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Wells, KC, Witek M, Flatau P, Kreidenwei SM, Westphal DL.  2007.  An analysis of seasonal surface dust aerosol concentrations in the western US (2001-2004): Observations and model predictions. Atmospheric Environment. 41:6585-6597.   10.1016/j.atmosenv.2007.04.034   AbstractWebsite

Long-term surface observations indicate that soil dust represents over 30% of the annual fine (particle diameter less than 2.5 mu m) particulate mass in many areas of the western US; in spring and summer, it represents an even larger fraction. There are numerous dust-producing playas in the western US, but surface dust aerosol concentrations in this region are also influenced by dust of Asian origin. This study examines the seasonality of surface soil dust concentrations at 15 western US sites using observations from the Interagency Monitoring of PROtected Visual Environments (IMPROVE) network from 2001 to 2004. Average soil concentrations in particulate matter less than 10 mu m in diameter (PM 10) were lowest in winter and peaked during the summer months at these sites; however, episodic higher-concentration events (> 10 mu g m(-3)) occurred in the spring, the time of maximum Asian dust transport to the western US. Simulated surface dust concentrations from the Navy Aerosol Analysis and Prediction System (NAAPS) suggested that long-range transport from Asia dominates surface dust concentrations in the western US in the spring, and that, although some long-range transport does occur throughout the year (1-2 mu g m(-3)), locally generated dust plays a larger role in the region in summer and fall. However, NAAPS simulated some anomalously high concentrations (> 50 mu g m(-3)) of local dust in the fall and winter months over portions of the western US. Differences between modeled and observed dust concentrations were attributed to overestimation of total observed soil dust concentrations by the assumptions used to convert IMPROVE measurements into PM(10) soil concentrations, lack of inhibition of model dust production in snow-covered regions, and lack of seasonal agricultural sources in the model. (c) 2007 Elsevier Ltd. All rights reserved.

Welton, EJ, Voss KJ, Quinn PK, Flatau PJ, Markowicz K, Campbell JR, Spinhirne JD, Gordon HR, Johnson JE.  2002.  Measurements of aerosol vertical profiles and optical properties during INDOEX 1999 using micropulse lidars. Journal of Geophysical Research-Atmospheres. 107   10.1029/2000jd000038   AbstractWebsite

[1] Micropulse lidar (MPL) systems were used to measure aerosol properties during the Indian Ocean Experiment (INDOEX) 1999 field phase. Measurements were made from two platforms: the NOAA ship R/V Ronald H. Brown, and the Kaashidhoo Climate Observatory (KCO) in the Maldives. Sun photometers were used to provide aerosol optical depths (AOD) needed to calibrate the MPL. This study focuses on the height distribution and optical properties (at 523 nm) of aerosols observed during the campaign. The height of the highest aerosols (top height) was calculated and found to be below 4 km for most of the cruise. The marine boundary layer (MBL) top was calculated and found to be less than 1 km. MPL results were combined with air mass trajectories, radiosonde profiles of temperature and humidity, and aerosol concentration and optical measurements. Humidity varied from approximately 80% near the surface to 50% near the top height during the entire cruise. The average value and standard deviation of aerosol optical parameters were determined for characteristic air mass regimes. Marine aerosols in the absence of any continental influence were found to have an AOD of 0.05+/-0.03, an extinction-to-backscatter ratio (S ratio) of 33+/-6 sr, and peak extinction values around 0.05 km(-1) (near the MBL top). The marine results are shown to be in agreement with previously measured and expected values. Polluted marine areas over the Indian Ocean, influenced by continental aerosols, had AOD values in excess of 0.2, S ratios well above 40 sr, and peak extinction values approximately 0.20 km(-1) (near the MBL top). The polluted marine results are shown to be similar to previously published values for continental aerosols. Comparisons between MPL derived extinction near the ship (75 m) and extinction calculated at ship level using scattering measured by a nephelometer and absorption using a particle soot absorption photometer were conducted. The comparisons indicated that the MPL algorithm (using a constant S ratio throughout the lower troposphere) calculates extinction near the surface in agreement with the ship-level measurements only when the MBL aerosols are well mixed with aerosols above. Finally, a review of the MPL extinction profiles showed that the model of aerosol vertical extinction developed during an earlier INDOEX field campaign (at the Maldives) did not correctly describe the true vertical distribution over the greater Indian Ocean region. Using the average extinction profile and AOD obtained during marine conditions, a new model of aerosol vertical extinction was determined for marine atmospheres over the Indian Ocean. A new model of aerosol vertical extinction for polluted marine atmospheres was also developed using the average extinction profile and AOD obtained during marine conditions influenced by continental aerosols.

