In-Situ Measurements of Aerosol Optical and Hygroscopic Properties at the Look Rock Site during SOAS 2013

One of the main goals of the Southern Oxidant and Aerosol Study (SOAS) is to characterize the climate-relevant properties of aerosols over the southeastern United States at the interface of biogenic and anthropogenic emissions. As part of the SOAS campaign, the UCD cavity ringdown/photoacoustic spectrometer was deployed to make in-situ measurements of aerosol light extinction, absorption and sub-saturated hygroscopicity at the Look Rock site (LRK) in the Great Smoky Mountains National Park, TN from June 1 to July 15, 2013. The site is influenced by substantial biogenic emissions with varying impacts from anthropogenic pollutants, allowing for direct examination of the optical and hygroscopic properties of anthropogenic-influenced biogenic secondary organic aerosols (SOA). During the experiment period, the average dry aerosol extinction (Bext), absorption (Babs) coefficients and single scattering albedo (SSA) at 532 nm were 30.3 × 16.5 Mm-1, 1.12 × 0.78 Mm-1 and 0.96 × 0.06. The Babs at 532 nm was well correlated (r2 = 0.79) with the refractory black carbon (rBC) number concentration determined by a single particle soot spectrometer (SP2). The absorption by black carbon (BC), brown carbon (BrC) and the absorption enhancement due to the 'lensing' effect were quantified by comparing the Babs of ambient and thermo-denuded aerosols at 405 nm and 532 nm. The optical sub-saturated hygroscopic growth factor was derived from extinction and particle size distribution measurements at dry and elevated relative humidity. In addition, to explore the extent to which ammonia mediated chemistry leads to BrC formation, as suggested in recent laboratory studies(1,2), we performed an NH3 perturbation experiment in-situ for 1 week during the study, in which ambient aerosols were exposed to approximately 100 ppb NH3 with a residence time of ~ 3hr. The broader implications of these observational data at LRK will be discussed in the context of the concurrent gas and aerosol chemical composition measurements as well as our previous findings for urban and biomass burning aerosols in other regions of the United States(3). 1. Bones, D. L. et al., Appearance of strong absorbers and fluorophores in limonene-O3 secondary organic aerosol due to NH4+-mediated chemical aging over long time scales, J. Geophy. Res., 115, D05203, 2010. 2. Updyke, K. M. et al., Formation of brown carbon via reaction of ammonia with secondary organic aerosols from biogenic and anthropogenic precursors, Atmos. Environ., 53, 22-31, 2012. 3. Cappa, C. D. et al., Radiative absorption enhancements due to the mixing state of atmospheric black carbon, Science, 337, 1078-1081, 2012.