The United Nations describes net zero as “cutting greenhouse gas emissions to as close to zero as possible, with any remaining emissions re-absorbed from the atmosphere, by oceans and forests for instance.” Climate change, the negative consequences of increased greenhouse gas emissions, is a rapidly growing threat to humanity and nature and immediate action is required.
However, there is a common misconception that any progress towards achieving net zero will also be beneficial in reducing local air pollution. Likewise, that misconception extends to believing that reducing local air pollution will always further the aim of achieving net zero.
This article looks at some of the areas where positive initiatives in one area can lead to unintended, negative consequences in another.
Local air pollutants vs greenhouse gases
Local air quality pollutants are often confused with greenhouse gases:
- In the UK, we have regulations that set out legally binding concentration limits for the following main local air pollutants: particulate matter, nitrogen oxides (NOx), nitrogen dioxide (NO2), sulphur dioxide (SO2), ozone (O3), carbon monoxide (CO), lead (Pb) and benzene, certain toxic heavy metals (arsenic, cadmium and nickel) and polycyclic aromatic hydrocarbons (PAHs).
- The World Meteorological Organization lists the main greenhouse gases of concern as: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and O3 in the lower atmosphere.
Looking at these two lists, it’s clear the pollutants are therefore very different with only ozone appearing on both lists.
The most obvious connection between net zero and air quality is the reduction in both greenhouse gases and local air quality pollutants (such as NOx, CO, particulate matter, SO2 and PAHs) achieved by avoiding the use of fossil fuels. It might be natural to conclude that a series of local air quality improvements would add up and be beneficial on a global scale. Reducing local air quality pollutants, however, especially particulate matter, can sometimes adversely affect the journey to halting global warming.
Aerosols, another name for particles suspended in the air, can be reflective, scattering solar radiation back to space, and have a negative/cooling radiative forcing effect on climate. If we improve air quality and reduce the pollutants that create those aerosols, there will be less reflection back to space, leading to increases in temperatures. But to complicate things, not all aerosols are reflective. Black carbon (or soot) absorbs solar radiation. A reduction in black carbon therefore may actually decrease temperatures.
Another factor is the role that suspended particles play in creating clouds. Aerosols lead to the formation of cloud droplets and ice crystals by acting as cloud condensation nuclei and ice nuclei. Aerosols created from burning fossil fuels increase the number of droplets. When there are high numbers of particles, the droplets that form are smaller, and the clouds last longer as the lighter droplets are less likely to fall out of the air as precipitation. Clouds reflect sunlight and the net effect of the radiative properties of clouds is generally cooling. Fewer clouds may mean less reflection of sunlight, and this is another example where reducing air pollution may unintentionally warm the climate.
“some policies and proposals could result in significant negative air quality impacts”
Location, location, location
Local air pollutants affect health and plant production at the bottom of the troposphere, but greenhouse gases are of greatest concern when they are high in the troposphere.
For local air pollutants, in contrast to greenhouse gas emissions, the location of the source of emission matters. If a net zero initiative causes the source of an emission to air to move closer to a populated area, this could have adverse health consequences, even if total national emissions decrease.
The Government’s ‘Net Zero Strategy: Build Back Greener’ acknowledges this by stating that: “…some policies and proposals could result in significant negative air quality impacts at both regional and local scales, for example emissions of fine particulate matter from biomass combustion, ammonia from the use of anaerobic digestion, and NOx emissions from hydrogen combustion in domestic or industrial settings.“
Burning biomass releases carbon that has only been locked-up for a relatively short period and helps to move closer to net zero. If the biomass is burned close to a populated area, then the associated particulate matter emissions could have adverse local air quality effects.
Similarly, the biogas produced by anaerobic digestion can be used to generate heat and power providing an alternative to fossil fuels. This helps towards achieving net zero. The digestate, a by-product of anaerobic digestion, needs to be disposed of and can be used as a soil improver. However, ammonia emissions from the spreading of the digestate can negatively affect human-health if it is close to populated areas and may cause biodiversity loss close to sensitive ecological sites.
While the combustion of hydrogen provides energy without producing any CO2 emissions, the process still leads to emissions of NOx. This is true for any combustion process as the high temperatures cause the oxidation of nitrogen in the atmosphere to produce NOx.
Again, there may be a reduction in fossil fuel use, but if the location is close to a populated area, there could be negative air quality effects.
The switch to electric vehicles may eliminate tail-pipe emissions, but there may still be emissions at the central electrical power plant. Battery and hydrogen-powered vehicles will also still generate particulate matter emissions from tyre and brake wear.
