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As part of climate change commitments, the United Kingdom is considering an incremental transition from natural gas to hydrogen for domestic heating, blending up to 20% of hydrogen (by volume) into the national gas network. We consider the possible impacts of this policy on nitrogen oxides (NOx) emissions, a minor waste by-product from combustion. A meta-analysis of changes in NOx emissions from hydrogen/natural gas blends used in gas burners is undertaken, with focus on mixtures between 5% and 20% v/v. Literature reports are highly variable: for a 5% hydrogen blend, changes in NOx emissions, when compared to burning pure natural gas, vary over the range –12% to þ39%, with a mean change across 14 studies of þ8%. These estimates required an important assumption to be made that, when not explicitly described, all literature data on changes in NOx emissions and/or concentrations were suitably corrected for the reduced energy density and heat output arising once hydrogen is added. A NOx increase can be rationalized through the increased adiabatic flame temperature generated from hydrogen combustion. The associated range of plausible damage costs of a 5% hydrogen blend is estimated to fall within the range –117 million GBP to þ362 million GBP per year; 20% hydrogen (the maximum that could be accommodated with existing infrastructure) would lead to a change in emissions in the range –50 to þ154% with a change in damage costs of between –467 million GBP and þ1,146 million GBP per year. The mean change is estimated at 292 million GBP per year. For existing poor performing boilers, an economic case can be made for scrappage and replacement based primarily on NOx damage costs avoided. The response of older boilers to added hydrogen is a critical evidence gap that needs filling before further decisions on hydrogen as a heating fuel are made.

Air pollutants in the exhaust emissions from internal combustion engine (ICE) vehicles result from unburnt fuel, high temperature combustion of fuel in the presence of air, and from exhaust after-treatment processes. As many of these pollutants are harmful to human health, impact climate as greenhouse gases, or both, emissions from vehicles have been subject to regulation for many years. Europe-wide standards on vehicle exhaust were introduced in the early 1990s and have been progressively made more stringent, extending the range of regulated pollutants included and reducing the permissible levels of emissions in new vehicles introduced to the fleet. A review of the development and evolution of exhaust emissions standards of relevance to UK vehicles is provided in Chapter 2 of this report.To comply with these standards, technologies for reducing emissions of regulated pollutants have been developed and fitted to vehicle exhaust systems. In combination with advances in engines, these have substantially reduced emissions of many of the regulated pollutants, and a review of the key technologies has been undertaken. Whilst there has been considerable success in substantially reducing exhaust emissions of pollutants such as carbon monoxide (CO), hydrocarbons (HCs) and particulate matter (PM), exhaust emissions from ICE vehicles remain a major source of nitrogen oxides (NOx) and an important contributor to poor urban air quality.Ambient atmospheric measurements have previously revealed important disparities between urban and roadside concentrations of nitrogen dioxide (NO2) and those that would be projected based on the prevailing vehicle emissions standards of NOx. It is now understood that exhaust gas after-treatment technologies introduced on diesel vehicles have not always been effectively implemented and not delivered the anticipated reductions in emissions when in real-world use. The on-road effectiveness of exhaust emissions controls has been found to be highly variable even between vehicles that are technically compliant with the same regulatory standards. There is now greater appreciation of the need to evaluate and demonstrate vehicle emissions performance under real-world driving conditions. On-road emissions have been higher than those measured during laboratory based tests, although in the past five years that gap has closed for some vehicles, and improvements continue. Future European standards will move close to demanding equivalence between on-road and laboratory performance. Real-world driving emissions are reviewed for various vehicle types in Chapter 3.The dynamic nature of the vehicle fleet means that vehicles with a range of different exhaust control technologies and historic standards are in use on UK roads. Between 1990 and 2015, the UK vehicle fleet transitioned to a greater proportion of diesel cars, compared with gasoline, a change that was motivated by predicted fuel economy savings and lower CO2 emissions per mile driven. The relative contributions