Executive summary

Effects of climate change on air quality

Before examining impacts of net-zero policies, the report considers how climate change itself is expected to affect air quality in the UK by influencing emissions, atmospheric processing and transport of many pollutants. Some of these effects are likely to slow or temporarily reverse improvements in air quality. They are also likely to lead to changes in the seasonal and geographical variations in air quality.

• In summertime, more frequent and intense heat waves are likely to lead to more episodes of O₃ and PM. This reflects the build-up of local emissions under stagnant meteorological conditions along with increases in precursor emissions from vegetation and soils, more moorland fires, and inflow of pollutants from mainland Europe. In contrast, wintertime air quality is likely to improve as the cold stagnant conditions that lead to pollutant accumulation are expected to become less frequent.
• The responses of air quality to climate change will vary across the UK, with the southeast more exposed to stagnant meteorological conditions, high temperatures and continental inflow. The difference between urban and rural air quality is expected to narrow in response to changes in both emissions and meteorology.
• Changes in temperature, humidity and precipitation will alter the emissions, formation, processing and removal of PM, affecting its composition and distribution. While increased removal by rainfall is beneficial, greater formation of PM from organics and NH₃ is a concern.
• Emissions of NH₃, CH₄, BVOCs and soil NO_x are expected to increase with temperature, creating an additional motivation to reduce such emissions now. Reducing NH₃ emissions would decrease PM concentrations and benefit biodiversity by reducing the deposition of reactive nitrogen on sensitive ecosystems.
• O₃ will remain an important global and regional pollutant throughout the period to 2050 and beyond. Climate-driven increases in input from the stratosphere, increased formation from climate-sensitive precursor emissions, especially of CH₄ from wetlands, and increased wildfires will lead to increased O₃ in many parts of Europe. In the UK this will be offset by greater O₃ destruction in air from the Atlantic, a consequence of a warmer, more humid atmosphere. International action to reduce CH₄ and other O₃ precursors would therefore be a win-win for climate and air quality.

Effects of net-zero measures on air quality

The transition to net-zero will deliver significant improvements in air quality as a co-benefit. The policy challenge is to maximise these improvements while retaining the GHG mitigation. Many of the actions taken to achieve net-zero are unequivocally positive for air quality. In other areas, action can be taken to enhance the air quality benefits, often through small changes. In a small number of areas, net-zero measures may have adverse impacts on air quality that require mitigation or swapping those measures for others that have no adverse effects on air quality. The short atmospheric lifetimes of most air pollutants mean that positive health benefits start as soon as the source of pollution is removed, providing a rationale for the prioritisation of net-zero measures that also deliver significant air quality co-benefits.

Opportunities with clear co-benefits

• Replacing fossil fuel derived electricity with decarbonised electricity will lead to substantial reductions in emissions of NO_x and sulphur dioxide (SO₂) and hence in PM_2.5 and O₃.
• ‘Active travel’ measures that encourage a shift away from car use to walking, cycling and public transport provide both decarbonisation and improvements in air quality, as well as health benefits that extend beyond improving air quality. Reducing demand can decrease emissions that are challenging to address through technology alone, such as non-exhaust PM from road vehicles and aviation jet turbine emissions.
• Improved efficiency in the management of nitrogen in the agricultural system has the potential to reduce NH₃, nitrous oxide (N₂O) and NO_x emissions in most cases. This would feed through into lower concentrations of PM_2.5 on a regional scale and a reduction in nitrogen deposition onto ecosystems. Measures to reduce dairy and red meat consumption would reduce NH₃ and CH₄ emissions, contributing to cleaner air and the net-zero target, as well as benefiting health.

Opportunities to enhance net-zero policies to benefit air quality

A transition to a fully battery electric vehicle fleet should bring significant improvements in urban air quality, benefiting many disadvantaged areas. However, emissions of non-exhaust particles from friction and abrasion such as from tyre, brake and road surface wear, and the resuspension of road dust, will continue to be a significant source of PM_2.5 emission, even from a fully electric vehicle fleet. These emissions could increase if average vehicle mass and numbers were to increase, as it may with larger batteries. This ongoing air quality issue can be mitigated by use of regenerative braking, smoother driving through vehicle autonomy, and the use of new pollution control technologies such as particle capture from brake callipers and low emission tyres.

