Towards a molecular understanding of atmospheric nucleation (Funded by National Science Foundation)

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Atmospheric aerosol particles range from as small as a nanometer to as large as few microns. These particles can affect visual air quality, climate and human health. Nucleation, or the process of the formation of these particles from gas phase precursors and their subsequent growth by the uptake of gas phase chemicals, has been shown to contribute significantly to the increase in atmospheric aerosol number concentrations and also affects aerosol number size distributions. When aerosol particles grow beyond 50 to 100 nm, these can serve as cloud condensation nuclei, and can alter the cloud albedo and ultimately the earth’s radiation balance. This effect of aerosols is the largest source of uncertainty in current global climate models. These uncertainties add to our imperfect understanding of nucleation process.

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Neutral particles are especially important in atmospheric nucleation, as claimed by a number of previous studies (Iida et al. 2006, Kulmala et al. 2007, Iida et al. 2008). For instance, Kulmala et al. (2007), based on their measurements of atmospheric concentrations of both neutral and charged nanometer-sized clusters conclude that a pool of numerous neutral clusters in the sub-3 nm size range is continuously present and neutral nucleation might dominate over the ion-induced mechanism. However, our preliminary observations lead us to believe on the contrary. While it is certainly true that the atmosphere has a very high concentration of small neutral particles ( ~ 1- 4 nm) during a nucleation event, this is not necessarily the case during a non-nucleation event. The goal of this study was to design a laboratory apparatus for continuous measurement of neutral particles in the atmosphere, during both a nucleation and non-nucleation event. This was followed by a month long atmospheric measurements at a research site at Lewes, DE.

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The measurements of particle concentrations in the small size range down to 1 nm is extremely challenging. Conventionally, the small size aerosol particles measurements are done using Condensation Particle Counters (CPCs). An obvious advantage of using CPCs over other electrical methods is that it can detect neutral particles as well. However, CPC detection efficiency decreases with decreasing particle size. Further, CPC size-dependent counting efficiency are affected by instrument design, particle concentration, particle charge and the 'working fluid'. The introduction of Di-Ethylene Glycol (DEG) as the working fluid has reduced the smallest size that can be detected by the CPCs. The accurate measurement of atmospheric neutral particle concentrations thus required three research objectives to be met:  

(1) Laboratory Studies : 

• Quantifying detection efficiencies of the CPCs in the sub 10 nm size range for charged/neutral particles. 

• Quantifying Sampling efficiency of charged and neutral particles for the experimental setup used.

(2) Field Studies : 

• Designing an equipment for neutral particle measurement. 

• Carrying out the continuous atmospheric measurements.

(3) Data Inversion and Closure : 

• Combining field and laboratory studies to infer actual neutral particle concentrations in the atmosphere.

• Compare the neutral particle concentrations inferred from different instruments to obtain a 'closure'.

The designed equipment for the field study consisted of two DEG-CPCs lined in parallel with different saturator temperatures (which affects the detection efficiency of different size particles), with the inlet of the two CPCs attached to an electrostatic precipitator (ESP). The ESP automatically alternated between 'ON' and 'OFF' positions, giving the measurements of total particle concentrations and the neutral particle concentrations as measured by the two CPCs in alternating cycles. Along with this newly designed equipment, the particle concentrations were also measured separately by a DEG-SMPS system. The closure condition required that the neutral particle concentrations measured in ~ 1-4 nm size range as predicted by both the equipment match within the limits of experimental error.

 

Reference:

Zhao, J., Han, H. S., Karn, A., McMurry P. (2015),  Development of an ultrafine Condensation Particle Counter (UCPC) for measurements of particles down to 1 nm in diameter, 34th American Association for Aerosol Research Conference, Minneapolis, Minnesota.