of exhaust emissions from diesel, gasoline, and more recently hybrid electric vehicles continue to evolve as new technologies develop, and the regulatory framework and consumer preferences (in part driven by economic policy such as fuel tax) change. The vehicle fleet will continue to change over the next decade with older more polluting vehicles leaving the fleet and newer cleaner vehicles joining. This report provides, in Chapter 4, modelled estimates of the vehicle fleet composition over time and the resulting exhaust emissions using best available estimates from inventories used for international reporting. Understanding how the future ensemble of vehicles on the roads will contribute to air pollution emissions is significant for policy development since this information can support the evaluation of interventions such as low emissions zones, and the likely compliance with air quality directives. This report was drafted prior to the COVID-19 crisis and projections of future vehicle use are based on knowledge and understanding at that time. Since March 2020 traffic volumes have varied significantly depending on national restrictions and this has influenced roadside concentrations of many vehicle-derived pollutants. AQEG provided an initial analysis of these effects to Defra in June 2020. The post-COVID fall in new vehicle sales and economic conditions which may affect transport in the future are not assessed here.Ambient observations of air pollution, particularly at the roadside, provide an independent assessment of vehicle exhaust emissions, trends in these emissions over time, and by extension whether emissions controls are working as intended. Methods to assess vehicle exhaust emissions using UK ambient air quality data for a range of different pollutants are presented in Chapter 5, which also includes examples of where ambient observations have detected unanticipated trends in emissions not predicted based on laboratory tests.Chapter 6 identifies possible future interventions and technologies to reduce exhaust emissions and their impacts, including after-treatment approaches. The longer-term impacts on vehicle exhaust emissions from increased use of ICE-electric hybridisation propulsion and driver automation technologies are considered, as are potential approaches to retrofitting as a means to improve emissions performance for existing vehicles. Potential regulatory approaches to managing exhaust emissions are also reviewed, including the possible impacts of changed vehicle inspection regimes, new fuels standards and external traffic management controls on vehicle movements and on idling.Chapter 7 provides the AQEG recommendations.

Ozone near the planetary surface is considered an air pollutant because of its adverse effects on human health, crop yields, and ecosystems more broadly. In addition, ozone in the free troposphere is a greenhouse gas with climate impacts. Ozone is a relatively long-lived air pollutant so can be transported for periods of days to several weeks over large distances. The concentrations that arrive over the UK in the prevailing westerly winds from the Atlantic are influenced by the cumulative effects of emissions that occur throughout the mid-latitudes of the Northern Hemisphere.UK population and ecosystem exposure to ozone is frequently lower than the north Atlantic baseline due to dominant ozone sink processes of removal by reaction with local emissions of NO and deposition to the terrestrial surface. In spring and summer, net ozone production from UK emissions, in addition to advection from mainland Europe, leads to episodes of elevated concentrations.Control of surface ozone is complex since it is not emitted directly from any source but is formed in the atmosphere in a non-linear manner from reactions involving nitrogen oxides (NOx), methane, volatile organic compounds (VOCs) and sunlight. Air quality policies have been in place to reduce surface ozone since the 1980s, through control of emissions of NOx and VOCs from sources such as vehicles, combustion, fuels and solvents. The 1990s saw reductions in peak concentrations and fewer severe ozone pollution events – a consequence of substantial reductions in emissions implemented internationally through agreements such as the Gothenburg protocol. Overall, since the UK is a net sink for ozone, changes in ozone concentration are controlled by the changes in the chemical and deposition sinks as well as changes in baseline concentration and local chemical production.The distribution of ozone across the UK shows highest concentrations over upland and rural locations, with annual average concentrations of >60 μg m-3 widespread over rural areas in Scotland, Northern Ireland, Wales, south-west England and the Pennines. Urban and suburban concentrations are typically ~15 μg m-3 lower than rural values, although this is very sensitive to local emissions from road