Preprint, Proceedings of the A&WMA/AGU Specialty Conference on
Visual Air Quality, Aerosols, and Global Radiation Balance
September 9-12, 1997, Bartlett, New Hampshire

P. B. Russell¹, P. V. Hobbs², and L. L. Stowe<sup>3

1</sup>NASA Ames Research Center, Moffett Field, CA 94035
²University of Washington, Seattle, WA 98195
³NOAA/NESDIS, Satellite Research Laboratory, NSC, Washington, DC 20233


Abstract. Aerosol effects on atmospheric radiation are a leading source of uncertainty in predicting future climate. TARFOX was designed to reduce this uncertainty by measuring and analyzing aerosol properties and effects in the US eastern seaboard, where one of the world's major plumes of industrial haze moves from the continent over the Atlantic Ocean.
The TARFOX Intensive Field Campaign was conducted July 10-31, 1996. It included coordinated measurements from four satellites (GOES-8, NOAA-14, ERS-2, LANDSAT), four aircraft (ER-2, C-130, C-131A, and a modified Cessna), land sites, and ships. A variety of aerosol conditions was sampled, ranging from relatively clean behind frontal passages to moderately polluted with aerosol optical depths exceeding 0.5 at mid-visible wavelengths. Gradients of aerosol optical thickness were sampled to aid in isolating aerosol effects from other radiative effects and to more tightly constrain closure tests, including those of satellite retrievals.
Early results from TARFOX include demonstration of the unexpected importance of carbonaceous compounds and water condensed on aerosol in the US mid-Atlantic haze plume, chemical apportionment of the aerosol optical depth, measurements of the downward component of aerosol radiative forcing, and agreement between forcing measurements and calculations. A wide variety of closure studies is currently in progress.



1. Introduction

Aerosol particles can change the Earth’s radiation budget both directly by scattering and absorption and indirectly by affecting cloud properties. Changing the net flux of radiation above or within the atmosphere changes the energy available for driving climatic processes. Hence, such a net flux change is termed a radiative forcing of climate [IPCC, 1995]. Negative forcings tend to cool the climate, and positive forcings tend to warm it. Current estimates of the global, annually-averaged, direct radiative forcing by anthropogenic aerosols (e.g., sulfates, soots, mineral dust, biomass smokes) range from about -0.3 to -1.0 W m^-2, with an uncertainty factor of about two. Analogous, but even less certain, estimates for the indirect effect are 0 to -1.5 W m^-2 [IPCC, 1996]. These values are comparable in magnitude, but opposite in sign, to the current estimates of +2.1 to +2.8 W m^-2 for the forcing caused by increases in greenhouse gases over the past century.

Because of the great spatial variability in aerosol concentrations that results from their short lifetime, there are many regions--principally over and downwind of major source areas--where the best estimates of aerosol negative forcing exceed the greenhouse positive forcing [e.g., Charlson and Heintzenberg, 1995; Charlson et al., 1992; Kiehl and Briegleb, 1993]. Some studies show that aerosol effects appear to be present in global and regional twentieth-century temperature records, and that inclusion of aerosol effects in numerical models improves agreement with observed temperature patterns in both time (decadal and diurnal) and space [Engardt and Rodhe, 1993; Hunter et al., 1993; Karl et al., 1995; Li Xiaowen et al., 1995; Schwartz, 1995; Santer, et al., 1995; Hansen et al., 1995; Meel et al., 1996; Tett et al., 1996]. Although these studies suggest that anthropogenic aerosols can play an important role in determining current and future climates, their results are far from conclusive. Major questions remain about the realism with which models represent the great diversity of actual aerosol properties, processes, and radiative effects. Error analyses show that the uncertainty in the aerosol radiative forcing is unacceptably large--larger, in fact, than the uncertainty in climate forcing by all greenhouse gases released over the past century [IPCC, 1996].

