Ionic composition of aerosol in Big Bend National Park, The

July 2002.Includes bibliographical references.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.Funding agency: National Park Service #CA238099001

Numerical and experimental performance evaluation of two multi-stage cloud collectors

January 1999.Also issued as Derek J. Straub's thesis (M.S.) -- Colorado State University, 1999.Includes bibliographical references.An evaluation of the collection characteristics of two new multi-stage cascade inertial impactors designed for size-resolved cloud drop collection has been performed. The FROSTY supercooled cloud collector is intended for the collection of supercooled cloud drops in a winter environment in three independent size fractions with stage 50% cut diameters of 15 μm, 10 μm, and 4 μm . The CSU 5-Stage cloud collector is designed for sampling warm clouds in five distinct fractions on five stages that have desired 50% cut diameters of 30, 25, 15 , 10, and 4 μm. Two approaches were selected for the evaluation of the FROSTY and CSU 5-Stage cloud collectors. Numerical simulations provided a visualization of the air flow patterns and drop trajectories through the collectors while experimental laboratory calibrations provided a quantitative analysis of true collection performance. For each of these methods, 50% cut diameters, efficiency curves, and wall losses for each stage of the FROSTY and CSU 5-Stage collectors were derived. The experimental calibration work indicated that distinct fractions of cloudwater are collected in each stage of the FROSTY and CSU 5-Stage collectors. At laboratory conditions, the experimentally determined 50% cut diameters for the three stages of the FROSTY supercooled cloud collector were 19, 11.5, and 5 μm. Drop losses to the interstage wall surfaces in the FROSTY collector peaked at approximately 35% for 16 μm drops and were lower for larger and smaller drop sizes. For operation at design conditions of 3000 m elevation and -4° C, the 50% cut diameters are expected to decrease to 17, 10.5, and 4.5 μm. The experimentally determined 50% cut diameters, measured at laboratory conditions, for the CSU 5-Stage cloud collector were 25.5, 29, 17.5, 10.5, and 4.5 μm for stages 1 through 5, respectively. Wall losses tended to be higher than those for the FROSTY cloud collector across the drop size range under consideration. Losses peaked at nearly 45% for drops between 10 and 18 μm in diameter and decreased to about 20% at the largest and smallest drop sizes. 50% cut diameters are expected to remain essentially unchanged for CSU 5-Stage collector operation at sea level design conditions. Numerical modeling of the air flow patterns as well as drop trajectories through the FROSTY and CSU 5-Stage cloud collectors was performed with the commercially available Computational Fluid Dynamics (CFO) software package FLUENT, from Fluent, Inc. FLUENT offered two alternatives for the calculation of drop trajectories. Trajectory simulations based on the average continuous phase (air) velocity field as well as trajectory simulations which included the effects of statistically derived turbulent velocity fluctuations on drop motion were performed. Drop collection patterns based on these types of trajectory calculations were used to generate collection efficiency curves. Comparisons were made between the numerically predicted collection efficiency curves and efficiency curves established through experimental calibration. These comparisons indicated that the inclusion of turbulent fluctuation effects on drop motion provided better agreement with experimental observations than trajectories based only on average flow field velocities. However, the use of velocity fluctuations defined by default parameters also produced unrealistic losses to wall surfaces for small drop sizes. The parameters controlling turb lent velocity fluctuation effects on drop motion were examined in an effort to provide better agreement between the numerical and experimental results. Despite this shortcoming, numerically derived 50% cut diameters and overall collection efficiency curve shapes, for drop trajectories including turbulent velocity fluctuations, agreed reasonably well with experimental observations in most cases.Sponsored by the National Science Foundation ATM-9509596 , and the U.S. Environmental Research and Quality Assurance R82-3979-010

