History of Electrical Aerosol Measurements

ABSTRACT Early studies of atmospheric electricity suggested that the electrical conductivity of the atmosphere should be sufficient to dissipate the charge on the surface of the earth in a matter of minutes. Efforts to understand how substantial electric fields could be maintained globally in spite of the high dissipation rates were propelled into the forefront of physics research at the turn of the century when it was observed that the newly discovered X-rays produced ions that behaved much like those in the atmosphere. Many of the approaches that are now employed in electrical measurements of aerosols were first conceived during the first three decades of this century. Initially the focus was on gas ions, but they were found to consist of charged clusters of water molecules that exhibited a number of distinct mobilities that were substantially lower than those that resulted after long efforts to dry the gas. The coaxial condenser mobility analyzer, introduced by McClelland in 1898 and enhanced by Zeleny in 1900, was used to measure atmospheric ions as early as 1901 by Ebert. Based upon atmospheric measurements with this device in 1905, Langevin reported on the existence of ions with mobilities 3000 times lower than those observed in the laboratory studies. These so-called large ions correspond to particles in what we now know as the accumulation mode of the atmospheric aerosol. The aspiration condenser dominated measurements of atmospheric ions for six decades even though Erikson developed a differential mobility analyzer by 1921, and Rohmann produced a differential mobility sampler in 1923. Only after electronics was improved in the 1950s and 1960s were these instruments reintroduced. It was based upon condenser measurements of atmospheric “ions” that Junge first described the structure of the ultrafine particle size distribution in 1955. The development of the Whitby Aerosol Analyzer in 1966 built upon earlier developments, including the Faraday cup electrometer that was used by both Zeleny and McClelland at the turn of the century, and a long history of mobility analyzers. This instrument represented a breakthrough nonetheless since it was the first mobility aerosol analyzer that was sufficiently refined and robust to be commercially produced. That instrument was refined into the electrical aerosol analyzer (EAA) that became the primary tool for characterizing ultrafine aerosol particles in the atmosphere for a number of years. At the same time that the EAA was developed, the differential mobility analyzer was reintroduced in a form that quickly became the standard for production of submicron calibration aerosols. Early efforts to transform that instrument from a calibration tool to a measurement device met with limited success due to the lack of a suitable detector. The introduction of a continuous flow condensation nucleus counter was followed quickly by the development of computerized differential mobility analysis of particle-size distributions and later accelerated through the introduction of scanning mode operation. Efforts to extend differential mobility analysis to smaller and larger particle sizes than have been accessible with the instruments developed during the 1970s continue to produce enhancements in the instrumentation and the understanding of its performance.