Aerosol light absorption plays a critical role in influencing the Earth’s climate and atmospheric conditions. Understanding the interactions between aerosols, radiation, and haze formation is essential for comprehending how pollutants impact the environment on both local and global scales. Aerosols, which are tiny particles suspended in the air, can absorb solar radiation, contributing to changes in atmospheric temperature and the overall heat balance between the atmosphere and the Earth’s surface. These particles influence climate patterns through two main effects: the aerosol direct effect and the aerosol indirect effect.
The aerosol direct effect occurs when aerosols absorb or scatter solar radiation, either warming or cooling the atmosphere depending on the nature of the aerosols. Aerosols that absorb solar radiation, known as light-absorbing aerosols, can lead to a warming of the atmosphere. On the other hand, aerosols that scatter sunlight can cause a cooling effect by reflecting sunlight back into space. The aerosol indirect effect, however, refers to aerosols acting as cloud condensation nuclei (CCN) or ice nuclei (IN), which can influence cloud formation and properties. By affecting cloud cover and cloud properties such as reflectivity, aerosols can further impact local and global climate conditions.
These two effects—direct and indirect—play a crucial role in regulating the heat balance of the atmosphere and contribute to regional and global climate changes. The interactions between aerosols, radiation, and photolysis (the chemical decomposition of substances due to sunlight) add an additional layer of complexity to understanding aerosol effects on climate, particularly when it comes to haze formation. Haze is a form of air pollution that occurs when particles in the atmosphere scatter and absorb light, reducing visibility and impacting air quality. Light-absorbing aerosols are key contributors to haze, as they play a central role in the formation and persistence of haze in polluted regions.
A recent study published in the journal Proceedings of the National Academy of Sciences (PNAS) provides new insights into the role of light-absorbing aerosols in haze formation, particularly during the winter months. In this study, the researchers used an advanced radiative transfer model that considered the multi-component, full-size distribution of aerosols in the atmosphere. By coupling this model with a regional atmospheric chemical transport model and integrating observational data, they were able to quantitatively analyze the interactions between aerosol light absorption, radiation, and photolysis in wintertime haze formation. The findings from this study challenge previous assumptions about the role of aerosol light absorption in haze formation and offer valuable insights into atmospheric processes.
One key finding from the study is that previous research may have overestimated the positive role of aerosol light absorption in haze formation. According to Prof. Li, one of the lead researchers in the study, the new insights suggest that light-absorbing aerosols may not contribute to haze formation in the way previously thought. In earlier studies, it was suggested that aerosols that absorb light worsen pollution at the urban scale by promoting haze formation. However, the new study reveals that on a larger, regional scale, the effects of light-absorbing aerosols are more complex and not necessarily as detrimental to air quality as previously assumed.
The study highlights the uneven heating rates that occur in the atmosphere due to light-absorbing aerosols. These aerosols cause heating at the top of the boundary layer—the region of the atmosphere closest to the Earth’s surface. The heating at the top of the boundary layer creates a “warm bubble” effect, which has significant implications for air movement. In polluted areas, this effect intensifies the upward movement of air, which can help disperse pollutants and reduce their concentration near the surface. In contrast, in cleaner areas, the warm bubble effect promotes the downward movement of air, which can contribute to the clearing of pollutants and the reduction of haze formation.
Another important finding is that light-absorbing aerosols can decrease atmospheric oxidation, which in turn suppresses the formation of secondary aerosols. Secondary aerosols are particles that form in the atmosphere through chemical reactions involving gases, such as sulfur dioxide or nitrogen oxides. By reducing oxidation, light-absorbing aerosols can limit the formation of these secondary particles, which are a significant contributor to the overall concentration of PM2.5—fine particulate matter that poses serious health risks. This suppression of secondary aerosol formation further contributes to the reduction of PM2.5 levels in areas affected by light-absorbing aerosols.
The implications of these findings are far-reaching, as they provide important insights into the complex role of light-absorbing aerosols in atmospheric processes. The study challenges previous assumptions and offers a more nuanced understanding of how these aerosols influence the formation of haze, air quality, and climate. These insights are crucial for improving climate models and developing effective strategies for managing air pollution.
The findings of this study are particularly important in the context of global efforts to reduce aerosol emissions and improve air quality. Aerosol pollution is a major environmental issue, particularly in rapidly industrializing regions and urban centers. Understanding the mechanisms by which aerosols influence climate and air quality is critical for developing policies and strategies that can mitigate the negative effects of pollution on human health and the environment.
The study also underscores the importance of considering aerosols at larger spatial scales and over extended time periods. While previous research focused on the effects of aerosols at the urban scale, the new study demonstrates that the impact of aerosols on haze formation can vary significantly depending on the scale of observation. On a regional scale, the interplay between aerosol absorption, radiation, and atmospheric dynamics can lead to complex effects that may reduce the concentration of pollutants, contrary to earlier expectations.
Furthermore, the study highlights the importance of integrating observational data with advanced modeling techniques to improve our understanding of aerosol-climate interactions. The use of radiative transfer models and regional atmospheric chemical transport models provides a more comprehensive view of how aerosols interact with radiation and atmospheric processes on a large scale. This approach allows researchers to better quantify the impact of aerosols on climate and air quality, providing a stronger scientific basis for policymaking.
Reference: Jiarui Wu et al, Aerosol light absorption alleviates particulate pollution during wintertime haze events, Proceedings of the National Academy of Sciences (2024). DOI: 10.1073/pnas.2402281121