Carbon capture and storage (CCS) technologies may involve the consumption of large volumes of chemicals needed for the CO2 stripping process. Possible fugitive emissions of volatile chemicals used in the CCS process can affect nearby human-health and ecosystems. These can be controlled through the application of process after-treatment and by selecting the materials based on low toxicity and environmental impacts.
All about chemistry
Most tropospheric NOx are emitted as nitric oxide (NO) which photochemically equilibrates with NO2 within a few minutes. NOx do not directly affect the earth’s radiative balance but they create and destroy O3 in the troposphere through a series of reactions. As O3 has the most global warming potential after CO2 and CH4, changes in the emissions of local air pollutants also have the potential to affect the climate. The NO in NOx created by combustion reacts with O3 to produce NO2. NO2 in the presence of sunlight (hv) reacts with oxygen to produce O3. The chemical reactions happening in the atmosphere are very simplistically summarised as follows:
NO + O3 ‚ NO2 + O2 (1)
NO2 + hv (+ O2) ‚ O3 (2)
If local air quality initiatives are successful, then a consequence of lower NO emissions is less destruction of O3 (equation 1) and concentrations of O3 will increase. However, if there is now more sunlight reaching the earth through the clearer skies, then more O3 will be created (equation 2). The net effect is complicated and is a source of uncertainty in climate models.
Added to that, there’s substantial spatial and temporal variability in NOx concentrations across the globe. Concentrations of local air pollutants in an urban location are very different from concentrations over an ocean. The accuracy of global climate models is therefore affected by the accuracy and availability of data from across the world.
The effects of air pollution and climate change on ecosystems are complicated with many interacting processes. A key air quality concern for ecosystems is the deposition of nitrogen. Some plants wither and die back if too much nitrogen is deposited onto them.
Critical loads for nitrogen deposition are defined as thresholds below which significant harmful effects on sensitive habitats do not occur, according to present knowledge. When deposition exceeds the critical load, there is considered to be a risk of harmful effects on sensitive habitats. In 2019, nitrogen deposition rates exceeded the critical load across 97% of the area of sensitive habitats in the UK.1
There are also plants that thrive with more nitrogen. Nitrogen deposition, therefore, creates competition between plant species
and leads to reductions in the biodiversity of ecosystems. A further difficulty in assessing the long-term impacts of nitrogen deposition is that any positive effects early on may be offset in the long term by negative effects through nutrient imbalances and decreased tolerance to stress, once systems become saturated
“there’s substantial spatial and temporal variability in NOx concentrations across the globe”
Prior to saturation, if local air pollution improves and there’s less deposition, the growth of those plants that thrive on added nitrogen may start to slow. This decreased plant growth will have an associated decrease in carbon uptake.
Equally, plants that are negatively affected by high levels of nitrogen deposition may start to improve if nitrogen deposition decreases. The increased plant growth for those plants will mean more carbon uptake. The magnitude and direction of the net effects are very uncertain and will depend on the ecosystems affected.
Most action is currently focused on reducing the use of fossil fuels. Another pollutant to air with a local impact, ammonia (NH3), is emitted by manures and fertilisers. The amount of ammonia volatising increases with temperature so ammonia concentrations may increase as the planet gets warmer. The nitrogen in ammonia rapidly deposits onto ecosystems.
Therefore, unless there’s an effort to control emissions of ammonia from manures and fertilisers, global warming may lead to increased ammonia emissions and greater nitrogen deposition.
Building-in good design
Air quality consultants use dispersion models to predict the path of a pollutant from its point of emission, such as a vehicle tail-pipe or a stack, to a receptor (a human being or a location indicative of an ecosystem). The results of that modelling can help to determine whether the location of a development is likely to lead to local air quality impacts.
Involving an air quality specialist early on in the planning process can avoid the costly mistake of neglecting to consider potential local impacts, or implementing an initiative to further the aim of achieving net zero in the wrong place. Sometimes the location can’t be changed, and air quality consultants often assist in determining appropriate stack heights for facilities, with the potential to pollute local air quality. With an appropriate stack height, the plume of emitted pollutants should disperse, avoiding local air quality impacts.
There are options for producing energy where there are no emissions to air: renewable energy produced from wind turbines and solar panels generally has no local air quality impacts once operational. However, there will still be processes that emit pollutants to air for some time to come. While there are clear tensions between achieving net zero and improving air quality for some technologies, it’s still possible to achieve a positive outcome if consideration is given to both at the point of designing a solution.
For example, negative air quality impacts from the use of biomass, anaerobic digestion, and hydrogen can be avoided by considering the location of emissions in relation to populated areas. That consideration also needs to be extended to the local air quality impact of initiatives on ecosystems, particularly, where those ecosystem services affect humans.
1 England Biodiversity Indicators technical background document Advice on the trends in acidity and nutrient nitrogen critical load Exceedances Updated: December 2022