A Three-Wavelength Optical Extinction Cell for Measuring Aerosol Light Extinction and Its Application to Determining Light Absorption Coefficient

摘要

An optical extinction cell (OEC) was used to measure aerosol particle extinction coefficient sigma_(EP) at three wavelengths,467,530,and 660 nm.The details of the design and the results of its use in the Reno Aerosol Optics Study (RAOS) in June 2002 are presented.The OEC agreed well with the scattering coefficient sigma_(SP) measured using an integrating nephelometer for nonabsorbing aerosols.Linear regression of 1 min-averaged sigma_(EP) versus sigma_(SP) yielded slopes 1.00 +-0.02 and offsets <4 Mm~(-1) in the range approx 50 to 900 Mm~(-1).The difference,( sigma_(EP)- sigma_(SP)) was taken as a standard measure of absorption coefficient,sigma_(AP).The noise calculated as the standard deviation of (sigma_(EP) - sigma_(SP)) from 1 min-averaged white aerosol data was about 10,11,and 5 Mm~(-1) for the blue,green and red wavelengths,respectively.The absorption coefficients calculated from sigma_(EP) - sigma_(SP) were compared with the absorption coefficients measured using a pho-toacoustic instrument.In general,the two methods agreed better than 10%.The data gives also a way for estimating indirectly the relative uncertainty of the nephelometer.The theoretical relative uncertainties of the absorption coefficient were closest to the measured ones when the relative uncertainty of the scattering coefficient