Fog's Effect: Reducing Pollution Or A Myth?

Fog and haze are both low-visibility phenomena, but they are not the same thing. Haze is caused by fine particulate matter and has a negative impact on human health. Fog, on the other hand, is a natural occurrence, a cloud near the Earth's surface composed of tiny water droplets that can stay suspended in the air for hours. Interestingly, fog has a removal effect on air pollution. This is known as the scavenging effect, where fog droplets deposit onto pollutant particles, removing them from the atmosphere. However, fog can also be intensified by air pollution, and the two together can worsen visibility and impact human health. So, does fog bring down pollution? The answer is complex. While fog can remove some pollutants from the air, it is also influenced and worsened by pollution, and the resulting low visibility and health impacts can be significant.

Fog and haze are different

Fog and haze are both low-visibility phenomena, but they have different formation mechanisms. Fog refers to water droplets in the atmosphere that reduce visibility. The moisture in fog comes from nearby sources of water like rivers, lakes, marshes, or the ocean. The airline industry defines fog as when visibility is less than 3,280 feet (1 km). In contrast, haze is caused by fine particulate matter and is directly related to air pollution. Most of the pollutants that cause haze are human-made, including smoke from fires, dust, and visibility-reducing aerosols like sulfur dioxide gases emitted through combustion.

Fog and haze also differ in terms of duration, with fog normally lasting longer than haze. The suspension of haze is higher above the ground compared to mist, which is typically at ground level. While fog and haze both worsen visibility, the interaction between fog droplets and atmospheric water can lead to the removal of atmospheric particles, including pollutants. This process is known as wet scavenging, and it has a greater impact on carbon particles from certain sources and sulfate concentrations.

Urbanization and air pollution can influence the formation and characteristics of fog. Urbanization inhibits low-level fog by delaying its formation and advancing its dissipation due to higher temperatures and lower saturations. In contrast, upper-level fog can be enhanced due to updraft-induced vapour convergence. Air pollution can suppress fog formation through aerosols, which block solar radiation and reduce surface heat flux. However, under certain conditions, aerosols can also promote fog formation by invigorating convection through vapour condensation.

The occurrence of fog and haze can have significant impacts on outdoor air pollution levels and human health. During fog and haze events, it is recommended to avoid outdoor activities, especially for elderly individuals who are more susceptible to respiratory and cardiovascular issues. Indoor activities, such as aerobic exercise, are advised to minimize exposure to air pollutants. Additionally, measures such as using air purifiers, avoiding smoking indoors, and frying food can help reduce indoor air pollution. Overall, understanding the differences between fog and haze, as well as their interactions with urbanization and pollution, is crucial for mitigating their adverse effects on human health and the environment.

Fog and pollution coexist

Fog is a cloud that forms near the Earth's surface, composed of millions of tiny water droplets or ice crystals. It occurs when cold air near the Earth's surface condenses water vapour, reducing visibility to less than 1km. Urbanisation and air pollution can influence the formation, inhibition, and duration of fog. For instance, in eastern-central China, aerosol loading from pollution altered wind circulation, creating conditions favourable for fog formation.

Air pollution, particularly in the form of aerosol pollution, can have both suppressing and promoting effects on fog. Higher temperatures and lower saturations associated with urbanisation tend to inhibit low-level fog and advance its dissipation. In contrast, upper-level fog can be enhanced due to updraft-induced vapour convergence. Additionally, aerosols can intensify turbulence during the growth phase of fog while suppressing it during the dissipation phase.

The presence of fog can also impact the concentration and removal of certain pollutants. Fog droplets can act as a removal mechanism for atmospheric particles, including carbon and sulphate particles. The duration of fog plays a role in this process, with the strongest scavenging effect on PM2.5 concentration occurring when fog lasts between 4 and 8 hours. However, fog is often accompanied by high pollution levels, and the pollutant concentration in fog water can be higher than in rainwater, posing potential health risks.

The interplay between fog and pollution has significant implications for human health and well-being. The combination of fog and haze can worsen visibility, leading to an increased risk of traffic accidents. Additionally, the presence of pollutants in fog can have adverse health effects, similar to those associated with smog, which is a mixture of smoke and fog containing harmful gases and particulate matter. To minimise these impacts, individuals may need to take precautionary measures, such as limiting outdoor activities during foggy conditions and adopting lifestyle changes to curb air pollution.

