Fiona Prismall, Technical Director, delves deeper into the headlines surrounding lockdown’s impact on air quality and considers the implications for development projects as we begin to emerge from COVID-19.
Attributed to a reduction in activity during lockdown, improvements in air quality have been widely reported over recent weeks. However, while it’s true emissions have decreased since lockdown began, it’s important to tread with care when comparing pollutant concentrations during lockdown with the same period last year or in previous years. Not only can the measurements themselves be subject to a degree of uncertainty, but the weather conditions at the time can significantly affect the measurement taken.
Fortunately, several scientific studies have delved deeper (to effectively remove the effects of weather) and have provided evidence that pollutant concentrations have decreased during lockdown – but not to the same extremes that some articles have suggested.
It’s also important to remember that the most high-profile air quality impacts – those posing a risk to human health - are due to elevated pollutant concentrations over the long-term. What this means is an improvement in air quality over a couple of months is unlikely to have much of a health impact unless the reductions in emissions are sustained. In fact, Defra’s biggest air quality struggle is meeting the long-term target for nitrogen dioxide levels (annual-mean EU Limit Value). Whereas the short-term target (hourly-mean EU Limit Value) is widely met.
Adding further need for caution, the studies have shown an increase in ozone at roadside locations. There is a complicated chemical reaction between vehicle-related emissions and roadside ozone, meaning that when vehicle-related emissions decrease, it can result in an increase in ozone at roadside locations. At stratospheric levels, ozone protects humans from harmful short-wave radiation and most people are familiar with concerns about the hole in the ozone layer. However, high ozone concentrations at ground level can be harmful to humans, causing irritation to eyes and noses, reducing lung function and increasing respiratory diseases.
When we undertake air quality assessments, we are predominantly seeking to answer:
Taking each question in turn, if the improvements in air quality are sustained, we could see an increase in the number of sites that are suitable for development. Sites where levels of pollution were previously too high may now be considered suitable if concentrations decrease.
When considering question two, assessing the impacts on air quality in the surrounding area, we normally consider changes in concentrations attributable to the development and the sensitivity of the area, the future ambient concentration. There are numerous factors affecting how we might estimate these in the future. The table below sets out some initial thoughts and shows the difficulty in establishing the net effect.
A further complication is the political landscape. With governments feeling the pressure to reduce debts and unemployment, environmental protection may be diluted if it is seen as a barrier to progress towards economic growth.
Previously, air quality consultants have been able to use air quality measurements and published research to inform future trends in emissions and pollutant concentrations, but how valid is the evidence of any predicted trends now?
We use atmospheric dispersions models to forecast the impacts associated with hypothetical situations. For traffic-related impacts, the outputs of our models largely depend on the accuracy of the emissions factors that we use. If emission factor forecasts are cast into doubt, it could be a case of ’garbage in, garbage out.’ If the behavioural changes we are currently seeing are long-lasting, it might not be possible to simply extrapolate from trends and forecasts established before the pandemic.
We also use site-specific monitoring to verify our models through a comparison of model predictions and measured concentrations. Over the coming months, we expect travel patterns to continue to fluctuate as lockdown restrictions are eased and, potentially, localised lockdowns are imposed. A second wave may also require a return to national lockdown. Any data collected during this period, and perhaps for the rest of 2020, is therefore unlikely to be representative across a prolonged period.
Unless we take an extremely pessimistic view - that a vaccine will never be found and we will never resume our lives as they were pre-pandemic – an air quality monitoring study starting now, is unlikely to be a true and robust representation.
Two options are:
Given the amount of uncertainty we still face, and the number of unknowns, it’s not possible to say whether either (or both) of these options are conservative or optimistic.
We must deal with this in the same way that we deal with other uncertainties by undertaking sensitivity tests to see how the conclusions of our assessments are affected by changes in assumptions. Using professional judgement, we can then determine the most likely outcome. In short, to borrow a phrase from the WHO, we need to test, test, test.
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