transport, and in heavily-trafficked roadside locations ozone concentrations can locally be close to zero. Ozone typically reaches its highest concentrations in the UK during April and May, although anticyclonic weather patterns in summer can also lead to significantly elevated concentrations.Since 2000 there has been little observed change in UK rural ozone concentrations, or ozone in the air arriving from the wider Atlantic region to the UK. There have however been some upwards trends in suburban and urban ozone (average increases of the order 5 – 9 μg m-3 over the 20-year period 2000-2019). These upwards trends in suburban and urban ozone are a consequence of reductions in primary emissions of NO; the NO has the effect of locally suppressing ozone.All areas of the UK have been compliant with the EU target value for the protection of human health for at least the last 12 years (≤25 days per year with daily maximum 8-hour mean ozone concentration >120 μg m-3, averaged over 3 years). Substantial parts of the UK are however routinely not compliant with the EU long-term objective of zero exceedance of this metric, although the number and geographical extent of these exceedance days has generally been declining over the last 20 years. All areas of the UK have been compliant with the EU target value for the protection of vegetation for at least the last 12 years (AOT40 not exceeding 18000 μg m-3.hour per year, averaged over 5 years). The EU long-term objective for the protection of vegetation is lower (AOT40 The prediction of future surface ozone in the UK is complex and subject to multiple drivers. Wider hemispheric trends in ozone resulting from global trends in emissions, in particular of methane, will have a significant impact on UK concentrations, but the direction of change is unclear. Further UK reductions in NOx emissions are anticipated and these will likely lead to further increases in urban ozone but be beneficial more widely and reduce ozone at a transboundary scale. Projections indicate little change to UK VOC emissions in the coming decade and this will become a limiting factor for reducing ozone in an optimal manner. Climate change will have multiple effects on ozone precursor emissions, chemical production and loss, dispersion and deposition. Whilst climate change is likely to lead to periods of higher temperatures that are known to increase biogenic VOC emissions and to reduce the surface dry deposition of ozone, the net effect on ozone of all climate-change influences is not known.As other air pollutants in the UK continue to decline it seems likely that the importance of ozone will grow, especially as its contribution to climate provides further motivation for control measures.

Flux measurements of nitrogen oxides (NOx) were made over London using airborne eddy covariance from a low-flying aircraft. Seven low-altitude flights were conducted over Greater London, performing multiple overpasses across the city during eight days in July 2014. NOx fluxes across the Greater London region (GLR) exhibited high heterogeneity and strong diurnal variability, with central areas responsible for the highest emission rates (20-30mgm-2h-1). Other high-emission areas included the M25 orbital motorway. The complexity of London's emission characteristics makes it challenging to pinpoint single emissions sources definitively using airborne measurements. Multiple sources, including road transport and residential, commercial and industrial combustion sources, are all likely to contribute to measured fluxes. Measured flux estimates were compared to scaled National Atmospheric Emissions Inventory (NAEI) estimates, accounting for monthly, daily and hourly variability. Significant differences were found between the flux-driven emissions and the NAEI estimates across Greater London, with measured values up to 2 times higher in Central London than those predicted by the inventory. To overcome the limitations of using the national inventory to contextualise measured fluxes, we used physics-guided flux data fusion to train environmental response functions (ERFs) between measured flux and environmental drivers (meteorological and surface). The aim was to generate time-of-day emission surfaces using calculated ERF relationships for the entire GLR; 98% spatial coverage was achieved across the GLR at 400m2 spatial resolution. All flight leg projections showed substantial heterogeneity across the domain, with high emissions emanating from Central London and major road infrastructure. The diurnal emission structure of the GLR was also investigated, through ERF, with the morning rush hour distinguished from lower emissions during the early afternoon. Overall, the integration of airborne fluxes with an ERF-driven strategy enabled the first independent generation of surface NOx emissions, at high resolution using an eddy-covariance approach, for an entire city region.

