Tracking and controlling air pollution

With air quality issues at the Beijing Olympics making headlines around the world, Phil Branchflower, air quality associate at SLR Consulting, reviews the impact of pollutants on local air quality.

An inevitable consequence of our modern life-style is the release of pollutants into the atmosphere – via combustion process, including transport and domestic heating, manufacturing processes and even waste treatment and agriclture.

While much of the current media attention on air quality focuses on the consequences of the release of carbon dioxide (and other greenhouse gases), a more immediate and typically local impact comes from the release of pollutants which can directly effect human and ecosystem health.

The current Beijing Olympics have demonstrated this on a truly awesome scale – as have the dramatic improvements in air quality that can be achieved through abatement. Banning car use and stopping industry and construction would be beyond the acceptable sacrifice for most societies, but a similar improvement in air quality to that aimed for currently by the Chinese government occurred in the UK in the 1950’s, when a reduction in domestic coal use condemned the London smog to the pages of history.

The effect of atmospheric pollution on human health is well researched for several pollutants and stringent limits have been set through EU directives and the UK Air Quality Strategy.

This includes limits for PM10 and recently PM2.5, far lower than the interim World Health Organisation (WHO) standards currently being exceeded on a daily basis in Beijing, as well as for other pollutants such as metals and organic compounds.

Environmental Assessment Levels (EAL) limits derived from health based data are regularly reviewed and typically more stringent limits recommended; for example the current EAL for Chromium (VI) of 0.1µg/m3 is likely to be reduced (EPAQS consultation) to 0.0002µg/m3 a factor of 500 following a review of the health effects associated with exposure to Chromium (VI).

The sensitivity of ecosystems to atmospheric pollution is typically less well researched, although the realisation in the 1970’s and 80’s that acid rain was effecting Scandinavian woodland demonstrated its importance.

This resulted in the Convention on Long-Range Transboundary Air Pollution and the setting of Nation Emission Limits from Large Combustion Plants throughout Europe for acid gases (sulphur dioxide and nitrogen oxides).

Complying with these agreements has resulted in the reduction of emissions from large sources such as coal fired power stations and steel refineries.

However it has been said that the shift of British (and indeed European) industry overseas, to China among other locations, has merely resulted in a relocation of the problem, again apparent from the images we are seeing in Beijing.

Consequently when protected habitats, especially those protected under European Law (such as the Habitats Directive) , are affected by emissions from a facility, this can become the critical receptor, requiring lower levels of impact than for human receptors. For example highly sensitive habitats such as bogs (to acid) and heathlands (to nitrogen) can pose significant obstacles to development.

Typically studies of nitrogen and acid deposition are carried out undertaken in relation to larger combustion process, but the effect of the deposition of organic compounds and metals can also be required.

Careful consideration of this factor at the site selection stage can avoid the need for subsequent over-engineering (such as the stack height of an additional abatement plant).

Given the importance of the effect of airborne pollutants, the impacts of emissions to air associated with any development are typically assessed through both the planning and environmental permitting processes. The emission limits for industrial processes are set by the Environment Agency (or local authorities) in environmental permits.

While the emission limits may be set, the resultant impact on the surrounding environment will vary significantly depending on factors such as topography, local meteorology, sensitivity of receptors, and discharge characteristics such as height, vertical momentum (velocity and temperature) and downwash effects from nearby buildings.

All of these factors are incorporated into an atmospheric dispersion model in which algorithms are used to predict the dispersion (dilution) in the atmosphere between the source and receptor.

This can be an iterative process to determine the optimum location and height of release, or whether lower emission limits are required if particularly sensitive receptors such as nitrogen sensitive heathlands or acid sensitive bogs and the species which inhabit them are impacted.

Typically calculations are undertaken for every hour over a 5-year period for upto 10,000 receptor locations (up to 438 million calculations, although the models sensibly simplify the process by only undertaking calculations for receptors downwind of the source).

Modern dispersion models utilise different algorithm for different meteorological conditions and parts of the atmosphere to account for ‘inversion’ effects and some models even utilise dynamic meteorology to predict flow through complex terrain and over areas of 10’s of km.

The SLR air quality department is experienced in the use of a wide range of dispersion models such as AERMOD, ADMS-4, SCREEN-3, ADMS-ROADS, CALQ3HR and CALPUFF, and the factors which affect their performance.

Recent projects carried out by the team include, detailed assessments for brickworks, energy from waste (including gasification), Bio-energy plants (such as anaerobic digestion, biodiesel, biomass, landfill gas), CHP plants, paper mills, power stations, composting facilities, animal by-product (rendering and carcass incineration), food manufacturing, waste water treatment, metal refining, and even pet food production processes.

Returning in conclusion to the current situation in Beijing, it is interesting to note that in its bid for the 2008 Olympic Games seven years ago, China vowed to monitor daily movement of sulfur dioxide, carbon monoxide, nitrogen dioxide and inhalable particulates; strive for an annual improvement of air quality; and meet the national standards and WHO guiding values for the four major pollutants.

Today, the first two goals have been met, but as the second week of the Games approaches the third goal remains pending.

So while London 2012 may struggle to reach the heights of Beijing’s opening ceremony, at least there is a greater chance of the Olympians performing under clear skies (British summer permitting!)

For more information on dispersion modelling contact Phil Branchflower at SLR on 01225 309400.

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