The origin of non-sea-salt sulphate in the Mount Logan ice core

Abstract

IT has been suggested¹ that oxidized sulphur compounds might play an important part in influencing climate, as they serve as cloud condensation nuclei and thus could affect the radiative properties of clouds. Schwartz², however, finds no evidence for a climate response arising from the increased concentrations of oxidized sulphur compounds resulting from the burning of fossil fuels. He contends that such a response should be detectable if these compounds are indeed important, as they are distributed widely in the atmosphere. Measurements of sulphate concentrations^3,4 in an ice core from Mount Logan (5,951 m), in northwestern Canada, indicate that the background concentration of non-sea-salt sulphate in snow deposited on the ice cap during the past century has remained nearly constant. Here we report ²¹⁰Pb/¹³⁷Cs ratios measured in the ice core and in soil cores collected at nearby low-altitude sites. As the ²¹⁰Pb/¹³⁷Cs ratio in sub-micrometre aerosols decreases with altitude, and as the non-sea-salt sulphate to²¹⁰Pb concentration ratios in snow deposited on Mount Logan are lower than values reported⁵ for aerosol samples collected at nearby high-latitude, low-altitude sites, our data indicate that the ice core contains sub-micrometre aerosols scavenged from the middle or upper troposphere, or both. Thus, the apparent lack of a secular increase in the non-sea-salt sulphate concentrations at Mount Logan suggests that anthropogenic oxidized sulphur compounds probably have not significantly affected a large part of the middle or upper troposphere (or both) in the remote Northern Hemisphere.