report. Gas exchange is a bi-directional diffusive flux of volatile chemicals between the dissolved phase in surface waters and the gas phase in the atmosphere. Although it is possible to mathematically separate this net flux into gross deposition (dissolution of gaseous hydrocarbons) and gross volatilization (degassing of dissolved hydrocarbons), both processes are occurring simultaneously and the net gas exchange determines both the magnitude and direction of air-water exchange.
This analysis was conducted on two spatial scales. The earlier method used by Duce and Gagosian (1982) in the 1985 NRC report, in which the world’s ocean was divided into impacted (Case A) and remote (Case B) zones, was used to estimate hydrocarbon and PAH air-sea exchange worldwide. In addition, the North American coastline was divided into 17 zones, each of which was further divided into zones 0-3 miles and 3-200 miles from shore. As part of this analysis, each of these zones was described as urban-influenced or rural, and assigned consensus values for gas, aerosol particle, and dissolved hydrocarbon and PAH concentrations based on review of the literature. Deposition was assumed to be uniform within each North American zone, and the concentrations represented annual averages. Assessing seasonality, which certainly influences both the concentrations and depositional processes, was not considered in this analysis.
Ambient gas-phase, aerosol-bound, and dissolved concentrations of each hydrocarbon in the atmosphere and surface waters of each North American model segment and in the global background were estimated from the current literature. Due to the scarcity of data for the atmospheric petroleum hydrocarbons in the atmosphere bordering North America, the selection of representative distributions of PAHs and n-alkanes was developed from the currently available literature. For this assessment, petroleum hydrocarbons were defined as n-alkanes with carbon lengths ranging from C10 to C33. To develop an accurate assessment of the contaminant burden to the coastal waters via atmospheric deposition, the various coastal structure and representative contaminant loadings had to be determined. Five zones were assembled based on the degree of urbanization along the zone’s shoreline: (1) urban coastline 0-3 miles from shore (U0-3), (2) urban coastline 3-200 miles from shore (U3-200), (3) rural coastline 0-3 miles from shore (R0-3), (4) rural coastline 3-200 miles from shore (R3-200), and (5) background (BG) contaminant levels that would represent the open ocean. In most cases, adjoining 0-3 and 3-200 mile zones had the same designation (rural or urban) except along the west coast of North America, where the 3-200 zones were designated as “rural” to reflect the predominant westerly air flows off the Pacific Ocean.
Literature on atmospheric hydrocarbons in North America is sparse. This analysis began with the data used by Duce and Gagosian (1982). Published literature and known on-going studies were then used to update the estimates of hydrocarbon concentration (n-alkanes and PAH) in the marine atmosphere and in surface waters.
This compilation includes those endeavors that have measured concentrations in various selected areas on the United States (see Appendix H). Even fewer atmospheric n-alkane and PAH data were available for the North American coast that reported vapor phase alkanes per homologue (Hoff and Chan, 1987; Fraser et al., 1997, 1998). Sampling methods were somewhat consistent throughout the literature.
Details of the methodology, databases, and computations can be found in Appendix H. Deposition models were used to estimate depositional fluxes (mass deposited per unit area per year) from these concentrations, and these fluxes were integrated over the area of each model segment to calculate the annual loading. Equations used in these calculations have been used extensively to estimate exchange of semivolatile organic chemicals between the atmosphere and surface waters (see Appendix H for equations and references). Wet deposition results from the scavenging of gases and particles, which were modeled from the temperature-corrected Henry’s law constant and the aerosol scavenging ratios, respectively. Henry’s Law constants for the hydrocarbons are identical to those used by Duce and Gagosian in the 1985 NRC report and were corrected to 11ºC using reported enthalpies of phase change. Global annual precipitation was assumed to be 100 cm/year. While spatial and temporal variability in temperature and precipitation rate will alter atmospheric deposition rates, any bias resulting from using uniform global temperature (11ºC) and precipitation rates here is likely within the error of these estimates.
Dry aerosol deposition fluxes were calculated as the product of the estimated aerosol-bound hydrocarbon and the dry deposition velocity. Estimates of deposition velocity range from <0.01 cm/sec to >1 cm/sec and depend on particle size, relative humidity, and surface turbulence. Most studies of organic chemical dry aerosol deposition suggest that a deposition velocity in the range of 0.1 cm/sec is conservative. For this analysis, a deposition velocity of 0.1 cm/sec was used, corresponding to a 0.5-μm particle depositing under average wind conditions. Annual dry deposition velocity was assumed to be spatially invariant.
Gross gas absorption deposition fluxes were calculated by dividing the estimated gas phase hydrocarbon concentrations by their respective temperature-corrected dimensionless Henry’s law constants and multiplying the result by the air-sea exchange mass transfer coefficient. The mass transfer coefficient for each compound was estimated using the two-film model, applying relationships between wind speed and tracer exchange rates to parameterize surface turbulence and the compound-specific diffusivities and Henry’s Law constants (see Appendix H for details and references). Gross gas deposition fluxes are only one-half of the net bidirectional diffusive exchange of gases across the air-water interface. The corresponding gross volatilization fluxes for each compound were calculated as the product of the estimated