NORTH ATLANTIC AEROSOL PROPERTIES

AND DIRECT RADIATIVE EFFECTS:

KEY RESULTS FROM TARFOX AND ACE-2

[P. B. Russell]{.underline},^1^ J. M. Livingston,^2^ B. Schmid,^3^ R. A.
Bergstrom,^3^

P. Hignett,^4^ P. V. Hobbs,^5^ and P. A. Durkee^6^

^1^NASA Ames Research Center, Moffett Field, CA 94035-1000 USA

^2^SRI International, Menlo Park, CA 94025 USA

^3^Bay Area Environmental Research Institute, San Francisco, CA 94122
USA

^4^United Kingdom Meteorological Office, Meteorological Research Flight,
DRA Farnborough, Hampshire, GU146TD, UK

^5^University of Washington, Seattle, WA 98195 USA

^6^Naval Postgraduate School, Monterey, CA 93943-5114 USA

#### Introduction

Aerosol effects on atmospheric radiative fluxes provide a forcing
function that can change the climate in potentially significant ways.
This aerosol radiative forcing is a major source of uncertainty in
understanding the observed climate change of the past century and in
predicting future climate. To help reduce this uncertainty, the
International Global Atmospheric Chemistry Project (IGAC) has endorsed a
series of multiplatform aerosol field campaigns. The Tropospheric
Aerosol Radiative Forcing Observational Experiment (TARFOX) and the
second Aerosol Characterization Experiment (ACE-2) were the first IGAC
campaigns to address the impact of anthropogenic aerosols.

Both TARFOX and ACE-2 gathered extensive data sets on aerosol properties
and radiative effects. TARFOX focused on the urban-industrial haze plume
flowing from the eastern United States over the western Atlantic Ocean
\[*Russell et al.,* 1999a\], whereas ACE-2 studied aerosols carried over
the eastern Atlantic from both European urban/industrial and African
mineral sources \[*Raes and Bates*, 1999\]. These aerosols often have a
marked influence on the top-of-atmosphere radiances measured by
satellites, as illustrated in Figure 1. Shown there are contours of
aerosol optical depth derived from radiances measured by the AVHRR
sensor on the NOAA-11 satellite. The contours readily show that aerosols
originating in North America, Europe, and Africa impact the radiative
properties of air over the North Atlantic. However, the accurate
derivation of flux changes, or radiative forcing, from the
satellite-measured radiances or retrieved optical depths remains a
difficult challenge. In this paper we summarize key initial results from
TARFOX and, to a lesser extent, ACE-2, with a focus on those results
that allow an improved assessment of the flux changes caused by North
Atlantic aerosols at middle and high latitudes.

2\. TARFOX Measurements and Calculations

TARFOX made 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. Included were measurements of
aerosol-induced flux changes, made simultaneously with measurements of
the chemical, physical, and optical properties of the aerosols causing
those changes.

![](media/image1.pct){width="3.251388888888889in"
height="3.5694444444444446in"}

**Figure 1**. June/July/August map of aerosol optical depth derived from
NOAA/AVHRR satellite reflectance data over the oceans \[*Husar et al.*,
1997\]

Key initial results from TARFOX include:

• Carbonaceous material (organic and inorganic) and sulfates accounted
for essentially all of the independently derived total dry aerosol mass
in samples obtained at altitudes between 0.2 and 3.8 km \[*Hegg et al*.,
1997\].

• Averaged over the aerosol layer depth (typically \~3 km), carbonaceous
material contributed 50% of dry aerosol mass and 66% of dry aerosol
light scattering \[*Novakov et al*., 1997; *Hegg et al.,* 1997\]. Both
these fractions increased with altitude; fractions near the surface were
similar to some other results for the U.S. and Canadian east coasts.

> • The mean midvisible single-scattering albedo of the dry aerosol was
> 0.9, with a range of 0.76 to 0.98 \[*Hegg et al.,* 1997\].

• The aerosol humidification factor (ratio of light scattering at 80%
relative humidity (RH) to that at 30% RH) in the haze plume was on
average greater than values previously used in aerosol climate effect
calculations, and exceeded the value for sulfate 70% of the time
\[*Kotchenruther et al.*, 1999\].

• Aerosol optical depths derived from in situ scattering and absorption
profiles (using measured humidification factors) averaged \~70-90% of
airborne sunphotometer values \[*Hegg et al.,* 1997; *Ross et al.,*
1998\].

• Chemical apportionment of aerosol optical depths showed the dominant
importance of water condensed on aerosol, with carbonaceous material
second, and sulfate a close third \[*Hegg et al.,* 1997\].

