Aerosol-Induced Albedo Change: Measurement and Modeling of an Incident

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Abstract

Many theoretical studies have shown that aerosol-induced changes in the earth-atmosphere albedo might be an important climate change mechanism. However, there has been a lack of experimental documentation of albedo changes caused by actual aerosol layers with measured properties. Here we report an incident in which the measured surface-plus-atmosphere albedo was increased by about 0.01 (from 0.11 to 0.12) by a transient aerosol layer. We also report simultaneous measurements of the aerosol by a multi-wavelength sunphotometer, a lidar, a nephelometer and other radiometers, and we use these aerosol measurements to deduce an expected albedo change for comparison to the measurements.

Specifically, we combine the aerosol measurements with several assumed refractive indices to derive a time-dependent aerosol optical model for the day of the incident. We then use this model in a two-stream radiative calculation to compute the expected time-dependent aerosol-layer albedo. Finally, we compute aerosol-plus-surface albedos by modifying a familiar expression to account for changing solar zenith angle and the diffuseness of surface reflectivity. Use of the aerosol model in this expression yields a calculated time-dependent atmosphere-plus-surface albedo that agrees with the measurements, provided an aerosol refractive index of about 1.50−0.01i is assumed. This refractive index value is in accord with the aerosol backscatter-to-extinction ratios measured simultaneously by the lidar and sunphotometer.

To our knowledge, this incident is the first in which an aerosol-induced albedo change and the responsible aerosol have been simultaneously measured to this degree of detail. Although the incident was too brief to be climatically significant, the analysis is significant because it provides a practical methodology for incorporating measured properties of aerosol layers into efficient albedo-change calculations. This methodology, which uses ground-based measurements to characterize elevated aerosol layers, could be applied to more widespread and persistent (hence, climatically significant) aerosol layers. Moreover, the agreement between measured and calculated albedos in this incident provides an initial validation of the methodology for not uncommon surface and aerosol conditions. More general measurements, including better complex refractive index determinations, are required to further validate and apply the methodology.

Abstract

Many theoretical studies have shown that aerosol-induced changes in the earth-atmosphere albedo might be an important climate change mechanism. However, there has been a lack of experimental documentation of albedo changes caused by actual aerosol layers with measured properties. Here we report an incident in which the measured surface-plus-atmosphere albedo was increased by about 0.01 (from 0.11 to 0.12) by a transient aerosol layer. We also report simultaneous measurements of the aerosol by a multi-wavelength sunphotometer, a lidar, a nephelometer and other radiometers, and we use these aerosol measurements to deduce an expected albedo change for comparison to the measurements.

Specifically, we combine the aerosol measurements with several assumed refractive indices to derive a time-dependent aerosol optical model for the day of the incident. We then use this model in a two-stream radiative calculation to compute the expected time-dependent aerosol-layer albedo. Finally, we compute aerosol-plus-surface albedos by modifying a familiar expression to account for changing solar zenith angle and the diffuseness of surface reflectivity. Use of the aerosol model in this expression yields a calculated time-dependent atmosphere-plus-surface albedo that agrees with the measurements, provided an aerosol refractive index of about 1.50−0.01i is assumed. This refractive index value is in accord with the aerosol backscatter-to-extinction ratios measured simultaneously by the lidar and sunphotometer.

To our knowledge, this incident is the first in which an aerosol-induced albedo change and the responsible aerosol have been simultaneously measured to this degree of detail. Although the incident was too brief to be climatically significant, the analysis is significant because it provides a practical methodology for incorporating measured properties of aerosol layers into efficient albedo-change calculations. This methodology, which uses ground-based measurements to characterize elevated aerosol layers, could be applied to more widespread and persistent (hence, climatically significant) aerosol layers. Moreover, the agreement between measured and calculated albedos in this incident provides an initial validation of the methodology for not uncommon surface and aerosol conditions. More general measurements, including better complex refractive index determinations, are required to further validate and apply the methodology.

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