As a result of both the potential importance of aerosols and the large uncertainties in their radiative effects, the International Global Atmospheric Chemistry (IGAC) Project has established a Focus on Atmospheric Aerosols (FAA) and endorsed a series of aerosol field campaigns [Hobbs and Huebert, 1996]. The Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) is the second in the IGAC/FAA series. TARFOX was designed to reduce uncertainties by measuring and analyzing a wide range of aerosol properties and effects in the US eastern seaboard. This is the region where one of the world's major plumes of industrial haze moves from the continent over the Atlantic Ocean (see Section 3). Early planning for TARFOX is described by Stowe (1994a,b); a more detailed plan is given by Russell et al. (1996). The latter is also available on the TARFOX World Wide Web page, http://tarfox.arc.nasa.gov/.


2. TARFOX Goals

The overall goal of TARFOX is to reduce uncertainties in the effects of aerosols on climate by determining the direct radiative impacts, as well as the chemical, physical, and optical properties, of the aerosols carried over the western Atlantic Ocean from the United States. Subsidiary objectives of TARFOX are to:

• Perform a variety of closure studies by using overdetermined data sets to test the mutual consistency of measurements and calculations of a wide range of aerosol properties and effects.
  
  
• Use the results of the closure studies to assess and reduce uncertainties in estimates of aerosol radiative forcing, as well as to guide future field programs on this subject.

An important component of the closure studies is tests and improvements of algorithms that retrieve aerosol properties and effects from satellite and aircraft radiometers. The resulting validated algorithms will permit extensions of the TARFOX results to other times and locations that have aerosol properties similar to those of the TARFOX Intensive Field Campaign (IFC).


3. TARFOX Approach

The TARFOX field project focused on the plume of pollutant haze that moves off the US East coast over the Atlantic Ocean. This plume is readily evident in a variety of satellite measurements, as exemplified by Figure 1. Shown there are contours of aerosol optical depth derived from NOAA/AVHRR satellite reflectance data and averaged over the months of June, July, and August [Husar et al., 1997]. Daily and weekly satellite data show that the three-month average plume in Figure 1 is the result of much stronger episodic plumes interspersed with relatively clean periods. Statistical analyses of several years of satellite data [Stowe, 1994a] show that the probability of observing such episodic plumes under cloud-free conditions is greatest during the last three weeks of July, before the onset of cloudiness associated with tropical storms. For this reason, the TARFOX Intensive Field Campaign (IFC) was conducted July 10-31, 1996.

As shown in Figure 2, TARFOX included coordinated measurements from four satellites (GOES-8, NOAA-14, ERS-2, LANDSAT), four aircraft (ER-2, C-130, C-131A, and a modified Cessna), land sites, and ships. Aircraft were based at the NASA Wallops Flight Facility in Virginia. A variety of aerosol conditions was sampled, ranging from relatively clean behind frontal passages to moderately polluted with aerosol optical depths exceeding 0.5 at mid-visible wavelengths. The latter conditions included separate incidents of aerosol enhancements caused primarily by anthropogenic sources and another incident of enhancement apparently influenced by recent fog processing. Spatial gradients of aerosol optical thickness were sampled to aid in isolating aerosol effects from other radiative effects and to more tightly constrain closure tests, including those of satellite retrievals.

Coordination of the four TARFOX aircraft was greatly aided by (1) near-realtime imagery of aerosols, clouds, and fog from the GOES-8 satellite and the AVHRR radiometer on the NOAA-14 satellite, (2) special 24- and 48-hour forecasts of the areas most likely to be cloud-free during TARFOX flights, and (3) forward and backward trajectory calculations at several altitudes. Aircraft flights were coordinated with overflights of the NOAA-14, ERS-2, and LANDSAT satellites, as well as with other times and locations of interest. These included lidar and multi-spectral diffuse and direct scanning shortwave radiometer measurements on Wallops Island and identical radiometer measurements at four other East Coast sites, on two cruise ships, and on Bermuda. Realtime vertical profiles of backscatter from the LASE lidar on the ER-2 were used to direct the other aircraft to altitudes of aerosol layers for intensive sampling, as well as to document cloud and water vapor layers and cloud-free regions. The MODIS Airborne Simulator (MAS) on the ER-2 provided multispectral images for post-mission analyses of ocean reflectance, aerosol optical depth and particle size information, and cloud properties (including cirrus correction of aerosol products). ER-2 cameras also documented cloud presence in study scenes.

