A31F-0178
Aerosol Optical Properties Observed during CHAPS
During the CHAPS, the DOE Gulfstream-1 aircraft was used to make in-situ measurements of aerosol optical properties. The flight pattern was designed to allow for measurements below cloud, within the cloud layer, and above the clouds in the vicinity of Oklahoma City. Two different inlets were used on the G-1: an isokinetic inlet for sampling dry aerosols smaller than approximately 2 μm in diameter, and a Counterflow Virtual Impactor (CVI) that excluded unactivated aerosols, but which allows cloud droplets to enter. A suite of paired instruments, including a nephelometer, Particle Soot Absorption Photometer (PSAP), and Aerosol Mass Spectrometer (AMS), was used to measure the aerosol optical properties from both sampling streams. Below the clouds, the single-scattering albedo measured inside the Oklahoma City plume was generally smaller than that observed outside of the plume. Within the cloud layer, but far from the clouds, there is little difference in the aerosol scattering measured inside and outside of the plume. These observations indicate that the vertical transport by the shallow clouds is very localized. Both aerosol extensive and intensive properties are discussed. For example, the total aerosol scattering and the mass-scattering efficiency measured inside the clouds was slightly larger for clouds that have roots within the Oklahoma City plume. Using data from the AMS in conjunction with the CVI inlet reveals that these in-cloud aerosols also have a relatively large amount of nitrate. Possible explanations for this increase nitrate will be discussed.
A31F-0179
Aerosol and Trace Gas Processing by Clouds During the Cumulus Humilis Aerosol Processing Study (CHAPS)
Clouds play an active role in the processing and cycling of atmospheric constituents. Gases and particles can partition to cloud droplets by absorption and condensation as well as activation and pact scavenging. The Cumulus Humilis Aerosol Processing Study (CHAPS) aimed at characterizing freshly emitted aerosols above, within and below fields of cumulus humilis (or fair-weather cumulus) in the vicinity of Oklahoma City. The experiment took place in June 2007. Evolution of aerosol and cloud properties downwind of the Oklahoma City is of particular interest in this project. These observations of a mid-size and mid-latitude city can be used in the development and evaluation of regional-scale and global climate model cumulus parameterizations that describes the transport and transformations of these aerosols by fair-weather cumulus. The Department of Energy (DOE) G-1 aircraft was one of the main platforms used in CHAPS. It carried a suite of instruments to measure properties of interstitial aerosols behind an isokinetic inlet and a set of duplicate instruments to determine properties of activated particles behind a counter-flow virtual impactor (CVI). The sampling line to the Aerodyne Aerosol Mass Spectrometer was switched between the isokinetic inlet and the CVI to allow characterization of interstitial particles out of clouds in contrast to particles activated in clouds. Trace gases including ozone, carbon monoxide, sulfur dioxide, and a series of volatile organic compounds (VOCs) were also measured as were key meteorological state parameters including liquid water content, cloud drop size, and dew point temperature were measured. This presentation will focus on results related to the transformation and transport of aerosols and trace gases observed in fair-weather cumulus and compare these results with concurrent observations made outside these clouds. Our interest will focus on the differences in particle size and composition under varying conditions. The role of sulfur (IV) oxidation as a potential path for sulfate formation in cloud will be considered, since it is known to be an important mechanism for new particle formation. Cloud water deposition as an alternative avenue for sulfate formation will also be evaluated.
