Quantifying aerosol direct effects from broadband and spectral irradiance observations

 

Authors

Torreon N Creekmore — National Geospatial-Intelligence Agency
Chuck N. Long (deceased) — NOAA- Earth System Research Laboratory

Category

Aerosol-Cloud-Radiation Interactions

Description

We outline a methodology using broadband and spectral irradiance observations to quantify aerosol direct effects on the diffuse shortwave (SW) surface irradiance. The data span an 11-year (January 1998–May 2010) timeframe at the Department of Energy Atmospheric Radiation Measurement (ARM) Climate Research Facility Southern Great Plains (SGP) site. Identified irradiances (Long and Ackerman 2000) and aerosol optical depth (τa), for solar zenith angles ≤ 65° are used to derive an empirical formulation to estimate clear-sky diffuse irradiance. Our approach estimates aerosol effects using ancillary input regarding changes in τa; Long and Ackerman (2000) excludes such input. The method is verified using irradiance observations from SGP’s Basic Radiation System (BRS). We evaluated the degree of accuracy of the considered correlation to fit the BRS observed data using the root mean square error (RMSE; non-systematic error) and mean bias error (MBE; systematic error). Resulting BRS diffuse irradiances were in accordance with estimates, producing a RMSE and MBE of 4 W/m2 and 1.4 W/m2, respectively. Frequency histograms of the absolute difference between the BRS and estimated diffuse show 99% of estimates are within ±10 W/m2 of BRS observations. The clear-sky diffuse estimates are used to derive quantitative estimates of aerosol radiative effects as a difference between measured and clear-sky (i.e., background aerosol conditions; τa = 0.008) amounts, represented as aerosol diffuse irradiance (ADI). The diurnal mean ADI ranges from about -50 W/m2 to 120 W/m2, implying notable altering in the relative proportion of the diffuse fraction due to scattering processes from changes in τa. Negative estimates are due to possible misidentification of clear skies, measurement errors, vagaries in fitting, and errors in τa retrieval. The estimated slope for the ADI as a function of τa was 229.59 W/m2 per unit τa, indicating an increase of ~23 W/m2 in diffuse SW per unit τa. This positive relationship suggests significant increases in the diffuse fraction in the presence of aerosols could possibly increase photosynthesis at SGP (Xia et al. 2007). Estimates of aerosol effects on the diffuse fraction are required to understand how much direct forcing is canceled out, how increased scattering at the surface leads to more efficient use of sunlight by plant canopies, and how much of the diffuse fraction increases under varying aerosol conditions (e.g., urban, rural, and oceanic).