The University of Washington's integrated airborne system aboard the C-131A was used extensively to measure aerosol precursor and other gases, CN and CCN, aerosol composition and size distribution, total scattering, backscattering and absorption coefficients, graphitic and organic carbon, aerosol hygroscopic growth factors, and a variety of cloud/fog properties using in situ sensors and samplers, as well as optical depth spectra, backscatter profiles, aerosol and cloud absorption and scattering, and surface reflectivity using a sunphotometer, lidar, and scanning radiometer. These airborne measurements were obtained during a total of 72 C-131A research hours of flying. The measurements included vertical and horizontal profiles on twelve occasions beneath NOAA-14 satellite overpasses, on two occasions beneath an ERS-2 satellite overpass, and on one occasion beneath a LANDSAT overpass. Seven sets of vertical profile measurements were obtained beneath the ER-2 (some with satellite overpasses), and seven vertical profiles above two sunphotometers and the Raman lidar at Wallops Island.

The UK Meteorological Research Flight C-130 measured upward and downwrad irradiances in the 0.3 to 3.0 and 0.7 to 3.0 micron wavebands. Additionally the downward irradiances could be divided into direct and diffuse components. A narrow-band, narrow field-of-view multi-cahannel radiometer operating from mid-visible through to the infrared was used to measure the angular distribution of sky radiance. Aerosol particle size distributions, absorption coefficients and information on chemical composition from volatility analyses were also obtained. During about 60 hours of experimental flying several flights were achieved in the essentially cloud-free conditions necessary for the shortwave radiative forcing measurements. Each flight included horizontal legs at typically ten altitudes with vertical profiles between legs. The measurement periods were coordinated with overpasses of NOAA-14, at which point the C-130 measured the irradiances at the top of the aerosol layer. Observations were also coordinated with the C-131A and profiles were flown over the Wallops Island surface instruments.

The Pelican (a modified Cessna) of the Center for Interdisciplinary Remotely Piloted Aircraft Studies measured aerosol size spectra, and spectral radiances and radiative fluxes in a variety of spectra bandwidths. The Pelican made measurements at numerous altitudes up through the top of the main haze layer. Six flights were flown under NOAA-14 overpasses and one flight was flown under an ERS-2 overpass. The Pelican flew in close coordination with the C-131A on three occasions during the final six flights. The first measurements with a new Ames tracking sunphotometer aboard the Pelican were collected during the final several flights of the Pelican in TARFOX.

At Wallops Island a Raman lidar measured vertical profiles of aerosol extinction and backscatter, and a variety of sun and sky radiometers measured optical depths and sky radiances used to derive scattering phase functions and aerosol size distributions. Two sunphotometers measured multiwavelength optical depth. Sun and sky radiometers were also operated at several other East Coast sites and Bermuda, as well as on cruise ships transiting between Bermuda and New York. Numerous aircraft flight legs and vertical profiles were flown over the surface sites to permit detailed intercomparisons. Other flight segments were devoted to intercomparisons among aircraft sensors.


4. TARFOX Meteorology, Operations, and Example Measurements

During TARFOX, Hurricane Bertha closed the Wallops Flight Facility for three days, and a persistent upper-level trough created extensive cloudiness on many days and highly variable haze conditions due to frequent shortwave weather systems passing over the area. Nonetheless, one or more of the aircraft collected aerosol in situ and radiation data on 14 of those days. Fortunately, in those fourteen, ten provided data coincident with AVHRR overpasses, eight with MAS (MODIS Airborne Simulator on ER-2 aircraft) images, two with ATSR-2 (Along Track Scanning Radiometer-2) overpasses, and one with a LANDSAT-TM (Thematic Mapper) overpass. Coincident GOES-8 images, which have been converted into aerosol optical thickness using the algorithm operational with NOAA/14 AVHRR data, have been archived.

