How Does Pollution Influence Fog Formation?

Fog is a natural phenomenon that can be exacerbated by human activity. Urbanization and air pollution, particularly in large cities, can cause and worsen fog events. The combination of cold air, increased fuel usage, and trapped pollutants can lead to the formation of killer fog, as seen in the Great Smog of London in the 1950s, which resulted in thousands of deaths. This occurs when cold polluted fog and smoke are created as a result of pollutants trapped at the ground level.

• Glasgow and Edinburgh, Scotland, in 1909.
• Beijing, Shanghai, Guangzhou, and Xi'an, China, in January 2013.
• Eastern China in December 2013.
• Mexico City. | | Health risks | - Particulate matter with an aerodynamic diameter of less than 2.5 micrometers (PM2.5) can penetrate deep into the small airways, alveoli, and bloodstream, leading to inflammation, vasoconstriction, malignancies, and premature death.
• Patients with cardiovascular or chronic lung diseases may suffer dyspnea and hypoxia when wearing N95 masks. | | Prevention | - Wear N95 masks.
• Minimize outdoor activities, especially for those with respiratory or cardiovascular diseases.
• Use public transportation instead of driving or riding a bike.
• Wash your face, gargle, and clean your nasal cavity after coming indoors. | | Mitigation | - Improve air quality by increasing the use of renewable energy, avoiding unnecessary consumption, and reducing waste.
• Planting trees and clean-up projects can help combat air pollution. |

Urbanization and air pollution

Urbanization refers to the process of expansion in the proportion of the population that resides in urban areas. The world is currently experiencing the biggest wave of urban growth in history. Currently, over 50% of the global population lives in urban areas, and this number is expected to rise to 5 billion by 2030. Urban areas are centers of resource utilization and are a major contributor to air pollutant and greenhouse gas emissions. Urbanization can increase urban land use and anthropogenic emissions, which affect the concentrations and health risks of air pollutants.

A study by Zhan et al. in 2023 examined the impacts of urbanization on air quality and related health risks in the city of Chengdu, a highly urbanized city with severe air pollution and complex terrain. The study found that air pollution in Chengdu was mainly caused by PM2.5 and O3 from 2015 to 2021. PM2.5 pollution tended to appear in the colder months due to blocked air and a stable atmospheric layer, while O3 pollution was more likely to occur in the warmer months with high temperatures and strong sunlight conditions. The health risks associated with urbanization and air pollution are significant, with lives cut short by heart disease, stroke, lung disease, and cancer triggered or exacerbated by air pollutants.

Another study by ACP investigated the effects of urbanization and air pollution on fog. The study found that urbanization suppresses low-level fog but promotes upper-level fog, while additional aerosols generally promote fog. Urbanization was found to affect fog to a much larger extent than aerosols. The development of China, in particular, has caused rapid urbanization and severe air pollution, with satellite images showing the formation of fog holes over urban areas.

While urbanization can have negative consequences on air quality and health, it is important to note that it also brings about social and economic progress. Urban residents may benefit from improved sanitation, infrastructure, and access to health services. However, they may also face issues such as unhealthy lifestyles and environmental pollution. The association between urbanization and health is complex and requires further study, especially in the context of global health and the role of air pollution.

Satellite images show fog holes over urban areas

Satellite images have revealed the existence of "fog holes" over urban areas. These images, captured by NASA's Terra satellite, show peculiar holes in a layer of fog that correspond with the locations of major cities. The phenomenon was first observed over northern India in 2014 and again in 2016, with the largest fog holes occurring over Delhi, the region's largest urban hotspot. Similar fog holes have also been observed in California, Europe, South Asia, and China.

The cause of these fog holes was initially a mystery, but scientists now believe that they are the result of a combination of additional aerosols and the urban heat island effect. Aerosol particles from pollution can serve as "seeds" that make it easier for fog to form. However, warm urban areas can reduce relative humidity, making less moisture available for fog formation and leading to fog holes. This effect is known as the "urban heat island" effect, where cities trap heat more effectively than the areas around them due to their denser infrastructure. This effect can speed up the dissipation of fog, leading to the formation of fog holes.

The urban heat island effect is in contrast to the effect of aerosols on fog. While pollution can amplify fog formation, urban heating can cause fog to dissipate faster. This results in a dynamic where pollution increases fog coverage, but urbanization reduces fog frequency. This relationship was demonstrated in a study by Ritesh Gautam, a climate scientist at the Environmental Defense Fund, who found a clear connection between the size of cities and the size of the fog holes.

The impact of urbanization and air pollution on fog has been further investigated through numerical experiments and modeling. These studies have found that urbanization suppresses low-level fog while promoting upper-level fog. The effects of urbanization are also found to be more significant than those of aerosols. For example, in eastern China, fog days have decreased since the 1990s due to the suppressing effects of urbanization outweighing the promoting effects of aerosols.

