Absorption and scattering characterization of airborne microparticulates by a cavity ringdown technique

Abstract

Nonresonant cavity ringdown laser absorption spectroscopy (CRLAS) was applied for detection and characterization of airborne particulates. Sensitive detection of a variety of aerosols under ambient conditions was achieved. The method provides, for the first time, time-resolved absolute aerosol concentration, with spatial resolution (along a line). The first report on absorption spectroscopy of monodispersed aerosols (in the size range 100-200 nm) is provided, and comparisons are made with the bulk data. The results indicate the possibility of applying CRLAS for selective analysis of aerosols. A new method for estimating the aerosol refraction index is also obtained from the ringdown data.

Sources of Error and Uncertainty in the Use of Cavity Ring Down Spectroscopy to Measure Aerosol Optical Properties

“…The experimental system used in this work consists of the same components as the majority of those to be found in the literature (Bulatov et al 2002(Bulatov et al , 2006Riziq et al 2007;Beaver et al 2008Beaver et al , 2010Dinar et al 2008;Freedman et al 2009;Lang-Yona et al 2009Radney et al 2009;Adler et al 2010;Nakayama et al 2010) and has been described in detail in previous publications (Miles et al 2010a(Miles et al , 2010b. Briefly, particles were generated from an aqueous suspension/solution of the aerosol medium (polystyrene beads or sodium chloride) using a constant output atomizer (TSI Model 3076), before being passed through a Nafion dryer (Perma Pure PD-100T-12SS, http://www.permapure.com/) to remove residual water.…”

“…Several different methods of drying have been reported in the cavity ring down literature to date, including mixing of the aerosol flow with large volumes of dry air (Radney et al 2009), use of Nafion dryers (Butler et al 2009;Miles et al 2010aMiles et al , 2010b or those containing molecular sieves, activated alumina or silica gel crystals (Baynard et al 2006;Bulatov et al 2006;Garland et al 2007;Riziq et al 2007;Spindler et al 2007;Riziq et al 2008;Thompson et al 2008;Adler et al 2010), or a combination of several of the above (Freedman et al 2009;Khalizov et al 2009;Xue et al 2009;Beaver et al 2010). The absorption crosssection of water at λ = 560 nm is very low (∼5 × 10 −27 cm 2 molec −1 (HITRAN)); thus, the effect of any residual solvent in our study is likely to be minimal; for τ 0 = 14 µs, a relative humidity of 50% at 20…”

“…It is assumed that within this region, aerosol particle number density is uniform both longitudinally and radially. The design of the RDC used in this work, analogous to the majority of those described to date (Bulatov et al 2002(Bulatov et al , 2006Garland et al 2007;Riziq et al 2007;Spindler et al 2007;Beaver et al 2008Beaver et al , 2010Dinar et al 2008;Riziq et al 2008;Freedman et al 2009;Khalizov et al 2009;LangYona et al 2009;Radney et al 2009;Xue et al 2009;Adler et al 2010), poses a challenge in determining the exact cavity length occupied by aerosol. To prevent mirror contamination, the aerosol flow cannot be introduced into or removed from the RDC directly over the mirrors, and consequently, the distance between the aerosol inlet and outlet ports is less than the total cavity length.…”

“…This highlights an important issue often neglected in publication of A-CRDS data, where the length of the cavity physically occupied by aerosol is frequently either omitted or stated without any reference as to whether it has been confirmed experimentally (Bulatov et al 2006;Garland et al 2007;Beaver et al 2008Beaver et al , 2010Thompson et al 2008;Freedman et al 2009;Baynard et al 2007;Riziq et al 2007;Spindler et al 2007;Dinar et al 2008;Riziq et al 2008;Khalizov et al 2009;Radney et al 2009;Xue et al 2009;Nakayama et al 2010). As the above discussion clearly shows, any uncertainty in the length of the cavity occupied by aerosol has a large impact on the values of the extinction efficiency retrieved by experiment and consequently on the refractive index returned by the fitting process.…”

“…A-CRDS is now routinely used in laboratory (Sappey et al 1998;Vander Wal and Ticich 1999;Smith and Atkinson 2001;Bulatov et al 2002Bulatov et al , 2003Bulatov et al , 2006Strawa et al 2003;Pettersson et al 2004;Baynard et al 2006;Garland et al 2007;Riziq et al 2007;Rudić et al 2007;Dinar et al 2008;Riziq et al 2008;Butler et al 2009;Freedman et al 2009;Khalizov et al 2009;Lang-Yona et al 2009Radney et al 2009;Xue et al 2009;Adler et al 2010;Hasenkopf et al 2010;Miles et al 2010aMiles et al , 2010b and field studies (Thompson et al 2002(Thompson et al , 2003Strawa et al 2006;Garland et al 2008;Nakayama et al 2010) to measure extinction by both aerosol ensembles (Vander Wal and Ticich 1999;Thompson et al 2002Thompson et al , 2003Thompson et al , 2008Strawa et al 2003;Pettersson et al 2004;Baynard et al 2006Baynard et al , 2007Garland et al 2007;Riziq et al 2007;Beaver et al 2008;…”