What are ultrafine aerosols?
Close to the earth's surface, continental air typically contains from 103 to 105 particles per cubic centimeter. These particles have diameters (dp) ranging from 1 nm to 100 μm.
Plots of particle number, surface area, and volume versus dp, such as those shown in the adjacent image for a typical rural setting, usually show that these particles are distributed over three or more modes. As the above plot of partical distribution shows, most particles are smaller than 0.1 μm in diameter. We call these ultrafine aerosols and this mode of the distribution is called the Aitken or nucleation mode (shown as the yellow shaded area). The other two modes, which are more clearly defined when we plot the surface area and volume distributions, are the accumulation mode (0.1 < dp < 2.5 μm, the blue shaded area) and the coarse mode (dp > 2.5 μm, the green shaded area). For particles with diameters smaller than 0.050 μm, many properties of the condensed phase, most notably vapor pressure, become quite different from those of the bulk phase: 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, and for total aerosol volume or mass, it is the course mode that contributes the most.
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.
Smog in Tecamac Mexico on a day when air from the city was blowing into town. The sunlight is scattered so much by the aerosols that it looks like a foggy day!
Goals
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 is unique in it's ability to characterize the chemical composition of particles as small as 5 nm in real-time.