Most of the measurements of light-absorbing aerosol particles made previously have been in non-cloudy air and therefore provide no insight into aerosol effects on cloud properties. Here, researchers describe an experiment designed to measure light absorption exclusively due to substances inside cloud droplets, compare the results to related light absorption measurements, and evaluate possible effects on the albedo of clouds. The results of this study validate those of Twomey and Cocks and show that the measured levels of light-absorbing material are negligible for the radiative properties of realistic clouds. For the measured clouds, which appear to have been moderately polluted, the amount of elemental carbon (EC) present was insufficient to affect albedo. Much higher contaminant levels or much larger droplets than those measured would be necessary to significantly alter the radiative properties. The effect of the concentrations of EC actually measured on the albedo of snow, however, would be much more pronounced since, in contrast to clouds, snowpacks are usually optically semi-infinite and have large particle sizes

Charleson, R. J.

Clarke, A. D.

Radke, L. F.

Twohy, C. H.

Warren, Stephen G.

English

NASA Technical Reports Server

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193, . Clarke2, S. arrent, L. F. Radkel, and R. J. Charlsonl 
The question of how aerosol pollutants affect the radiative properties of clouds was posed by 
Twomey [1977l, who pointed out that aerosols have two competing effects: they increase the 
number of particles which might act as cloud condensation nuclei (CCN), but also include materials 
such as elemental carbon (EC) which are essentially black to solar radiation. Elevated levels of 
CCN, assumed to result in greater numbers of smaller droplets, would lead to brighter, more 
reflective clouds, while enhanced absorption due to carbon particles could potentially make clouds 
darker. Based on the global distribution of cloud cover and absorption measurements which 
Twomey et al. [ 19841 used in a later paper, the overall climatic effect of increasing pollution over 
time was shown by their model to be cooling, of the same order of magnitude as predicted warming 
from the greenhouse effect of CQ2 alone. 
Cloud absorptance values measured directly by aircraft Reynolds et al., 1975; Stephens et al., 
19781 are frequently much higher than those which are theoretically calculated from measured 
microphysical parameters. One possible explanation of this phenomenon is absorption of radiation 
by aerosol particles within the cloud. In a comparison of measured and computed reflectance spectra 
at various wavelengths, Twomey and Cocks [ 19823 concluded that this process could not be the 
major explanation since the observed enhanced absorption predominated not at visible wavelengths, 
but in the near infrared where absorption by particles should be negligible relative to that by liquid 
water and ice. 
Most of the measurements of light-absorbing aerosol particles made previously have been in 
non-cloudy air and therefore provide no insight into aerosol effects on cloud properties. In this 
paper, we describe an experiment designed to measure light absorption exclusively due to 
substances inside cloud droplets, compare the results to related light absorption measurements, and 
evaluate possible effects on the albedo of clouds. The results of this study validate those of 
Twomey and Cocks and show that the measured levels of light-absorbing material are negligible for 
the radiative properties of realistic clouds. 
Elemental carbon (EC) is the most efficient light-absorbing material present in the atmosphere, 
and except in unusual cases absorption by aerosols is dominated by elemental carbon Rosen et al., 
1978; Heintzenberg, 1982; Clarke and Charlson, 19851. It is possible for hydrophobic carbon 
particles to obtain a hygroscopic coating and be incorporated into clouds by nucleation scavenging. 
Measurement of EC in cloud droplets, however, is problematic due to the difficulty in collecting 
cloud droplets separately from interstitial aerosol particles and the tendency of EC to stick to surfaces 
of collection vessels [Ogren, 19831. In addition, identification of EC requires relatively long 
sampling times in many environments. In order to eliminate the first difficulties, cloud droplets 
were sampled in this experiment with a counterflow virtual impactor (CVI), a probe which by 
inertial impaction collects only droplets above a certain minimum radius while effectively rejecting 
interstitial aerosol particles [Ogren et al., 1985; Noone et al., 19881. The CVI evaporates droplets 
(leaving the non-volatile aerosol residue) before they touch any surface, minimizing sample 
contamination and loss of carbon particles to the walls of the sampler. The long sampling time 