Design, construction and evaluation of the CSU optical fog detector

July 2001.Also issued as Scott E. Emert's thesis (M.S.) -- Colorado State University, 2001.Includes bibliographical references.The goal of this project was to develop an inexpensive cloud/fog detector that could be used to automate sampling equipment at remote (unmanned) cloud/fog research sites. A secondary objective was to test the ability of this sensor to measure/track trends in fog/cloud liquid water content (LWC). This characteristic is important because LWC is a significant indicator of a cloud's ability to process aerosols and gases and changes in LWC often correspond to changes in fog/cloud solute concentration. The following actions were taken to help realize these objectives. An evaluation of the use of commercially available optical components for fog detection has been performed. The research reinforced the need to have an inexpensive cloud/fog detector that could be used to automate sampling equipment at remote (unmanned) cloud/fog research sites. No such instrument is currently available commercially. Requirements for components of the CSU Optical Fog Detector (OFD) were defined. Important factors included transmitter wavelength and modulation characteristics, detector sensitivity, and component stability/durability over a range of environmental conditions. Readily available commercial components were utilized to ensure the sensor could be built economically. Laboratory tests in a glove box filled with artificially generated fog proved that optical components purchased from Banner Engineering were capable of monitoring changes in fog liquid water content (L WC) when operated in a light attenuation mode. After an initial calibration, the signal from the CSU OFD was found to correlate strongly with LWC measured by a Gerber Scientific Particulate Volume Monitor (PVM-100). Theoretical calculations of attenuation of 880 run light passing through a population of fog drops were completed. The results indicated extinction decreases as the drops are shifted to larger sizes (with a fixed LWC and lognormal distribution breadth). Accordingly, the response of the CSU OFD is expected to vary with mean fog/cloud drop size. Numerous fog detector design configurations were tested and the current attenuation design of the CSU optical fog detector was deemed successful in that it provides, at a minimum, an inexpensive switch capable of automating remote fog sensing equipment. It also provides useful information concerning fog LWC. Two calibrated OFD's were compared to PVM LWC measurements during initial field tests of orographic clouds at Storm Peak Laboratory (SPL) in Steamboat Springs, Colorado. The combined results from both OFD's overall time periods yield a regression equation of LWCofd = 0.99 * LWCpvm with a correlation coefficient of 0.92. Tests performed in the absence of fog on top of our laboratory in Fort Collins provided a measure of OFD baseline noise. Analysis of the observed noise yielded a minimum detection limit of 4.4 mg m·3 for the OFD and a comparable value (5.6 mg m·3) for the PVM. The OFD was incorporated in several automated fog sampling systems deployed in California's San Joaquin Valley as part of the California Regional Particulate Air Quality Study (CRP AQS). The OFD performed well as a fog detector and provided some insight into fog LWC. LWC measurements by a PVM and a co-located OFD showed good correlation (R2 = 0.91) and only modest bias (LWCofd = 1.16 LWCpvm) during an extended radiation fog episode.Sponsored by the National Science Foundation ATM-9980540, and the San Joaquin Valleywide Air Pollution Study Agency

Design and testing of a new aircraft-based cloud water sampling system

December 2002Also issued as Derek J. Straub's dissertation (Ph.D.) -- Colorado State University, 2002.Includes bibliographical references.Experimental studies of cloud processing mechanisms necessitate the collection of representative samples of cloud water for chemical analysis. In order to provide samples from clouds that are inaccessible from ground-based sampling stations, a new aircraft-based cloud water collection system has been developed . The objective of the design process was to produce an automated collector that can acquire well-characterized cloud water samples and is portable between multiple research aircraft. Issues such as cloud drop shatter and re-entrainment, structural integrity, system size and weight, material compatibility with the anticipated chemical analyses, and ease of use during field operation w re all considered during the design process. The new cloud water collection system utilizes an axial-flow cyclone to centrifugally separate cloud drops from the air stream. Up to seven individual samples can be stored over the course of a single research flight. An analysis of the axial-flow cyclone was performed with a finite volume based computational fluid dynamics (CFD) code. Solutions were obtained for air flow patterns and cloud drop trajectories. The predicted continuous phase (air) velocity field indicates that the axial-flow cyclone generates a strong rotational ow field with a tangential velocity of 85 ms-'. Based on simulations of cloud drop trajectories, centrifugal force in the rotational flow field is sufficient to quickly move entrained cloud drops to the wall of the axial-flow cyclone duct where they can be removed for storage. Collection efficiency as a function of drop size was ascertained and the 50% cut diameter was determined to be approximately 8 microns. An experimental laboratory calibration involving monodisperse fluorescein-tagged drops verified the numerical modeling results. The system was deployed during the Dynamics an Chemistry of Marine Stratocumulus, Phase II (DYCOM -II) field project in July 2001. The DYCOMS-II campaign served as a testing and evaluation program for the system as well as an opportunity to study the chemical composition of stratocumulus clouds in the remote marine environment. Over the course of the project, 50 samples were obtained during seven nighttime and two daytime flights. Sample pH was measured on-site after each flight. Peroxide, formaldehyde, S(IV), trace metals and major ions (Cr, NO3-, so/-, Na+, NH/, K+, ca2+, and Mg2+) were preserved on site and analyzed after the field campaign. The analyses were used to characterize the composition of the sampled clouds and to investigate cloud processing mechanisms, including the potential for rapid aqueous phase oxidation of S(IV) to sulfate.Sponsored by the National Science Foundation ATM-0084696, and the National Center for Atmospheric Research Advanced Study Program

Wood smoke contribution to ambient aerosol in Fresno during winter 2003-2004

Includes bibliographical references.Sponsored by San Joaquin Valley-Wide Air Pollution Study Agency NSF ATM-0222607

