Air quality in winter

Fiona Prismall, Technical Director, looks at the effects we see in winter.

Fiona Prismall, Technical Director
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27 Jan 2021

Behavioural impacts

The UK is now well into winter and, as shown by historical records and real-time measurements, levels of outdoor air pollutants are highest in this season. This may not come as a surprise; the emissions that are fed into the air are generally higher during winter than any other season with our demand for heat and power reaching its peak. We are still a long way from meeting this demand using renewable energy sources alone, so combustion of fuel tends to peak in this season.

In recent years, the use of domestic wood burning stoves and open fires has increased. Carbon is stored in wood for extremely short periods when compared with the carbon stored in fossil fuels. When wood is burned, the carbon used only relatively recently by the growing plants is returned to the air. Whereas when coal is burned, the carbon released into the air was originally removed millions of years ago. Burning wood, rather than fossil fuels, can therefore be good for our climate but bad for local air quality. Defra’s 2019 Clean Air Strategy quoted studies that show that wood burning in London contributes over 30% of the total emissions of small particles. These are the particles that are small enough to penetrate deeper into our lungs and are therefore of great concern. Burning wood can still play a useful role in providing us with heat and power but it is important that consideration is given to whether it’s appropriate in each location.

Temperature impacts

In autumn, we looked at how the changing weather affected the dispersion and – importantly – the dilution of pollutants once they’ve been emitted to our air. These effects become more pronounced as the weather worsens. In addition to that, this is the season with the shortest days and longest nights.

During the day, the air is usually warmest near to the ground and gets colder the further upwards we go. As hot air rises, the pollutants at ground level tend to be dispersed upwards and away from the air that we breathe. However, after sunset, this situation tends to reverse and the air close to the ground becomes colder than the air above it. This is known as a ‘temperature inversion’. When this happens, the air stops rising and the stagnant conditions trap pollutants in a shallow layer where we breath. In short, the longer the nights, the more pollution stays close to the ground and in the air that we breath.

How can we help?

As our demand for heat and power reaches its peak in winter, power outages are also most likely to happen in winter: Cold temperatures affect the working of key components in the system, ice and snow weigh down on power lines until they break and high winds can knock out power lines. This tension between the supply and demand of energy is therefore greatest in winter. It is key that systems are in place to ensure a continuous supply of power to allow critical services to function and to avoid blackouts.

Here at RPS, we assist developers in assessing the air quality impacts from ‘peaking plant’ or short-term operating reserves (known as STOR). These are plant that are quickly put into operation when demand on the network becomes extreme and causes short-term outages. We help clients by using a dispersion model to predict the air quality impacts from these peaking or STOR plant in several different ways including:

  • Modelling the impacts for a variety of site layouts with the exhausts or stacks in different locations until we arrive at a layout with the least environmental impact
  • Modelling the impacts for a range of exhaust or stack heights so that we can advise our clients on the optimum height for emissions to be released

We similarly assist developers in designing back-up power provision for data centres. As more and more vital information is stored electronically, we need to ensure continued access to data even at times when central power networks are under strain, such as in winter. We carry out similar assessments for back-up power at data centres as we do for peaking or STOR plant. As this back-up provision can be relatively large, we also assist developers to understand the short-term impacts that arise when the generators supplying this back-up power are tested or used during an emergency. We use statistical methods to determine the probability that the generators will be tested or used when weather conditions are likely to hinder dispersion.

The verdict

In summary, air quality in winter can be poor due to the combined effect of an increase in emissions and weather conditions that hinder dispersion. Nevertheless, with good upfront design and ensuring that air quality is considered early in the planning process, it is possible to mitigate impacts.

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