TARFOX operations, including aircraft flight paths, satellite overpasses, and selected initial results, are described by Whiting et al. (1996) and also on the TARFOX World Wide Web page, http://tarfox.arc.nasa.gov/.

Figure 3 shows an illustrative composite of data acquired from satellite, aircraft, and surface sensors on one day of the TARFOX IFC. The top frame shows ER-2, C-131A, and Pelican flight tracks superimposed on a GOES image that has been processed to show contours of aerosol optical depth (as well as fog and cloud in white). The middle frames show atmospheric cross sections obtained from the Wallops Island and ER-2 lidars and from in situ scattering and sunphotometer measurements on the C-131A. The bottom frames show aerosol optical thickness obtained both from solar transmission and from integrated lidar extinction at Wallops Island.

As suggested by the number of flights and measurements described in Section 3, the TARFOX data set is extensive and diverse. Further examples of the types of data acquired are given by Hobbs [1996] and on the TARFOX World Wide Web page, http://tarfox.arc.nasa.gov/.


5. Key Initial Findings

Initial scientific results from TARFOX were presented in a collection of papers at the TARFOX Special Sessions of the 1997 Spring Meeting of American Geophysical Union (Baltimore, MD, May 27-30, 1997). Some of these have been submitted for journal publication (see below), and further publications are planned for a special issue of J. Geophys. Res. Although space does not permit a comprehensive description of findings to date, the following list illustrates the scope and significance of the key findings to date:

1. Demonstration of the unexpected importance of carbonaceous compounds in the U.S. mid-Atlantic haze plume, and of the increase of the carbonaceous fraction with increasing height [Novakov et al., 1997a,b].
   
   
2. Demonstration that aerosol humidification factors were greater than values previously used in aerosol climate effect calculations [Kotchenruther and Hobbs, 1997].
3. Chemical mass closure between (1) total aerosol and (2) water, carbonaceous material (organic and inorganic) and sulfate [Hegg et al., 1997a,b].
   
   
4. Agreement between optical depth measured by airborne sunphotometer and computed from airborne nephelometer [Hegg et al., 1997a,b].
   
   
5. Chemical apportionment of the optical depth, showing the dominant importance of water condensed on aerosol, with carbonaceous material second, and sulfate a close third [Hegg et al., 1997a,b].
   
   
6. The first airborne measurements of the downward component of radiative forcing (i.e., the change in the net input of radiative energy) by the U.S. mid-Atlantic haze plume [Hignett and Taylor, 1997].
   
   
7. Agreement between radiative forcings computed from measured aerosol properties and those measured from aircraft [Hignett and Taylor, 1997; Russell et al., 1997b].
   
   
8. Numerous comparisons between aerosol optical depths derived from satellite scattered-radiance measurements and determined by transmission measurements (from aircraft, ships, and land) [Durkee et al., 1997; Ignatov et al., 1997].
   
   
9. Characterization of climatically important aerosol optical properties (optical depth, single scatter albedo, asymmetry parameter) and radiatively important gases (e.g., water vapor, ozone) from a variety of remote measurements [Remer et al., 1997; Smirnov et al., 1997; Tanre et al., 1997; Ferrare et al., 1997; Ismail et al., 1997; Livingston and Russell, 1997] as well as from in situ measurements [Hobbs, 1997; Novakov et al., 1997; Hegg et al., 1997; Kotchenruther and Hobbs, 1997; Taylor and Hignett, 1997]. These extensive overdetermined data sets are now being used to conduct a wide variety of the tests of closure (i.e. consistency) mentioned above.

6. Planned Future Analyses.

The combined surface, air, and space data sets obtained in TARFOX permit a wide variety of closure analyses. Specifically, in-situ measurements of aerosol light-scattering, absorption and forward/backscatter ratios at a given height are being used to derive the basic quantities needed to determine the effects of aerosols on solar radiation, namely, extinction, single-scattering albedo, and the asymmetry parameter. "Internal closure" is being assessed by comparing the quantities thus derived with those deduced from the simultaneous in-situ measurements of aerosol size distribution and chemical composition. The chemical apportionment of aerosol extinction [Hegg et al., 1977a,b] has provided insights into the sources of this extinction.