A31F-0180
Digital Inversion and Initial Analysis of Nephelometer Data from the CHAPS Campaign
The Cumulus Humilis Aerosol Processing Study (CHAPS) was carried out downwind of Oklahoma City during June 2007 with the objective of investigating the evolution of freshly nucleated particles from an urban plume as a consequence of initial activation in clouds. As part of this campaign, the G-1 research aircraft operated by Pacific Northwest National Laboratory (PNNL) sampled particle properties below, within, and above the layer of fair-weather cumulus that frequently capped the daytime atmospheric boundary layer. These measurements were executed just downwind of Oklahoma City both within and outside urban plume. As part of its instrument complement, the G-1 carried a pair of three-wavelength nephelometers to measure optical scattering by the atmospheric aerosol. One nephelometer sampled from an isokinetic inlet to measure non- activated particles. The other nephelometer sampled activated particles from air that passed through a counterflow virtual impactor (CVI). We have determined that the effective time constants for the nephelometer signals are 2 s and 8 s for the isokinetic and CVI air streams, respectively. These time constants pose a challenge for comparing scattering with other cloud properties, such as liquid water, reported by instruments with much faster response, since scattering peaks from the nephelometer often occur after the trailing cloud edge in sampled time series. For isolated clouds, this is not too difficult to manage. However, in a field of cumulus, the nephelometer signal may not return to background values before the next cloud is encountered. This greatly complicates the interpretation of the data. In this presentation, we describe our determination of the time constants for the nephelometers on the G-1, and we use these time constants to invert the sampled time series. This reduces the effective time constants of the instruments at the cost of amplifying noise in the signals. We then use the inverted time series to infer optical properties of the aerosol in and out of the Oklahoma City urban plume. The data from the CVI show greater scattering outside versus inside the plume at the blue wavelength. This effect is less for the green wavelength and negligible at the red wavelength. The Angstrom exponent is smaller for cloud air that originated outside the plume, suggesting different particle size distributions for the two particle populations. Single-scattering albedo (SSA) measurements (adding data from a particle soot absorption photometer) also indicate that activated particles originating outside the urban plume are darker than those, presumably more recently formed, originating inside the plume.
A31F-0181
Evidence for New Particle Formation in Isoprene Plumes Near Oklahoma City During the 2007 Cumulus Humilis Aerosol Processing Study (CHAPS)
IPCC has recognized that a major uncertainty in climate prediction is associated with aerosols. Current climate models are highly dependent on information about the aerosol component of the atmosphere, and in particular, about the size, optical properties and composition of atmospheric aerosols, and how these properties evolve under conditions with various trace-gas precursors. One area of particular uncertainty involves modeling the formation of secondary organic aerosol (SOA) formed from the oxidation of VOC species. The ability to distinguish between SOA formed from biogenic and anthropogenic sources is also crucial in assessing the impact of manmade contributions to the global aerosol budget and its impact on climate change. Although a number of laboratory simulations have demonstrated the capacity of biogenic VOC species such as isoprene to form SOA by direct oxidation, the low natural levels of isoprene and other biogenic aerosols make observation of particle formation from these species difficult. During The Department of Energy's Cumulus Humilis Aerosol Processing Study (CHAPS) , conducted in June of 2007 near Oklahoma City, a Proton transfer Reaction Mass Spectrometer (PTR-MS) flew on the Department of Energy (DOE) Gulfstream 1 (G-1) research aircraft and detected plumes of isoprene and its oxidation products some distance from from the urban plume of Oklahoma City, which in turn was characterized by emissions of CO, benzene, toluene and other anthropogenic emissions. Concentrations within these isoprene plumes were on the order of several parts per billion, much higher than might be expected for biogenic emissions in the Oklahoma City area. Preliminary results were presented at the 2007 AGU Fall meeting indicating that these plumes of isoprene were also correlated with new particle formation as determined by the Passive Cavity Aerosol Spectrometer Probe (PCASP) also aboard the G-1. Here, we report the results of continued investigation into the new particle formation correlated with the isoprene plume. We will present results that include correlations with other observations made on board the G-1, including a newly developed Fast Integrated Mobility Spectrometer (FIMS). Back-trajectories and correlations with other aerosol instrumentation including the Aerodyne aerosol mass spectrometer (AMS), nephelometer, cloud condensation nuclei (CCN) counter and optical instrumentation will also be presented, to identify the source of the isoprene plume and assess its consequence relative to aerosol formation.
A31F-0182
Airmass source analysis during CLASIC using simultaneous CO and CO2 measurements on the CIRPAS Twin Otter
The CLASIC campaign deployed several aircraft to the ARM SGP site in June of 2007. The NASA Ames Research Center Argus instrument made high time resolution measurements of CO and a non-dispersive infrared gas analyzer made CO2 measurements on the CIRPAS Twin Otter. We present case studies for several CLASIC flights of the CIRPAS Twin Otter to discern the relationship between CO and CO2 in order to understand the land surface air exchange and transport. Data was acquired on the CIRPAS Twin Otter over the SGP site and on flights over and around Oklahoma City sampling urban, industrial and agricultural airmasses. CO, with a lifetime of months in the troposphere, serves as an excellent tracer of the sampled air mass over the duration of the flight. Information on air mass history is developed by examining the CO/CO2 ratio along the flight track of each case study. In addition to the use of CO as a tracer, we have carried out back trajectories to help in air mass identification. Special attention is paid to the flight location relative to cloud and to the top of the boundary layer.
