Marine organic particles released from the ocean may contribute more to cloud formation and natural cloud brightening than some climate models currently estimate, according to a study published in National Science Review.
Marine primary organic aerosols are emitted with sea spray when bubbles burst at the ocean surface, particularly in biologically active waters. These particles can act as cloud condensation nuclei, providing surfaces on which water vapor condenses to form droplets. Increasing the number of droplets can make marine clouds more reflective, affecting how much solar energy reaches Earth’s surface.
Wei Xu, and colleagues of the Institute of Urban Environment at the Chinese Academy of Sciences, analyzed five years of aerosol measurements collected between 2009 and 2015 at the Mace Head Atmospheric Research Station on Ireland’s west coast. The site frequently receives relatively clean air from the Northeast Atlantic.
The researchers combined several instruments to characterize the particles. A scanning mobility particle sizer measured particle size and number, while a humidified tandem differential mobility analyzer assessed how particles responded to moisture. A high-resolution time-of-flight aerosol mass spectrometer measured chemical composition, and a cloud condensation nuclei counter evaluated cloud-forming activity.
Black carbon concentrations and wind direction were used to select clean marine air and reduce the influence of land-based pollution. The researchers distinguished marine primary organic aerosols from more water-attracting secondary marine particles using their characteristic response to humidity.
The resulting number-size distribution had two main peaks: one at approximately 60 to 70 nanometers and another between 100 and 200 nanometers. The smaller particles belong to the Aitken mode, a size range that is poorly represented in a parameterization used by several atmospheric models.
Compared with the field measurements, this established parameterization underestimated the total number of marine primary organic aerosol particles larger than 35 nanometers by a factor of approximately 3.1. For particles between 30 and 70 nanometers, the underestimate was five- to sevenfold.
The revised distribution approximately doubled the estimated number of cloud condensation nuclei originating from these aerosols across representative marine cloud conditions. Their ability to lower droplet surface tension could further increase activation, although the size of this effect remains uncertain.
Using GEOS-Chem simulations and satellite cloud observations, the researchers estimated that the overlooked small particles could produce a cooling effect of approximately 0.03 to 0.04 watts per square meter across the global ocean. Larger effects were estimated over biologically active waters. The authors emphasized that these calculations represent a sensitivity analysis rather than a complete estimate of climate forcing.
The findings indicate that particle number and size may provide information that mass measurements alone do not capture. This is particularly relevant because small particles contribute little mass but may be numerous enough to influence cloud droplet formation.
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