Special Issue on Aerosol Measurements in the 1 nm Range

Nucleation leads to the formation of new aerosol particles in systems that include the atmosphere, semiconductor processing reactors, aerosol synthesis reactors, and flames. Nucleation can occur by the spontaneous condensation of supersaturated vapors or by reactions of gases to form nonvolatile products. Despite the importance of nucleation, our ability to accurately predict nucleation rates and subsequent particle growth in aerosol systems of practical importance is limited by our inadequate understanding of processes that lead to the formation of molecular clusters and their subsequent growth. In his pioneering work on atmospheric nucleation, John Aitken pointed out a century ago (Aitken 1911): “The great difficulty in investigations of this kind is the extremely minute quantities of matter which produce surprising results and make the work full of pitfalls for the hasty.” In the atmosphere, for example, sulfuric acid participates in nucleation at typical mole fractions of 10⁻¹⁴–10⁻¹¹, and the concentrations of molecular clusters formed by nucleation are several orders of magnitude lower than that. Our inability to detect and identify such “minute quantities of matter” has been a major impediment to advancing our understanding of nucleation in aerosol systems.

This special issue focuses on the convergence of mass spectrometers, ion mobility spectrometers, and condensation particle counters towards measurements in the 1 nm size range. It is around this size that aerosol particles are born and begin their growth toward larger sizes. The high mass resolution that can be achieved by mass spectrometry offers the promise of identifying the chemical makeup of nucleated clusters, which in principle can contain as few as two molecules. Ion mobility spectrometers allow for fast, routine measurements of ion mobility distributions down to molecular dimensions. Condensation particle counters detect individual particles and are therefore the sensitivity champions, but vapor condensation onto particles below ∼2 nm depends on both vapor and particle composition and charge. Establishing relationships among these measurements requires reconciling sizes based on mass, mobility and geometry. Also, in order to achieve size separation particles, as well as molecules and clusters, must be charged. This adds an additional challenge when detecting particles that are initially neutral, since charging processes are also composition-dependent. Because particle and ion composition are not always known, charged fractions are frequently uncertain. The articles in this special issue describe significant progress on several of these topics, and point out where additional work is needed.

When used to measure particles in the 1 nm range, mass spectrometers, ion mobility spectrometers, and condensation particle counters may or may not detect the same “particles.” In this special issue we adopt the term “nano-CN” when referring to sub 3 nm particles that can form larger particles; i.e., species that connect “molecules” and “clusters” (few molecules) to larger nanoparticles. By this definition, sub 3 nm particles detected with condensation particle counters are nano-CN, although it does not follow that all nano-CN can be detected with condensation particle counters.

The experimental challenge is to relate different measurements of nano-CN size and composition. For example, condensation particle counters detect nano-CN in both neutral and charged states while direct sizing techniques require charging for size separation and often for detection. Thus the experimental challenge is to determine the relationship between neutral and charged molecules, clusters and particles; i.e., when different measurement principles are measuring the same particles and when they are not. Some of the articles in this special issue address those relationships, but not always definitively. Improving our ability to carry out quantitative measurements to elucidate the detailed chemistry and physics of nano-CN remains a priority for the future.