Thermal Desorption Chemical Ionization Mass Spectrometer
What’s the composition of a tomato, and what does this have to do with atmospheric chemistry?
Atmospheric new particle formation is the name given to the spontaneous formation of new nanometer-sized particles in the atmosphere. Often these events extend for hundreds of square kilometers and last for hours. While the impacts of these events are not well understood, they are the dominant source of particles in the atmosphere. This could be very important for phenomena such as cloud formation. Clouds form when water vapor condenses on atmospheric particles, and clouds have a significant impact on the earth’s radiation balance. Because new particle formation can increase concentrations of these cloud condensation nuclei, it may have a significant impact on the earth’s radiation balance. The key for understanding the impact of new particle formation on clouds is understanding the growth rate of newly formed nanoparticles: if these particles grow too slowly they deposit on other surfaces in the atmosphere (such as other particles) and disappear. Particles that quickly grow from nanometer-sized clusters to 100 nm diameter particles have a much better chance of surviving to form a cloud droplet.
For decades, it has been known that gaseous sulfuric acid is a key species in both the formation of nanoparticles by “nucleation,” as well as their subsequent growth. Researchers in ACD, along with Peter McMurry’s research group at the University of Minnesota, were the first to perform size-by-side measurements of newly formed particles and sulfuric acid. They and other investigators who have since performed similar measurements uncovered a new mystery: there is typically not nearly enough sulfuric acid in the atmosphere to account for the observed nanoparticle growth rates.
A new tool was needed to directly probe the chemical composition of particles formed by nucleation. Thus in 2000, an instrument was developed by the NCAR/UMN team to measure the composition of the smallest particles in the atmosphere. Under the leadership of ACD’s Jim Smith this instrument, the Thermal Desorption Chemical Ionization Mass Spectrometer, or TDCIMS , is now being deployed throughout the world in order to answer the question: What chemical species are responsible for the growth of atmospheric nanoparticles?
In March 2006 the UMN/NCAR team made measurements with the TDCIMS at a ground-based site located in Tecamac, Mexico, about 40 km N of the center of Mexico City as part of the MIRAGE-Mexico field study. The particle formation events observed in Tecamac were unlike any they had seen. New particle formation was so vigorous that sub-10 nm particles were constantly being formed even as the particles were growing rapidly by condensation of low-volatility vapors. This phenomenon, looks like a tomato when plotted as a time series of particle size distribution, so we refer to these as “tomato events.”. Figure 1 shows one such tomato event that occurred on 16 March 2006. The shape of this size distribution made it particularly challenging to calculate the growth rates, since the peak of the distribution of the growing nanoparticles was obscured by the new sub-10 nm particles that were simultaneously being formed. A new technique, developed by UMN graduate student Kenjiro Iida, allowed the accurate determination of the growth rate during these tomato events that employed an additional measurement, the fraction of ambient nanoparticles that were charged. The result was a growth rate for the 16 March event of 22 nm hr-1, among the highest growth rates reported anywhere.
Figure 1. Contour plot of the particle size distribution on 16 March 2006 in Tecamac, Mexico, during the MILAGRO campaign, show a new particle formation “tomato” event that started just before 10am local time. Also plotted is the particle diameter used for TDCIMS chemical composition measurements.
During the 16 March event, the TDCIMS was tuned to measure the composition of particles in the size range from 10 – 35 nm, depicted by the black line plotted over the size distribution in Figure 1. The result, shown in Figure 2, is that about 84% of the detected ions are organic, comprising of organic acids, hydrocarbon-like species, and nitrogen-containing organic compounds. Particulate sulfate which arises from the condensation of sulfuric acid vapor constituted only 10% of the detected ions, and nitrate comprised 6%. Measurements of gaseous sulfuric acid, performed by Greg Huey’s research group at the Georgia Institute of Technology, showed that sulfuric acid levels were insufficient to account for the observed growth rate of 22 nm hr-1. The calculated growth rates based on these measurements of sulfuric acid, in which we assume that every sulfuric acid molecule that collides with a particle sticks to the particle and becomes particulate sulfate, amounted to 10% of the observed growth rate. This result that sulfuric acid contributes to 10% of particulate mass, based only on measurements of particle growth rate and sulfuric acid vapor concentration, is the exact same conclusion that one might draw from TDCIMS measurements if the detected ions correspond to the actual species present in particles. This latter issue, the extent to which detected ions correspond to the species in the particles, is the primary focus of current laboratory development with the TDCIMS.
What is the consequence of this research? Current models that include the growth of newly formed particles account for the 10% contribution of sulfuric acid to growth rates but entirely miss the 90% contribution of the organics. The result is that these models underestimate the impact of new particle formation on cloud processes. Furthermore, the recognition of the importance of organic compounds in these particles highlights the need to understand the processes by which organic compounds transform into compounds with extremely low volatilities.
Figure 2. TDCIMS measurements of the molar ratio of sulfate, nitrate, and organics during 16 March 2006 event. Index above plot indicates the mass-weighted geometric mean diameter, in nanometers, of measured particles.