Pollution worsens fog

Fog and haze are both low-visibility phenomena, but they have different formation mechanisms. The decrease in visibility in fog is caused by the extinction of small water droplets, while that of haze is caused by fine particulate matter. Haze is known to negatively impact human health, and urban fog is often accompanied by high pollution. The concentration of atmospheric pollutants is usually higher at night than during the day.

Air pollution is a serious problem that has worsened with industrial development and urbanization in China. In January 2013, a large-scale fog and haze event covered major Chinese cities, including Beijing, Shanghai, Guangzhou, and Xi'an. Eastern China experienced an even longer and wider-ranging fog and haze event in December of the same year. The health risks associated with particulate matter, specifically PM2.5, are well-documented.

Outdoor air pollution is significantly worse when fog and haze occur. The interaction between aerosol and atmospheric water affects the removal of atmospheric particles. Fog droplets have been observed to have a removal effect on almost all species of pollutants. For example, fog water has been found to have a stronger sulfate removal effect than precipitation, with sulfate concentrations in fog water in the Kinki area being six times higher than in rainwater.

Aerosols affect convective clouds through two mechanisms: invigorating convection by promoting vapour condensation, and suppressing convection by blocking solar radiation and reducing surface heat flux. Under polluted conditions, the suppressing effect outweighs the invigorating effect. While fog formation is not impacted by this suppressing effect because it typically occurs at night, it can delay the turning point in fog formation.

In conclusion, while fog can have a removal effect on certain pollutants, the presence of fog and haze worsens overall air pollution and has negative health impacts.

Fog droplets remove pollutants

Fog and haze are both low-visibility phenomena, but they are formed differently. The decrease in visibility in fog is caused by the extinction of small water droplets, while in haze, it is caused by fine particulate matter. Haze has been found to negatively impact human health, and urban fog is usually accompanied by high pollution.

Fog acts as an agent for removing pollutants from the atmosphere. The mechanisms involved in this process are nucleation, coagulation, and the uptake of gases depending on their concentration in fog droplets. The presence of fog can alter the microphysical and chemical properties of the aerosol. The oxidation of aerosol particles increases their hydrophilic property, while scavenging through coagulation and condensation increases particle size.

Fog is particularly good at scavenging acidic particles and gases, and its formation is dependent on neutralized CCN. Hygroscopic chemical species with water-soluble chemical species attached to SO42−, NO3−, and NH4+ are washed down more readily. As fog develops, LWC rises, diluting the acid concentration in the droplets.

The removal effect of fog on air pollution has been observed in several studies. For example, during a dense fog event in Jiangsu Province, China, the PM2.5 concentration decreased from 159 μg m−3 to 38 μg m−3 after 11 hours of fog. Another study in Tianjin, China, found that fog brought down PM2.5 concentrations during a severe haze and fog episode.

The duration of fog also affects its scavenging ability. The positive correlation between fog duration and scavenging effect turns negative when the fog starts to dissipate. When the duration of fog is between 4 and 8 hours, the scavenging effect on PM2.5 is strongest, with an average PM2.5 concentration decrease of >70 μg m−3.

Urbanization affects fog

Urbanization and air pollution have been shown to have a significant impact on fog. The remarkable development of China has resulted in rapid urbanization and severe air pollution. These two factors have been found to have either suppressing or promoting effects on fog.

A study in 2020 investigated the effects of urbanization and aerosols on fog by changing the land use and emission intensity around SX. The experiment revealed that urbanization inhibits low-level fog, delays its formation, and advances its dissipation due to higher temperatures and lower saturations. In contrast, upper-level fog could be enhanced due to updraught-induced vapour convergence.

Further experiments have shown that the current pollution level in China is still below the critical aerosol concentration that suppresses fog. However, large-scale aerosol pollution can change weather patterns and affect large-scale fog formation. Additionally, urbanization influences fog to a larger extent than aerosols. When urbanization and aerosol pollution occur together, the weaker aerosol-promoting effect is counteracted by the stronger urbanization-suppressing effect on fog.

The impact of urbanization and air pollution on fog has important implications for climate change, air quality, and human health. Urban fog is usually accompanied by high pollution, which can negatively affect human health. Therefore, understanding the effects of urbanization and air pollution on fog is crucial for developing strategies to mitigate their adverse impacts.

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