The effects of aerosols on fog

Aerosols are suspensions of solid or liquid particles in the air. They are classified as primary or secondary. Primary aerosols contain particles introduced directly into the gas, while secondary aerosols form through gas-to-particle conversion. Aerosols can be natural or anthropogenic, with key groups including sulfates, organic carbon, black carbon, nitrates, mineral dust, and sea salt.

Aerosols have been shown to have an impact on fog. The interaction between aerosols and clouds, known as aerosol-cloud interaction (ACI), and the interaction between aerosols and radiation, known as aerosol-radiation interaction (ARI), can both influence fog formation and duration.

In the case of ACI, clouds form on aerosols when they absorb sufficient water vapour. This process can lead to the formation of fog, a type of near-surface cloud. ARI, on the other hand, refers to the scattering and absorption of radiation by aerosols, which can affect meteorological conditions and influence fog formation.

Studies have found that increasing aerosols can promote fog formation. This is due to the higher liquid water content (LWC), more fog droplets, higher liquid water path (LWP), and longer persistence. The decreased droplet size associated with aerosols slows down autoconversion and sedimentation rates, favouring the maintenance of fog.

In East China, for example, fog frequency has been decreasing while fog duration has been increasing. This trend has been linked to the increase in aerosol pollution. Numerical simulations have shown that ACI significantly advances fog formation, delays dissipation, and increases fog duration by about one hour. On the other hand, ARI has been found to have a negligible effect on fog.

The health risks of particulate matter

Fog can be caused by a combination of urbanization and air pollution. Urbanization suppresses low-level fog but can promote upper-level fog. Air pollution, on the other hand, generally promotes fog. The effects of air pollution on fog are less significant than those of urbanization. However, the individual and combined effects of these factors on fog are not yet fully understood.

Now, let's discuss the health risks associated with particulate matter:

Particulate matter, or particle pollution, refers to a mixture of tiny solid and liquid particles suspended in the air we breathe. These particles vary in size, with some being one-tenth the diameter of a human hair, while others are so small that they can only be seen under an electron microscope. The size of these particles is directly linked to their potential for causing health issues.

Fine particles, or particulate matter 2.5 (PM2.5), are particularly harmful. These particles are 2.5 microns or smaller in width, and they can penetrate deep into the respiratory tract, reaching the lungs. Exposure to PM2.5 can cause short-term health effects such as eye, nose, throat, and lung irritation, coughing, sneezing, a runny nose, and shortness of breath. It can also aggravate pre-existing conditions like heart disease and asthma and increase the risk of heart attacks. Long-term exposure to fine particles has been linked to increased mortality from heart disease, higher rates of chronic bronchitis, reduced lung function, and lung cancer.

Ultrafine particles, which are smaller than 0.1 microns in diameter, are even more concerning. These particles can pass through the lungs and directly enter the bloodstream. This allows them to circulate throughout the body, potentially causing systemic health issues.

Particle pollution can originate from both indoor and outdoor sources. Outdoor sources include vehicle exhaust, burning wood, wildfires, agricultural fires, and the burning of fossil fuels in factories and power plants. Indoor sources include tobacco smoke, frying or broiling food, burning candles, fireplaces, and fuel-burning space heaters.

The health risks associated with particulate matter are particularly pertinent for sensitive groups, including children, teenagers, older adults, pregnant women, and individuals with pre-existing heart or respiratory conditions. It is crucial for these individuals to monitor air quality and take appropriate measures to limit their exposure to particle pollution.

The impact of fog on traffic accidents

Fog can be caused by a combination of urbanization and air pollution. Urbanization suppresses low-level fog but promotes upper-level fog, while additional aerosols, which are a result of air pollution, generally promote fog.

Fog that impacts visibility and safety on the road is a common phenomenon, especially during the late fall and winter months. From 2001 to 2021, there were a total of 8,410 fatal crashes on US roadways when fog was present. These crashes are more likely to occur during the early morning hours, with 38% taking place between 5 and 8 am. The worst county in the United States for fog-related fatal crashes is Hendry County, Florida, where nearly 8% of all fatal crashes occurred in foggy conditions.

When fog sets in, it is best to wait it out and avoid driving until the fog lifts. If driving in foggy conditions is necessary, it is important to take several precautions to reduce the risk of accidents. These include:

• Slowing down to give more time to react to sudden changes
• Using low beams or fog lights, as high beams can reflect off the fog and worsen visibility
• Increasing the following distance to allow for more time to react if the car in front stops suddenly
• Avoiding sudden movements, as gradual lane changes and gentle braking are safer in low-visibility conditions
• Using road markings to stay oriented

Checking forecasts before traveling can also help drivers be prepared for potential fog and other adverse weather conditions, enabling them to plan routes or delay travel until conditions improve.

Frequently asked questions