• Remarkable year-to-year consistency was observed in the
characteristics of the aerosol in the TARFOX region during episodes of
heavy haze. \[*Remer et al.*, 1999\].

• Agreement was obtained between aerosol optical thickness magnitudes
and wavelength dependences obtained by the MODIS Airborne Simulator
(MAS) on the ER-2 and by sunphotometers on the C-131A and the surface
\[*Tanre et al*., 1999\].

• First validation of a dual-view algorithm that retrieves
two-wavelength aerosol optical depth over land and ocean from radiances
measured by the Along-Track Scanning Radiometer-2 (ATSR-2) on the
European ERS-2 Satellite \[*Veefkind et al.,* 1998, 1999\].

• Agreement between shortwave radiative flux changes measured from
aircraft and those calculated from measured aerosol properties, provided
the calculations use midvisible single scattering albedos for the
ambient (humidified) aerosol of \~0.89 to 0.93 \[*Hignett et al.,* 1999;
*Russell et al*., 1999b\]. An example of this agreement is shown in
Figure 2. These single scattering albedo values are consistent with the
independent measurements of the dry TARFOX aerosol particles by *Hegg et
al.* \[1997\] and *Novakov et al.* \[1997\], cited above.

**3. ACE-2 Measurements and Preliminary Calculations**

Early results from ACE-2 have been presented at special sessions of the
IGAC conference in Seattle (August 1998) and the Fifth International
Aerosol Conference in Edinburgh (September 1998); many are included in
several papers at this Symposium \[e.g., *Schmid et al.,* 1999\]. These
results show some similarities between ACE-2 and TARFOX results for the
polluted marine boundary layer aerosol, as well as some significant
differences. An example of a significant difference is:

• ACE-2 measurements yield consistently smaller organic aerosol mass
fractions than were obtained in TARFOX and in some other measurements on
the U.S. and Canadian East coasts.

Examples of similarities include:

• Sunphotometer-derived aerosol optical depths usually exceeded values
derived from aircraft-measured profiles of scattering and absorption
coefficients, humidification factor, and/or size distribution \[e.g.,
*Schmid et al.,* 1999\].

• Aerosol optical depths (AODs) retrieved from AVHRR satellite radiances
agreed with values measured by A/C and ship sunphotometers when African
dust was absent. However, agreement degraded when African dust was
present \[e.g., *Durkee et al.*, 1998; *Livingston et al.,* 1998;
*Schmid et al.,* 1999\].

We have applied to ACE-2 data the methods used to compute the TARFOX
flux changes shown in Figure 2. The results are preliminary, in that
they use literature values for refractive

![](media/image2.pct){width="6.656944444444444in"
height="3.1430555555555557in"}

**Figure 2.** Comparison between aerosol-induced changes in shortwave
downward flux determined from TARFOX C-130 measurements (data points)
and calculations (curves) for size distributions retrieved from
sunphotometer optical depth spectra for two days.

indices of polluted marine boundary-layer and soil-dust aerosols, rather
than values based on the ACE-2 size-resolved chemical composition
measurements. However, these refractive indices from the literature do
yield midvisible single-scattering albedos (\~0.94) that are in the
range measured for ACE-2 aerosols, both in the

polluted marine boundary layer and in elevated African dust layers. The
preliminary calcu- lations, which use ACE-2 measured spectra of aerosol
optical depth (AOD), yield radiative flux sensitivity (∆Flux/∆AOD)
values for ACE-2 aerosols (boundary layer and African dust) over the
ocean that are similar to values for TARFOX boundary layer aerosols
(e.g., ∆F↓~24-hr~/ AOD(550 nm)≈-70 W m^-2^).

**4. North Atlantic Flux-Change Calculations**

Noting this similarity in flux sensitivities, especially for the
polluted boundary-layer aerosols in TARFOX and ACE-2, we have used the
aerosol optical model that yielded the agreement in Figure 2 to estimate
shortwave radiative flux changes caused by aerosols over the North
Atlantic Ocean at middle and high latitudes (i.e., excluding latitudes
strongly impacted by African dust aerosols). Our calculations use the
North Atlantic AOD values obtained over a two-year period from AVHRR
radiances by *Husar et al*. \[1997; cf. Figure 1\]. Results will be
shown for seasonal and annual-average flux changes. To illustrate the
sensitivity of flux changes to aerosol absorption, we show results both
for the TARFOX best-fit single-scattering albedo, ω(550 nm)=0.9, and for
non-absorbing aerosols, ω=1. To illustrate the sensitivity to clouds, we
show results calculated without clouds and with cloud fractional
occurrence from the ISCCP data base.

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