Size corrections based on refractive index for particle measuring systems active scattering aerosol spectrometer probe (ASASP-X)

January 1996.Includes bibliographical references.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.Funding agency: National Park Service #1443-CA0001-92-006 96.5

Large particle characteristics over the southern ocean during ACE 1

December 1998.Also issued as Janel T. Davis's thesis (M.S.) -- Colorado State University, 1998.Includes bibliographical references.The Aerosol Characterization Experiment (ACE-1) in November and December 1995 was designed to characterize aerosol physical, chemical, and optical properties in remote marine regions in the Southern Hemisphere. Data from six ACE-1 research flights were used to examine concentrations of large particles in two size ranges: those having diameters, Dp, 0.5 Dp 50 µm (N1) and those with 2.0 Dp 50 µm (N2). Reported here are observations of vertical profiles of N1 and N2 for heights, z, from ~30 to 7000 mover the ocean surface. Number concentrations near the surface (z 900 m) varied from 0.8 to ~30 cm-3, while maximum N2 concentrations were ~ 2.0 cm-3. Above altitudes of 2400 m, N1 concentrations were found to vary from greater than 0.07 to 1.2 cm-3. Significant concentrations (> 0.02 cm-3) of N2 particles aloft were usually associated with regions of deep convection, cloud outflow, and cloud dissipation. Calculated dry mass concentrations for N1 particles near the surface (z 100 m) assumed to be primarily sea salt, showed dependence on wind speed. Computed dry sea salt mass concentrations varied from 2.0 to 30.0 µg m-3 and varied with wind speed similarly to previously proposed relationships. Aerosol size distributions were used to compute particle light scattering coefficients and aerosol visible optical depths. The light scattering coefficient for N1 particles ranged from 0.002 to 0.08 1an-1 at altitudes less than 900 m, and from 0.00005 to 0.05 km·1 at higher altitudes. For N2 particles, the light scattering coefficient ranged from 0.001 to 0.05 km-1 for z 900 m. The large particles are a significant contribution to the total aerosol light scattering coefficient. Optical depths for these particles ranged from 0.043 to 0.085 for N1 and from 0.019 to 0.039 for N2.Sponsored by the National Science Foundation (NSF), Significant Opportunities in Atmospheric Research and Science (SOARS)

Aerosol hygroscopicity and visibility estimates in the Great Smoky Mountains National Park

National Park Service.Includes bibliographical references (pages 79-82).Summertime visibility in the National Parks in the Eastern United States is often very poor, due to high particulate mass concentrations and high relative humidities. As a part of the Southeastern Aerosol and Visibility Study (SEAVS) in the Great Smoky Mountains National Park during the summer of 1995, aerosol size distributions (Dp = 0.1-3 µm) were measured with an Active Scattering Aerosol Spectrometer (ASASP-X). A relative humidity (RH) controlled inlet allowed for both dry and humidified measurements. The objective of this experiment was to examine the aerosol size distribution and its variation with RH to characterize its effect on visibility in the region. The ASASP-X was calibrated with polystyrene latex spheres (PSL) (m = 1.588), however, the instrument response was sensitive to the refractive index of the measured particles, which was typically much lower than that of PSL. An inversion technique accounting for varying particle real refractive index was developed to invert ASASP-X data to particle size. Dry (RH < 15%) particle refractive indices were calculated using the partial molar refractive index method and 12-hour fine aerosol (<2.5 µm) chemical compositions from the National Park Service Interagency Monitoring of Protected Visual Environments (IMPROVE) filter samples. A study average dry refractive index of m = 1.49 ± 0.02 was determined. The dry aerosol number distributions inverted using the scaling method were fit with single mode lognormal curves, resulting in dry accumulation mode size parameters. A study average total volume concentration of 7 ± 5 µm3 cm-3 was determined, with a maximum value of 26 µm3 cm-3. The large variability was due to extremes in meteorological situations occurring during the study. The study average volume median diameter was 0.18 ± 0.03 µm, with an average geometric standard deviation of 1.45 ± 0.06. A newly-developed iteration method was used to determine wet refractive indices, wet accumulation mode volume concentrations and water mass concentrations as a function of relative humidity. Theoretical predictions of water mass concentrations were determined using a chemical equilibrium model assuming only ammonium and sulfate were hygroscopic. Comparisons of predicted and experimental water mass showed agreement within experimental uncertainties. To examine the effects of particles on visibility, particle light scattering coefficients, bsp, were calculated with derived size parameters, refractive index and Mie theory. Dry scattering agreed well with nephelometer measurements made at SEAVS, with an average bsp of 0.0406-km-1. Estimates of particle light scattering growth (b/b0) were determined from ratios of wet and dry light scattering coefficients, and also agreed with nephelometer results. The new inversion techniques were compared to earlier, simpler methods which ignored variations in aerosol chemical composition. The simpler method yielded smaller mean diameters, however, hygroscopicity estimates were comparable to those derived using daily varying chemical composition. This suggests that although the aerosol chemical composition is needed to determine aerosol size parameters, it may not be critical for deriving hygroscopicity (or other ratios of size parameters). This result may be specific to this study, as the variation in refractive index with RH assumed by previous models appears to be a good estimate for that observed during SEAVS.Funding agency: National Park Service # 1443-CA0001-920006, AMD#5/SUB#3

