Satellite data can help UK cities better manage pollution reduction strategy

Since the 1950s, Ricardo has been a world leader in air quality measurement and management strategy, helping cities around the world to improve air quality. In a recent study, Ricardo emissions and modelling scientists have proven that satellite data can provide city authorities with critical insight into how best to design their pollution management strategies. Here, lead consultant, Matthieu Pommier tells us more.

Q. What’s the challenge?

A. The potential impacts of poor air quality on human health is an important issue that governments worldwide are looking to address in order to protect the health of citizens. The urgency of this issue is reflected by the World Health Organisation’s (WHO) issue of new guidelines¹ on air quality levels to protect the health of populations.

The new WHO guideline level for nitrogen dioxide (NO₂) (10 µg/m3 as an annual average), corresponds to a 75% reduction from the previous WHO guideline issued in 2005. To put things into perspective, in the UK, 33 zones (defined as required by the EU ambient air directive) out of 43 exceeded the previous WHO annual limit of NO₂ concentrations back in 2019². This highlights the scale of the challenge faced by the UK (and all European) government in meeting the new lower annual limit.

Q. What is the standard method for quantifying air pollution emissions?

A. Currently, approaches for quantifying air pollution emissions are usually limited to calculating annual values only for pollutant emissions. This because the estimation of emissions for a region traditionally relies on a “bottom-up” method that is based on quantification of total fuel use coupled with averaged emission factors for different emitting sectors, technologies and processes. The problem with this approach is that it is subject to uncertainties due to there being an incomplete understanding of sectoral activity, real-world operating conditions, and spatial distributions of sources. Additionally, estimates of emissions may become outdated when fuel consumption and emission factors change, and they are often limited to an annual basis.

The exceptional situation of the COVID-19 pandemic in 2020, that caused the world to come to a standstill is a good illustration of sudden changes in our emitting activities (e.g. industrial processes, road transport etc). Such unforeseen changes are not well represented in emission inventories due to them largely relying on a static representation of the annual totals or using simple assumptions to represent their temporal variation. For example, these temporal variations often do not vary spatially or are sometimes even fixed throughout the years.
 

Q. Why did Ricardo carry out the satellite data study?

A. Conventional methods for estimating nitrogen oxides (nitric oxide (NO) and nitrogen dioxide (NO₂)), collectively known as NO_x emissions, are still uncertain. For example the NAEI does not routinely provide temporal profiles for emission sources. An accurate estimate of the emissions (in amount of pollutant emitted, their temporal variation and their geographical distribution) is crucial to better model NO₂ concentrations, especially for polluted episodes and to inform efficient strategy on air pollution reduction. 

At Ricardo, we support a number of local authorities and public and private sector organisations to monitor and understand the sources of air pollution and the wider impacts to local communities and the environment. The satellite data study was conducted to test whether satellite data could offer better insight into pollution concentration changes than standard annual values data. Satellite data (made available by the European Space Agency) was chosen for the study since these observations are complementary of surface observations. Indeed, surface observations are not homogenously distributed over the UK and for some pollutants satellite observations provide better coverage than current ground-based monitoring and thus enable improved spatial distributions of the pollutants. However, it is worth noting that satellite observations are not perfect and also suffer some drawbacks such as a limited detection over cloudy scenes. It is important to bear this in mind when analysing data.

Q. What were the results of the study?

A. Our study showed that satellite instruments are key observers of the changes in emitting activities and the data collected can allow for the quantification of the changes in emissions.

This new approach provides better insight into total annual NO_x emissions, average NO₂ lifetime levels, seasonal variation, and estimated weekday/weekend emissions.

We found satellite data is also useful in understanding seasonal climatic changes of emissions and lifetime (residual) estimates, especially regarding variations in pollution quantities between seasons and on a day-to-day basis e.g. weekdays vs. weekends. This level of insight and granularity is not normally possible using annual emission values alone.

The satellite data method opens the door for more granular reporting and improved insight into emissions concentrations that can be used to better inform pollution abatement strategies in future.

Q. What are the implications of the study findings?

A. Insight into ambient pollution changes is helpful to understand the knock-on effects on personal pollution exposure and where to target local mitigation efforts to be most effective. For instance, our findings suggest that the mitigation of NO₂ concentrations in Manchester could require a different emission management strategy than London and Birmingham. Policies in Manchester could be focused in a targeted manner on reducing weekend emissions (where both emission rate and NO₂ lifetime are longest), whereas in London and Birmingham focusing mitigation action on weekday emissions may be more beneficial to lowering pollution levels, as there is less seasonal variation in emissions and weekday emissions are much greater than weekend levels.

In Manchester, a seasonal approach can be beneficial to reach different objectives. For example, targeting summer emissions might help to reduce the number of consecutive hours of exposure to NO₂ exceedance caused by an increased presence of NO₂ in the air, as found in this study. Similarly, targeting the reduction of autumnal emissions might help decrease NO₂ concentrations given average NO_x emissions tend to be higher around this time compared to other seasons.

Q. How can this new approach be applied in practice?

A. Even for a country with a well-documented emission inventory such as the UK, the study demonstrates the added value satellite observations can offer in providing greater insight into the design of better emission management strategy. For regions or countries where these inventories are incomplete, our satellite data method offers a valuable way forward in improving understanding of the behaviour of emissions and how they are affected by local temporal conditions.

At Ricardo, our emissions and modelling team is involved in a number of projects that could benefit from this approach to emissions quantification. Our experts are currently supporting air quality projects in Abu Dhabi, Saudi Arabia and across the UK, to help local government and policy makers to better understand local emissions. The team is excited to explore opportunities to use this new method where appropriate to provide better support to ongoing projects.

For full details on Ricardo’s satellite data pollution study, visit: https://pubs.rsc.org/en/content/articlelanding/2023/ea/d2ea00086e

For more information on satellite data observations or how our emissions team can support your organisation, get in touch.

Footnotes

1. W. H. Organization, “WHO global air quality guidelines: particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide,” 2021.
2. D. M. Brookes, J. R. Stedman, A. J. Kent, S. L. Whiting, R. A. Rose, C. J. Williams, K. L. Pugsley, J. V. Wareham and A. Pepler, Technical report on UK supplementary assessment under The Air Quality Directive (2008/50/EC), The Air Quality Framework Directive (96/62/EC) and Fourth Daughter Directive (2004/107/EC) for 2019, 2021: https://uk-air.defra.gov.uk/assets/documents/reports/cat09/2102111100_2019_PCM_technical_report.pdf