technical guidance documents - Institute for Health and Consumer ...
technical guidance documents - Institute for Health and Consumer ...
technical guidance documents - Institute for Health and Consumer ...
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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