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In Situ Observations of Contrail Microphysics and Implications for Their Radiative Impact Michael R

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In Situ Observations of Contrail Microphysics and Implications for Their Radiative Impact Michael R JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D10, PAGES 12,077–12,084, MAY 27, 1999 In situ observations of contrail microphysics and implications for their radiative impact Michael R. Poellot Department of Atmospheric Sciences, University of North Dakota, Grand Forks W. Patrick Arnott and John Hallett Atmospheric Science Center, Desert Research Institute, Reno, Nevada Abstract. In this study we present the microphysical characteristics of 21 jet contrail clouds sampled in situ and examine the possible effects of exhaust on natural cirrus and radiative effects of contrails. Microphysical samples were obtained with Particle Measuring Systems (PMS) 2D-C, 1D-C, and FSSP probes. About one half of the study contrails were generated by the sampling aircraft, a Cessna Citation, primarily at times of 3–15 min after generation; the source and age of the others is unknown. On average, the contrails contained particles of mean diameter of the order of 10 mm in concentrations exceeding 2 10,000 L 1. Contrails embedded in natural cirrus appeared to have little effect on the natural cloud microphysics. Anomalous diffraction theory was used to model radiative properties of sampled contrails. The contrail cirrus showed considerably more spectral variation in extinction and absorption efficiencies than natural cirrus because of the large numbers of small crystals in contrails. Embedded contrails also displayed greater emissivity and emission than natural cirrus and a greater spectral variation in transmission. 1. Introduction Illinois also showed locally significant radiative effects [Wend- land and Semonin, 1982]. The radiative impact of an aircraft Increasing levels of air traffic have raised concerns about the contrail is dependent to a large degree on the cloud micro- potential effects of aircraft exhaust on the climate. One possi- physics as the infrared and solar forcing are strongly dependent ble mechanism for effect is through changes in the radiative on particle numbers and sizes. In one of the few in situ micro- budget of the atmosphere resulting from the generation of physical studies to date, Knollenberg [1972] studied the char- contrail clouds or the modification of natural cirrus clouds by acteristics of a persistent contrail and its effects on the local exhaust products. The net effect of contrail clouds would de- water vapor budget. Measurements of particle sizes and num- pend on their duration, areal coverage, and radiative proper- bers were limited to particles larger than ;100 mm. A more ties. With the rapid growth of commercial jet traffic in the recent sampling of four contrails by Gayet et al. [1996a] found 1960s, the frequency and areal coverage of persistent contrail significant differences between the microphysical characteris- cirrus became quite marked [e.g., Changnon et al., 1980]. The tics of natural and contrail cirrus. Using probes with a mini- generation of “artificial cirrus” has even been proposed as a mum size resolution of 25 mm, large concentrations of small means of intentionally reducing surface temperatures [Johnson particles (,100 mm in diameter) were detected in the contrails. et al., 1969]. A possible contribution by contrails to climate Near-field measurements of contrail microphysics by Baum- change has been noted by several authors [e.g., Liou et al., gardner and Cooper [1994] characterized particle distributions 1990; Sassen, 1997; Smith et al., 1998], but it is not clear to submicron sizes with evidence for significant concentrations whether the net effect, if any, would be warming or cooling. of particles between 1 and 10 mm. Other evidence for small The modification of natural cirrus may result from the nucle- mean particle sizes in contrails has been seen in remote sensing ation of additional ice particles on exhaust aerosols or through by satellite [e.g., Gothe and Grassl, 1993] and by visual obser- increased competition between contrail and natural ice for vation [Sassen et al., 1989]. Most recently, the Subsonic Air- available water vapor. For contrails that form within natural craft Contrail and Cloud Effects Special Study (SUCCESS) cirrus clouds, the competition for atmospheric water vapor [Toon and Miake-Lye, 1998] was conducted in 1996 to learn may limit the maximum size of particles within the plume, as it did within the core of a contrail plume reported by Heymsfield more about the character of aircraft exhaust and contrail et al. [1998]. clouds. Analyses of both in situ [e.g., Heymsfield et al., 1998] The uncertainty surrounding climatic effects of contrails is and remote sensing measurements [e.g., Minnis et al., 1998] related in part to a dearth of adequate sampling of their radi- found that contrails initially contain large numbers of small ative impact and microphysical characteristics. Airborne radio- particles. Under favorable (humid) conditions the particles metric measurements by Kuhn [1970] indicated