requirement was avoided through the use of a very sensitive technique for measuring the absorption 
of light by the droplet residue particles. The "Integrating Sandwich Method" {Clarke, 19821, 
enhances light absorption by particles in the sample by confining them between two highly reflecting 
surfaces. 
lDepartment of Atmospheric Sciences, University of Washington, Seattle, Washington 98 195. 
2Hawaii Institute of Geophysics, University of Hawaii 
3 C u ~ n t l y  at National Center for Atmospheric Rese 
03 
https://ntrs.nasa.gov/search.jsp?R=19910001215 2020-03-19T20:29:27+00:00Z
droplets were sampled by the CV 
ect off the Southern ealiforni 
t radius) were collected on five different days for light absorption analysis of the 
e particles, which were impacted onto Nuclepore polycarbonate filters. The C-I 31 
at cloud base, above the cloud, or inside the cloud for periods of twenty to thirty 
ashington's C- 13 1 aircraft in 
987. Droplets above five pm 
minutes at an airspeed of 80-85 meters s-* with occasional shallow descents or ascents through the 
stratus deck. Sampling times in cloud ranged from 22 to 143 minutes, resulting in collection of 
droplets from about 3-20 cubic meters of cloudy air. 
Absorption coefficients representing the absorption per unit length of droplet-laden air were 
calculated directly from the amount of light absorption by the filter sample and the sample volume of 
air. Model trajectories [ Kloesel et al., 19883 revealed that air reaching the 925 mb level had been 
located west, northwest, or north of the project area on the days prior to the flights, and windspeeds 
were highly variable. Examination of the droplet size spectrum measured by the Forward Scattering 
Spectrometer Probe (FSSP) on the aircraft indicated that some occasions, the air appeared to be of 
marine origin but many times the spectra showed continental influence. Four of the measured 
absorption coefficients for the droplet residue ranged from 6 to 2 0 ~ 1 0 - ~  m-l,  while one (from July 
7, 1987) exhibited much less absorption. The absorption coefficients were generally higher than 
those measured in aerosol at remote locations such as Mauna Loa Observatory [Clarke and 
Charlson, 19851 and the South Pole [Clarke et al., in preparation], and fell within the range 
measured at Hurricane Ridge on the western coast of Washington State IClarke, 19821. They were, 
however, much lower than those typical of extremely polluted air at industrially influenced locations 
like St. Louis, Missouri IWeiss, 19801. 
The mass concentration of elemental carbon inside the droplets was estimated from the measurcd 
absoiption coefficients and the liquid water content, with the assum tion that all absorption was duc 
EC in cloud water were 23-79 ng EC g-1 for four of the five cases. These values were comparable 
with those measured in rainwater [Ogren, 19831 at Seattle, Washington and at less polluted sites in 
Sweden. Samples of interstitial particles were also taken during the project, but were analyzed for 
light absorption by a less sensitive method, and all were below the detection limit. From these 
results an absolute upper bounh on the concentration of interstitial elemental carbon (IEC) was 
established for each flight. 
The maximum possible effect of the measured EC on cloud albedo was detexmined by 
calculating albedo at wavelength 0.475 pm where liquid water has its weakest absorption [Wale and 
Querry, 19731, by using the largest measured in-cloud EC concentrations, and by assessing the 
effect upon an optically "semi-infinite" cloud. In another scenario but still at the most sensitive 
wavelength, the effect of EC on the albedo of a more realistic (thinner) cloud was evaluated. The 
surfdce-area-weighted or "effective" mean radius [Iiansen and Travis, 19741, reff, which determines 
the effect of the droplet size distribution on the single-scattering bedo, o, was calculated and 
averaged for each sampling period from the FSSP data, and the ie program of Wiscombe [ 1980) 
was used to obtain values for o, asymmetry factor, and extinction efficiency. 
Figures 1 and 2 show the calculated effect of  elemental carbon on the albedo of clouds with 
different reff values. Figure 1 applies to a semi-infinite cloud (optical thickness T* = 00) with the 
shaded area representing the range of EC concentrations under consideration, explained in more 
detail in the figure caption. Curved lines show the effect of different reff values on albedo, with a 
greater albedo reduction resulting when EC is present in larger droplets. If an reffof 10 pm 
(slightly larger than the ones calculated) is assumed, the change in the albedo of this semi-infinite 
cloud due to the carbon concentrations considered ranges from -0.014 to -0.029. EC has less effect 
