@article{royalty_phillips_dawson_reed_meskhidze_petters_2017, title={Aerosol Properties Observed in the Subtropical North Pacific Boundary Layer}, volume={122}, ISSN={["2169-8996"]}, DOI={10.1002/2017jd026897}, abstractNote={AbstractThe impact of anthropogenic aerosol on climate forcing remains uncertain largely due to inadequate representation of natural aerosols in climate models. The marine boundary layer (MBL) might serve as a model location to study natural aerosol processes. Yet source and sink mechanisms controlling the MBL aerosol number, size distribution, chemical composition, and hygroscopic properties remain poorly constrained. Here aerosol size distribution and water uptake measurements were made aboard the R/V Hi'ialakai from 27 June to 3 July 2016 in the subtropical North Pacific Ocean. Size distributions were predominantly bimodal with an average integrated number concentration of 197 ± 98 cm−3. Hygroscopic growth factors were measured using the tandem differential mobility analyzer technique for dry 48, 96, and 144 nm particles. Mode kappa values for these were 0.57 ± 0.12, 0.51 ± 0.09, and 0.52 ± 0.08, respectively. To better understand remote MBL aerosol sources, a new algorithm was developed which decomposes hygroscopicity distributions into three classes: carbon‐containing particles, sulfate‐like particles, and sodium‐containing particles. Results from this algorithm showed low and steady sodium‐containing particle concentrations while the sulfate‐like and carbon‐containing particle concentrations varied during the cruise. According to the classification scheme, carbon‐containing particles contributed at least 3–7%, sulfate‐like particles contributed at most 77–88% and sodium‐containing particles at least contributed 9–16% to the total aerosol number concentration. Size distribution and hygroscopicity data, in conjunction with air mass back trajectory analysis, suggested that the aerosol budget in the subtropical North Pacific MBL may be controlled by aerosol entrainment from the free troposphere.}, number={18}, journal={JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES}, author={Royalty, T. M. and Phillips, B. N. and Dawson, K. W. and Reed, R. and Meskhidze, N. and Petters, M. D.}, year={2017}, month={Sep}, pages={9990–10012} }
 @article{dawson_petters_meskhidze_petters_kreidenweis_2016, title={Hygroscopic growth and cloud droplet activation of xanthan gum as a proxy for marine hydrogels}, volume={121}, ISSN={["2169-8996"]}, DOI={10.1002/2016jd025143}, abstractNote={AbstractKnowledge of the physical characteristics and chemical composition of marine organic aerosols is needed for the quantification of their effects on cloud microphysical processes and solar radiative transfer. Here we use xanthan gum (XG)—a bacterial biopolymer—as a proxy for marine hydrogels. Measurements were performed for pure XG particles and mixtures of XG with sodium chloride, calcium nitrate, and calcium carbonate. The aerosol hygroscopicity parameter (κ) is derived from hygroscopic growth factor measurements (κgf) at variable water activity (aw) and from cloud condensation nuclei activation efficiency (κccn). The Zdanovskii, Stokes, and Robinson (ZSR) hygroscopicity parameter derived for multicomponent systems (κmix, sol) is used to compare measurements of κgf and κccn. Pure XG shows close agreement of κgf (at aw = 0.9) and κccn of 0.09 and 0.10, respectively. Adding salts to the system results in deviations of κgf (at aw = 0.9) from κccn. The measured κgf and ZSR‐derived hygroscopicity parameter (κmix, sol) values for different solutions show close agreement at aw > 0.9, while κgf is lower in comparison to κmix, sol at aw < 0.9. The differences between predicted κmix, sol and measured κgf and κccn values are explained by the effects of hydration and presence of salt ions on the structure of the polymer networks. Results from this study imply that at supersaturations of 0.1 and 0.5%, the presence of 30% sea salt by mass can reduce the activation diameter of pure primary marine organic aerosols from 257 to 156 nm and from 87 to 53 nm, respectively.}, number={19}, journal={JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES}, author={Dawson, K. W. and Petters, M. D. and Meskhidze, N. and Petters, S. Suda and Kreidenweis, S. M.}, year={2016}, month={Oct}, pages={11803–11818} }
