Precipitation events scavenge contam inant-laden particles by incorporating the particles into rain drops, either du rin g droplet form ation or as the d ro p let falls through the air colum n (Ligocki et a l, 1985a). V apour phase contam inants are rem oved from the atm osphere as a result of partitioning across the w ater droplet surface followed by dissolution into the bulk liquid. N o n -re ac tiv e v a p o u rs (i.e. v a p o u r p h ase SOCs) are scav en g ed by precipitation according to H enry's Law, if equilibrium betw een the vapour an d aq u eo u s phases is attained (Ligocki et al., 1985a, b). Slinn (1978) p red icted th at a falling raindrop should attain equilibrium w ith a trace
organic vapour in a ~10 m fall. As an example from the w ork of Leister and
Baker (1994) w et depositional fluxes (Fwet - pg m -2 m o n th -1) can be calculated from a volum e-weighted m onthly S O C concentration ( [ S O C J v w m ,
pg m -3):
[ S O C ] v w m = [ X C [ S O C ] iV i] /[ X V i]
F w e t = [ S O C j v w m (x)
W here C[S0C]i is the SOC concentration in each individual rain event, V i is
the volum e of precipitation and x is the precipitation rate (m 3 m -2 m o n th -1). The extent of w et scavenging for a com pound is given by the ’w ashout ratio' (W) w hich is the concentration in rain (ng L-1), d iv id ed by the concentration in air (ng m -3). The particle w ashout ratio is given by Wp and the vapour phase w ashout ratio is given by Wv expressed as:-
Wv = R T /H
W here R is the gas constant and H is the H enry's Law constant for that co m p o u n d a t a certain tem perature, T. If the fraction p resen t in the particulate phase is defined as 0, then the overall w ashout ratio is given by
(Ligocki et al. 1985b):-
In general, w ashout efficiency is enhanced as the volatility of a species is red u ced , probably due to particle w ashout being the p rim ary rem oval m echanism for these compounds. Eitzer and Hites (1989) calculated gas and particle w ash o u t ratios for the dioxin and furan hom ologue gro u p s in Bloomington, IN. Plots of Wv and Wp against com pound vapour pressure show ed a strong correlation for Wv and a lack of correlation for W p, the latter indicating th at particle w ashout is a physical process acting on the particle and, therefore, all the compounds bound to the particle are affected sim ilarly. W v w as found to increase significantly w ith decreasing v apour pressure i.e. as the H enry's constants decreased, com pounds became more susceptible to washout.
SOCs are rem oved from the atm osphere during dry periods by dry particle deposition and by vapour exchange between the atm osphere and w ater and terrestrial surfaces. Dry particle deposition rates of SOCs depend upon the aerodynam ic size distribution and u pon m icrom eteorological conditions (Slinn, 1983). A lthough dry particle deposition rates are very difficult to m easure directly, m odelled and experimentally derived particle deposition velocities have been derived for various SOCs including PAHs (McVeety and Hites, 1988), PCDD/Fs (Koester and Hites, 1992) and PCBs (Holsen et al.,
1991). M onthly dry particle fluxes (Fpart. dry, Hg m -2 m o n th -1) can be estim ated by:-
W here [SOC] part is the m easured particle SOC concentration (pg m -3) and Vd is the estim ated dry deposition velocity (m m onth-1).
G aseous contam inants actively exchange between the atm osphere and w ater an d atm osphere and terrestrial surfaces at a rate pro p o rtio n al to their concentration gradients (Mackay, 1986). Gaseous fluxes of SOCs have been exam ined over w ater bodies such as in the Great Lakes region of N orth Am erica. Leister and Baker (1994) used the following equation to derive g aseous fluxes to the C hesapeake Bay area on the eastern seaboard (M aryland)
F g a s, d r y — K O L ([S O C ]d is s . ■ [ S O C ] g a s / H )
W here F g a s , d r y is the S O C flux resulting from gas exchange (pg m -2 m onth-
* ), K o l is a m ass transfer coefficient expressed on a liq u id phase
concentration basis (cm m onth-1), H is the H enry's Law constant (Pa m -3
m ol-1) and [ S O C ] d i s s . and [ S O C ] g a s are the concentration of S O C dissolved in
su rface w a te r (pg L-1) and in the atm ospheric gas p hase (ng m -3) respectively. Baker and Eisenreich (1990) and Hoff et al. (1992a) found that the vap o u r exchange of SOCs is highly dynamic, with volatilisation during the w arm er sum m er m onths offsetting efficient deposition d u rin g the cooler, w inter m onths.
M onitoring deposition for SOCs in urban areas is less extensively studied, b u t for the PCBs and PAHs depositional fluxes are expected to be orders of m ag n itu d e higher. H olsen et al. (1991) determ ined dry deposition PCB fluxes in Chicago air and found them to be three orders of m agnitude higher than in rem ote areas of the USA. In this thesis bulk deposition (wet an d dry) was collected m onthly from both urban sites and a rural location. By calculating deposition fluxes simple mass balances could be derived for the UK environm ent, annual release data for PAHs and PCBs was obtained from contem porary source inventories.