A Long-Term Record of Aerosol Optical Depth from TOMS Observations and Comparison to AERONET Measurements

TLDR

TOMS backscattered near‑UV observations from Nimbus‑7 and Earth Probe satellites have been used to produce a long‑term record of aerosol optical depth over oceans and continents. The retrieval exploits near‑UV’s low land reflectivity for continental coverage and its high sensitivity to UV‑absorbing aerosols, enabling separation of carbonaceous and mineral particles from scattering aerosols, and is validated against AERONET ground‑based measurements. TOMS aerosol optical depth over land (1996‑2000) agrees reasonably with AERONET across diverse aerosol types, with UV‑absorbing values within 30 % and non‑absorbing within 20 %, establishing the first long‑term nearly global climatology from 1979 to present.

Abstract

Observations of backscattered near-ultraviolet radiation from the Total Ozone Mapping Spectrometer (TOMS) on board the Nimbus-7 (1979–92) and the Earth Probe (mid-1996 to present) satellites have been used to derive a long-term record of aerosol optical depth over oceans and continents. The retrieval technique applied to the TOMS data makes use of two unique advantages of near-UV remote sensing not available in the visible or near-IR: 1) low reflectivity of all land surface types (including the normally bright deserts in the visible), which makes possible aerosol retrieval over the continents; and 2) large sensitivity to aerosol types that absorb in the UV, allowing the clear separation of carbonaceous and mineral aerosols from purely scattering particles such as sulfate and sea salt aerosols. The near-UV method of aerosol characterization is validated by comparison with Aerosol Robotic Network (AERONET) ground-based observations. TOMS retrievals of aerosol optical depth over land areas (1996–2000) are shown to agree reasonably well with AERONET sun photometer observations for a variety of environments characterized by different aerosol types, such as carbonaceous aerosols from biomass burning, desert dust aerosols, and sulfate aerosols. In most cases the TOMS-derived optical depths of UV-absorbing aerosols are within 30% of the AERONET observations, while nonabsorbing optical depths agree to within 20%. The results presented here constitute the first long-term nearly global climatology of aerosol optical depth over both land and water surfaces, extending the observations of aerosol optical depth to regions and times (1979 to present) not accessible to ground-based observations.