Sulfate Versus Carbonaceous Materials on the East Coast of the United States: Results from TARFOX

Contributed by P.V. Hobbs, University of Washington, USA
Reprinted from IGACtivities Newsletter No. 11, January 1998.

Introduction
One of the main goals of the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX), carried out just off the East Coast of the United States from 10-31 July 1996, was to measure the direct effects of tropospheric aerosols on regional radiation budgets while simultaneously measuring their chemical and physical properties (Russell et al., 1996a). An earlier article in this NewsLetter described the conduct of the TARFOX field experiment (Russell et al., 1996b). In this article, a summary is given of some important (and surprizing) results from TARFOX concerning the relative contributions of various chemical species to the total aerosol mass and the aerosol optical depth (a determinant of aerosol radiative forcing). Full accounts of these results will appear shortly in the Journal of Geophysical Research (Novakov et al., 1998; Hegg et al., 1998). All of the measurements described here were obtained aboard the University of Washington's (UW) Convair C-131A research aircraft.

Most studies of aerosol radiative forcing have dealt with just a single component of the atmospheric aerosol, namely, sulfate. However, chemical species other than sulfate contribute to the sub-micrometer particles in the air that play a role in radiative forcing. Of particular importance in this regard are carbonaceous species, which derive from biogenic sources (organic carbon) and from the incomplete combustion of fossil and biomass fuels (black carbon as well as organic carbon). Both organic and black carbon affect the extinction of solar radiation, and therefore aerosol radiative forcing. Black carbon is the principal light-absorbing aerosol species in the atmosphere, while both organic and black carbon scatter solar radiation.

Measurements at ground level show considerable variability in the relative amounts of sulfate and carbonaceous material (e.g., Malm et al., 1994). However, aerosol radiative forcing is determined by aerosol properties throughout the vertical column, about which much less is known. Therefore, in TARFOX, the UW aircraft was used to obtain information on the composition and physical properties of aerosols in vertical columns in the lower troposphere (up to altitudes of about 3 km, which encompassed the main aerosol layers). At the same time, the total aerosol optical depth above the aircraft was measured continuously with a sunphotometer mounted on the aircraft. The techniques used to obtain the various measurements, and the methods of data analyses, are described in detail by Novakov et al. (1998) and Hegg et al. (1998).

Figure 1 shows point measurements of total (dry) aerosol mass versus the sum of the masses of sulfate and carbonaceous materials. (Sulfate was the only significant inorganic constituent of the aerosol by mass.) It can be seen from this figure that sulfate and carbonaceous materials generally accounted for virtually all of the aerosol mass. On average, carbonaceous materials contributed about 50% to the total (dry) aerosol mass.

Components of the Total Aerosol Mass

Figure 1.
Total dry aerosol mass (from weighing of filters and PMS PCASP measurements) versus the sum of the dry masses of carbonaceous materials and sulfate. (From Hegg et al., 1998.)

Altitude Dependence of the Carbonaceous Mass Fraction
Figure 2 shows measurements of the ratio of the mass of carbonaceous materials to the total aerosol mass (the carbonaceous mass fraction or CMF) as a function of altitude for the TARFOX data. It can bee seen that the CMF generally increased with increasing altitude. Near the surface, the CMF was 10-40%, in agreement with previous ground-based measurements.


Figure 2.
Altitude dependence of the carbonaceous aerosol mass fraction. (From Novakov et al., 1998.)

However, aloft the CMF is substantially larger, reaching values as large as 90% at 3 km. One possible cause for this altitude dependence could be that sulfate is removed from the atmosphere by cloud and precipitation processes more efficiently than carbonaceous materials. In this case, carbonaceous materials would have longer lifetimes in the atmosphere than sulfate, and therefore be carried to higher altitudes.

Contribution of Carbonaceous Materials to the Light-Scattering Coefficient of the Dried Aerosol
Figure 3 shows the frequencies with which carbonaceous materials contributed various fractions to the (dried) aerosol light-scattering coefficient. The average fraction of the light-scattering coefficient contributed by carbonaceous materials was 66+/-16%. Therefore, on average, the majority of the light scattering of the dried aerosol derived from carbonaceous materials.


Figure 3.
Frequency distribution of the fractional contribution of carbonaceous materials to the dry aerosol light scattering. (From Hegg et al., 1998.)

