Analyzing Surface Solar Flux Data in Oregon for Changes Due to Aerosols

The radiative impact of anthropogenic aerosol concentrations could be an important forcing to the earth's surface energy budget. That impact is uncertain, however, in large part because of a lack of aerosol measurements. The Solar Radiation Monitoring Lab has collected over 25 years of five minute resolution surface shortwave flux measurements at three sites in Oregon. Cloud-free measurements from these time-series are used to characterize the direct effect of aerosols on surface radiation in Oregon. This dataset is particularly valuable for aerosol studies because direct normal measurements are available in addition to total shortwave fluxes. Cloud- free days have been determined using the clear-sky identification method of Long and Ackerman (2000). The clear sky direct fluxes are then examined for the impact of aerosol changes over time. Measurements from low solar elevation angles are analyzed because of the higher sensitivity to scattering and absorbing by aerosols along the longer atmospheric path lengths at these angles. The impacts of events which are known to have high aerosol concentrations can be clearly seen. For example, the volcanic eruptions of El Chichón and Mt. Pinatubo reduce monthly averages of low solar elevation direct normal fluxes by 100-200 W/m2, a 12-25% reduction from typical values. Very little change is seen in the average background aerosol since 1980 when the measurements began. Limits are set on the possible amount of that change within the uncertainty of the measurements used. This analysis is strengthened by retrieving aerosol optical depths from the fluxes using radiative transfer calculations.