October 4, 2022
Research Highlight

Understanding How Gas-Phase Reaction Conditions Affect Organic Aerosol Formation

Oxidant and monoterpene structure more strongly influence secondary organic aerosol formation from monoterpenes than nitric oxides and hydroperoxy radical concentrations

Inside of aerosol chamber with cool looking blue lights

An environmental chamber at Pacific Northwest National Laboratory was used to conduct secondary organic aerosol formation experiments.

(Photograph by Andrea Starr | Pacific Northwest National Laboratory)

The Science                                 

Observations show that cloud patterns transform from elongated rolls to broken cells along the pathway of a cold-air outbreak (CAO). However, the internal cloud processes that cause these changes are poorly understood. Researchers selected two cases of cloud pattern transitions seen during the Atmospheric Radiation Measurement (ARM) Cold-air Outbreak in Marine Boundary Layer Experiment (COMBLE) campaign to characterize the evolution of cloud morphology (e.g., size, horizontal aspect ratio) along cold-air pathways and how the cloud shape transition correlates with different environmental properties. They used an object segmentation method to identify individual clouds from satellite images and obtained cloud morphology statistics. They then analyzed how cloud morphology changes with the environment, with a focus on the region where roll clouds break up. The methodologies are being used in ongoing research to quantify cloud morphology in numerical simulations and examine how the roll-to-cell transition is modulated by surface fluxes and cloud microphysics.

The Impact

Marine CAO events over the Arctic play an important role in the region’s climate. While clouds are ubiquitous during marine CAOs, they are poorly represented in weather and climate models. Simulating CAO clouds is difficult due to their small size relative to the model grid. This size difference makes it hard to resolve the processes happening in the clouds. This study characterizes how cloud sizes evolve with the changing environment along the fetch of CAOs. The obtained quantitative results are critical for evaluating and improving cloud morphology simulations in next-generation models.


Cloud morphology, including cloud size and spatial distribution, directly affects how clouds can absorb and emit radiation. In this work, researchers used two cases observed during the COMBLE field campaign to study the evolution of cloud size distributions as an air mass moves from the Arctic over a comparatively warm ocean and to characterize cloud mesoscale organization as it changes from rolls to cells. They identified clouds from satellite images and found the roll-to-cell transition region from the homogeneity of the cloud water paths.

A local minimum in wind shear, a local maximum in cloud size, and a local maximum in marine CAO index accompany the roll-to-cell transition. Cloud horizontal aspect ratios (length divided by width) have weak fetch dependency, and the mean values are almost the same in roll, transition, and cell regimes. That means clouds tend to have the same shape along the cold air pathway, whether they are big or small. Most clouds have small sizes (e.g., < 10 km2), regardless of where the cold air is in its pathway, while a few large clouds (e.g., > 100 km2) contribute most of the cloud cover in the transition and cell regimes (our eyes usually capture those large clouds rather than the numerous small clouds). After roll clouds breakup, cloud size distributions show bimodality with an additional peak at ~100 km2, compared to the single peak at ~1 km2 before breakup. The presented statistical results pave the way for evaluating model simulated cloud organizations during CAO events.

PNNL Contact

Jerome Fast, Pacific Northwest National Laboratory, jerome.fast@pnnl.gov


Funding for scientific analysis was provided through the ICLASS scientific focus area by the Department of Energy, Office of Science, Biological and Environmental Research program as part of the ASR program.

Published: October 4, 2022

Liu, J, EL D’Ambro, BH Lee, S Schobesberger, DM Bell, RA Zaveri, A Zelenyuk, JA Thornton and JE Shilling. 2022. “Monoterpene photooxidation in a continuous-flow chamber: SOA yields and impacts of oxidants, NOx, and VOC precursors,” Environmental Science and Technology, 56, 12066-12077. [DOI: 10.1021/acs.est.2c02630]