Characterization of the Real Part of Dry Aerosol Refractive Index Over North America From the Surface to 12 km

Abstract

This study reports a characterization of the real part of dry particle refractive index (n) at 532 nm based on airborne measurements over the United States, Canada, the Pacific Ocean, and the Gulf of Mexico from the 2012 Deep Convective Clouds and Chemistry (DC3) and 2013 Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC 4 RS) campaigns. Effective n values are reported, with the limitations and uncertainties discussed. Eight air mass types were identified based on criteria related to gas-phase tracer concentrations, location, and altitude. Average values of n for these air types ranged from 1.50 to 1.53. Values of n for the organic aerosol (OA) fraction (n OA ) were calculated using a linear mixing rule for each air mass type, with 1.52 shown to be a good approximation for all OA. Case studies detailing vertical structure revealed that n and n OA increased with altitude, simultaneous with enhancements in the mass fraction of OA. Values of n OA were positively (negatively) correlated with the O:C (H:C) ratio in the absence of biomass burning influence; in contrast, the cumulative data set revealed a slight decrease in n OA as a function of the O:C ratio. The performance of parametric (multiple linear regression) and nonparametric (Gaussian process regression) methods in predicting n based on aerosol composition data is discussed. It is shown that even small perturbations in n values significantly impact aerosol optical depth retrievals, radiative forcing, and optical sizing instruments, emphasizing the importance of further improving the understanding of this important aerosol property.

Retrievals of Aerosol Size Distribution, Spherical Fraction, and Complex Refractive Index From Airborne In Situ Angular Light Scattering and Absorption Measurements

“…In most cases these properties are not known exactly and the OPC is calibrated with a generic reference aerosol, polystyrene latex spheres ( n 532 nm >1.6; Jones et al, ) in the case of all of the aforementioned studies. The particles sampled during SEAC 4 RS and DC3 have been shown, in this work and others (Aldhaif et al, ), to have a consistently lower real refractive index than that of polystyrene and will therefore produce a weaker scattering signature in the LAS. Moreover, most OPC calibration standards, including polystyrene spheres, are nonabsorbing, which is not the case for many natural aerosols.…”

“…A DMA was also used, this time in combination with a polar nephelometer, by Dick et al () to retrieve a value of n =1.49 for aerosols located in the SEUS during July and August. Measurements made during DC3 and SEAC 4 RS by the Differential Aerosol Sizing and Hygroscopicity Spectrometer Probe (DASH‐SP; Sorooshian et al, ) provided rarely obtained values of aerosol real refractive index above the surface layer (Aldhaif et al, ; Shingler et al, ; Sorooshian et al, ). The DASH‐SP instrument also paired a DMA with an OPC to obtain values of n typically ranging from 1.5 to 1.54.…”

Biomass Burning Plumes in the Vicinity of the California Coast: Airborne Characterization of Physicochemical Properties, Heating Rates, and Spatiotemporal Features

“…A Mie theory calculation (Bohren & Huffman, ) was used to estimate the aerosol scattering cross section and phase function for each PCASP size bin. The real refractive index is assumed to be 1.53 at all wavelengths consistent with airborne observations (at 532 nm) in BB plumes (Aldhaif et al, ; Shingler et al, ). The results were then combined with the observed aerosol size distributions to produce estimated profiles of total scattering and asymmetry factor across the 0.175–4.0 μm wavelength range.…”

Atmospheric Research Over the Western North Atlantic Ocean Region and North American East Coast: A Review of Past Work and Challenges Ahead

“…This type of effort can be enhanced even further by incorporating the effects of hygroscopic growth as well as refractive index determination (Sawamura et al, 2017). Refractive indices have been shown to be undercharacterized over the WNAO and other regions in general (e.g., Aldhaif et al, 2018;Ferrare et al, 1998;Shingler et al, 2016). These novel techniques represent an advance upon the prior methods that related column-integrated AOD to aerosol and CCN concentrations (Andreae, 2009;Shinozuka et al, 2015) or passive remote sensing techniques requiring strong assumptions as to cloud properties (Rosenfeld et al, 2016).…”