Broadband measurements of aerosol extinction in the ultraviolet spectral region

Abstract

Aerosols influence the Earth's radiative budget by scattering and absorbing incoming solar radiation. The optical properties of aerosols vary as a function of wavelength, but few measurements have reported the wavelength dependence of aerosol extinction cross sections and complex refractive indices. We describe a new laboratory instrument to measure aerosol optical extinction as a function of wavelength, using cavity enhanced spectroscopy with a broadband light source. The instrument consists of two broadband channels which span the 360–390 and 385–420 nm spectral regions using two light emitting diodes (LED) and a grating spectrometer with charge-coupled device (CCD) detector. We determined aerosol extinction cross sections and directly observed Mie scattering resonances for aerosols that are purely scattering (polystyrene latex spheres and ammonium sulfate), slightly absorbing (Suwannee River fulvic acid), and strongly absorbing (nigrosin dye). We describe an approach for retrieving refractive indices as a function of wavelength from the measured extinction cross sections over the 360–420 nm wavelength region. The retrieved refractive indices for PSL and ammonium sulfate agree within uncertainty with the literature values for this spectral region. The refractive index determined for nigrosin is 1.78 (&plusmn; 0.03) + 0.19 (&plusmn; 0.08)<i>i</i> at 360 nm and 1.63 (&plusmn; 0.03) + 0.21 (&plusmn; 0.05)<i>i</i> at 420 nm. The refractive index determined for Suwannee River fulvic acid is 1.71 (&plusmn; 0.02) + 0.07 (&plusmn; 0.06)<i>i</i> at 360 nm and 1.66 (&plusmn; 0.02) + 0.06 (&plusmn; 0.04)<i>i</i> at 420 nm. These laboratory results support the potential for a field instrument capable of determining ambient aerosol optical extinction, average aerosol extinction cross section, and complex refractive index as a function of wavelength

Error Analysis and Uncertainty in the Determination of Aerosol Optical Properties Using Cavity Ring-Down Spectroscopy, Integrating Nephelometry, and the Extinction-Minus-Scattering Method

“…Using the derivation presented in the SI, an R L value of 1.086 ± 0.009 was found. This results in a relative error of 0.8%, which is somewhat larger than <0.5% uncertainty suggested by Miles et al (2011) but smaller than the 2% suggested by Washenfelder et al (2013). While the mechanically measured value of 1.073 was close to this (within two standard deviations), greater confidence can be attributed to this measurement because it has a definite and propagatable error, and it takes into account any mixing of particle-laden air and purge gas.…”

“…An alternative method was used in this work, and is documented here in the SI. Though previously mentioned (Washenfelder et al 2013), it has not been described in any detail. The ring-down time is measured in the absence of an absorber, in the presence of an absorber under normal flow conditions, and when allowing the absorber to fill the cavity.…”

“…Accurate broadband measurements of aerosol optical properties over the entire solar spectrum are technologically challenging (Ramanathan and Carmichael 2008;Bond et al 2009Bond et al , 2013. We conclude that other cavityenhanced techniques, where a broadband light source is used in a wavelength-resolved (Washenfelder et al 2013) or unresolved (LED-based) manner, do not need a wavelength-dependent correction to reconcile scattering and extinction.…”

A broadband cavity enhanced absorption spectrometer for aircraft measurements of glyoxal, methylglyoxal, nitrous acid, nitrogen dioxide, and water vapor

“…Unlike in previous designs (Wagner et al, 2011;Washenfelder et al, 2013), the ACES instrument does not include mirror purges of dry gas for mirror cleanliness. The mirror purges were found to be unnecessary during SENEX, SONGNEX, and UBWOS due to a combination of reduced cell pressure, aerosol filtration, and cell temperature (controlled at 30 • C), and their absence eliminates the small uncertainties from flow dilution and the relative sample length occupied by sample gas and purge gas over the length of the cavity.…”

“…K.-E. Min et al: Aircraft broadband cavity enhanced absorption spectrometer Venables et al, 2006;Gherman et al, 2008;Washenfelder et al, 2008;Langridge et al, 2009;Watt et al, 2009;Thalman and Volkamer, 2010;Axson et al, 2011;Chen and Venables, 2011;Kahan et al, 2012), as well as wavelength-dependent aerosol extinction (Thompson and Spangler, 2006;Washenfelder et al, 2008Washenfelder et al, , 2013Thalman and Volkamer, 2010;Varma et al, 2013). Groundand ship-based BBCEAS field instruments have been used to measure CHOCHO and NO 2 (Washenfelder et al, 2011a;Coburn et al, 2014), HONO and NO 2 (Washenfelder et al, 2011a;Young et al, 2012), NO 3 , and N 2 O 5 (Benton et al, 2010).…”