Understanding air pollution in urban areas is difficult – they are densely populated and contain many different indoor and outdoor pollution sources – but the impacts of climate change add an extra challenge.

We asked researchers at the National Centre for Atmospheric Science to explain why towns and cities make predicting how much pollution we breathe in on a day-to-day basis complicated, and what climate change might mean for air quality now and in the future.

Clean air is important for healthy people and a healthy environment.

Air pollution is a local, regional and global problem caused by the emission of gases, chemicals and small particles. It has negative impacts on human health and the natural environment, and for the most part, is invisible to the naked eye.

Examples of air pollutants include nitrogen dioxide (NO2), ozone (O3), sulphur dioxide (SO2), methane (CH4), carbon monoxide (CO), volatile organic compounds (VOCs) and particulate matter (PM). These pollutants are emitted by a wide range of human activities and natural sources such as fossil fuel burning, vehicle exhausts and tyres, household gas combustion and woodburners, evaporation from petrol stations, paint and chemical factories, cleaning and personal care products, plants and trees, wildfires, and farming practices – the list goes on.

We can measure the source, amount, and type of air pollutants – but we also need to consider how air pollution moves around the environments we spend time in, and how weather and the impacts of climate change can affect people’s exposure to harmful emissions.

Air pollution levels are closely linked to the weather. Hot spells often arrive hand-in-hand with poor air quality as the sun turns up the heat on a melting pot of airborne chemicals. Wind can affect the flow of airborne pollutants in and around cities, and rain can “washout” particulate matter – changing how much air pollution is in one place at a given time on a daily basis.

For example, depending on the direction the wind is blowing, pollution from road vehicles can become trapped between buildings. Tall buildings on either side of a street create a ’street canyon’. Street canyons can shelter streets from wind, restricting air flow down the road and creating conditions where air pollution can build up.

Extreme weather like heatwaves, strong winds, and heavy rainstorms, can create, worsen, or reduce the effects of air pollution.

And certain air pollutants can also contribute to climate change. Chemicals and particles like ozone and black carbon have sunlight-absorbing properties, like the well-known greenhouse gases carbon dioxide and methane, which means they too can contribute to a warming global climate.

Increased global temperatures are leading to drier dry spells and hotter heatwaves, both of which can worsen air pollution. During a dry spell in a city, surface dust on roads can become airborne easier and contribute to particulate pollution. Increased particle pollution lowers the quality of the air we breathe outdoors.

As global temperatures continue to rise, dry weather and heatwaves create conditions in which extreme wildfires are more likely to occur. It is anticipated that the frequency and intensity of wildfires will increase because of global warming, therefore it is important that we understand the impacts of wildfire smoke on air quality.

But why is this a problem for urban areas?

In June 2023, many cities across North America were engulfed by smoke from nearby wildfires. The air pollution caused by the smoke led to health officials distributing facemasks and advising people to stay indoors.

Heatwaves also increase natural volatile organic compound emissions (VOCs) to be emitted from plants and increase photochemistry – this is when nitrogen oxides and VOCs react to sunlight – which in turn increases ozone and secondary particulate matter concentrations, lowering air quality.

Heavy rain, during storms, reduces particulate matter concentrations which can improve air quality in cities. Some air pollutants even have a role in weather formation, acting as “cloud seeds” and leading to subsequent rainfall.

We know that climate change is likely to make winters milder and wetter in the UK, largely because warmer air can hold much more water. But it is important to remember that extremely cold weather can still happen in a warming climate.

Increased demand on heating in the wintertime can cause severe pollution events, particularly during the night or early morning. During extremely cold and calm weather conditions, less electricity can be generated from wind power – meaning the UK may have to rely on coal or biomass for electricity generation, which are more polluting.

The UK’s transition to net zero greenhouse gas emissions has the potential to minimise future climate change – as well as reducing air pollution in nearly every sector, ranging from transport, and home heating to energy production, but the scale of benefits often depends on which replacement low-carbon technologies and processes are used and how they are managed.

Some actions to deliver net zero could counteract progress being made on air quality – through introducing new sources of air pollutants. One example is biogas production, which may increase emissions of ammonia during the handling of materials. Also burning solid biomass – such as wood chips – as a replacement for natural gas may reduce net carbon emissions, but may increase emissions of health-impacting air pollutants.

Particular attention needs to be paid to the use of hydrogen as a replacement fuel. While not producing carbon dioxide, hydrogen-burning engines can potentially produce oxides of nitrogen (NOx) and very small ‘ultrafine’ particles, so these must be understood and engineered appropriately.

Some low carbon technologies eliminate combustion emissions, but still retain pollutants derived from friction. For example, electric vehicles will still create particle pollution from road surface abrasion and brake wear. Because of that, walking, cycling and public transport remain the cleanest options for transitioning to net zero emissions.

Researchers are working to improve estimates of how air pollution is changing in towns and cities by monitoring real-time emissions and by forecasting future conditions with high-resolution computer models.

Recognising the connection between air pollution, weather, and climate change is important for people living and working in urban areas, policymakers who need to weigh-up local air quality improvement schemes, for weather forecasting teams, and for people taking actions that aim to slow down global warming.