Further development and testing of a bimodal aerosol dynamics model

April 1994.Also issued as Debra A. Youngblood's thesis (M.S.)- Colorado State University, 1994.Includes bibliographical references.A previously reported bimodal monodisperse aerosol model is further developed and tested. The starting point is the BImodal MOnoDisperse Aerosol Model (BIMODAM I) which was developed to model the formation of ammonium sulfate ((NH4) 2SO4) particles from sulfuric acid (H2SO4) vapor. The model follows the evolution of two monodisperse modes where each mode, i, is characterized by a unique mean diameter and the number of particles with that mean diameter. The aerosol distribution is assumed to undergo typical atmospheric processes such as condensational growth, coagulation, nucleation, and deposition. In BIMODAM I, the effect of each process on the aerosol distribution is represented as a rate equation. The prognostic equations are coupled, so a variable time step differential equation solver is utilized to simultaneously solve the system of equations to predict the mass and number concentration in each mode. The diameter of each mode is diagnosed from the mass and number concentrations. In the first part of this work, two new parameterizations were developed for BIMODAM I. First, a condensation rate factor was developed to account for the lack of polydispersity in the model. Second, a criterion was developed which dictates when the two modes may be merged without generating large errors. In the second part of this work, a new version of the model (BIMODAM II) was developed to give the same accurate results as BIMODAM I without using the variable time step differential equation solver. A key development in BIMODAM II is a parameterization for the process of homogeneous nucleation. This parameterization is based on the approximation of the time-dependent nucleation rate with a triangular function; using this approach, only two parameters are needed to predict the total number of particles resulting from a nucleation event The two parameters are correlated to chemical source rate and relative humidity. Therefore, prediction of the number concentration of particles resulting from a nucleation burst depends on knowing the relative humidity and determining the chemical source rate. This development has been shown to perform well in the presence and absence of preexisting particles and over short and long time scale simulations. Further developments in BIMODAM II include simple analytical solutions of the differential equations for coagulation and deposition. Using a mass balance equation, a simple solution was also derived to predict the amount of sulfuric acid in the vapor phase at any time during the simulation. From this calculation, the amount of mass in the aerosol phase is calculated by subtracting the amount in the vapor phase from the total amount of sulfuric acid produced during any given time step. By using the simplifications and parameterizations mentioned above, computational time is saved by eliminating the variable time stepping differential equation solver. This model is shown to perform well when compared against a simulation which uses a more detailed description of the aerosol size distribution.Sponsored by the Western Regional Center of the National Institute for Global Change W/GEC91-114, and Colorado State University Graduate School

Optical measurements of aerosol size distributions in Great Smoky Mountains National Park: particle hygroscopicity and its impact on visibility

