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Cumulus Humulis Aerosol Process Study OVERVIEW OF THE CUMULUS HUMILIS AEROSOL PROCESSING STUDY BY LARRY K. BERG, CARL M. BERKOWITZ, JOHN A. OGREN, CHRIS A. HOSTETLER, RICHARD A. FERRARE, MANVENDRA K. DUBEY, ELISABETH ANDREWS, RICHARD L. COULTER, JOHNATHAN W. HAIR, JOHN M. HUBBE, YIN-NAN LEE, CLAUDIO MAZZOLENI* JASON OLFERT\ AND STEPHEN R. SPRINGSTON During the summer of 2007, CHAPS investigated changes in the chemical and optical properties of aerosols due to their interaction with shallow cumuli. erosols influence climate directly by scattering framework, the Cumulus Humilis Aerosol Processing and absorbing radiation and indirectly through Study (CHAPS) is a stage 1 activity, that is, a detailed Atheir influence on cloud microphysical and process study. The specific focus of CHAPS was to dynamical properties. The Intergovernmental Panel provide concurrent observations of the chemical com- on Climate Change (IPCC) concluded that the global position of the activated [particles that are currently radiative forcing due to aerosols is large and in gen- serving as cloud condensation nuclei (CCN)] and eral cools the planet (Forster et al. 2007). But the nonactivated aerosols, the scattering and extinction uncertainties in these estimates are also large due to profiles, and detailed aerosol and droplet size spectra our poor understanding of many of the important in the vicinity of Oklahoma City, Oklahoma, during processes related to aerosols and clouds. To address June 2007. this uncertainty, Ghan and Schwartz (2007) proposed Numerous campaigns have examined aerosol an integrated strategy for addressing issues related to properties downwind from large pollution sources, aerosols and aerosol processes. Using this conceptual including the Megacity Initiative: Local and Global AFFILIATIONS: BERG, BERKOWITZ, AND HUBBE—Pacific Northwest "CURRENT AFFILIATION: The University of Alberta, National Laboratory, Richland, Washington; OGREN—NOAA Edmonton, Alberta, Canada Earth System Research Laboratory, Boulder, Colorado; HOSTETLER, CORRESPONDING AUTHOR: Dr. Larry K. Berg, P.O. Box FERRARE, AND HAIR—NASA Langley Research Center, Hampton, 999, MSIN K9-30, Richland, WA 99352 Virginia; DUBEY AND MAZZOLENI—LOS Alamos National Laboratory, E-mail: [email protected] Los Alamos, New Mexico; ANDREWS—NOAA Earth System Research The abstract for this article can be found in this issue, following the Laboratory, and Cooperative Institute for Research in Environmental table of contents. Sciences, University of Colorado, Boulder, Colorado; COULTER— DOIilO.I I75/2009BAMS2760.I Argonne National Laboratory, Argonne, Illinois; LEE, OLFERT, AND SPRINGSTON—Brookhaven National Laboratory, Upton, New York. In final form 30 June 2009 * CURRENT AFFILIATION: Michigan Technological University, ©2009 American Meteorological Society Houghton, Michigan AMERICAN METEOROLOGICAL SOCIETY NOVEMBER 2009 BAfft | 1653 Brought to you by NASA LANGLEY RESEARCH CENTER | Unauthenticated | Downloaded 06/16/21 08:01 PM UTC Research Observations (MILAGRO) campaign online at www.asp.bnl.gov) is to improve the under- (Molina et al. 2008) and the two of the three Aerosol standing of aerosol radiative effects on climate. This Characterization Experiments, ACE-2 and ACE-Asia, objective encompasses not only clear sky observations which are described by Raes et al. (2000) and Huebert but also studies relating the effects of both aerosols et al. (2003), respectively. Other studies conducted on clouds and clouds on aerosols—in particular, how near cities have examined changes in both aerosols clouds affect the chemical and optical properties and clouds downwind of urban areas. For example, of aerosols. The latter was the science driver in the Alkezweeny et al. (1993) found that wintertime design of CHAPS. stratiform clouds associated with the urban plumes The measurement strategy for CHAPS was in- of Denver, Colorado, and Kansas City, Missouri, have tended to provide measurements relevant to four a larger number concentration and smaller median questions associated with the aerosol radiative forcing volume diameter of droplets than clouds that had not issues of interest to the ASP: been affected by the urban plume. Likewise, Jirak and Cotton (2006) found a decrease in precipitation in 1) How do the below-cloud and above-cloud aerosol polluted regions along the Front Range of the Rocky optical and cloud nucleating properties downwind Mountains. In a modeling study, Van Den Heever and of a typical North American city differ from the Cotton (2007) found that precipitation downwind of optical and nucleating properties of aerosols in urban areas may be influenced by changes in aero- air unperturbed by urban emissions? Our interest sols as well as the convergence pattern caused by the is in the differences in