Hi Mat
I can answer the air dispersion factor question:
The air dispersion factor is based on a mass flux calculation - the
bigger your source area, the more dust you have from your source getting
suspended in the air.
We can probably assume that someone standing on the down wind side of a
big site will be breathing the same overall PM10 concentration as
someone standing on the down wind side of a small site. However, the
proportion of the PM10 that comes from the site itself will be greater
for the big site than the small site (due to the higher overall flux of
soil to airborne dust). If you assume that only the on-site PM10 is
contaminated (unlikley, but this is what is assumed in CLEA) then the
outdoor dust inhalation exposure for someone on a big site is greater
than someone on a small site.
It should also be noted that the method CLEA uses for assessing PM10
creation from soils is based on a US EPA screening method (Cowherd et
al) and this makes lots of simplifying assumptions. In reality, the
amount of PM10 arising from soils on a site is not just a function of
area of exposed soil, but also on how easily the soil is eroded (will
depend on nature of soil, particle size, moisture, vegetation etc.) and
wind speed + turbulence adjacent to the soil surface (which will depend
on how open the landscape is - i.e. if there are lots of garden walls +
bushes, then the soil surface will not be a windy place).
I think it makes logical sense that the concentration of PM10 derived
from soil on a large public open space is greater than that for a garden.
Hope this helps
Simon
On 26/11/2010 10:27, Mathew Rouge wrote:
> Dear All,
> Does anybody know why the air dispersion factors in CLEA decrease with the source area, as provided within Table 9.1 of SC050021/SR3.
>
> The effect of this is that for the outdoor air inhalation pathway, the volatilisation factor increases as the source area increases, resulting in more stringent assessment criteria.
>
> For example, if only the outdoor air inhalation pathway is considered for the default residential land-use and a site specific public open space of 2 hectares size (with the exposure frequencies, respiration rates and outdoor occupancy time set to the default residential settings and everything else being equal) significantly more stringent criteria are derived for the public open space than the residential land-use.
>
> Obviously setting more stringent criteria for public open space than residential land use raises a few eyebrows.
>
> Surely the air dispersion factor should be constant, irrespective of the size of the source area. Does anybody know how it has been calculated?
>
> Also, does anybody know how the respiration rates for an allotment use have been calculated. The manual says that the occupany period for allotments is 3 hours per day and provides respiration rates for the activities undertaken in the 3 hours. The manual, however, does not say what the respiration rates are for the remaining 21 hours.
>
> I initially calculated the average hourly inhalation rate from the default values for a residential land- use, multiplied by 21 hours and added the respiration rate that may be calculated for the 3 hours of allotment activity. This, however, does not provide the default values for an allotment land use.
>
> For determining site specific respiration rates, it would be useful to know how the allotment rates were calculated.
>
> Many Thanks in advance for ideas
> Mat Rouge
>
>
--
Simon Firth
Firth Consultants Ltd
Windsor House
Greville Road
Bristol
BS3 1LL
Tel: 0117 953 2096
Mob: 07766 224 363
www.firthconsultants.co.uk
Company registered in England and Wales: 6382069
|