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ENVIRONMENTAL EXPOSURE ASSESSMENT<br />

2.3.8.2 Calculation of PEClocal <strong>for</strong> the atmosphere<br />

In this section, the following parameters are derived:<br />

• local concentration in air during emission episode;<br />

• annual average local concentration in air;<br />

• total deposition flux (annual average).<br />

The air compartment receives its input from direct emission to air, <strong>and</strong> volatilisation from the<br />

sewage treatment plant. The most important fate processes in air, are schematically drawn in<br />

Figure 8.<br />

PEClocal <strong>for</strong> air cannot be compared with<br />

the PNEC <strong>for</strong> air because the latter is<br />

usually not available. The PEClocal <strong>for</strong> air<br />

is used as input <strong>for</strong> the calcu-lation of the<br />

intake of substances through inhalation in<br />

the indirect exposure of humans.<br />

Deposition fluxes are used as input <strong>for</strong> the<br />

calculation of PEClocal in soil. There<strong>for</strong>e,<br />

both deposition flux <strong>and</strong> concentration are<br />

calculated as annual average values.<br />

Many air models are available that are<br />

highly flexible <strong>and</strong> can be adjusted to take<br />

specific in<strong>for</strong>mation on scale, emission<br />

wet deposition<br />

dry deposition<br />

sources, weather conditions etc. into<br />

account. For new substances, as well as<br />

very often <strong>for</strong> existing substances, this<br />

Figure 8 Fate processes in the air compartment<br />

type of in<strong>for</strong>mation is normally not available. Hence a st<strong>and</strong>ardised exposure assessment is<br />

carried out making a number of explicit assumptions <strong>and</strong> using a number of fixed default<br />

parameters. The gaussian plume model OPS, as described by Van Jaarsveld (1990) is proposed<br />

using the st<strong>and</strong>ard parameters as described by Toet <strong>and</strong> de Leeuw (1992). These authors used the<br />

OPS model <strong>and</strong> carried out a number of default calculations in order to describe a relationship<br />

between the basic characteristics of substances (vapour pressure <strong>and</strong> Henry's Law constant) <strong>and</strong><br />

the concentration in air <strong>and</strong> deposition flux to soil near to a point source. The following<br />

assumptions/model settings are made:<br />

• realistic average atmospheric conditions are used, obtained from a 10-year data set of<br />

weather conditions <strong>for</strong> The Netherl<strong>and</strong>s;<br />

• transport of vaporised <strong>and</strong> aerosol-bound substances is calculated separately. The<br />

partitioning between gas <strong>and</strong> aerosol is determined by means of the equation of Junge (see<br />

equation (19));<br />

• the atmospheric reaction rate is set at a fixed value of 5% per hour. However, on the spatial<br />

scale that is regarded (i.e. a distance of 100 m from the source), atmospheric reactions do<br />

not play any role in the removal of the substance (even at very high reaction rates) (Toet <strong>and</strong><br />

De Leeuw, 1992);<br />

• losses due to deposition are neglected <strong>for</strong> estimation of the concentration <strong>and</strong> deposition<br />

fluxes at this short distance from the source;<br />

• assumed source characteristics are:<br />

72<br />

wind<br />

rainwater<br />

partitioning<br />

air<br />

gas phase<br />

degradation<br />

partitioning<br />

aerosol

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