Non-resonant scattered sunlight at twilight in Alaska

Abstract

The color and intensity of non-resonantly scattered sunlight in the sky at twilight is sensitive to the presence of upper atmospheric dust, or haze layers. Multi-spectral photometric measurements of the twilight sky at 20° elevation angle, made in central Alaska, indicate that a stratospheric dust layer formed by the eruption of Mt. Fuego in Guatemala (eruption date, October 1974) reached Alaska in late January 1975. Optical effects from the Fuego dust veil maximized in mid-February, then subsided slowly with a time constant of roughly one year. It is estimated that the Fuego dust layer, as observed in Alaska, was located in the lower sub-arctic stratosphere at a median elevation of 8 to 12 km and had a maximum optical thickness of about 0.015 in the red (700 nm), a columnar mass load of 4 ± 1 mg m-2 (assuming density of 1.6 g cm-3) and a vertical columnar number density of 107 particles cm-2. Analysis of the twilight radiance records (made continuously down to solar depression angles of 15°) was carried out with a theory of first order scattering with corrections introduced to account for multiple scattering. It was found that the twilight probing method is capable of yielding a somewhat smoothed vertical profile of dust layers having optical thicknesses larger than about 0.001 and located between about 5 to 60 km elevation. The inference of possible optically thin haze layers near the mesopause, which may be involved in the formation of noctilucent clouds, is complicated by resonant scattering processes and direct emission in the airglow, and sometimes by auroral emissions. The study suggests that the occurrence of periods with high volcanic activity may be capable of causing global climatic changes since the volcanic dust veils exist for relatively long periods (one to two years), reach all latitudes, and have sufficient optical depth to radiatively interact and cause climatically significant changes in the earth-atmosphere radiation budget. Calculations indicate a new global cooling caused by these volcanic dust veils originated in the stratosphere that, in the case of Fuego, could possibly reach ΔT = 0.2°C.