 @article{meskhidze_johnson_hurley_dawson_2016, title={Influence of measurement uncertainties on fractional solubility of iron in mineral aerosols over the oceans}, volume={22}, ISSN={["2212-1684"]}, DOI={10.1016/j.aeolia.2016.07.002}, abstractNote={The atmospheric supply of mineral dust iron (Fe) plays a crucial role in the Earth's biogeochemical cycle and is of specific importance as a micronutrient in the marine environment. Observations show several orders of magnitude variability in the fractional solubility of Fe in mineral dust aerosols, making it hard to assess the role of mineral dust in the global ocean biogeochemical Fe cycle. In this study we compare the operational solubility of mineral dust aerosol Fe associated with the flow-through leaching protocol to the results of the global 3-D chemical transport model GEOS-Chem. According to the protocol, aerosol Fe is defined as soluble by first deionized water leaching of mineral dust through a 0.45 μm pore size membrane followed by acidification and storage of the leachate over a long period of time prior to analysis. To estimate the uncertainty in soluble Fe results introduced by the flow-through leaching protocol, we prescribe an average 50% (range of 30–70%) fractional solubility to sub-0.45 μm sized mineral dust particles that may inadvertently pass the filter and end up in the acidified (at pH ∼ 1.7) leachate for a couple of month period. In the model, the fractional solubility of Fe is either explicitly calculated using a complex mineral aerosol Fe dissolution equations, or prescribed to be 1% and 4% often used by global ocean biogeochemical Fe cycle models to reproduce the broad characteristics of the presently observed ocean dissolved iron distribution. Calculations show that the fractional solubility of Fe derived through the flow-through leaching is higher compared to the model results. The largest differences (∼40%) are predicted to occur farther away from the dust source regions, over the areas where sub-0.45 μm sized mineral dust particles contribute a larger fraction of the total mineral dust mass. This study suggests that different methods used in soluble Fe measurements and inconsistences in the operational definition of filterable Fe in marine environment and soluble Fe in atmospheric aerosols are likely to contribute to the wide range of fractional solubility of aerosol Fe reported in the literature.}, journal={AEOLIAN RESEARCH}, author={Meskhidze, Nicholas and Johnson, Matthew S. and Hurley, David and Dawson, Kyle}, year={2016}, month={Sep}, pages={85–92} }
 @article{dawson_meskhidze_josset_gasso_2015, title={Spaceborne observations of the lidar ratio of marine aerosols}, volume={15}, ISSN={["1680-7324"]}, DOI={10.5194/acp-15-3241-2015}, abstractNote={Abstract. Retrievals of aerosol optical depth (AOD) from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite sensor require the assumption of the extinction-to-backscatter ratio, also known as the lidar ratio. This paper evaluates a new method to calculate the lidar ratio of marine aerosols using two independent sources: the AOD from the Synergized Optical Depth of Aerosols (SODA) project and the integrated attenuated backscatter from CALIOP. With this method, the particulate lidar ratio can be derived for individual CALIOP retrievals in single aerosol layer, cloud-free columns over the ocean. Global analyses are carried out using CALIOP level 2, 5 km marine aerosol layer products and the collocated SODA nighttime data from December 2007 to November 2010. The global mean lidar ratio for marine aerosols was found to be 26 sr, roughly 30% higher than the current value prescribed by the CALIOP standard retrieval algorithm. Data analysis also showed considerable spatiotemporal variability in the calculated lidar ratio over the remote oceans. The calculated marine aerosol lidar ratio is found to vary with the mean ocean surface wind speed (U10). An increase in U10 reduces the mean lidar ratio for marine regions from 32 ± 17 sr (for 0 < U10 < 4 m s−1) to 22 ± 7 sr (for U10 > 15 m s−1). Such changes in the lidar ratio are expected to have a corresponding effect on the marine AOD from CALIOP. The outcomes of this study are relevant for future improvements of the SODA and CALIOP operational product and could lead to more accurate retrievals of marine AOD.
                    }, number={6}, journal={ATMOSPHERIC CHEMISTRY AND PHYSICS}, author={Dawson, K. W. and Meskhidze, N. and Josset, D. and Gasso, S.}, year={2015}, pages={3241–3255} }