"External column closure" is being assessed by comparing aerosol extinction values computed from the in situ measurements on the C-131A with those derived from the airborne sunphotometers, satellite radiometers, and the ER-2 imaging spectrometer. Another aspect of column closure is comparisons of aerosol radiative forcing, or radiative flux changes, determined by: (1) airborne flux radiometer measurements, (2) satellite flux retrievals from radiance measurements, and (3) flux calculations from (a) in-situ measured aerosol scattering, absorption and asymmetry factors, (b) the same properties derived from aerosol size distribution and composition measurements, and (c) sunphotometer- and satellite-derived optical depths with ancillary single-scattering albedos and asymmetry factors. Results from some of these analyses have been reported per the list in Section 5; other analyses are in progress or planned. The sensitivity of radiative forcing to changes in aerosol optical thickness is being derived from the detailed in situ measurements and compared to the empirical sensitivity obtained by regressing satellite-derived radiative forcing vs. satellite-derived optical depth.

Such closure analyses yield critically needed assessments and are expected to reduce the uncertainties in derived values of anthropogenic aerosol radiative forcing. The closure analyses that use satellite optical depth and flux results will provide tests and, where necessary, improvements of satellite retrieval algorithms. The resulting validated algorithms will permit extensions of the TARFOX results beyond the TARFOX period and to other locations dominated by similar aerosols.


Acknowledgments. This research was conducted as part of the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX), which is a contribution to the International Global Atmospheric Chemistry (IGAC) core project of the International Geosphere-Biosphere Programme (IGBP). Financial support for the measurements and analyses was provided by the US National Aeronautics and Space Administration, National Science Foundation, Office of Naval Research, and National Oceanographic and Atmospheric Administration, by the UK Meteorological Office, and by the French Science Foundation (Centre National de la Recherche Scientifique, CNRS) and the French Space Administration (Centre National d'Etudes Spatiales, CNES). Data in Figure 3 were furnished by Philip Durkee, Richard Ferrare, Syed Ismail, Dean Hegg, John Livingston, and Lorraine Remer.

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Durkee, P. A., B. B. Brown, K. E. Nielsen, P. B. Russell, and J. Livingston, Aerosol optical properties from NOAA AVHRR and GOES-9 measurements during TARFOX, EOS, Trans. Amer. Geophys. Union, 78, S87, 1997.

Engardt, M., and H. Rodhe, A comparison between patterns of temperature trends and sulfate aerosol pollution. Geophys. Res. Lett., 20, 117-120, 1993.

Ferrare, R. A., G. Schwemmer, S. H. Melfi, D. N. Whiteman, D. Guerra, and D. Wooten, Scanning Raman lidar measurements of aerosol backscatter and extinction profiles during TARFOX, EOS, Trans. Amer. Geophys. Union, 78, S81, 1997.

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Hegg, D. A., J. Livingston, P. V. Hobbs, T. Novakov, and P. B. Russell, Chemical apportionment of aerosol column optical depth off the Mid-Atlantic coast of the United States, EOS, Trans. Amer. Geophys. Union, 78, S82, 1997a.

Hignett, P., and J. P. Taylor, Aircraft observations of direct aerosol forcing during TARFOX, EOS, Trans. Amer. Geophys. Union, 78, S88, 1997.

Hobbs, P. V., Summary of types of data collected on the University of Washington's Convair C-131A aircraft in the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) on the East Coast of the United States from July 10-31, 1996. Report from the Cloud and Aerosol Research Group, University of Washington, Seattle, WA, October 1996.

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Kotchenruther, R.A., P. V. Hobbs, and D. A. Hegg, Humidification factors for aerosols off the mid-Atlantic coast of the United States, EOS, Trans. Amer. Geophys. Union, 78, S82, 1997.

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Livingston, J. M., and P. B. Russell, Aerosol optical depth spectra, vertical profiles, and horizontal transects derived from TARFOX airborne sunphotometer measurements, EOS, Trans. Amer. Geophys. Union, 78, S92, 1997.

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