Spatial and temporal variations in precipitation and cloud interception in the Sierra Nevada of central California

Spatial and temporal variations in patterns of precipitation and cloud interception were studied for a period of 14 months in the Sierra Nevada of central California. 14 fully automated sampling stations, located at elevations from 800 to 2400 m, were utilized in the study. Both precipitation and cloud interception were observed to increase with elevation. Cloudwater deposition increased at higher elevations due both to a greater frequency of cloud interception and higher wind speeds. Cloudwater deposition, caused primarily by the interception of clouds associated with cold fronts approaching from the north or north-west, is most important at elevations above 1500 m; however, the interception of highly polluted winter “Tule” fogs, lifting above the floor of the San Joaquin Valley, appears to be an important mechanism for cloudwater deposition at lower elevation sites. Observed and estimated hydrological and chemical inputs to the passive cloudwater collectors used in the study were substantial, suggesting that cloud interception may contribute significantly to the same inputs for exposed conifers in the region

Characterization of carbonaceous aerosol during the Big Bend Regional Aerosol and Visibility Observational study

December 2001.Includes bibliographical references.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.Funding agency: National Park Service #CA2350-97-001 T098-07, #CA2380-99-001 T001-52

Single-object Imaging and Spectroscopy to Enhance Dark Energy Science from LSST

Single-object imaging and spectroscopy on telescopes with apertures ranging from ~4 m to 40 m have the potential to greatly enhance the cosmological constraints that can be obtained from LSST. Two major cosmological probes will benefit greatly from LSST follow-up: accurate spectrophotometry for nearby and distant Type Ia supernovae will expand the cosmological distance lever arm by unlocking the constraining power of high-z supernovae; and cosmology with time delays of strongly-lensed supernovae and quasars will require additional high-cadence imaging to supplement LSST, adaptive optics imaging or spectroscopy for accurate lens and source positions, and IFU or slit spectroscopy to measure detailed properties of lens systems. We highlight the scientific impact of these two science drivers, and discuss how additional resources will benefit them. For both science cases, LSST will deliver a large sample of objects over both the wide and deep fields in the LSST survey, but additional data to characterize both individual systems and overall systematics will be key to ensuring robust cosmological inference to high redshifts. Community access to large amounts of natural-seeing imaging on ~2-4 m telescopes, adaptive optics imaging and spectroscopy on 8-40 m telescopes, and high-throughput single-target spectroscopy on 4-40 m telescopes will be necessary for LSST time domain cosmology to reach its full potential. In two companion white papers we present the additional gains for LSST cosmology that will come from deep and from wide-field multi-object spectroscopy.Comment: Submitted to the call for Astro2020 science white paper

Assessing contributions of agricultural and nonagricultural emissions to atmospheric ammonia in a Chinese megacity

Ammonia (NH3) is the predominant alkaline gas in the atmosphere contributing to formation of fine particles—a leading environmental cause of increased morbidity and mortality worldwide. Prior findings suggest that NH3 in the urban atmosphere derives from a complex mixture of agricultural (mainly livestock production and fertilizer application) and nonagricultural (e.g., urban waste, fossil fuel-related emissions) sources; however, a citywide holistic assessment is hitherto lacking. Here we show that NH3 from nonagricultural sources rivals agricultural NH3 source contributions in the Shanghai urban atmosphere. We base our conclusion on four independent approaches: (i) a full-year operation of a passive NH3 monitoring network at 14 locations covering urban, suburban, and rural landscapes; (ii) model-measurement comparison of hourly NH3 concentrations at a pair of urban and rural supersites; (iii) source-specific NH3 measurements from emission sources; and (iv) localized isotopic signatures of NH3 sources integrated in a Bayesian isotope mixing model to make isotope-based source apportionment estimates of ambient NH3. Results indicate that nonagricultural sources and agricultural sources are both important contributors to NH3 in the urban atmosphere. These findings highlight opportunities to limit NH3 emissions from nonagricultural sources to help curb PM2.5 pollution in urban China

Synchrotron X-Ray Study of the Thickness Dependence of the Phase Diagram of Thin Liquid-Crystal Films

The phase diagram of freely suspended thin films of heptyloxybenzylidene-heptylaniline shows dramatic changes for thicknesses below 22 layers. The most surprising feature of the phase diagram is the inclusion of two phases lacking long-range crystalline order (smectic-F and hexatic-B phases) between two crystalline phases (crystalline smectic B and smectic G). Neither the smectic F nor the hexatic B occurs in bulk samples. Between sixteen and ten layers the width, in temperature, of the hexatic-B phase increases.Engineering and Applied Science