National Park Service.Includes bibliographical references (pages 70-73).Aerosol size distributions were measured during the 1995 Southeastern Aerosol and Visibility Study (SEAVS) in Great Smoky Mountains National Park using a PMS ASASP-X optical aerosol spectrometer. Ambient aerosol was conditioned in a relative humidity (RH) controlled inlet before sampling. 130 dry (RH ~ 15%) and 112 humidified aerosol size distributions, plus 24 distributions at ambient RH, were recorded during daylight hours for aerosol in the size range 0.1 < Dp <2.5 µ. Particle light scattering from the ASASP-X was inverted to particle sizes using Mie theory and applying a refractive index of either 1.530-0i or 1.501-0i for dry conditions, depending on the ambient aerosol chemical composition. A dry aerosol volume concentration time line from this work, when compared with a similar time line of aerosol mass concentration from IMPROVE samplers, indicates the ASASP-X provided a reliable representation of temporal trends in the ambient aerosol loading. The median dry aerosol geometric mass mean diameter measured during SEAVS was 0.28 µm, with a range from 0.24 to 0.38 µm, and median geometric standard deviation of 1.64. Sequential dry and humidified aerosol size distributions were corrected for refractive index dependence on RH and used to derive ambient aerosol hygroscopicity as a function of RH. This work demonstrates that experimentally derived water absorption is equivalent to or less than predicted by theory, assuming ambient aerosol water uptake is dictated by ionic compounds that have a chemical composition consistent with the particle fine mass measured during SEAVS. In this work, special consideration is given to the uncertainty in derived aerosol water contents and the degree to which this uncertainty propagates to estimates of light scattering. An ultimate goal of this project is to augment visibility and radiative transfer models through a better understanding of how RH affects the ambient aerosol size distribution in the southeastern U.S.Funding agency: National Park Service # 1443-CA0001-92-0006 96.5

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

Modeling of global sulfate aerosol number concentrations

A two-moment, two-mode model of sulfate aerosol dynamics has been added to the University of Michigan three-dimensional chemical transport, transformation and deposition model, GRANTOUR. The two-moment model predicts both aerosol number and mass concentrations, and was chosen based on its computational efficiency and the small number of prognostic variables, which reduce storage requirements in the large-scale model. Simulations were performed to investigate the processes that control predicted number concentrations, and comparisons with observations were used to suggest additional features that should be added to the coupled gas-phase chemistry/aerosol model to bring the predictions more in line with observations. © 2000 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87551/2/677_1.pd

Critical Reflectance Derived from MODIS: Application for the Retrieval of Aerosol Absorption over Desert Regions

Aerosols are tiny suspended particles in the atmosphere that scatter and absorb sunlight. Smoke particles are aerosols, as are sea salt, particulate pollution and airborne dust. When you look down at the earth from space sometimes you can see vast palls of whitish smoke or brownish dust being transported by winds. The reason that you can see these aerosols is because they are reflecting incoming sunlight back to the view in space. The reason for the difference in color between the different types of aerosol is that the particles arc also absorbing sunlight at different wavelengths. Dust appears brownish or reddish because it absorbs light in the blue wavelengths and scatters more reddish light to space, Knowing how much light is scattered versus how much is absorbed, and knowin~ that as a function of wavelength is essential to being able to quantify the role aerosols play in the energy balance of the earth and in climate change. It is not easy measuring the absorption properties of aerosols when they are suspended in the atmosphere. People have been doing this one substance at a time in the laboratory, but substances mix when they are in the atmosphere and the net absorption effect of all the particles in a column of air is a goal of remote sensing that has not yet been completely successful. In this paper we use a technique based on observing the point at which aerosols change from brightening the surface beneath to darkening it. If aerosols brighten a surface. they must scatter more light to space. If they darken the surface. they must be absorbing more. That cross over point is called the critical reflectance and in this paper we show that critical reflectance is a monotonic function of the intrinsic absorption properties of the particles. This parameter we call the single scattering albedo. We apply the technique to MODIS imagery over the Sahara and Sahel regions to retrieve the single scattering albedo in seven wavelengths, compare these retrievals to ground-based retrievals from AERONET instruments and compute error bars on each retrieval. The results show that we can retrieve single scattering albedo for pure dust to within +/-0.02 and mixtures of dust and smoke to within +/-0.03. No other space based instrument has achieved a retrieval of single scattering albedo that spans the spectrum from 0.47 microns to 2.13 microns and produces regional maps of aerosol absorption showing gradients and changes. Applied in a more operational fashion, such information will narrow uncertainties in estimating aerosol forcing on climate