Satellite remote sensing of H₂SO₄ aerosol using the 8‐ to 12‐μm window region: Application to Mount Pinatubo

Abstract

Monitoring stratospheric aerosols containing H 2 SO 4 using the brightness temperature (BT) difference between 11 and 8.3 μm (BT 8 –BT 11 ) spectral channels is demonstrated using theoretical calculations and satellite observations. Assuming an aqueous solution of 50% and 75% sulfuric acid, radiative transfer calculations indicate that over oceans an increase in the optical depth of the stratospheric aerosol results in an increase in BT 8 –BT 11 . Theoretical simulations suggest that the technique is sensitive to visible optical depths greater than approximately 0.15. The simulations also demonstrate a lack of sensitivity to the particle size distribution. Changes in pre‐ and post‐Pinatubo observations by the HIRS2 on board the NOAA 10 are consistent with observed optical depth measurements and confirm the sensitivity of these channels to the presence of the aerosol. The technique is also applied to cold tropical convective clouds and desert regions where the signal, though evident, is less conclusive. Time series analysis is applied to the NOAA 10 and NOAA 12 combined BT 8 –BT 11 observations to detect the periodicity of the spread of the volcanic aerosol. Over a region of the southern Pacific a 18‐ to 26‐day period is present. Model simulations were conducted to demonstrate a trispectral technique with observations near 8, 11, and 12 μm. The trispectral approach has high potential in that the spectral signature of cirrus, water vapor, and H 2 SO 4 aerosols are different. Observations from NOAA 10 and NOAA 11 are combined to demonstrate the capabilities of these infrared wavelengths of detecting the aerosol. The signal is clearly evident when a region of the South Atlantic is compared for pre‐ and post‐Pinatubo conditions.