How long do carbon black particles stay in the atmosphere?
Researchers are discovering how carbon black evolves from hydrophobic particles to nucleation sites in clouds, ultimately removing heat-absorbing particles from the sky.
There is a stubborn particle that absorbs the heat that floats in the earth’s atmosphere: it initially doesn’t like water, it absorbs light, and it takes a long time to move forward. Black carbon in the atmosphere tends to linger until it finally absorbs enough water to fall from the sky. During this time, black carbon absorbs energy from the sun and heats the surrounding air, creating a radiative effect.
Cool, youthful carbon black tends to be water resistant. Over time, the particles age and become more hygroscopic, or able to absorb water from the air. But when does carbon black start to absorb water, act like cloud nuclei, and recede from the atmosphere?
Researchers previously studied the hygroscopic conditions of carbon black in the laboratory, with limited conditions on chemical sources and water vapor conditions. In all of these studies, the cloud nucleation values of carbon black were indirect measures.
In a new study by Hu et al., the researchers simultaneously measured the concentration of cloud condensation nuclei and carbon black particles. The sampling site was near busy roads and industrial centers in Wuhan, China, an urban mega-city in the center of the country.
They first corrected for particle size, then measured cloud condensation nuclei and individual carbon black particles in certain levels of water supersaturation in the atmosphere. The team found that the activation diameter, or the size of the black carbon particle where half of the particles will nucleate and precipitate, was 144 ± 21 nanometers at 0.2% supersaturation. According to the authors, how these carbon black-containing particles might act as cloud nuclei is determined by their size combined with their coatings, and in general, the less saturated the air, the more particles should be. large to nucleate.
In addition, the team found that a particle itself can influence the size of the nucleation. For example, the amount of organic content in a particle or any coating on carbon black can alter the hygroscopicity and hence the activation.
The research team noted that their work may help improve estimates of the longevity of black carbon particles suspended in the atmosphere and therefore the radiative impacts these particles can have.
Reference: “Direct quantification of droplet activation of ambient carbon black under water supersaturation” by Dawei Hu, Dantong Liu, Shaofei Kong, Delong Zhao, Yangzhou Wu, Siyuan Li, Shuo Ding, Shurui Zheng, Yi Cheng, Kang Hu, Zhaoze Deng, Yunfei Wu, Ping Tian, Quan Liu, Mengyu Huang and Deping Ding, June 25, 2021, JGR: Atmospheres.
DOI: 10.1029 / 2021JD034649