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Title: Simple model for simulation of wet deposition of chloride at a near-coastal site in Florida

item Sigua, Gilbert

Submitted to: Florida Scientist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2005
Publication Date: 10/17/2005
Citation: Sumner, D.M., Sigua, G.C. 2005. Simple model for simulation of wet deposition of chloride at a near-coastal site in Florida. Florida Scientist. 68(4):236-246.

Interpretive Summary: A simple method, requiring readily available data was shown to have utility to explain most of the variation in weekly composites of measured wet chloride deposition. The method developed in the study for the estimation of chloride deposition may have values for other environmentally important atmospheric constituents such as sulfur dioxide and oxides of nitrogen that contribute to acid rain. This method offers the potential to enhance measurements of wet deposition by placing these measurements within a cause-and-effect framework to: (1) identify individual source area contributions to wet atmospheric deposition at a given point (for example, land versus sea or urban versus agricultural); (2) predict wet deposition under hypothetical scenarios not encountered in the data collection period; and (3) better define the aerial distribution of atmospheric deposition. The method could be further tested and improved through comparison of high-resolution (less than hourly) measurements of wet atmospheric deposition to simulated values. Also, improvements are needed to better define the atmospheric 're-loading' with constituents following rainfall cleansing.

Technical Abstract: A simple empirical model, requiring only readily available data, was developed to estimate weekly composites of wet chloride deposition in a near-coastal setting. The scavenging of atmospheric chloride by rainfall was conceptualized as intense during the initial part of a rainfall event, but decreasing in intensity as the event progresses and the atmosphere becomes progressively depleted in chloride. This conceptualization was quantified with a parameterized relation between an index of removal efficiency of atmospheric chloride and the cumulative rainfall within a storm event. Distinctive source areas of chloride (land and sea) were incorporated in this method by using individual removal efficiency index functions for each of the source areas. In addition, the role of seaward winds and distance from measurement point to marine sources of chloride was considered. The method was applied to data collected during July 1997-September 1998 at a site about 20 kilometers inland near Daytona Beach, Florida. Data used included hourly precipitation (all as rainfall) totals, chloride concentrations of weekly rainfall composites, and hourly values of wind speed and direction. The atmospheric scavenging model explained about 69 percent of the variation in measured weekly atmospheric deposition. Model results indicated that more than 90 percent of rainfall-deposited chloride is derived from the Atlantic Ocean.