the radiative properties, city. Recently, the New England Air Quality Study chemical composition, hygroscopic properties, (NEAQS), and the 2004 International Consortium for and size distributions below and above cloud, and Atmospheric Research on Transport and Transforma- upwind and downwind of such a city. tion (ICARTT; Fehsenfeld et al. 2006), which were 2) How does the distribution of aerosol extinction conducted during the summer of2004, examined the vary in relation to the proximity to individual transport of pollutants and aerosols eastward from clouds and fields of clouds and why? New England over the Atlantic Ocean. The Texas Air 3) What are the differences, in terms of both size Quality Study/Gulf of Mexico Atmospheric Compo- distributions and chemical composition, between sition and Climate Study (TexAQS/GoMACCS) also activated aerosols within the urban plume and looked at relationships between clouds and aerosols those outside the urban plume? This question in polluted conditions around Houston, Texas (e.g., is also intended to encompass issues related to Sorooshian et al. 2007; Lu et al. 2008). In contrast to differences between particles that have not been these recent studies near large or very dirty cities, activated inside and outside of the urban plume. CHAPS was conducted near a moderately sized 4) To what extent can models with state-of-the-art city that is representative of a large number of cities cloud parameterizations capture the statistical around the United States. features of the below-above-cloud aerosols? CHAPS was also one of the first times that a Aerodyne aerosol mass spectrometer (AMS; Jayne The material in this article presents results from et al. 2000) was used in conjunction with a counter- CHAPS illustrating the observations and discussing flow virtual impactor (CVI) inlet (Noone et al. 1993) their relevance to questions 1 and 3 listed above. on an aircraft. The AMS provides information on the CHAPS provides a rich set of observations that is nonrefractory (i.e., materials that are chemically and publically available at the ASP data archive (available physically unstable at high temperatures) composi- online at ftp://ftp.asd.bnl.gov/pub/ASP%20Field%20 tion of aerosols, while the CVI uses a counterflow Programs/2007CHAPS/). relative to the main incoming airstream to exclude small droplets and nonactivated particles from the EXPERIMENTAL APPROACH. CHAPS utilized inlet, allowing only larger cloud droplets to enter the a sampling strategy closely linking the DOE G-l, inlet. The combination of the CVI and AMS allow the the National Aeronautics and Space Administration examination of the chemical composition of the dried (NASA) B200 aircraft and primary and secondary aerosol kernel from the cloud droplets. surface sites. The G-l made in situ measurements of particle concentrations, composition, particle and EXPERIMENTAL GOALS. A key objective of the cloud droplet size distributions, optical properties, U.S. Department of Energy's (DOE)'s Atmospheric and cloud nucleating properties below, within, and Sciences Program (ASP; more information is available above clouds downwind of Oklahoma City, while 1654 I BAF15- NOVEMBER 2009 Brought to you by NASA LANGLEY RESEARCH CENTER | Unauthenticated | Downloaded 06/16/21 08:01 PM UTC the instrumentation on the B200 provided remotely pass through. The cloud droplets are then dried in a observed profiles of aerosol extinction, backscatter, heated tube, leaving the aerosol kernel for analysis and depolarization over the same air space as that downstream. The water vapor from the evaporated sampled by the G-l. Observations of boundary layer drops was measured using a Maycomm tunable diode aerosols and meteorological conditions were made laser (TDL) hygrometer. The CVI inlet used during at the primary surface site just north of Oklahoma CHAPS was the same unit deployed by Hayden City and sky cover was measured at a nearby sec- et al. (2008), who reported high levels of organics ondary surface site. Both surface sites were under during ICARTT 2004. They attributed these high the airspace in which observations were made by the levels to the siloxane sealant used on the tip of the two aircraft. CVI. Aware of this problem, extensive cleaning and The in situ aircraft sampling strategy consisted of testing was conducted before CHAPS at the NOAA/ crosswind legs made below, within, and above fields Earth System Research Laboratory to ensure that the of fair-weather clouds (FWCs). The lengths of these results collected during CHAPS would be free of this transects were designed to provide sufficient statistics contamination. for testing the fidelity of parameterizations used in The sampling streams from both inlets fed large-scale models describing aerosol transport over into
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