Imperial College London


Faculty of Natural SciencesCentre for Environmental Policy

Research Associate in Energy and Air Pollution Policy







307Weeks BuildingSouth Kensington Campus





Publication Type

8 results found

Mehlig D, Staffell I, Stettler M, ApSimon Het al., 2023, Accelerating electric vehicle uptake favours greenhouse gas over air pollutant emissions, Transportation Research Part D: Transport and Environment, Vol: 124, ISSN: 1361-9209

The rapid uptake of new vehicle technologies will change the environmental impact of road transport. The emissions produced in power plants supplying electric vehicles (EVs) and vehicular non-exhaust PM2.5 emissions leaves the benefits of EVs unclear. We develop a fleet turnover model to assess how different vehicle technologies, the rate of technological change, and changing transport demand impact vehicle and power station CO2eq and air pollutant emissions. By 2050, the transition to EVs reduces yearly CO2eq emissions by 98% and cumulative CO2eq emissions by over 50%; accelerating or delaying EV uptake by 5 years changes these results by 1% and 17%, respectively. By 2050, EVs reduce annual NOx emissions by 97%, but have little impact on PM2.5 due to vehicular non-exhaust emissions. Accelerating or delaying EV uptake had little impact on air pollution emissions. Reducing vehicle kilometres has the potential to reduce non-exhaust PM2.5 emissions by 20% in the long-term.

Journal article

Mehlig D, 2023, Electricity: powering vehicles & reducing pollution

There is growing promise that electric vehicles (EVs) may lead to a reduction in the pollution caused by road transport. This thesis begins by assessing the current link between EVs and the electricity system; how EVs are charged. The emissions produced in power plants for charging EVs have significantly decreased over the past decade due to the decarbonation of the electricity system, reaching 41 g CO2, 27 mg NOx and 0.7 mg PM2.5 per kilometre in 2019. But as marginal demand is still met by natural gas power plants there is a small emission increase for NO¬x and CO2 of up to 25%, for new EVs entering the system. In 2020, the COVID-19 pandemic highlighted how additional decarbonisation of the electricity system should continue and will bring additional emission reductions for EVs in the future. This thesis then looks to the future, illustrating a path towards a fully electric fleet and decarbonised electricity system. Emissions from road transport will approach net zero in the 2040s if the current government road transport and power generation targets are met. Under this scenario, air quality may significantly improve over the next decade with exposure to NOx and PM2.5 reducing by 6.7 µgm-3 and 0.5 µgm-3, respectively, but this is only marginally better than without EVs due to the improvements in conventional vehicles. Beyond 2030, a fleet of EVs with a decarbonised electricity system may further reduce NOx and PM2.5 exposure by 1.6 µgm-3 and 0.1 µgm-3 compared to a conventional fleet by 2050. The long-term barrier for improving air quality for EVs is now non-exhaust emissions where exposure to PM2.5 may remain at a significant level (0.4 µgm-3) until 2050. Non-exhaust emissions are highly uncertain and may increase in the future with heavier and cheaper to run EVs.

Thesis dissertation

ApSimon H, Oxley T, Woodward H, Mehlig D, Holland M, Reeves Set al., 2023, Integrated assessment modelling of future air quality in the UK to 2050, and synergies with net-zero strategies, Atmosphere, Vol: 14, Pages: 1-21, ISSN: 2073-4433

Integrated assessment modelling (IAM) has been successfully used in the development of international agreements to reduce transboundary pollution in Europe, based on the GAINS model of IIASA. At a national level in the UK, a similar approach has been taken with the UK Integrated Assessment Model, UKIAM, superimposing pollution abatement measures and behavioural change on energy projections designed to meet targets set for the reduction of greenhouse gas emissions and allowing for natural and imported contributions from other countries and shipping. This paper describes how the UKIAM was used in the development of proposed targets for the reduction of fine particulate PM2.5 in the UK Environment Act, exploring scenarios encompassing different levels of ambition in reducing the emissions of air pollutants up to 2050, with associated health and other environmental benefits. There are two PM2.5 targets, an annual mean concentration target setting a maximum concentration to be reached by a future year, and a population exposure reduction target with benefits for health across the whole population. The work goes further, also demonstrating links to social deprivation. There is a strong connection between climate measures aimed at reducing net GHG emissions to zero by 2050 and future air quality, which may be positive or negative, as illustrated by sectoral studies for road transport where electrification of the fleet needs to match the evolution of energy production, and for domestic heating, where the use of wood for heating is an air quality issue. The UKIAM has been validated against air pollution measurements and other types of modelling, but there are many uncertainties, including future energy projections.

Journal article

Oxley T, Vieno M, Woodward H, ApSimon HM, Mehlig D, Beck R, Nemitz E, Reis Set al., 2023, Reduced-form and complex Actm modelling for air quality policy development: a model inter-comparison, Environment International, Vol: 171, Pages: 1-13, ISSN: 0160-4120

Simulation models can be valuable tools in supporting development of air pollution policy. However, exploration of future scenarios depends on reliable and robust modelling to provide confidence in outcomes which cannot be tested against measurements. Here we focus on the UK Integrated Assessment Model, a fast reduced-form model with a purpose to support policy development with modelling of multiple alternative future scenarios, and the EMEP4UK model which is a complex Eulerian Atmospheric Chemistry Transport Model requiring significant computing resources. The EMEP4UK model has been used to model selected core scenarios to compare with UKIAM, and to investigate sensitivity studies such as the interannual variability in response to meteorological differences between years. This model intercomparison addresses total PM2.5, primary PM2.5 and Secondary Inorganic Aerosol concentrations for a baseline of 2018 and selected scenarios for projections to 2040. This work has confirmed the robustness of the UK Integrated Assessment Model for assessing alternative futures through a direct comparison with EMEP4UK. Both models have shown good agreement with measurements, and EMEP4UK shows an ability to replicate past trends. These comparisons highlight how a combination of reduced-form modelling (UKIAM) and complex chemical transport modelling (EMEP4UK) can be effectively used in support of air pollution policy development, informing understanding of projected futures in the context of emerging evidence and uncertainties.

