Lidar data. Lidar measurements from Nagoya, Japan (35.13°N, 136.88°E) 2, 6, 13, 19, and 27 December showed similar altitudes and peak backscattering ratios, but small secondary peaks were detected at 35-36 km only on the 13th and 19th. Both the altitude and the strength of peak backscatter measured at Garmisch-Partenkirchen, Germany were significantly higher on 1 January than for the very consistent readings of 10, 18, and 29 January. Late February-early March lidar data from Mauna Loa, Hawaii, Fukuoka, Japan, and Hampton, Virginia were similar to data at the same locations a month earlier. Integrated aerosol backscatter was considerably higher in Virginia than in Hawaii, suggesting that the bulk of the cloud had moved from the low latitudes where it was concentrated for several months after the March-April 1982 eruption.

Unusual sunrises and sunsets. From Tsukuba, Japan (36°N, 140°E) Toshio Fujita observed unusual twilight glows through January. Evening glows in December appeared redder than those in November, but by mid-January the red twilight colors were rapidly becoming lighter. A twilight photograph taken 24 January showed much paler colors than one taken 7 December at a time of similar solar depression angle. Despite the difference in color, the peak backscattering ratio measured 26 January was very close to the 8 December value. The maximum backscattering ratio increased from 17 (at 23 km altitude) on 8 December, to 28 on 28 December, but both heights of the strongest aerosol layers and their peak backscattering ratios were gradually descending by mid-January, and maximum backscattering was 16 on the 26th. Fujita attributed the differences in color at times of similar lidar readings to varying turbidity in the lower atmosphere. In mid-February, lidar at Tsukuba again measured relatively weak backscattering and the strongest aerosol layer had descended farther to about 20 km altitude.

Edward Brooks noted considerable variation in dawn and dusk colors from Jeddah, Saudi Arabia in February. Colorful early dawns 6-9 February indicated the presence of higher stratospheric aerosols, but the absence of unusual late dawn colors suggested that lower stratospheric aerosols were absent. This pattern reversed early 10 February, when no early dawn was evident but a colorful late dawn resulted from illumination of volcanic layers near the tropopause, visible as faint N-S bands. Similar bands were visible that evening, when aerosols could be observed both at the tropopause and higher in the stratosphere. Few unusual colors were visible early 11 February, but on the 12th both the upper and lower aerosol layers were illuminated. For the next several days, bands of material, generally trending SSW-NNE, were often observed at dawn and twilight with both early and late colors. Only higher aerosols were illuminated at dawn 19 February; some lower-altitude material appeared to be present early 20 February, but no unusual colors were evident that evening.

From Norwich, England, H. H. Lamb reported that on all cloudless days the sun continued to be surrounded by a white sheen of diffused light that seemed to be increasing steadily in extent, from about 20° in angular radius in mid-January to 25-30° as of 10 February. The sun itself often appeared nearly white at elevations of 5-15° and on partly cloudy days the sky was a paler gray than usual. Richard Keen observed no unusual sunsets from Boulder, Colorado between 13 January and 17 February.

Fred Schaaf observed increased optical effects in February from Millville, New Jersey after a notable weakening in January. By 15 February, late dawn colors (lower altitude aerosols) had returned to moderate levels. During the afternoon of 18 February, the sun was surrounded by a red-brown ring with a radius of about 30° that remained visible until shortly after sunset, when weather clouds obscured further observations. The next evening, twilight glow was stronger and Schaaf calculated that later glows seen for the first time since 31 January were produced by aerosols as high as about 16 km. Twilights were less impressive for the next few days, but similar effects were seen 23 February. On the 26th, the length of twilight glows indicated aerosols to 16-19 km. On 2 March, moderate to strong early twilight colors from material at about 16 km were followed by a very weak secondary glow that may have indicated the presence of aerosols to 32-40 km.

Balloon data - Wyoming. David Hofmann reported that balloon launches from Laramie, Wyoming continued to penetrate remnants of the extensive cloud of tiny aerosols at 29-35 km altitude first detected 28 January, and encountered a newly formed cloud of similar particles 2 March. The average radius of the 28 January particles was about 0.015 µm, with a few as large as 0.05-0.06 µm. Given the particle size distribution and a 30-50% drop in electrical conductivity measured within the cloud, Hofmann calculated a particle concentration of about 1,200/cm³. During the next week, this concentration dropped to about 100/cm³, a rate of decay corresponding closely to the expected rate of particle coagulation. About 90% of the particles disappeared when heated to 150°C, indicating that the cloud was composed of H₂SO4 and H₂O droplets. Remnants of this cloud were still present 2 March but had diffused and coagulated considerably, extending from 25-35 km altitude with a maximum concentration of about 50 particles per cm³. Although coagulation had increased the size of individual particles, they remained too small to be detected by lidar at these concentrations. Superimposed on the remnants of the 28 January cloud, a new cloud was detected 2 March. Sharply constrained between 31 and 34 km altitude, the new cloud reached concentrations of about 300 particles per cm³, and appeared to be about 4-5 days old.

Hofmann noted that between 25 and 35 km altitude, liquid H₂SO4 is vulnerable to vaporization if its temperature is raised slightly. If cooled again, it would then recondense into tiny droplets. High-altitude wind data indicated that the 28 January cloud originated in the Alaska-Siberia area, in a zone of 30-40°C stratospheric warming. From this warm area, the cloud reached Wyoming in about 30 hours, carried by 200 km/hour winds. Cooling of about 40°C probably occurred during transport, sufficient to recondense the H₂SO4. Similar clouds have been detected for the past several years, usually in the spring, but the 28 January cloud had particle concentrations 15 times as high as clouds seen in 1982, which in turn were 5 times as concentrated as 1981 clouds (Rosen and Hofmann, 1983).

The much larger particles from the original El Chichón cloud remained evident over Wyoming. During the most recent fully-analyzed sounding on 11 February, a very broad layer extended from the tropopause (11 km) to about 27 km with a peak concentration of 10 particles (larger than 0.15 µm) per cm³ at 19 km altitude. A sounding in an equatorial airmass, on 10 March, showed a similar profile, with a peak concentration of 8 particles per cm³ at 19-20 km altitude and the top of the cloud at roughly 27 km. The profile included a 1 km-thick zone of very clean air (about 1/2 particle per cm³) centered at about 15 km altitude, probably tropospheric in origin.

Reference. Rosen, J. M., and Hofmann, D. J., 1983, Unusual behavior in the condensation nuclei concentration at 30 km: JGR, v. 88, p. 3725-3731.

Information Contacts: D. Hofmann, Univ. of Wyoming; W. Fuller, NASA; T. DeFoor, MLO; M. Hirono, Kyushu Univ., Japan; R. Reiter, Garmisch-Partenkirchen, W. Germany; E. Brooks, Saudi Arabia; H. Lamb, Univ. of East Anglia, England; T. Fujita, Meteorological Research Inst., Japan; S. Hayashida, Nagoya Univ., Japan; F. Schaaf, Millville NJ; R. Keen, Univ. of Colorado.