In addition, one consequence of the reduction in urban NO_x emissions is an increase in O₃ concentrations because of a reduction in the chemical suppression of O₃ that takes place via reaction with nitric oxide (NO). The benefits of NO₂ reduction are likely to outweigh any O₃ disbenefit at the roadside, but this effect should be recognised, and regional O₃ pollution mitigated through policies that also reduce O₃ precursor emissions of volatile organic compounds (VOCs).

Net-zero measures that may require mitigation to protect air quality

• Carbon capture and storage (CCS) technologies may involve the consumption of large volumes of chemicals needed for the carbon dioxide (CO₂) stripping process. Possible fugitive emissions of volatile chemicals used in CCS can be controlled through the application of process after-treatment, and by selecting the materials on the basis of low toxicity and environmental impacts.
• While the expansion of decarbonised and nuclear infrastructure to replace fossil fuel assets will lead to air quality improvements, the transition period may have air quality impacts as a result of the temporary use of back-up power facilities, such as diesel farms, to supply capacity in peak periods, as well as construction activities. Mitigations can be introduced to manage such impacts, including enhanced requirements for after-treatment of combustion sources and dust suppression during construction.
• In the residential sector, technologies such as heat pumps and photovoltaics lead to unequivocal local air quality improvements. However, use of hydrogen or biogas boilers would likely lead to some emissions of NO and NO₂, which could be mitigated through enhanced requirements for emissions control and possibly new after-treatment technologies. Minimising leakage of hydrogen will maximise the climate and air quality benefits.
• Indoor air quality can be influenced, either positively or negatively, by net-zero measures. Delivering better indoor air quality in homes, workplaces and public buildings will require independent strategies as it is also influenced by human behaviours, product standards for buildings and furnishing materials, and the use of consumable products.
• Increased cultivation of fast-growing crops for biofuels and the planting of trees to create green urban spaces could lead to increased biogenic VOC emissions, leading to additional O₃ and secondary organic aerosol formation. These impacts can be significantly reduced through use of low-emitting species, such as cultivars of beech and lime, while avoiding large-scale planting of high emitting cultivars of species such as willow and oak.
• Actions on agricultural emissions should avoid ‘pollution swapping’ to deliver air quality benefits. For example, a switch away from ammonium nitrate fertilisers to urea could increase NH₃ emissions, although partial mitigation is possible through reduced overall consumption and improved farming practices.
• Combustion of biomass can reduce net GHG emissions relative to fossil fuels but could lead to air pollution emissions. After-treatment of emissions from biomass is likely to be cost effective at industrial scale, but possibly less effective at reducing emissions from domestic wood burning stoves or pellet boilers. Avoiding the use of biomass combustion in areas of high population density will be a key mitigation.
• Many of the key pollutants are secondary in nature, being formed in the atmosphere rather than emitted directly, including much of PM_2.5 and all O₃. These pollutants often have a non-linear relationship to their precursor emissions. In general, the term ‘non-linearity’ refers to a less than proportionate decrease in the secondary pollutant when the precursor emissions fall. Reducing the secondary emissions thus depends on continued action on the relevant primary emissions. For example, sustained reductions in emissions of NH₃ and NO_x are likely to be needed before substantial UK-wide reductions in resulting secondary PM are experienced. As PM_2.5 has a longer atmospheric lifetime than NH₃ or NO₂, this will demand continued international cooperation to reduce transboundary transport of pollution.
• Aviation has limited options for decarbonisation and thus for the foreseeable future it seems likely that airports will remain hotspots for air pollution due to emissions of PM, NO_x and VOCs from aircraft, as well as from road traffic and ground operations.

Areas for ongoing investigation

Effects of poor air quality on human health due to PM will likely remain despite the gradual reduction in exposure. There is strong evidence of adverse effects at exposures well below current levels and no identified safe lower concentration limit. The differential toxicity of particles is currently not well understood, which makes it challenging to determine the health impacts of changes in PM composition.

As emissions of pollutants continue to decline from historically dominant sectors such as power generation and road transport, where the emissions are generally well-quantified, a larger proportion of the remaining emissions will originate from diffuse sources where emissions are often not well quantified, such as cooking, ad-hoc burning and agricultural emissions. These changes will require further work on the UK National Atmospheric Emission Inventory (NAEI).

The changes in pollutant concentrations expected through to 2050 will have implications for their measurement in terms of the appropriate locations of monitoring sites and the relative importance of different source types. For example, there may be less need for roadside monitoring sites as vehicle exhaust emissions diminish.