Stratospheric aerosol characteristics from space-borne observations: extinction coefficient and Ångström exponent

Abstract

Abstract. Stratospheric aerosols are of a great importance to the scientific community, predominantly because of their role in climate, but also because accurate knowledge of aerosol characteristics is relevant for trace gas retrievals from remote-sensing instruments. There are several data sets published which provide aerosol extinction coefficients in the stratosphere. However, for the instruments measuring in the limb-viewing geometry, the use of this parameter is associated with uncertainties resulting from the need to assume an aerosol particle size distribution (PSD) within the retrieval process. These uncertainties can be mitigated if PSD information is retrieved. While occultation instruments provide more accurate information on the aerosol extinction coefficient, in this study, it was shown that limb instruments are more sensitive to the smaller particles in the visible–near-infrared spectral range. However, the sensitivity of occultation instruments improves if the UV part of the wavelength spectrum is considered. A data set containing PSD information was recently retrieved from SCIAMACHY limb measurements and provides two parameters of the unimodal lognormal PSD for the SCIAMACHY operational period (2002–2012). In this study, the data set is expanded by aerosol extinction coefficients and Ångström exponents calculated from the retrieved PSD parameters. Parameter errors for the recalculated Ångström exponents and aerosol extinction coefficients are assessed using synthetic retrievals. For the extinction coefficient the resulting parameter error is within ±25 %, and for the Ångström exponent, it is better than 10 %. The SCIAMACHY aerosol extinction coefficients recalculated from PSD parameters are compared to those from SAGE II. The differences between the instruments vary from 0 % to 25 % depending on the wavelength. Ångström exponent comparison with SAGE II shows differences between 10 % at 31 km and 40 % at 18 km. Comparisons with SAGE II, however, suffer from the low number of collocated profiles. Furthermore, the Ångström exponents obtained from the limb-viewing instrument OSIRIS are used for the comparison. This comparison shows an average difference within 7 %. The time series of these differences do not show signatures of any remarkable events (e.g., volcanic eruptions or biomass burning events). In addition, the temporal behaviour of the Ångström exponent in the tropics is analyzed using the SCIAMACHY data set. It is shown that there is no trivial relation between the Ångström exponent value at a single wavelength pair and the PSD because the same value of Ångström exponent can be obtained from an infinite number of combinations of the PSD parameters.

Australian Lidar Measurements of Aerosol Layers Associated with the 2015 Calbuco Eruption

“…For comparison, our value was 18 ± 3%. Judging from the distributions of δ p provided by Prata et al [28] in panels a-c of their Figure 2, our observations were at the upper end of the distribution for Kasatochi and less than the lower end of the distribution for Puyehue. Ansmann et al [77] and Gasteiger et al [78] observed much higher δ p values of 35-40% for the Eyjafjallajökull eruption.…”

“…For the bright region of the middle layer at 14.6 km, S aer = 71 sr which is consistent with volcanic aerosol [28], but less than the lidar ratio for the upper layer (S aer = 92 sr), and higher than for cirrus [45]. Considering the relatively high humidity over ice at the height of the layer, and the appearance of weak aerosol scattering in its vicinity, we suggest that the feature represented a mix of volcanic aerosol and ice, with water vapor potentially undergoing ice nucleation because of the presence of the aerosol.…”

“…For the same Icelandic eruption, Kokkalis et al [27] found that the range-resolved Ångström exponent obtained between 355 nm and 532 nm ranged from 0.7 to 1.7 and varied as the volcanic layers travelled to the Eastern Mediterranean. Background stratospheric aerosols, on the other hand, show values of α between~2.4 and~3.4 [28], but we assume that the low extinction of these aerosols did not have a significant effect on our retrieval. The first step in the retrieval was the determination of the calibration constant C in Equation (1) at each wavelength.…”

“…Prata et al [28] provided a comprehensive evaluation of lidar ratio and depolarisation for the volcanic plumes associated with the Puyehue, Kasatochi and Sarychev eruptions using CALIOP data. Their analysis assumed η = 0.90 ± 0.05.…”

Issues related to the retrieval of stratospheric-aerosol particle size information based on optical measurements

“…Although satellite occultation instruments and groundbased lidar instruments typically take measurements at different wavelengths -e.g., 1020, 525 and 385 nm for SAGE II (Mauldin et al, 1985) compared to 1064, 532 and 355 nm for the ALOMAR (Arctic Lidar Observatory for Middle Atmosphere Research) Rayleigh-Mie-Raman lidar ) -we assume for simplicity the same pair of wavelengths for all observation geometries studied here, i.e., λ 1 = 1064 and λ 2 = 532 nm.…”