frequently more than half of the OC observed during BRAVO was secondary in origin. Findings from this investigation reveal the limitations to performing quantitative source apportionment of carbonaceous aerosols in remote locations, where concentrations are low and significant conversion to secondary organics has taken place. To refine the method, future work is needed to evaluate the stability of common organic source markers during long-range transport and the production of secondary organic carbon. More details on the experiment and the data analyses described above are provided in an article by Brown et al. (2002). 5.2.4 Particle Size Distribution Dry particle size distributions were measured with 15 min resolution over the size range of 0.05 < Dp < 20 µm from July to October 1999 at the K-Bar site in Big Bend National Park. Three instruments were used to cover this size range: a differential mobility analyzer (0.05–0.87 µm), an optical particle counter (0.1–1.0 µm), and an aerodynamic particle sizer (0.5–20 µm). A new method was developed to align the individual instrument size distributions in overlap regions, thereby producing a single, merged size distribution from the data (Hand and Kreidenweis, 2002). As part of this alignment method, estimates of dry aerosol refractive index (study average, 1.566 for the fine mode) and effective dry particle density (study average, 1.56 g cm –3 ) were retrieved. The retrieved estimates agreed well with properties computed from 24-hour particulate chemical composition measurements. Statistics for the particle size distributions were computed for accumulation, coarse, and occasionally giant, modes, where the modes were distinguished by minima in the observed distributions. As shown in Figure 5-8, coarse mode particulate matter volume concentrations contributed significantly to total particulate matter volume concentrations during July and August, when episodic North African dust outbreaks were detected. During the first half of the study, the geometric mean volume diameter for the coarse mode was smaller than during the second half, reflecting the non-local sources of the dust. The studyaverage particulate matter volume concentrations were 3 ± 2 and 4 ± 4 µm 3 /cm 3 for the accumulation and coarse modes, respectively, with peak values of accumulation mode 5-12
Final Report — September 2004 volume concentrations occurring from mid-August through mid-October. Geometric volume mean diameters for the accumulation and coarse particle modes were 0.26 ± 0.04 and 3.4 ± 0.8 µm, respectively. Further details about the size distribution measurements are contained in the article by Hand et al. (2002). 25 20 Aug 1 Sept 1 Oct 1 accumulation mode coarse mode volume, µm 3 cm -3 15 10 5 0 182 202 222 242 262 282 302 Day of Year, 1999 Figure 5-8. Timelines of dry particulate matter volume concentrations (in µm 3 /cm 3 ) at Big Bend National Park for the accumulation and coarse modes, as computed from measured size distributions. Day 182 is July 1, and day 304 is October 31. 5.2.5 Very Fine PM as a Source Tracer Primary particles from high temperature sources tend to form very fine and ultra-fine particles that reflect the source materials. Normally, very fine (D p
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