Assess whether climate-model representations of marine primary organic aerosols capture their observed particle sizes, abundance, and potential cloud effects.
Approach:
Measurements: Researchers analyzed five years of aerosol measurements collected from 2009 to 2015 at Ireland’s Mace Head Atmospheric Research Station, using instruments to measure particle size, number, humidity response, chemical composition, and cloud-forming activity.
Analysis: Black carbon concentrations and wind direction were used to select clean marine air. The researchers compared observed particle distributions with an established model parameterization and used GEOS-Chem simulations and satellite cloud observations to estimate the cooling effect of the overlooked particles.
Key Findings:
Observed marine primary organic aerosol distributions had two main peaks: approximately 60–70 nanometers, in the Aitken mode, and 100–200 nanometers.
The parameterization underestimated the number of particles larger than 35 nanometers by about 3.1-fold, and particles between 30 and 70 nanometers by five- to sevenfold.
Using the revised distribution approximately doubled estimated cloud condensation nuclei from these aerosols across representative marine cloud conditions.
The sensitivity analysis estimated a global-ocean cooling effect of approximately 0.03–0.04 watts per square meter, with larger effects over biologically active waters.
Interpretation:
The authors report that small marine organic particles may be more numerous and contribute more cloud condensation nuclei than the examined parameterization represents. They noted that possible enhancement of particle activation through lower droplet surface tension remains uncertain.
Limitations:
The estimated cooling effect was a sensitivity analysis, not a complete estimate of climate forcing.
The magnitude of the effect of lower droplet surface tension on particle activation remains uncertain.
Conclusion:
The findings indicate that particle number and size may provide information that mass measurements alone do not capture.