A31F-0183
Closure Study of Modeled and Measured Cloud Condensation Nuclei During CLASIC 2007
A Differential Mobility Analyzer/Tandem Differential Mobility Analyzer (DMA/TDMA) was operated aboard the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter during The Cloud and Land Surface Interaction Campaign (CLASIC) in June of 2007 to characterize the size-resolved concentration and hygroscopicity of the submicron aerosol. A Droplet Measurement Technologies (DMT) Cloud Condensation Nuclei Counter (CCNc) was also operated on the aircraft to measure the concentration of CCN active at supersaturations of 0.165, 0.305, and 0.60 %. The campaign included 14 research flights designed to study how land surface processes affect atmospheric aerosol loading and chemistry and the resulting effects on the microphysical and macrophysical properties of cumulus cloud fields. Over 1300 size distributions were measured along with 110 hygroscopic growth factor distributions for each dry diameter of 0.025, 0.050, 0.100, 0.200, and 0.400 micrometer. The CCN number concentration at each specified supersaturation was predicted from the combined size-resolved concentration and hygroscopicity measurements. Here we present a comparison of the predicted concentrations with those directly measured with the CCNc.
A31F-0184
Objective Analysis of CLASIC data
We implemented a constrained variational analysis approach to process data collected from the Cloud and LAnd Surface Interaction Campaign (CLASIC), which was conducted at Southern Great Plains (SGP) in June 2007 by the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) Program and other agencies and institutions, in order to produce the large-scale forcing data for single-column and cloud- resolving models. One unique feature of the analysis method is to use measurements at the surface and the top of the atmosphere (TOA) as constraints to adjust atmospheric state variables from soundings by the smallest possible amount to conserve column-integrated mass, moisture, static energy, and momentum so that the final analysis data are dynamically and thermodynamically consistent. The required observational constraints include surface and TOA radiative fluxes, surface latent and sensible latent fluxes, and surface precipitation. We will discuss several important technical and scientific issues related to the data analysis and present preliminary results at the meeting.
A31F-0185
Measurements of the Atmospheric Boundary Layer with the Duke University Helicopter Observation Platform (HOP) and the University of Iowa Lidar
The primary goal of CLASIC was to collect a dataset that can be used to improve parameterizations of cumulus convection and associated parameterizations of land surface processes. An important part of the experiment was to decipher the respective roles of local and regional forcing on the observed cloud structure. Thus, the spatial and temporal observation of turbulent fluxes in the atmospheric boundary layer (ABL) was essential for the overall success of this experiment. The necessary observations to compute these fluxes were carried out on-board the Duke University Helicopter Observation Platform (HOP), which collected about 100 hours of data during June and August of 2007. A description of the HOP and its sensors is provided at http://hop.pratt.duke.edu. A triangular flight pattern (10-km on the side) at different altitudes in the ABL was performed at four sites where clusters of ground observations operated (i.e., Little Washita, Fort Cobb, ARM/CART "Central Facility", and Okmulgee – a forested site located in the eastern part of Oklahoma). A mobile lidar mounted on a truck (developed and operated by the University of Iowa) was also operating simultaneously with the HOP. In addition, the HOP participated in two coordinated flights aimed at providing a special coverage of the land and the atmosphere during the overpasses of the A-Train satellites. In these cases, the HOP flew near the ground surface to produce surface fluxes following exactly the track of the satellites. Flights started about 15 minutes before the arrival of the satellites and ended about 15 minutes after their departure. Finally, the HOP provided near-ground surface fluxes in two CO2 Lagrangian experiments that were performed jointly with the Cessna 206 in August 2007, near Ponca City. Some of the data collected during different flights and with the mobile lidar are used to illustrate the development of the ABL under different meteorological conditions and at the various sites that are characterized by different land-cover heterogeneity and topography.