Atmospheric heating due to carbonaceous aerosol in northern Australia—confidence limits based on TOMS aerosol index and sun-photometer data

Abstract

The aerosol index (AI) derived from backscattered UV radiances measured by the total ozone mapping spectrometer (TOMS) provides global coverage spanning nearly 20 years. Published maps of significant global sources of UV-absorbing aerosol produced from AI do not include northern Australia, despite the extensive grass fires that persist throughout the dry season, a result that could be interpreted to mean that radiative heating by smoke in northern Australia is not significant. In this paper, we show that such is not the case. The radiative heating is intense, persists throughout most of the dry season, and is likely to moderate the local climate. Numerical simulations to determine confidence limits for radiative heating based solely on AI data show that large uncertainties remain, because AI does not characterize either the vertical distribution or optical properties of aerosol uniquely. However, when AI is combined with sun-photometer observations of aerosol optical depth (AOD) at Jabiru in Kakadu National Park, the uncertainty is reduced markedly. Analysis of the combined data sets shows that moderate amounts of aerosol (with AOD≈0.5) persist throughout September and October. The associated radiative heating cannot be neglected, even in comparison with the heating produced by water vapour. The dominant sources of uncertainty, namely, the single-scattering albedo and the vertical distribution of the aerosol, are quantified. Simulations show that the uncertainty may be reduced significantly if sun-photometer and AI data are analyzed in conjunction with LIDAR data to determine the vertical distribution of aerosol.