Effect of aerosols on surface UV at Socorro, New Mexico: measurements based on global irradiances and a direct sun photometer

E ect of aerosols on surface UV at So corro, New Mexico:Measurements based on global irradiances and a direct sunphotometerKen Minschwaner, Leah Varney, and Virginia StarkeDepartmentof Physics, New Mexico Institute of Mining and Technology, So corro, NMABSTRACTMeasurements of aerosol UV optical depths are describ ed as part of an ongoing study of surface ultraviolet irradiancesover the southwestern United States. Global UV irradiances are continuously monitored using a mo derate-bandwidthradiometer (Biospherical, GUV-511), which has b een in op eration since June 1997. Irradiances at 305 and 320 nm areused to derive column ozone; erythemal doses are determined with the additional consideration of 340 and 380-nmirradiances. The clear sky relationship b etween ozone and UV dose is well characterized bya poer lawwithanexp onent that decreases with increasing solar zenith angle, from 1.12 to 0.99 b etween solar zenith angles of 20 and 60degrees. Most recently, aerosol optical depths at 340 nm have b een estimated using standard direct sun techniquesapplied to mo del corrected global measurements. These are compared with direct sun measurements (Solar Light,Microtops I I) over a vemonth p erio d. Mean values agree well, but daily observations show di erences in aerosoloptical depths of up to 0.1, with direct sun measurements indicating larger variability. Aerosol optical depths inferredfrom global irradiances vary b etween a minimum of ab out 0.03 in winter and a maximum of 0.10 in summer.Keywords:UV Radiation, Aerosols, Ozone1. INTRODUCTIONThere are many factors that determine the intensity of solar ultraviolet radiation reaching the Earth's surface, primar-ily ozone absorption, solar zenith angle, molecular, aerosol and cloud scattering, and cloud and aerosol absorption.1Ground-based measurements have established the exp ected anticorrelation b etween ozone and UV exp osure underclear skies.2,3Reductions in surface UV due to clouds (typically 20 to 60%) are more variable b oth temp orallyand spatially,4,5and in some situations clouds mayeven induce lo cal increases in UV.6The UV reduction in thepresence non-absorbing aerosols is exp ected to b e smaller on average, ab out 10% for an aerosol optical thickness ofone.7Absorbing aerosols and dust may pro duce larger reductions, however. Under certain circumstances, changesin surface UV alb edo can play an imp ortantrole.8We rep ort here on measurements of global UV irradiances at So corro, New Mexico, lo cated in the U.S. desertsouthwest. The high frequency of clear skies p ermit a baseline to b e established for examining e ects of clouds andaerosols. A technique is describ ed for estimating aerosol optical thickness using global UV irradiances.2. MEASUREMENTSContinuous measurements of UV irradiances at 305, 320, 340, and 380 nm, with subsequent determination of columnozone and erythemal UV, have b een made from So corro, NM (34N, 107W), from June 1997 to present. Data iscollected using a mo derate-bandwidth UV radiometer (Biospherical GUV 511C) which senses downwelling, global(direct plus di use) irradiances incident on a horizontal Teon-quartz di user.The radiometer is temp eraturestabilized at 50C. Nominal sp ectral bandwidths of the four UV lter/detectors are 10 nm.Detector outputs are prop ortional to the integral of the pro duct of lter resp onses and incident sp ectral irra-diances. Voltages from each sensor are converted to irradiances at a xed waelength through the use of a linearcalibration function, with calibration constants determined through a solar intercomparison to a reference GUV in-strument at Biospherical Instruments, which is ultimately tied to a NIST-traceable 200W FEL Standard of Sp ectralIrradiance. The So corro GUV was initially calibrated in April 1997, with recalibrations in Septemb er 1998 and AprilFurther author information: (Send corresp ondence to Ken Minschwaner, Departmentof Physics, New Mexico Tech,So corro, NM, 87801.) E-mail: krm@kestrel.nmt.edu