Particle phase chemistry enables soot to better seed clouds
New study challenges current understanding of the formation mechanism
New study challenges current understanding of the formation mechanism of atmospheric secondary organic aerosol – Particle chemistry appears to have a strong influence on the climate impact of soot and organic aerosols from wildfires or anthropogenic combustion.
Peer-Reviewed PublicationIMAGE: THE FIGURE SHOWS THAT HIGHLY OXYGENATED ORGANIC MOLECULES WITH HIGH UNSATURATION (HU-HOMS) ARE FORMED BY THE MULTIGENERATIONAL PHOTO-OXIDATION OF LARGE POLYCYCLIC AROMATIC HYDROCARBONS (PAHS) ON SOOT. view more
CREDIT: HTTPS://DOI.ORG/10.1016/J.CHEMPR.2022.06.011
Highly oxygenated organic molecules are a key component of atmospheric secondary organic aerosol. However, the origin and formation mechanism of highly oxygenated organic molecules with high unsaturation (HU-HOMs), remain unknown. But now an international team of researchers has found that photooxidation of large polycyclic aromatic hydrocarbons (PAHs) on soot by singlet oxygen and superoxide anion radicals can be an important source of the unexplained HU-HOMs widely observed in the atmosphere. The team was led by Yafang Cheng from the Max Planck Institute for Chemistry and Chuncheng Chen from the Institute of Chemistry, Chinese Academy of Sciences. Their results are based on molecular-level investigations of the photochemical aging of soot by O2. The PAH-derived HU-HOMs exhibit lactone and anhydride functional groups and can substantially increase the hydrophilicity of soot.
The increase in the hydrophilicity of soot after photochemical aging is expected to further influence the fate and effects of soot aerosols in the atmosphere: e.g., becoming better cloud condensation nuclei, more easily being involved in aqueous phase chemistry and aging, altering its wet deposition process etc.
Deciphering the molecular formulae
The researchers characterized the evolution of molecular composition during the photoaging of soot by applying laser desorption ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry (LDI FT-ICR MS), an ultrahigh-resolution mass spectrometry technique allowing confident assignment of the molecular formulae. In situ attenuated total internal reflection IR (ATR-IR) was used to investigate the evolution of functional groups during soot oxidation. They find that highly oxygenated organic molecules with high unsaturation (HU-HOMs) are formed through a multigenerational photochemical oxidation pathway, where ketones, aldehydes, and acids are produced by the photooxidation of large polycyclic aromatic hydrocarbons on soot in the initial stage, followed by the formation and accumulation of lactones and anhydrides on further oxidation.
“In this heterogeneous photochemical oxidation, O2 molecules are the initial oxidant, which is further photosensitized to form reactive oxygen species such as singlet oxygen and superoxide anion radicals,” said Meng Li, postdoc in Yafang Cheng’s group and the first author of the study. “Considering the abundance of O2 in the troposphere, this oxidation pathway should be a very important aging process for PAHs and soot particles, especially in clean and remote regions”, added Meng Li.
“This new HU-HOM formation pathway could be a characteristic evolution pathway of primary organic aerosols from various combustions, due to the widespread existence of PAHs there, thus contributing to a more comprehensive understanding of the chemical evolution of organic aerosols”, said Yafang Cheng who leads the Minerva Independent Research Group at the MPIC.
JOURNAL
Chem
METHOD OF RESEARCH
Computational simulation/modeling
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Highly Oxygenated Organic Molecules with High Unsaturation Formed upon Photochemical Aging of Soot
Revealing how different parts of mixed convective-stratiform clouds distinctly respond to cloud seeding
IMAGE: FIGURE (A) AND (B) SHOW MACRO- AND MICRO-PHYSICAL CHARACTERISTICS OF MIXED CONVECTIVE-STRATIFORM CLOUDS BEFORE AND AFTER CLOUD SEEDING, RESPECTIVELY. THE HIGHER CLOUD TOP IS THE CONVECTIVE REGION, AND THE LOWER CLOUD TOP IS THE STRATIFORM REGION. view more
CREDIT: DEJUN LI
Cloud seeding is a method of placing condensation nuclei into clouds that induce rain or snowfall. Seeding is a complex process, and its effectiveness depends on the target cloud type. To better understand the mechanisms behind seeding, in a recent study, scientists showed how a connected mixture of both convective and stratiform clouds responds to cloud seeding.
Using data from Ka-band cloud profiling radar (KPR) measurements onboard aircraft, along with the remote sensing observations from SNPP and Himawari-8 satellites, members of the Hubei Weather Modification innovation team (HBWM), under the guidance of Prof. Chuanfeng ZHAO from Beijing Normal University (Currently at Peking University), published their findings in Advances in Atmospheric Sciences.
“A mixed convective-stratiform cloud is one type of precipitating cloud with complex structure, composed of both cumulus clouds and stratiform clouds.” said the study’s first author, Dejun LI, Chief Expert on Weather Modification in Hubei Province.
LI elaborated that the successful launch of NPP in 2011, Himawari-8 in 2014, and the new high-resolution instrument components of KPR developed by satellite scientists at the University of Wyoming in 2014, all make observing the fine structure of cloud properties possible. This study measures the properties of mixed convective-stratiform clouds using KPR onboard aircraft in China for the first time.
Based on the KPR measurements before and after seeding, along with multi-source data including satellite observations, researchers now have the ability to analyze the characteristics of icing seeding tracks within the stratiform region and cloud seeding mechanisms in the convective region. This wealth of data gives important clues that help determine the differences in the cloud’s macro- and micro-physical properties.
“Few researchers have carried out studies regarding the characteristics and mechanisms of cloud responses to seeding at different parts of clouds.” said Dr. LI. “This is mainly due to the scarcity of observation instruments for cloud macro- and micro- physical properties, the insufficiency of confidence from single source of cloud observations, and so on.”
According to corresponding author Prof. ZHAO, the distinct seeding responses of stratiform and convective regions within a stratocumulus cloud are quite different. In the convective region, the cloud top is higher, which is associated with more released heat and stronger convection due to water-ice conversion. This results in both a greater number of and larger precipitation particles.
Within the stratiform region, the radar echo becomes weaker near cloud tops, indicating that liquid droplets are converting to ice crystals faster than in the convective region, causing icing seeding tracks to occur.
“Now that we have discovered how these mechanisms between the convective and stratiform regions respond to cloud seeding, future studies should further investigate application into mesoscale weather model simulations.” adds Prof. ZHAO.
JOURNAL
Advances in Atmospheric Sciences
ARTICLE TITLE
Macro- and Micro-physical Characteristics of Different Parts of Mixed Convective-stratiform Clouds and Differences in Their Responses to Seeding
ARTICLE PUBLICATION DATE
23-Jul-2022
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