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Browsing Publications by Subject "Aerosols -- Measurement"

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    Characterization of carbonaceous aerosol during the Big Bend Regional Aerosol and Visibility Observational study
    (Colorado State University. Libraries, 2001-12) Brown, Steven G., author; Herckes, Pierre, author; Kreidenweis, Sonia M., author; Collett, Jeffrey L., Jr., author
    The Big Bend Regional Aerosol and Visibility Observational (BRAVO) study was a four month field campaign (July-October 1999) to investigate aerosol particle properties, sources, and impacts on regional visibility in Big Bend National Park, Texas. Daily PM2.5 aerosol samples were collected on pre-fired quartz fiber filters for detailed molecular analysis of the aerosol organic carbon fraction. Aerosol black carbon concentrations during BRAVO were measured with an aethalometer. The molecular characterization of the organic carbon fraction of aerosol present during the BRAVO study was performed using gas chromatography - mass spectroscopy (GC-MS). Organic carbon concentrations on individual days were too low for a detailed analysis by GC-MS. Therefore, multi-day composite samples, selected based on common air mass trajectories and temporal proximity, were extracted and analyzed for numerous compounds, including n-alkanes, polycyclic aromatic hydrocarbons (PAH), and alkanoic acids. Low alkane Carbon Preference Indices (CPIs) during July through September reflect similar concentrations of n-alkanes containing odd and even numbers of carbon atoms and indicate that anthropogenic emissions were important contributors to carbonaceous aerosol during this period, when air masses generally were advected from the east over Texas and Mexico. In October, CPIs increased, reflecting increased influence of odd carbon numbered alkanes and suggesting a predominant biogenic aerosol influence with air masses arriving from the north and the south. Plant wax contributions to odd carbon number alkanes (C25-C33) were estimated to range between 26% and 78%, with the highest contributions occurring in October with air masses arriving from the north and south. Periods with transport from eastern Texas and northeastern Mexico had much smaller plant wax contributions. Alkanoic acids were the most abundant compound class, with CPIs that were high throughout the study. The high acid CPI suggests that the alkanoic acids may be largely biogenic in origin, a finding consistent with other studies. Caution is required in interpreting the acid CPI, however, as alkanoic acids can also be formed as secondary products of atmospheric reactions. Polycyclic aromatic hydrocarbons (P AH) were usually not found in abundance, suggesting that upwind combustion emissions were not important contributors to carbonaceous aerosol or that P AH were removed by reaction or deposition in transit. Higher P AH concentrations during one period indicated a more significant contribution from fresh combustion emissions. Molecular source tracer (hopanes for vehicle emissions, levoglucosan for wood combustion, cholesterol for meat cooking) concentrations were generally not detected. Based on analytical detection limits for these species, it was estimated that wood smoke contributed no more than 1% of the total Organic Carbon (OC) present, vehicle exhaust contributed no more than 4%, and smoke from meat cooking contributed less than 13%. The presence of other wood smoke tracer molecules, however, suggests a possibly greater influence from wood combustion and possible chemical instability of levoglucosan during multi-day transport in an acidic atmosphere. Several observations suggest that secondary production contributed significantly to BRAVO carbonaceous aerosol. Examination of ratios of aerosol organic carbon to elemental carbon indicates that secondary organic aerosol may have contributed between 45% and 90% of the total BRAVO aerosol organic carbon. High ratios of saturated/unsaturated C18 acids, an abundance of nonanoic acid, and high concentrations of 6,10,14 trimethylpentadecan-2-one (an indicator of secondary aerosol production from vegetation emissions) all support the conclusion that secondary aerosol formation was important in the region. Total black carbon (BC) concentrations ranged from below detection limit (71 ng/m3) to 267 ng/m3, averaging 129 ng/m3. Fine (< 1 μm) aerosol BC concentrations averaged 114 ng/m3, and comprised 89% of the total BC. BC concentrations correlated reasonably well with aerosol sulfate concentrations, suggesting similar source regions for these species.
