@article{johnson_meskhidze_2013, title={Atmospheric dissolved iron deposition to the global oceans: effects of oxalate-promoted Fe dissolution, photochemical redox cycling, and dust mineralogy}, volume={6}, ISSN={["1991-9603"]}, DOI={10.5194/gmd-6-1137-2013}, abstractNote={Abstract. Mineral dust deposition is suggested to be a significant atmospheric supply pathway of bioavailable iron (Fe) to Fe-depleted surface oceans. In this study, mineral dust and dissolved Fe (Fed) deposition rates are predicted for March 2009 to February 2010 using the 3-D chemical transport model GEOS-Chem implemented with a comprehensive dust-Fe dissolution scheme. The model simulates Fed production during the atmospheric transport of mineral dust, taking into account inorganic and organic (oxalate)-promoted Fe dissolution processes, photochemical redox cycling between ferric (Fe(III)) and ferrous (Fe(II)) forms of Fe, dissolution of three different Fe-containing minerals (hematite, goethite, and aluminosilicates), and detailed mineralogy of wind-blown dust from the major desert regions. Our calculations suggest that during the year-long simulation ~0.26 Tg (1 Tg = 1012 g) of Fed was deposited to global oceanic regions. Compared to simulations only taking into account proton-promoted Fe dissolution, the addition of oxalate and Fe(II)/Fe(III) redox cycling to the dust-Fe mobilization scheme increased total annual model-predicted Fed deposition to global oceanic regions by ~75%. The implementation of Fe(II)/Fe(III) photochemical redox cycling in the model also allows for the distinction between different oxidation states of deposited Fed. Our calculations suggest that during the daytime, large fractions of Fed deposited to the global oceans is likely to be in Fe(II) form, while nocturnal fluxes of Fed are largely in Fe(III) form. Model sensitivity simulations suggest Fed fluxes to the oceans can range from ~50% reduction to ~150% increase associated with the uncertainty in Fe-containing minerals commonly found in dust particles. This study indicates that Fed deposition to the oceans is controlled by total dust-Fe mass concentrations, mineralogy, the surface area of dust particles, atmospheric chemical composition, cloud processing, and meteorological parameters and exhibits complex and spatiotemporally variable patterns. Our study suggests that the explicit model representation of individual processes leading to Fed production within mineral dust are needed to improve the understanding of the atmospheric Fe cycle, and quantify the effect of dust-Fe on ocean biological productivity, carbon cycle, and climate.
                    }, number={4}, journal={GEOSCIENTIFIC MODEL DEVELOPMENT}, author={Johnson, M. S. and Meskhidze, N.}, year={2013}, pages={1137–1155} }
 @article{meskhidze_petters_tsigaridis_bates_o'dowd_reid_lewis_gantt_anguelova_bhave_et al._2013, title={Production mechanisms, number concentration, size distribution, chemical composition, and optical properties of sea spray aerosols}, volume={14}, number={4}, journal={Atmospheric Science Letters}, author={Meskhidze, N. and Petters, M. D. and Tsigaridis, K. and Bates, T. and O'Dowd, C. and Reid, J. and Lewis, E. R. and Gantt, B. and Anguelova, M. D. and Bhave, P. V. and et al.}, year={2013}, pages={207–213} }
 @article{johnson_meskhidze_kiliyanpilakkil_2012, title={A global comparison of GEOS-Chem-predicted and remotely-sensed mineral dust aerosol optical depth and extinction profiles}, volume={4}, ISSN={["1942-2466"]}, DOI={10.1029/2011ms000109}, abstractNote={Dust aerosol optical depth (AOD) and vertical distribution of aerosol extinction predicted by a global chemical transport model (GEOS‐Chem) are compared to space‐borne data from the Moderate‐resolution Imaging Spectroradiometer (MODIS), Multi‐Angle Imaging SpectroRadiometer (MISR), and Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) for March 2009 to February 2010. Model‐predicted and remotely‐sensed AOD/aerosol extinction profiles are compared over six regions where aerosol abundances are dominated by mineral dust. Calculations indicate that over the regions examined in this study (with the exception of Middle Eastern dust sources) GEOS‐Chem predicts higher AOD values compared to MODIS and MISR. The positive bias is particularly pronounced over the Saharan dust source regions, where model‐predicted AOD values are a factor of 2 to 3 higher. The comparison with CALIPSO‐derived dust aerosol extinction profiles revealed that the model overestimations of dust abundances over the study regions primarily occur below ∼4 km, suggesting excessive emissions of mineral dust and/or uncertainties in dust optical properties. The implementation of a new dust size distribution scheme into GEOS‐Chem reduced the yearly‐mean positive bias in model‐predicted AOD values over the study regions. The results were most noticeable over the Saharan dust source regions where the differences between model‐predicted and MODIS/MISR retrieved AOD values were reduced from 0.22 and 0.17 to 0.02 and −0.04, respectively. Our results suggest that positive/negative biases between satellite and model‐predicted aerosol extinction values at different altitudes can sometimes even out, giving a false impression for the agreement between remotely‐sensed and model‐predicted column‐integrated AOD data.}, journal={JOURNAL OF ADVANCES IN MODELING EARTH SYSTEMS}, author={Johnson, Matthew S. and Meskhidze, Nicholas and Kiliyanpilakkil, Velayudhan Praju}, year={2012}, month={Jul} }