Journal article

Mehlig D, ApSimon H, Staffell I, 2022, Emissions from charging electric vehicles in the UK, Transportation Research Part D: Transport and Environment, Vol: 110, Pages: 1-16, ISSN: 1361-9209

Understanding how to best integrate electric vehicles (EVs) into electricity systems is key to the success of both sectors. We pair national-scale EV charging data with high resolution electricity generation data for the UK to calculate the average and marginal emissions produced through charging EVs. Considering the average generation mix weighted by when charging occurs, a typical Battery EV (BEV) emitted 41 g CO2, 27 mg NOx and 0.7 mg PM2.5 per kilometre in 2019. A static analysis using annual averages underestimates these values by 4 %. The ‘marginal’ emissions from BEV charging are 25 % higher than average emissions for CO2 and NOx, and 50 % lower for PM2.5. Smart charging was found to reduce average CO2 emissions by 10 % when compared to the typically charged vehicle; however, smart charging strategies may increase marginal emissions. Future smart charging strategies should minimise marginal emissions and will require access to 24-hour opportunistic smart charging.

Journal article

Mehlig D, 2021, Electrification of road transport and the impacts on air quality and health in the UK, Atmosphere, Vol: 12, Pages: 1-15, ISSN: 2073-4433

Currently, many cities in Europe are affected by concentrations of PM2.5 and NO2 above the WHO guidelines on the protection of human health. This is a global problem in which the growth of road transport constitutes a major factor. Looking to the future, electric vehicles (EVs) are considered to be the choice technology for reducing road transport greenhouse gas emissions, but their impact on air quality needs to be considered. Taking the UK as a case study, this paper begins by understanding the trajectory of a future scenario without the introduction of EVs, reflecting on the latest emission control improvements in internal combustion engine vehicles (ICEVs). This is then compared to a 2050 scenario in which the introduction of EVs, based on the UK government’s Transport Decarbonisation Plan, is reviewed. This plan includes a ban on the sale of ICEV cars and LGVs, beginning in 2030, with the subsequent electrification of heavier vehicles. By 2030, population exposure to NOx was found to be significantly reduced in the ICEV scenario, with a marginal further reduction found for the EV scenario. The EV scenario further reduced NOx exposure by 2050, with most of the benefits being realized before 2040. For the ICEV and EV scenario, PM2.5 emissions were largely unchanged due to the primary contribution of non-exhaust emissions, suggesting that EVs are likely to yield relatively smaller changes in exposure to PM2.5 than for NOx.

Journal article

ApSimon H, Oxley T, Woodward H, Mehlig D, Dore A, Holland Met al., 2021, The UK Integrated Assessment Model for source apportionment and air pollution policy applications to PM2.5, Environment International, Vol: 153, ISSN: 0160-4120

Source apportionment and the effect of reducing individual sources is important input for the development of strategies to address air pollution. The UK Integrated Assessment Model, UKIAM, has been developed for this purpose as a flexible framework, combining information from different atmospheric dispersion models to cover different pollutant contributions, and span the range from European to local scale. In this paper we describe the UKIAM as developed for SO2, NOx, NH3, PM2.5 and VOCs. We illustrate its versatility and application with assessment of current PM2.5 concentrations and exposure of the UK population, as a case-study that has been used as the starting point to investigate potential improvement towards attainment of the WHO guideline of 10 µg/m3.

Journal article

Mehlig D, ApSimon H, Staffell I, 2021, The impact of the UK’s COVID-19 lockdowns on energy demand and emissions, Environmental Research Letters, Vol: 16, Pages: 1-9, ISSN: 1748-9326

Around the world, efforts to contain the COVID-19 pandemic have profoundly changed human activity, which may have improved air quality and reduced greenhouse gas emissions. We investigated the impact of the pandemic on energy demand and subsequent emissions from electricity and gas throughout 2020 in the UK. The daily pattern of electricity demand changed in both lockdowns, with weekday demand shifting to that of a typical pre-pandemic weekend. Energy demand in 2020 was modelled to reveal the impact of the weather and the pandemic. The first lockdown reduced demand by 15.6% for electricity and 12.0% for commercial gas, whereas the second lockdown produced reductions less than half. Domestic gas demand did not change during the first lockdown, but increased by 6.1% in the second, likely due to increased domestic heat demand. The changes in demand for gas resulted in little change to overall gas consumption emissions during the pandemic. For electricity, large emission reductions occurred during the two lockdowns: up to 22% for CO2, 47% for NO¬x ¬, and 29% for PM2.5. Yet, the largest CO2 emission reduction for electricity in 2020 (25%) occurred before the pandemic, which happened during a warm and stormy spell with exceptional wind generation. These observations suggest that future similar changes in activity may result in little change for gas demand and emissions. For electricity, emission reductions through changes in energy demand are made possible by the generation mix. To enable further emission reductions in the future, the generation mix should continue to decarbonise. This will yield emission reductions in both times of lowered energy demand, but more importantly, during times of high renewable output.

Journal article

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