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    Size corrections based on refractive index for particle measuring systems active scattering aerosol spectrometer probe (ASASP-X)
    (Colorado State University. Libraries, 1996-01) Hand, Jennifer L., author; Kreidenweis, Sonia M., author; Cooperative Institute for Research in the Atmosphere (Fort Collins, Colo.), publisher
    The response function for the ASASP-X is affected by the optical properties of atmospheric aerosols. The manufacturer calibration is based on polystyrene latex spheres (m=l.588-0i), therefore the size distributions derived from measurements taken with the ASASP-X should be corrected for particles of different refractive index. Corrections based on the manufacturer calibration and Mie theory are used to derive size corrections for different refractive indices. These corrections are applied to data and demonstrate the significant over and underestimation of aerosol volume distributions possible if no corrections to diameter are applied.
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    ItemOpen Access
    The ionic composition of aerosol in Big Bend National Park
    (Colorado State University. Libraries, 2002-07) Lee, Taehyoung, author; Collett, Jeffrey L., Jr., author; Cooperative Institute for Research in the Atmosphere (Fort Collins, Colo.), publisher
    The chemical compositions of PM2.5 and size-resolved aerosol particles were measured from July to October, 1999 in Big Bend National Park, Texas, during the Big Bend Regional Aerosol and Visibility Observational (BRAVO) Study. Daily PM2.5 samples were collected using a URG cyclone/annular denuder/filter pack sampling system consisting of a PM2.5 cyclone inlet, two coated annular denuders in series (for nitric acid and ammonia), and a filter pack. Aerosol particles collected on a Teflon filter were analyzed for major ions and a backup nylon filter was used to capture nitric acid volatilized from the collected particles. A Micro Orifice Uniform Deposition Impactor (MOUDI) was used to collect 24 hr size-resolved aerosol particle samples in 9 size categories (D50 = 18, 10, 5.6, 3.2, 1.8, 1.0, 0.56, 0.32 and 0.18 μm). 41 MOUDI sample days were selected for analysis of the ionic chemical composition as a function of particle size. PM2.5 and size-resolved aerosol concentrations of C1-, SO42-, NO3-, Na+, NH4+, K+, Mg2+, and Ca2+ were obtained through ion chromatographic analysis of the filter and impactor samples. Aerosol acidity was measured on-site in the daily PM2.s filter samples. The composition of the BRAVO PM2.5 aerosol was dominated by sulfate and ammonium. Daily average sulfate and ammonium concentrations were strongly correlated (R2=0.97). The ratio of ammonium to sulfate averaged 1.54 with standard deviation of 0.30. This ratio is consistent with the direct pH measurements of aerosol acidity. The highest concentrations of sulfate were observed from August to October, reaching as high as 8.5 μg/m3. Back-trajectories suggested long-range transport from regions along the Texas/Mexico border and east Texas was associated with peak sulfate concentrations in the park. The particle composition as a function of size obtained from the MOUDI samples suggests that most of the particulate nitrate is associated with coarse mode particles in the range of 4 - 5 μm diameter. Aerosol nitrate concentrations were correlated with the sum of aerosol Na+ and Ca2+ concentrations (R2 = 0.70 and 0.60 for MOUDI and URG, respectively), demonstrating the importance of sea salt and soil dust particles in providing non-acidic surfaces for the condensation of nitric acid. The MOUDI samples indicate that nitrate and sulfate are separated into supermicron (mode diameter 4 - 5 μm) and submicron (mode diameter 0.4 - 0.5 μm) particles, respectively. The MOUDI samples show that a 1 μm size cut would have provided a better division between the fine mode and the coarse mode aerosol during the BRAVO study. Comparison of ISORROPIA and SCAPE2 thermodynamic model predictions of solid phase sulfate species shows reasonable agreement between the models, although ISORROPIA sometime predicts higher concentrations of some species. ISORROPIA often predicts the presence of solid phase Na2SO4, while SCAPE2 seldom does. The difference between solid phase sulfate concentrations predicted by the two models largely reflects differences in predicted aerosol water content. Both models reasonably predict the observed phase partitioning of N(-III) but poorly predict the observed phase partitioning of N(V). The underprediction of aerosol nitrate by these bulk aerosol models reflects the fact that the PM2.5 aerosol is externally mixed, containing acidic submicron sulfate particles and supermicron nitrate particles.