 @article{gantt_johnson_meskhidze_sciare_ovadnevaite_ceburnis_o'dowd_2012, title={Model evaluation of marine primary organic aerosol emission schemes}, volume={12}, ISSN={["1680-7316"]}, DOI={10.5194/acp-12-8553-2012}, abstractNote={Abstract. In this study, several marine primary organic aerosol (POA) emission schemes have been evaluated using the GEOS-Chem chemical transport model in order to provide guidance for their implementation in air quality and climate models. These emission schemes, based on varying dependencies of chlorophyll a concentration ([chl a]) and 10 m wind speed (U10), have large differences in their magnitude, spatial distribution, and seasonality. Model comparison with weekly and monthly mean values of the organic aerosol mass concentration at two coastal sites shows that the source function exclusively related to [chl a] does a better job replicating surface observations. Sensitivity simulations in which the negative U10 and positive [chl a] dependence of the organic mass fraction of sea spray aerosol are enhanced show improved prediction of the seasonality of the marine POA concentrations. A top-down estimate of submicron marine POA emissions based on the parameterization that compares best to the observed weekly and monthly mean values of marine organic aerosol surface concentrations has a global average emission rate of 6.3 Tg yr−1. Evaluation of existing marine POA source functions against a case study during which marine POA contributed the major fraction of submicron aerosol mass shows that none of the existing parameterizations are able to reproduce the hourly-averaged observations. Our calculations suggest that in order to capture episodic events and short-term variability in submicron marine POA concentration over the ocean, new source functions need to be developed that are grounded in the physical processes unique to the organic fraction of sea spray aerosol.
                    }, number={18}, journal={ATMOSPHERIC CHEMISTRY AND PHYSICS}, author={Gantt, B. and Johnson, M. S. and Meskhidze, N. and Sciare, J. and Ovadnevaite, J. and Ceburnis, D. and O'Dowd, C. D.}, year={2012}, pages={8553–8566} }
 @article{johnson_meskhidze_solmon_gasso_chuang_gaiero_yantosca_wu_wang_carouge_2010, title={Modeling dust and soluble iron deposition to the South Atlantic Ocean}, volume={115}, ISSN={["2169-8996"]}, DOI={10.1029/2009jd013311}, abstractNote={The global chemical transport model GEOS‐Chem, implemented with a dust‐iron dissolution scheme, was used to analyze the magnitude and spatial distribution of mineral dust and soluble‐iron (sol‐Fe) deposition to the South Atlantic Ocean (SAO). The comparison of model results with remotely sensed data shows that GEOS‐Chem can capture dust source regions in Patagonia and characterize the temporal variability of dust outflow. For a year‐long model simulation, 22 Tg of mineral dust and 4 Gg of sol‐Fe were deposited to the surface waters of the entire SAO region, with roughly 30% of this dust and sol‐Fe predicted to be deposited to possible high nitrate low chlorophyll oceanic regions. Model‐predicted dissolved iron fraction of mineral dust over the SAO was small, on average only accounting for 0.57% of total iron. Simulations suggest that the primary reason for such a small fraction of sol‐Fe is the low ambient concentrations of acidic trace gases available for mixing with dust plumes. Overall, the amount of acid added to the deliquesced aerosol solution was not enough to overcome the alkalinity buffer of Patagonian dust and initiate considerable acid dissolution of mineral‐iron. Sensitivity studies show that the amount of sol‐Fe deposited to the SAO was largely controlled by the initial amount of sol‐Fe at the source region, with limited contribution from the spatial variability of Patagonian‐desert topsoil mineralogy and natural sources of acidic trace gases. Simulations suggest that Patagonian dust should have a minor effect on biological productivity in the SAO.}, journal={JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES}, author={Johnson, Matthew S. and Meskhidze, Nicholas and Solmon, Fabien and Gasso, Santiago and Chuang, Patrick Y. and Gaiero, Diego M. and Yantosca, Robert M. and Wu, Shiliang and Wang, Yuxuan and Carouge, Claire}, year={2010}, month={Aug} }