Little things matter: Particulate air pollution 2.5 micrometers or smaller in size (PM2.5) is a major cause of human mortality, and controlling its production is a health policy priority. Nitrogen oxides are an important precursor of PM2.5 and have been a focus of pollution control programs. However, Gu et al . now show that abating ammonia emissions is also an important component of PM2.5 reduction, and the societal benefits of abatement greatly outweigh the costs (see the Perspective by Erisman). Reducing ammonia emissions thus would be a cost-effective complement to nitrogen oxides and sulfur dioxide controls. —HJS

Currently, many cities in Europe are affected by concentrations of PM_2.5 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 NO_x 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 NO_x exposure by 2050, with most of the benefits being realized before 2040. For the ICEV and EV scenario, PM_2.5 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 PM_2.5 than for NO_x.

The burden of diseases attributable to air pollution is comparable to those of global health risks such as unhealthy diets and tobacco smoking, with many air pollution sources also emitting climate heating gases. In this UK study we estimated the co-benefits of Net Zero (NZ) climate policy on the health benefits of air pollution reduction, increased active travel, outdoor exposure inequalities and indoor air pollution changes. The study focused on two of the largest UK sources, road transport and building heating, with comparisons made between NZ and UK existing policy, referred to as Business as Usual (BAU). Particulate matter (PM_2.5), Nitrogen Dioxide (NO₂) and Ozone (O₃) projections were made between 2019 and 2050, with emphasis placed upon the NZ co-benefits in 2030 and 2040. We compared the UK BAU scenarios with the Climate Change Committee's (CCC) Balanced Net Zero Pathway (BNZP) and Widespread Innovation (WI) pathway. Compared to BAU predictions, BNZP assumptions lead to more electric vehicles, reduced vehicle km, more low carbon building heating, and reduced emissions of NO₂ and PM_2.5. By 2040 under BNZP, relative to BAU, the buildings sector was predicted to be three times more effective at reducing PM_2.5 than road transport. To help reduce the inequality gap the NZ building transition was tailored toward those most in need. Outdoor air pollution exposure inequalities prevailed across the socioeconomic spectrum, especially for NO₂, but were less pronounced due in part to NZ policies. Core air quality health benefits for the BNZP buildings sector were £21.3 billion (16.4 to 26.2) by 2050 and £98.4 billion (75.7 to 121.1) by 2154. For the transport sector the health benefits were £9.1 billion (7.0 to 11.2) by 2050 and £36.5 billion (28.1 to 44.9) by 2154. NZ building sector operating costs did not achieve break-even via efficiency savings, but with Greenhouse Gas (GHG) (lower benefits) break-even was achieved in 2052. With additional air pollution health benefits, building-sector time to break-even improved by between 3.1 (2.5 to 4.7) and 6.3 (4.7 to 7.6) years to between 2046 and 2049. Analysis found that removing gas cooking at home, for NZ, may result in greater concentration reductions than outdoor air pollution for NO₂. Net Zero health and economic co-benefits are large, as are the changes needed, requiring political leadership and public engagement. Our findings are relevant to other countries facing the NZ transition.

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 PM_2.5 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 PM_2.5 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.

Background: The UK’sClimate Change Act 2008(CCA; Great Britain.Climate Change Act 2008. Chapter 27. London: The Stationery Office; 2008) requires a reduction of 80% in carbon dioxide-equivalent emissions by 2050 on a 1990 base. This project quantified the impact of air pollution on health from four scenarios involving particulate matter of ≤ 2.5 µm (PM_2.5), nitrogen dioxide (NO₂) and ozone (O₃). Two scenarios met the CCA target: one with limited nuclear power build (nuclear replacement option; NRPO) and one with no policy constraint on nuclear (low greenhouse gas). Another scenario envisaged no further climate actions beyond those already agreed (‘baseline’) and the fourth kept 2011 concentrations constant to 2050 (‘2011’).Methods: The UK Integrated MARKAL–EFOM System (UKTM) energy system model was used to develop the scenarios and produce projections of fuel use; these were used to produce air pollutant emission inventories for Great Britain (GB) for each scenario. The inventories were then used to run the Community Multiscale Air Quality model ‘air pollution model’ to generate air pollutant concentration maps across GB, which then, combined with relationships between concentrations and health outcomes, were used to calculate the impact on health from the air pollution emitted in each scenario. This is a significant improvement on previous health impact studies of climate policies, which have relied on emissions changes. Inequalities in exposure in different socioeconomic groups were also calculated, as was the economic impact of the pollution emissions.Results: Concentrations of NO₂declined significantly because of a high degree of electrification of the GB road transport fleet, although the NRPO scenario shows large increases in oxides of nitrogen emissions from combined heat and power (CHP) sources. Concentrations of PM_2.5show a modest decrease by 2050, which would have been larger if it had not been for a significant increase in biomass (wood burning) use in the two CCA scenarios peaking in 2035. The metric quantifying long-term exposure to O₃is projected to decrease, while the important short-term O₃exposure metric increases. Large projected increases in future GB vehicle kilometres lead to increased non-exhaust PM_2.5and particulate matter of ≤ 10 µm emissions. The two scenarios which achieve the CCA target resulted in more life-years lost from long-term exposures to PM_2.5than in the baseline scenario. This is an opportunity lost and arises largely from the increase in biomass use, which is projected to peak in 2035. Reduced long-term exposures to NO₂lead to many more life-years saved in the ‘CCA-compliant’ scenarios, but the association used may overestimate the effects of NO₂itself. The more deprived populations are estimated currently to be exposed to higher concentrations than those less deprived, the contrast being largest for NO₂. Despite reductions in concentrations in 2050, the most socioeconomically deprived are still exposed to higher concentrations than the less deprived.Limitations: Modelling of the atmosphere is always uncertain; we have shown the model to be acceptable through comparison with observations. The necessary complexity of the modelling system has meant that only a small number of scenarios were run.Conclusions: We have established a system which can be used to explore a wider range of climate policy scenarios, including more European and global scenarios as well as local measures. Future work could explore wood burning in more detail, in terms of the sectors in which it might be burned and the spatial distribution of this across the UK. Further analyses of options for CHP could also be explored. Non-exhaust emissions from road transport are an important source of particles and emission factors are uncertain. Further research on this area coupled with our modelling would be a valuable area of research.Funding: The National Institute for Health Research Public Health Research programme.

Poland is in the group of European countries with the most severe air quality and specific emission structure (over 80 % of PM2.5 emitted in residential combustion). In this work, we quantify the health impacts of PM2.5 concentrations in Poland using two chemical transport models (CTMs), WRF-Chem and EMEP4PL. The models were run for two years (2017 and 2018), which differed in meteorological conditions. The results show that the total number of premature deaths that could be avoided if the PM2.5 concentrations do not exceed the WHO recommended value of 5 μg m^-3 exceeds 22,000 deaths for all model runs and meteorological years considered, with the lowest estimate at 22,447 (WRF-Chem model run for the year 2017; 95 % confidence intervals CI (95 % CI) 17121-25045) and the highest at 29,227 (95 % CI 22343 - 32572; EMEP4PL for the year 2018). These numbers are underestimated, as both CTMs underestimate the measured PM2.5 concentrations at site locations. The underestimation is the highest during the heating seasons and can be reduced if the emission of primary PM2.5 from residential combustion increases. The total number of estimated premature deaths is sensitive to chemical transport model selection and meteorological year. The selection of the meteorological year for the analysis may change the estimated number of premature deaths by 10 %. This change is significant compared to the estimated change in health impact due to the envisaged implementation of the Clean Air Program in Poland.