A SIMPLE BOX MODEL FOR THE ANNUAL CYCLE OF PARTITIONING OF SEMIVOLATILE ORGANIC-COMPOUNDS BETWEEN THE ATMOSPHERE AND THE EARTHS SURFACE

A box model is developed to describe why the gas-phase concentrations of certain semi-volatile organic compounds (SOCs) can exhibit annual c ycles that peak in the summer. Decreasing partitioning to materials at the Earth's surface with increasing temperature (T) is invoked to acc ount for the summer increase. The gas-surface partition coefficient is formulated as K(m) congruent-to K(m)BARunderbare(Qm/RT). Within a giv en compound class, K(m)BARunderbar is assumed to be largely independen t of both compound and T; the compound dependence in K(m) is found in the enthalpy of desorption from surface materials, Q(m) (kJ mol-1). Fo r SOCs whose summer concentrations are only somewhat larger than their winter concentrations, the model predicts that the summer concentrati ons can give a direct estimate of q/V, the total mass of readily excha ngeable SOC per unit volume of the box. The annual cycle in the gas-ph ase concentration for such a compound can then be analysed in search o f best-fit values for Q(m) for the compound, and K(m)BARunderbar. for the compound class. The model can then be applied to less volatile SOC s in search of their best fit values of q/V. Multiplication of the q/V values by the effective height of the troposphere gives the readily e xchangeable mass per unit area of the box, q/a. Multiplication of the q/a values by an estimate of the area of the box in turn gives estimat es of q. The model is applied for the first time using the data of Hof f et al. (Envir. Sci. Technol. 26, 266-275, 1992) for samples collecte d at Egbert, Ontario, from July 1988 to September 1989. Highly prelimi nary estimates of q/V and q/a for a latitude range involving Egbert ar e obtained for alpha-HCH, p, p'-DDT, o, p'-DDT, p, p'-DDE, PCB-52 and PCB-101.