Comparison of Remote Sensing and In Situ Measurements of Aerosol Optical Depths
The optical depths of the aerosol in the strongly polluted lower layers of the atmosphere, which the aircraft profiled, were derived from the in situ measurements of the light-scattering and light-absorption of the dried aerosol, the vertical profile of relative humidity (RH), and the measured humidification factors of the aerosol (i.e., the change in aerosol light scattering with RH). These derived values can be compared with optical depths measured directly with the sunphotometer aboard the aircraft (Livingston and Russell, 1997). Since the sunphotometer measured the optical depth of the aerosol in the total vertical column above the aircraft, values measured at the top and bottom of the aircraft profile were differenced to obtain the optical depth of the layer sampled by the aircraft.

The results of these comparisons are shown in Figure 4, where it can be seen that the two independent measurements of layer optical depth are well correlated. The sunphotometer measurements are systematically slightly higher than the in situ measurements; this could be due to inefficient in situ sampling of the largest particles.


Figure 4.
Layer optical depths measured with the airborne sunphotometer versus those derived from in situ airborne measurements of aerosol properties in the same layer. (From Hegg et al., 1998.)

Contributions of Chemical Species to the Total Aerosol Column Optical Depth
Figure 5 shows the contributions of sulfate, carbonaceous materials, and water condensed on the aerosols, to the column aerosol optical depth derived from in situ measurements on each of fourteen flights made in TARFOX. These results show that:
  • Condensed water, carbonaceous materials, and sulfate accounted for most of the aerosol optical depth in the column sampled by the aircraft.

  • On average, scattering by condensed water accounted for about 35% of the aerosol optical depth. However, for optical depths greater than 0.4, condensed water accounted, on average, for 58% of the optical depth. (These results highlight the importance of accurate measurements of the hygroscopicity of various aerosol types, and the careful incorporation of such information in modeling studies of aerosol radiative forcing.)

  • Second to condensed water in contributing to the aerosol optical depth were carbonaceous materials, followed by sulfate.

  • Dry sulfate in the lower troposphere contributed, on average, only 16% to the total aerosol optical depth. Even using a generous value of 3 for the hygroscopic growth factor of sulfate, wet sulfate contributed less than 50% to the total aerosol optical depth. Using a more reasonable value of 2 for the humidification factor of sulfate, its contribution to the optical depth was, on average, only 30%.

Figure 5.
Contributions of the light scattering by water condensed on aerosols, carbonaceous materials and sulfate in the lower troposphere to the column aerosol optical depth for each of fourteen University of Washington flights off the East Coast of the United States. Also shown is the contribution to the optical depth of light absorption by the aerosols. (From Hegg et al., 1998.)

Conclusions
The widely held view that sulfate dominates aerosol column optical depths, and therefore aerosol radiative forcing, was not verified in TARFOX measurements on the East Coast of the United States, even though this is one of the most likely places on earth for this paradigm to hold. Instead carbonaceous materials were more important. While one data set collected in one location is by no means definitive, it indicates that much more attention needs to be placed on carbonaceous materials in the atmosphere, and that measurements similar to those obtained in TARFOX should be made in other airsheds around the world.

References
  1. Hegg, D.A., J.L. Livingston, P.V. Hobbs, T. Novakov, and P. Russell, J. Geophys. Res., 1998 (in press).

  2. Livingston, J.M. and P.B. Russell, EOS, Trans. Amer. Geophys. Union, 78, S93, 1997.

  3. Malm, W.C., J.F. Sisler, D. Huffman, R.A. Eldred, and T.A. Cahill, J. Geophys. Res., 99, 1345-1370, 1994.

  4. Novakov, T., D.A. Hegg, and P.V. Hobbs, J. Geophys. Res., 1998 (in press).

  5. Russell, P.B., W. Whiting, P.V. Hobbs, and L.L. Stowe, Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) Science and Implementation Plan, NASA Ames Research Center, Moffett Field, CA (also on the WWW site http://prometheus.arc.nasa.gov/~tarfox/), 1996a.

  6. Russell, P.B, P. Hignett, L.L. Stowe, and P.V. Hobbs, IGACtivities NewsLetter, No. 7., pp. 8-9, December 1996b.