on albedo as optical thickness decreases, and Figure 2 shows the results for a more realistic cloud 
with T* = 30. (The calculated optical thicknesses of the FIRE clouds during the five sampling 
periods were all smaller than 30, so their albedos should be slightly less affected.) Assuming the 
same reff and range of EC concentrations as before, the reduction in albedo in this case is only 
0.001 even under the most stringent circumstances. Due to the higher absorption of water at 
to elemental carbon with a mass absorption coefficient of 10 m 2 g -p Resulting concentrations of 
04 
an 0.475 pm, reduction in s 
the amount of EC present was insuffi 
r much larger droplets than those mea 
alter the radiative properties. The effect of the concen 
of snow, however, would be much more pronounced since in contrast to clouds, snowpacks are 
usually optically semi-infinite and have large particle sizes. 
86-07377 and the 
343 from Sandia 
National Laboratory. Radiative transfer calculations were made at NCAR and supported by NSF 
grant ATM-86-05134. We thank David Covert, Kevin Yoone, and ‘iteven Domonkos for their 
invaluable advice and aid in installing instruments aboard the aircraf,. Thanks also are due to John 
Ogren and Jost Heintzenberg for buildiry the first CVI. 
ote: The full version of this paper is in preparation for the Journal of Geophysical Research. 
p.446, Wiley-Interscience, New York, 1983. 
Geophys. Res., 88, 10837-10843, 1983. 
coefficient for atmospheric aerosols, &pi. Opt., 21,301 1-3020, 1982. 
Iy- average^ a lbe~os 
en&: This work was supported in part by NSF grant 
Agency, through funds made available under documen 
Bohren, C. F., and D. R. Hoffman, in Absorption and scattering of light by smallparticles, 
Chylek, P., Ramaswamy, V., and V. J. Srivastava, Albedo of soot-contaiminated snow, 9. 
Clarke, A. D., Integrating Sandwich: a new method of measurement of the light absorption 
Clarke, A. D., and R. J. Charlson, Radiative properties of the background aerosol: absorption 
t of extinction, Science, 229,263-265, 1985. 
, Warren, S. G., and T. C. Grenfell, Soot in the atmosphere and snow surface of 
, J. Ceophys. Res., in preparation. 
Hale, G.  M., and M. R. Querry, Optical constants of water in the 200-nm to 200-pm wavelength 
, 555-562, 1973. 
. Travis, Light Scattering in heres, Space Sci. Rev., 
Heintzenberg, J., Size-segregated particulate elemental carbon and aerosol light absorption at 
remote Arctic location 6, 2461-2469, 1982. 
Kloesel, K. A., Albrecht, Wy le, FIRE marine stratocumulus observations- 
summary of operation ditions, Penn State University, 1988. 
Noone, K. J., Charlson, R. J., Covert, D. S., Ogren, J. A. and J. E. Heintzenberg, Design 
and calibration of a countemow virtual impactor for sampling of atmospheric fog and cloud 
Accepted in Aer.Sci.and Tech., 1988. 
mental carbon in the atmosphere, Ph.5. dissertation, University of 
Ogren, J. A., Heintzenberg, J., and R. J. Charlson, In-situ sampling of cloud droplets with a 
to aerosol converter, Ceophys. Res. Lett., 12, 12 1 - 124, 1985. 
Hansen, D. A., Gundel, L., and T. Novakov, Identification of the optically absorbing 
ent in urban aerosols, Appl. Opt., 17,3859-3861, 1978. 
Stephens, G., Paltridge, G., and C. Platt, Radiation profiles in extended water clouds. 111: 
Twomey, S., The influence of pollution on the short-wave albedo ofcIouds, J. Amos Sci., 3 
Twomey, S . ,  and T. Cocks, Spectral r ctance of clouds in the near-infrared: comparison of 
measurements and calcufations, J. teor. SOC. Jap., 60,583-592, 1982. 
Twomey, S. A., Piepgrass, M., and Wolfe, An assessment of the impact of pollution on 
global cloud albedo, Tellus, 36B, 356-366, 1984. 
Weiss, R. E., The optical absorption properties of suspended particles in the lower troposphere at 
Observations, J.Atmos. Sci., 35, 2133-2141, 1978. 
1149-1 152, 1977. 
issertation, University of 
ie scattering algorithms, Appl. Opt., 19, 1505-1509, 1980. 
ashington, Seattle, 1980. 
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he effect of different concentrations of elemental carbon (EC) on the albedo of 
a semi-infinite cloud (optical thickness = 00) at h = 0.475 pm, with curved lines 
representing d~fferent values of effective droplet radii in pm. The shaded area represents 
the range of concentrations under consideration: from 1.6~10-7 which assumes the only 
EC in the cloud is the maximum amount measured inside droplets, ta 5.9~10-7 which 
includes the maximum possible value for interstitial carbon as well. Since calculations 
external mixture, the inside-droplet value was 
on of EC inside water droplets [Chylek, 1983 
bled to account for the 
06 
. 
. 
. Same as Figure 1 except with a ore realistic optical thickness of 30. 
407 


Measurements of the light-absorbing material inside cloud droplets and its effect on cloud albedo

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