Witek, ML, Flatau PJ, Teixeira J, Markowicz KM.  2011.  Numerical Investigation of Sea Salt Aerosol Size Bin Partitioning in Global Transport Models: Implications for Mass Budget and Optical Depth. Aerosol Science and Technology. 45:401-414.   10.1080/02786826.2010.541957   AbstractWebsite

In this study the importance of sea salt aerosol (SSA) size representation in a global transport model is investigated. For this purpose the Navy Aerosol Analysis and Prediction System (NAAPS) model is employed in a number of SSA simulations. A new dry deposition velocity parameterization is implemented into NAAPS in order to more physically represent deposition processes in the model. SSA size distribution is divided into size bins using two different partition procedures: the previously used iso-log method and the iso-gradient method, which relies on size-dependence of deposition processes. The global SSA simulations are analyzed in terms of the total sea salt mass and the average SSA optical thickness. The results indicate that there is a large dependence of the total mass and average aerosol optical depth on the number of size bins used to represent the aerosol size distribution. The total SSA mass is underestimated by 20% if 2 instead of 15 (reference) size intervals are used. The average aerosol optical depth underestimation is even higher and reaches over 35%. Such large differences can have substantial implications on the accuracy of SSA radiative forcing simulations in climate models. A comparison of the two division procedures shows that the simulations with the iso-gradient intervals are more accurate than the iso-log ones if at least 6 size bins are used. This result indicates that the more physically based division scheme can offer better performance and reduce computational cost of global aerosol transport models.

Witek, ML, Teixeira J, Flatau PJ.  2008.  On stable and explicit numerical methods for the advection-diffusion equation. Mathematics and Computers in Simulation. 79:561-570.   10.1016/j.matcom.2008.03.001   AbstractWebsite

In this paper two stable and explicit numerical methods to integrate the one-dimensional (1D) advection-diffusion equation are presented. These schemes are stable by design and follow the main general concept behind the semi-Lagrangian method by constructing a virtual grid where the explicit method becomes stable. It is shown that the new schemes compare well with analytic solutions and are often more accurate than implicit schemes. In particular, the diffusion-only case is explored in some detail. The error produced by the stable and explicit method is a function of the ratio between the standard deviation an of the initial Gaussian state and the characteristic virtual grid distance AS. Larger values of this ratio lead to very accurate results when compared to implicit methods, while lower values lead to less accuracy. It is shown that the sigma(0)/Delta S ratio is also significant in the advection-diffusion problem: it determines the maximum error generated by new methods, obtained with a certain combination of the advection and diffusion values. In addition, the error becomes smaller when the problem becomes more advective or more diffusive. (C) 2008 IMACS. Published by Elsevier B.V. All rights reserved.

Witek, ML, Flatau PJ, Teixeira J, Westphal DL.  2007.  Coupling an ocean wave model with a global aerosol transport model: A sea salt aerosol parameterization perspective. Geophysical Research Letters. 34   10.1029/2007gl030106   AbstractWebsite

[1] A new approach to sea salt parameterization is proposed which incorporates wind- wave characteristics into the sea salt emission function and can be employed globally and under swell- influenced conditions. The new source function was applied into Navy Aerosol Analysis and Prediction System model together with predictions from the global wave model Wave Watch III. The squared surface wind velocity U-10 and the wave's orbital velocity V-orb= pi H-s/ T-P are shown to be the key parameters in the proposed parameterization. Results of the model simulations are validated against multi- campaign shipboard measurements of the sea salt aerosol. The validations indicate a good correlation between V-orb and the measured surface concentrations. The model simulations with the new parameterization exhibit an improved agreement with the observations when compared to a wind- speed- only approach. The proposed emission parameterization has the potential to improve the simulations of sea salt emission in aerosol transport models.

Witek, ML, Flatau PJ, Quinn PK, Westphal DL.  2007.  Global sea-salt modeling: Results and validation against multicampaign shipboard measurements. Journal of Geophysical Research-Atmospheres. 112   10.1029/2006jd007779   AbstractWebsite

[1] Open-ocean measurements of sea-salt concentrations from five different campaigns are used to validate the sea-salt parameterization in numerical models. The data set is unique in that it is from open-ocean shipboard measurements which alleviates typical problems associated with onshore wave breaking on land stations ( surf zone). The validity of the sea-salt parameterizations is tested by employing a global forecasting model and transport model with detailed representation of dry and wet deposition, advection and diffusion, and other physical processes. It is shown that the inclusion of these processes leads to good agreement with shipboard measurements. The correlation coefficient of measured and modeled sea-salt mass concentrations for all data points was 0.76 and varied from 0.55 to 0.84 for different experiments. Average sea-salt mass concentration was 4.6 mu g/m(3) from measurements and 7.3 mu g/m(3) from the model, for all considered experiments. It was found that model-measurements discrepancies were affected by wet deposition uncertainties but also suggested was the influence of source uncertainties in the strong wind-speed regime, lack of a wind-speed threshold for emission onset, and lack of size differentiation in applied deposition velocity. No apparent relationship between the water temperature and the measured sea-salt concentration was found in the analyzed data set.