Sensitivity of Aerosol Refractive Index Retrievals Using Optical Spectroscopy

Abstract

Accurate refractive index values are required to determine the effects of aerosol particles on direct radiative forcing. Theoretical retrievals using extinction data alone or extinction plus absorption data have been simulated to determine the sensitivity of each retrieval. A range of aerosol types with a range of different refractive indices were considered. The simulations showed that the extinction-only retrieval was not able to accurately or precisely retrieve refractive index values, even for purely scattering compounds, but the addition of a simulated absorption measurement greatly improved the retrieval.

Assessing the accuracy of complex refractive index retrievals from single aerosol particle cavity ring-down spectroscopy

“…The value of jDnj from comparison to coarse mode data is 0.0045 ( §0.0015). This level of accuracy (»0.3%) remains better than typical ensemble measurements; Zarzana et al (2014) estimated the level of accuracy as 0.6%, while Miles et al (2011b). predicted at 2.5% level of accuracy when considering the systematic errors in particle sizing only.…”

“…However, a fraction of the particles in the ensemble are multiply charged with larger size but the same electrical mobility; thus, the resulting size distributions selected by the DMA can be described as multi-modal (Miles et al 2011b). The combination of imprecise s ext measurements with the presence of multiply charged particles of larger size can result in poor RI retrievals unless appropriately accounted for in the analysis of the E-CRDS data (Miles et al 2011b;Mason et al 2012;Zarzana et al 2014). Furthermore, analysis of E-CRDS measurements typically neglects the broadening of measured s ext values due to the effect of the CRDS standing wave Mason et al 2014).…”

“…Zarzana and co-workers address the question of how accurately the RI needs to be characterized for accurate calculations of RFE (Zarzana et al 2014). To calculate RFE with an uncertainty <1%, the error in n must be <0.003 (0.2% of true n) for nonabsorbing ammonium sulfate particles with radii of either 75 or 100 nm, and the error in k is required to be <0.0005 (1% of true k) for absorbing brown carbon particles.…”

“…Miles et al published a thorough review of the sources of errors and uncertainties in RI retrievals from E-CRDS measurements, and argued that errors in the measured particle number density can alone introduce an error of »2.5% in determined values of n for purely scattering compounds (Miles et al 2011b). Meanwhile, Zarzana and co-workers performed a comprehensive theoretical study to assess the lower-bound estimates of errors in retrieved RIs from E-CRDS measurements on both non-absorbing and absorbing aerosol (Zarzana et al 2014). In the case of non-absorbing ammonium sulfate aerosol, for which extinction measurements were performed at twelve discrete values of size, the error in the fitted n was »0.6%.…”

Fundamental Review to Ozone Gas Sensing Using Optical Fibre Sensors

“…While discussion of this paper is focus on absorption spectroscopy, there are other classes of optical sensors such as:  Reflection spectroscopy [33,34]  Luminescence intensity spectroscopy [35]  Fluorescence lifetime spectroscopy [36]  Refractive index spectroscopy [37]  Surface Plasmon resonance or ellipsometric spectroscopy [38] The classifications are meant to give a clear picture and are not discussed further. Ozone, the gas of interest in this article, absorbs light intensity and hence absorption spectroscopy is dwelled upon in other sections of this article.…”