Cloud Water Chemistry and Aerosol Processing: Assessing the Hydroxyl Radical Burst in Newly Formed Cloud Droplets

Aerosol

aerosols

Small particles or droplets, such as wildfire smoke, volcanic gases, or salty sea spray, that float in the air. They are emitted by both natural events and human activities.

Source: NASA Global Climate Change Source

particles are tiny liquid or solid particles in the atmosphere, ranging in size from a few nanometers to several micrometers in diameter. Aerosol-cloud interactions

aerosol-cloud interaction

The interaction of airborne particles (aerosols) with clouds. Scientists study how aerosols affect clouds and how these interactions can impact the Earth's climate.

Source: DOE Source

are the most uncertain aspect of the climate system, and models routinely under-predict aerosol particle formation. Clouds are well known to alter aerosol size, chemical composition and radiative properties, but the processes are poorly characterized. Organics and sulfates are the most abundant materials in particles, and for each of these are significant gaps in the understanding of the role of clouds in their formation and/or aging. Many of the cloud processes are driven by aqueous

aqueous

Having to do with water.

Source: National Cancer Institute Source

chemistry that depends on hydroxyl (OH) radicals, the subject of this project. 

We recently found a new, substantial and unrecognized source of hydroxyl radicals in cloud droplets. During the first few minutes following cloud droplet formation and in the presence of daylight, material in aerosols produces a burst of hydroxyl radicals. The estimated contribution of the ‘OH radical burst’ to total OH radical concentrations in droplets based on limited data collected so far ranges from about the same to up to 10 times larger than other known sources, significantly enhancing the ability of cloud droplets to process organics in the cloud condensation nuclei.

cloud condensation nuclei

Tiny airborne particles suspended in the atmosphere that serve as the seeds around which cloud droplets form.

Source: Source

Here, we plan to 1) Measure aerosol OH generation at the Department of Energy's Southern Great Plains (SGP) site and characterize the process for its dependence on size and chemical composition; 2) Investigate the ability of particle to regenerate OH burst precursors after cloud processing; and 3) develop a chemical kinetics model describing the process that could later be incorporated into larger modeling frameworks. At the Southern Great Plains field site, we will measure the ability of continental, biomass

biomass

Material that comes from living things, including trees, crops, grasses, and animals and animal waste. Some kinds of biomass, such as wood and biofuels, can be burned to produce energy.

Source: EPA Global Climate Change Source

burning and biogenic secondary organic aerosol particles, with a range of cloud processing histories, to produce an OH burst. This will greatly expand measurements of the phenomenon, which are currently limited to urban aerosol particles from transportation, industry and residential wood burning. We will deploy a new direct-to-liquid sampler that captures OH formation within ~0.1 seconds of cloud droplet formation and leverage many instruments available at SGP to characterize aerosols.  Additionally, we plan to make measurements of the size dependence of the aerosols that generate OH.

We also plan to investigate the impact of cloud processing on the ability of particles to produce an OH burst in the laboratory. Recognizing that aerosol particles can encounter five or more clouds before they are removed from the atmosphere and that the burst precursors appear to be quickly consumed when droplets form, it is important to know how rapidly (if at all) the precursors are regenerated. To address this question directly, we propose to simulate cloud evaporation/condensation cycles and aging on ambient aerosols in the lab using an environmental chamber we will construct.

Finally, we plan to develop a model parameterization of the new OH source for future implementation in a widely used atmospheric model known as the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem).

Once complete, the study will provide a clearer understanding of the OH burst phenomenon. This will change the understanding of the influence of cloud aqueous phase chemistry on aerosol physical and chemical properties and how these can feed back to clouds and radiation, addressing important gaps in understanding aerosol-cloud interactions in climate.