Image courtesy of Climate Visuals

TL;DR — Because air quality is impacted by factors such as temperature and weather patterns, certain aspects of the summer season lead to heightened air pollution levels. Increased sunlight levels can contribute to higher ground-level ozone, smog, and nitrogen dioxide levels — and, thus, harm human health. Summer weather can also be accompanied by air stagnation, which causes a buildup of air pollutants, as well as heat waves, drought, and increased susceptibility to wildfire — all of which are made worse by our changing climate. By understanding these complex interactions, we can work to minimize exposure and advocate for cleaner air.

The link between air quality, weather, and seasons

Air quality can vary due to a vast array of factors — including the season and its associated changes in temperature, weather patterns, and human behavior.

Some specific weather factors which can affect pollutant concentrations are:

  • Sunshine, which is most often linked to smog production
  • Rain, which generally results in lower pollution levels because it can “wash away” dissolvable pollutants
  • Temperature, which can play a role in pollutant formation and distribution
  • Wind speed and air turbulence, which can affect how concentrated pollutants are in a certain area and how they move from their original source

Seasonal patterns also exist in the human behaviors that affect air pollutant concentrations. For example, increased wood burning and car idling in the winter tends to result in higher particulate matter, carbon monoxide, and nitrogen dioxide levels.

Temperature and air quality affect not only the production of individual pollutants such as ground-level ozone, but also larger atmospheric patterns. 

Convection currents may occur, where warmer air near the ground moves upwards in the atmosphere as cooler, heavier air higher up in the troposphere sinks — resulting in pollutants moving upwards to higher altitudes.

However, during the winter, thermal inversions may occur where this warm air acts like a lid, trapping colder air and air pollution closer to the ground. Cities in mountain basins or valleys such as Los Angeles, Mexico City, and Denver are more likely to see thermal inversions.

The impacts of summer on air quality

The summer season affects air pollution levels in a number of ways — let’s take a look at its impacts on some individual air pollutants.

Ground-level ozone pollution

Hot, sunny summer weather can contribute to greater production of ground-level ozone pollution. Ozone is a secondary pollutant that is created by a chemical reaction that must occur in the presence of light, meaning that higher levels are often observed during the summer as well as during heat waves.

Ground-level ozone levels also depend on the level of precursor pollutants present — meaning those that are needed in the chemical reaction, namely nitrous oxides and volatile organic compounds — making it especially important to work to reduce these pollutants.

Ground-level ozone is also a main component of smog, which not only reduces visibility but can also significantly damage health. When thermal inversions occur, smog can become especially concentrated at the ground level — helping to explain why cities like Los Angeles are infamous for their smog.

Ozone is a dangerous pollutant, measured as one of the six criteria pollutants under the US EPA. Some of its health risks include:

  • Coughing
  • Difficulty breathing
  • Lung damage
  • Aggravation of lung disease
  • Increased frequency of asthma attacks
  • Increased risk of premature death due to heart or lung disease

To learn more about ground-level ozone pollution, read our air quality measurements series blog here.

Nitrogen dioxide air pollution

Nitrogen dioxide levels are also linked to the summer season due to the role of sunlight. 

Though a portion of nitrogen dioxide levels are emitted directly from combustion sources, the pollutant can also form in the atmosphere due to chemical reactions between nitric oxide and other air pollutants in the presence of sunlight. Thus, NO2 levels can vary in accordance with sunlight.

While this would suggest that NO2 levels might be highest in the summer, data finds that many places experience peaks in the winter for a variety of reasons, including increased use of combustion power plants for home heating and greater vehicle idling to “warm up” engines

Regardless of the season, nitrogen dioxide remains a harmful air pollutant. Read our air quality measurements series blog here to learn more.

How summer weather conditions affect overall air pollutant concentrations

Summer weather is often accompanied by stagnation events, which negatively impact overall air quality levels.

Hot, still, dry weather can lead to a build-up of pollutants in the air we breathe—a process called stagnation”

— Climate Central
The graphic above demonstrates the change in observed stagnant days from 1973 to 2021, with 83% of the U.S. cities analyzed experiencing an increase in the number of stagnant days during the summer. The state of California accounted for the top five cities in the country with the greatest increase in stagnant days. (Image source: Climate Central)

According to the same study, 98% of the analyzed cities had a positive correlation between the number of stagnant days and temperatures in the summer — suggesting that the increasing summer temperatures and periods of extreme heat that we are seeing as part of climate change will likely be accompanied by more stagnant days, posing a threat to air quality and human health.

To learn more about the links between air quality and climate change and the ways that we will have to adapt in the face of accelerating environmental disasters, read our blog here.

Heat waves can also be accompanied by drought conditions, which can lead to dry soil and make forest fires more common. Wildfires significantly degrade air quality, contributing especially to levels of particulate matter and carbon monoxide, and threatening human and environmental health.

Read our blog here to learn more about the ways that wildfires affect air quality and what can be done to protect communities from these impacts.

As we’ve seen firsthand this past year, heat waves are only becoming more frequent and intense across the globe. Just this past month of July, severe heat waves were observed across the American Southwest, Southern Europe, and China. 

One study finds that the combination of stagnant air and heat-trapping gases like carbon dioxide made heatwaves between 1.8 degrees Fahrenheit and 4.5 degrees Fahrenheit hotter than if these conditions had not occurred.

[R]esearch found the increase in heat-trapping gases, largely from the burning of coal, oil and natural gas has made another heat wave — the one in China — 50 times more likely with the potential to occur every five years or so”

— Time Magazine

Several climate scientists have said that the heat levels in July have likely been the hottest observed on Earth in the past 120,000 years, and that this would very likely not have occurred without climate change.

Climate change has complex ties to weather patterns, which in turn interact with air quality and can cause a slew of negative health outcomes due to exposure to polluted air. (Image source: De Sario et al.)

Because climate change can impact a variety of weather patterns such as temperature, wind, solar radiation, and precipitation, it is likely to impact how air pollutants are generated and dispersed. 

[C]hanges in wind patterns and desertification will modify the long-range transport of pollutants emitted by human activities and biomass burning” 

— De Sario et al.

Important practices to follow to reduce exposure

By understanding the ways that air quality can suffer in the summer season, individuals can take action to reduce their exposure and protect their health.

It is important to avoid doing strenuous activities outdoors during times of high air pollution, whether the pollution is from local sources or has traveled via wind from other areas. This includes avoiding exercising outdoors during rush hour when air pollution levels tend to spike.

This practice is especially important for individuals with existing medical conditions and for older adults.

Individuals should also avoid activity near major emission sources, such as industrial facilities and freeways, where pollution levels are higher.

While people can follow these practices to reduce their individual exposure, it is also essential that these issues are addressed systemically and that we advocate for cleaner air and a healthier climate at the policy level.

[Researcher Dr. Zhu] says though these are examples of individual-level reductions to exposure, policy makers must do more to protect the public – especially those unable to avoid poor air quality due to circumstance and historical and systemic racism in environmental policies”

— UCLA Health

Interested in measuring air quality for cleaner air, improved physical and mental health, and a healthier climate? Get in touch with our team to learn more about our Sensing-as-a-Service solution for governments, businesses, and community organizations, using our Clarity Node-S monitors and Modules that do not depend on infrastructure like WiFi or power — making them especially resilient during environmental disasters.