Atmospheric aerosol and the cloud droplets and ice crystals that grow on them remain major sources of uncertainty in global climate models. A subset of aerosol, ice nucleating particles, catalyze the freezing of water droplets at temperatures warmer than −38 °C. Here we show that RuBisCO, one of the most abundant proteins in plants and phytoplankton, is one of the most efficient known immersion ice nucleating particles with a mean freezing temperature of −7.9 ± 0.3 °C. Further, we demonstrate RuBisCO is present in ambient continental aerosol where it can serve as an ice nucleating particle. Other biogenic molecules act as immersion ice nucleating particles, in the range of −19 to −26 °C. In addition, our results indicate heat denaturation is not a universal indicator of the proteinaceous origin of ice nucleating particles, suggesting current studies may fail to accurately quantify biological ice nucleating particle concentrations and their global importance.
Pollen grains significantly contribute to the aerosol population, and levels are predicted to increase in the future. Under humid atmospheric conditions, pollen grains can rupture creating pollen grain fragments referred to as subpollen particles (SPPs) which are dispersed into the atmosphere with wind. In this laboratory study, SPP emission factors were determined for ryegrass, Lolium sp., and giant ragweed,Ambrosia trifida, in terms of the number of SPPs produced per pollen grain and the number of SPPs produced per m 2 , which were compared to previously measured live oak,Quercus virginiana, emission factors. The SPP emission factors were 4.9 × 10 13 ± 4.3 × 10 13 SPPs per m 2 for ryegrass, 1.3 × 10 15 ± 1.1 × 10 15 SPPs per m 2 for giant ragweed, and 1.1 × 10 15 ± 1.6 × 10 15 SPPs per m 2 for live oak. SPPs and whole pollen grains from these species were evaluated for their ice nucleation efficiency in immersion and contact mode freezing. Measurements of the ice nucleation efficiency indicate that SPPs are weakly effective INPs in immersion mode, but that pollen grains represent a source of moderately efficient INPs in immersion and contact modes.
Ocean aerobiology is defined here as the study of biological particles of marine origin, including living organisms, present in the atmosphere and their role in ecological, biogeochemical, and climate processes. Hundreds of trillions of microorganisms are exchanged between ocean and atmosphere daily. Within a few days, tropospheric transport potentially disperses microorganisms over continents and between oceans. There is a need to better identify and quantify marine aerobiota, characterize the time spans and distances of marine microorganisms’ atmospheric transport, and determine whether microorganisms acclimate to atmospheric conditions and remain viable, or even grow. Exploring the atmosphere as a microbial habitat is fundamental for understanding the consequences of dispersal and will expand our knowledge of biodiversity, biogeography, and ecosystem connectivity across different marine environments. Marine organic matter is chemically transformed in the atmosphere, including remineralization back to CO2. The magnitude of these transformations is insignificant in the context of the annual marine carbon cycle, but may be a significant sink for marine recalcitrant organic matter over long (∼104 years) timescales. In addition, organic matter in sea spray aerosol plays a significant role in the Earth’s radiative budget by scattering solar radiation, and indirectly by affecting cloud properties. Marine organic matter is generally a poor source of cloud condensation nuclei (CCN), but a significant source of ice nucleating particles (INPs), affecting the formation of mixed-phase and ice clouds. This review will show that marine biogenic aerosol plays an impactful, but poorly constrained, role in marine ecosystems, biogeochemical processes, and the Earth’s climate system. Further work is needed to characterize the connectivity and feedbacks between the atmosphere and ocean ecosystems in order to integrate this complexity into Earth System models, facilitating future climate and biogeochemical predictions.
To date, the relative contribution of primary marine organic matter to the subset of atmospheric particles that nucleate cloud droplets is highly uncertain. Here, cloud condensation nuclei (CCN) measurements were conducted on aerosolized sea surface microlayer (SML) samples collected from the North Atlantic Ocean during the NASA North Atlantic Aerosols and Marine Ecosystems Study (NAAMES), κ values were predicted for three representative high molecular weight (HMW) organic components of marine aerosol: 6-glucose, humic acid, and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). The predicted κ values for pure organic aerosols varied by only ±0.01 across all of the organics chosen. For the desalted SML samples, calculations assuming an organic composition of entirely RuBisCO provided the closest predicted κ values for the desalted SML samples with a mean κ value of 0.53 ± 0.10. These results indicate that it is the sea salt in the SML which drives the cloud formation potential of marine aerosols. While the presence of organic material from the ocean surface waters may increase aerosol mass due to enrichment processes, cloud formation potential of mixed organic/salt primary marine aerosols will be slightly weakened or unchanged compared to sea spray aerosol.
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