Determining the index of refraction of polar stratospheric clouds above Andoya (69°N) by combining size‐resolved concentration and optical scattering measurements

Abstract

Observations within two polar stratospheric clouds (PSCs) of aerosol scattering and size‐resolved aerosol concentration were compared to infer the index of refraction of the PSC particles. The observations were completed in situ with balloon‐borne aerosol counters and a laser scatterometer (692, 830 nm) and remotely with an ozone (308, 353 nm) and Rayleigh (532, 1064 nm) lidar. A Monte Carlo analysis, accounting for the errors of the individual measurements, indicates the comparison method has a precision of ±0.03 for an index of refraction range of 1.30–1.60. Measurements from all instruments were obtained in one PSC with relatively little vertical or horizontal structure. The comparison suggested that the index of refraction of the PSC particles was near 1.47±0.01 in the nondepolarizing region of the cloud and 1.52–1.56±0.04 in the depolarizing region. These values were consistent for the observations at 308, 353, 692, and 830 nm. The comparisons with the Rayleigh lidar were not as consistent. Aerosol volumes inferred from the particle measurements agree closely with volumes expected for liquid ternary aerosol (LTA) at the base of the cloud, with nitric acid trihydrate (NAT) above 23 km, in the depolarizing region, and with both LTA and NAT in the bulk of the nondepolarizing portion of the cloud. A much more limited set of measurements was obtained in a second PSC with strong vertical structure, evident in the temperature and aerosol profiles. Comparisons in this cloud were difficult because of the inherent problems in comparing in situ and remote measurements in clouds with strong vertical and horizontal structure. In this PSC the comparisons of in situ aerosol size distribution and remote aerosol scattering did not converge to a clear index of refraction.