locally strong grow through vapor diffusion as they age and take on the (12%) reductions in net downwelling radiation below contrails. characteristics of natural cirrus. A rather extensive surface-based study of contrail effects over The impact of the exhaust aerosol upon the atmosphere is nearly instantaneous when conditions are favorable for contrail Copyright 1999 by the American Geophysical Union. formation. Karcher et al. [1996] argue that the nuclei for the Paper number 1999JD900109. contrail ice particles are formed primarily through the inter- 0148-0227/99/1999JD900109$09.00 action of soot and sulfuric acid produced by combustion. How- 12,077 12,078 POELLOT ET AL.: CONTRAIL MICROPHYSICS AND RADIATIVE IMPACT Table 1. Citation Instrumentation hygrometer was operating near its lower-temperature limit and will not be presented here. The CN counter data were adjusted Parameter Probe Range for diffusional losses, and a correction was applied to account Temperature Rosemount total temperature 2658Cto1508C for reduced counting efficiency at low pressures reported by Dew point EG&G cooled mirror 2508Cto1708C Zhang and Liu [1991]. Because of electronic response at the Pressure Rosemount 0–1034 mbar relatively high sampling speeds, the Particle Measuring System Cloud particles PMS 2D-C optical array 33–1056 mm PMS FSSP-100 2–47 mm (PMS) 2D-C probe had an effective lower size threshold of 66 PMS 1D-C optical array 20–600 mm mm and the 1D-C a threshold of 40 mm. 2 Condensation nuclei TSI 3760 alcohol condensing 0–104 cm 3 The FSSP is designed and calibrated to measure liquid water . m (D 0.01 m) spheres, and its response to ice particles is uncertain [e.g., Gardiner and Hallett, 1985], especially in high concentrations of larger vapor-grown crystals. However, while it is unlikely that ever, Twohy and Gandrud [1998] and Jensen et al. [1998] sug- the contrails contained any liquid water, there is evidence that gest that a significant number of ambient aerosol particles may the FSSP can provide quantitative estimates of the numbers also act as ice nuclei within the plume. As the exhaust plume and sizes of small ice particles when few large particles are ages, the nucleating effects of soot or other exhaust products present. This has been suggested in a previous study of natural are less certain. Schuman et al. [1996] and Hudson and Xie cirrus by Heymsfield and Platt [1984] and in the use of modified [1998] suggest that these aerosols may remain active as cloud versions of the FSSP-100 [Knollenberg et al., 1993; Baumgard- condensation or ice nuclei in the upper troposphere and influ- ner et al., 1992]. Pueschel et al. [1997] used FSSP data to size ence the subsequent formation of cirrus clouds. Pueschel et al. and count small ice crystals in tropical cirrus, supported by [1997] have argued that even in the absence of soot, sulfuric comparisons with replicator data. In polar stratospheric clouds, acid aerosol may later take on water, dilute, and then freeze Baumgardner et al. [1989] estimated errors in FSSP-derived homogeneously. These small ice particles may then be of a concentrations of around 40% and in sizing of about 20%. sufficient size and number to have a significant effect on radi- Gayet et al. [1996b] also concluded that the FSSP appears to ative transfer. provide quantitative information about contrail cloud micro- The present study examines the in situ data from 21 contrails physics, probably because the particles tend to be compact and sampled at altitudes from 9.3 to 12.5 km and temperatures of quasi-spherical. They caution, however, that significant sizing 2478Cto2668C. The microphysical characteristics of the con- and counting errors may occur when sampling larger, non- trails, which occurred in both clear and cloudy air, are pre- spherical ice crystals; the quality of overlap with 2D-C spectral sented and compared with natural cirrus properties. Distribu- data is a measure of the reliability of the FSSP data. Although tions of atmospheric aerosol are also examined along with the the FSSP and 2D-C size spectra from the Citation do not cloud microphysics to look for possible exhaust effects. Com- directly overlap, the 2D-C data can be extrapolated with the putations of the wavelength-dependent radiative properties of help of data from the 1D-C probe to estimate the fit with the the sampled particle distributions are then presented. Finally, FSSP. the climatic implications of these findings are discussed. The above studies support using the FSSP data to quantita- tively describe the sampled contrails, with consideration for the measurement uncertainties. Figure 1 shows a size distribution 2. Data from FSSP data for contrail F. This contrail was isolated (in The measurements were taken with probes mounted on the clear air), and no larger particles were detected by either the University of North Dakota Cessna Citation aircraft as it was 2D-C or the 1D-C probes. It is uncertain whether the roll-off flown on two separate field programs. The first program was of counts in the first channel (2.8–5.5 mm) is real or is due to the First International Satellite Cloud Climatology Program probe response.
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