Epidemiological studies have consistently linked exposure to PM2.5 with adverse health effects. The oxidative potential (OP) of aerosol particles has been widely suggested as a measure of their potential toxicity. Several acellular chemical assays are now readily employed to measure OP; however, uncertainty remains regarding the atmospheric conditions and specific chemical components of PM2.5 that drive OP. A limited number of studies have simultaneously utilised multiple OP assays with a wide range of concurrent measurements and investigated the seasonality of PM2.5 OP. In this work, filter samples were collected in winter 2016 and summer 2017 during the atmospheric pollution and human health in a Chinese megacity campaign (APHH-Beijing), and PM2.5 OP was analysed using four acellular methods: ascorbic acid (AA), dithiothreitol (DTT), 2,7-dichlorofluorescin/hydrogen peroxidase (DCFH) and electron paramagnetic resonance spectroscopy (EPR). Each assay reflects different oxidising properties of PM2.5, including particle-bound reactive oxygen species (DCFH), superoxide radical production (EPR) and catalytic redox chemistry (DTT/AA), and a combination of these four assays provided a detailed overall picture of the oxidising properties of PM2.5 at a central site in Beijing. Positive correlations of OP (normalised per volume of air) of all four assays with overall PM2.5 mass were observed, with stronger correlations in winter compared to summer. In contrast, when OP assay values were normalised for particle mass, days with higher PM2.5 mass concentrations (µg m−3) were found to have lower mass-normalised OP values as measured by AA and DTT. This finding supports that total PM2.5 mass concentrations alone may not always be the best indicator for particle toxicity. Univariate analysis of OP values and an extensive range of additional measurements, 107 in total, including PM2.5 composition, gas-phase composition and meteorological data, provided detailed insight into the chemical components and atmospheric processes that determine PM2.5 OP variability. Multivariate statistical analyses highlighted associations of OP assay responses with varying chemical components in PM2.5 for both mass- and volume-normalised data. AA and DTT assays were well predicted by a small set of measurements in multiple linear regression (MLR) models and indicated fossil fuel combustion, vehicle emissions and biogenic secondary organic aerosol (SOA) as influential particle sources in the assay response. Mass MLR models of OP associated with compositional source profiles predicted OP almost as well as volume MLR models, illustrating the influence of mass composition on both particle-level OP and total volume OP. Univariate and multivariate analysis showed that different assays cover different chemical spaces, and through comparison of mass- and volume-normalised data we demonstrate that mass-normalised OP provides a more nuanced picture of compositional drivers and sources of OP compared to volume-normalised analysis. This study constitutes one of the most extensive and comprehensive composition datasets currently available and provides a unique opportunity to explore chemical variations in PM2.5 and how they affect both PM2.5 OP and the concentrations of particle-bound reactive oxygen species.

Flux measurements of nitrogen oxides (NOx) were made over London using airborne eddy-covariance from a low flying aircraft. Seven low altitude flights were conducted over Greater London performing multiple over-passes across the city during eight days in July 2014. NOx fluxes across the Greater London region exhibited high heterogeneity and strong diurnal variability, with central areas responsible for the highest emission rates (20–30 mg m−2 h−1). Other high emission areas included the M25 orbital motorway. The complexity of London’s emission characteristics makes it challenging to pinpoint single emission sources definitively using airborne measurements. Multiple sources, including road transport and residential, commercial and industrial combustion sources are all likely to contribute to measured fluxes. Measured flux estimates were compared to scaled National Atmospheric Emissions Inventory (NAEI) estimates, accounting for; monthly, daily and hourly variability. Significant differences were found between the flux-driven emissions and the NAEI estimates across Greater London, with measured values up to two times higher in Central London than those predicted by the inventory. To overcome the limitations of using the national inventory to contextualise measured fluxes, we used physics-guided flux data fusion to train environmental response functions (ERF) between measured flux and environmental drivers (meteorological and surface). The aim was to generate time-of-day emission surfaces using calculated ERF relationships for the entire Greater London region (GLR). 98 % spatial coverage was achieved across GLR at 400 m2 spatial resolution. All flight leg projections showed substantial heterogeneity across the domain, with high emissions emanating from Central London and major road infrastructure. The diurnal emission structure of the GLR was also investigated, through ERF, with the morning rush-hour distinguished from lower emissions during the early afternoon. Overall, the integration of airborne fluxes with an ERF-driven strategy enabled the first independent generation of surface NOx emissions, at high resolution using an eddy-covariance approach, for an entire city region.