Image courtesy of Bodor et al., 2020
TL;DR — Air pollution levels are far from constant — they can vary by season, time of day, across large spans of time, based on meteorological factors, and in connection to climate change. Trends also vary by type of pollutant due to different pollutants’ unique relationship with these meteorological factors and human behavior. By better understanding these pollution patterns and trends, we can better tailor policy change and action to address the times of the worst air quality.
Air pollution levels can fluctuate based on a variety of parameters, including across historical measurements, between seasons, between pollutant types, and even at different times of the day.
By better understanding the complex ways that air quality can shift over time, we can better understand how to tackle the air pollution crisis and achieve cleaner air.
Types of pollutants and their effects
The US EPA measures and regulates six criteria pollutants that research has shown to have harmful impacts on both human and environmental health. These pollutants are:
- Particulate matter
- Ground-level ozone
- Carbon monoxide
- Sulfur dioxide
- Nitrogen dioxide
- Lead
Research has also demonstrated other types of pollutants, such as hazardous air pollutants and air toxics, to have negative effects.
To learn more about these pollutants and the vast ways that they harm human health, read our blog here.
Air quality fluctuations across time
Historical data trends show that air pollution levels have changed significantly over time.
In the United States, data from the US EPA shows that air quality levels have largely improved as compared to their levels decades ago.
However, this pattern of decreasing pollution levels has not been observed worldwide. In many low- and middle-income countries, air pollution has instead been on the rise.
Trends over time in specific air pollutants
We can also examine air pollution patterns and fluctuations by looking at specific pollutants.
US EPA data shows improvements in PM and nitrogen dioxide levels across the country, as compared to levels twenty years ago.
Seasonal and weather-related variations in air quality
Different pollutions often exist in different concentrations depending on the season and various meteorological factors at play.
One study examining pollution levels of PM, nitrous oxides, ozone, and carbon monoxide in Transylvania found that there were lower pollutant concentrations in the summer season and significantly higher levels in the winter, when biomass burning is used for heating. Such differences in the winter season can also be due to inversion layers.
However, it is important to note that the relationship between air quality and meteorological factors is a complex one with many factors at play, making it difficult to make general statements about whether air quality is better in the summer or winter.
Air pollution can also fluctuate due to meteorological factors that are specific to a geographical area. One study collecting data during the monsoon season in Sri Lanka found different air quality variations in two different urban cities depending on the specific variables relevant in each area.
Differences in weather patterns — such as levels of sunshine and rain, temperatures, wind speed, air turbulence, and mixing depths — all impact air pollution levels. To learn more about the interaction between weather and air quality, particularly in the context of the summer season, read our blog here.
Interestingly, air pollution levels themselves can also impact weather.
This largely happens due to aerosols, which are airborne particles. Aerosols can increase cloud formation, resulting in thicker or more numerous clouds, which can help to reflect sunlight back into space and reduce surface warming.
However, if there is not a sufficient amount of water vapor in the air, these aerosols cannot make clouds and instead contribute to haze, heavy thunderstorms, or hailstorms depending on the type of aerosol. Because there is a lot of uncertainty around exactly how aerosols will impact atmospheric conditions, they are one of the most unpredictable factors when it comes to what the future of climate change will look like.
Daily fluctuations in air quality
Many air pollutants also fluctuate at different times of the day depending on their sources. For example, nitrogen dioxide and ground-level ozone often peak in the morning and in the evening in urban areas in connection with rush hour traffic.
Changes in air pollution levels due to climate change
Air pollution fluctuations over time have become increasingly complex due to the influence of our changing climate.
Research suggests that worsening climate change will result in increased outdoor air pollution levels.
In many regions of the United States, climate-driven changes in weather conditions, including temperature and precipitation, are expected to increase ground-level ozone and particulate matter (such as windblown dust from droughts or smoke from wildfires). These changes worsen existing air pollution.”
— US EPA
Climate change contributes to increasingly frequent and severe wildfires, in addition to a longer wildfire season, thereby also worsening air quality through this avenue — not only in the region surrounding the wildfire but also in geographical regions hundreds or thousands of miles away that resulting pollution can travel to downwind.
To learn more about how climate change is increasing the severity of wildfires, read our blog here.
Understanding these air pollution patterns
Air pollution levels can vary based on a variety of factors, from season to time of day, and also present interesting historical trends.
Because of this multitude of factors, exactly when air pollution levels are higher or lower depend on the complex relationships between air quality and human behavior, meteorological factors, and climate change.
However, some major trends do emerge, such as:
- Air pollutants such as ground-level ozone tend to be higher in the summer due to sunny, hot weather that supports ozone formation, while pollutants like particulate matter and carbon monoxide tend to increase in cold winter months due to heating and the impact of inversion layers.
- Overall air quality level has tended to increase in high-income regions like Europe, the U.S., and Canada while many low- and middle-income countries have seen trends of consistent or worsening pollution levels.
- Climate change’s worsening of environmental disasters, such as wildfires, negatively impacts air quality, both locally and at larger spatial scales during and in the aftermath of these environmental events.
- Human behavior, like vehicle use and biomass burning for heating, lead to worsened air quality — thus, times where these behaviors are more frequent, like during rush hour traffic or in cold winter months, see higher pollution levels.
By increasing our understanding of these trends and patterns in air quality levels, we can better target policy and action to address pollution levels.
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