What are ultrafine aerosols?
Close to the earth's surface, the air typically contains anywhere from hundreds to millions of particles in a cubic centimeter. These particles have diameters ranging from one nanometer to as large as 100 micrometers (about the diameter of human hair).
At any instant in time we can plot the number of particles, their surface area, and their volume as a function of diameter. The plots to the right show how these distributions might appear for a typical rural setting. As the plots show, typically particle distributions in the atmosphere show three or more peaks. The largest numbers of particles are usually smaller than 100 nm in diameter, which is shown in the yellow bands in the plots (note that the particle number distribution, which is the top plot, has a peak in this band. We call these ultrafine aerosols. This range of particle diameters is also known as the nucleation mode, which refers to the atmospheric process that creates most of these particles, and also as the Aitken mode in honor of John Aitken, a pioneer in aerosol science. The other two size ranges shown in the plots are more clearly defined when we plot the surface area (middle plot) and volume (lower plot) distributions. These are called the accumulation mode (with diameters around 100 nm to 2.5 micrometers, the blue shaded area) and the coarse mode (with diameters larger than 2.5 micrometers, the green shaded area). For particles with diameters smaller than 50 nanometers, many properties of the condensed phase, most notably the volatility of the molecules that comprise these particles, become quite different from those in a flat surface: particles in this size range are called nanoparticles.
The plots of particle number, surface area, and volume demonstrate important points about potential influences of particles in the various modes of the distributions. When considering the particle numbers, it is often the ultrafine aerosols that contribute the most (since the peak of the number distribution is in the ultrafine size range). For surface area, the accumulation mode is most influential: this can be important for chemical processes that take place on aerosol surfaces in the atmosphere (atmospheric chemists call this “heterogeneous chemistry”). Lastly, the course mode dominates total aerosol volume or mass.
Forget the spray can!
Many of us think of spray cans when we hear the word aerosol but here we define the word as particles suspended in air. Aerosols are the major constituents of the haze that we perceive as smog. They can impact human health (e.g., asthma, bronchitis) and climate (e.g., directly scattering sunlight and modifying cloud properties).
The goals of the ultrafine aerosols group are to understand the processes by which ultrafine aerosols form and grow in the atmosphere and to study the impacts of ultrafine aerosols on the chemistry and climate.
We are particularly interested in the way in which ultrafine particles are created in the atmosphere by nucleation, which is the process by which the smallest stable molecular clusters are formed, and how these stable clusters grow into larger sized particles. Interesting questions along these lines are:
- What are the chemical compounds that are responsible for the formation of the stable clusters and for the growth of these clusters into "new particles"?
- Are these compounds formed in the atmosphere or in the particles/clusters themselves?
Another interest that we have is in the impacts of ultrafine particles. Particles of this size form the inner "seed" upon which water condenses to form cloud and fog droplets. When we change the composition of ultrafine aerosols through the pollutants we emit, we may be impacting cloud droplet formation, which in turn could affect the amount of sunlight we receive as well as precipitation.
To study these phenomena, the UA group has teamed with university investigators to develop a unique set of instruments. Among these is the Thermal Desorption Chemical Ionization Mass Spectrometer, or TDCIMS, which can characterize the chemical composition of particles as small as 5 nm.