Evolution of tropospheric ions observed by an ion mobility spectrometer with a drift tube

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<jats:p>The evolution of tropospheric ions has been studied by investigating the change in the mobility spectrum of ions generated in laboratory air as a function of aging time. An ion mobility spectrometer with a drift tube was constructed to get high‐resolution measurements of mobility spectra of ions aged for about 0.03–5 s. Mobility spectra and their changes with ion age obtained in the present study showed the following features of tropospheric ion evolution near the ground: The positive ion mobility spectrum changes significantly with aging time, in which the peak mobility value shifts from 1.8 to about 1.0 cm<jats:sup>2</jats:sup> V<jats:sup>−1</jats:sup> s<jats:sup>−1</jats:sup>. Six peaks are usually distinguished in the mobility range between 2.0 and 1.0 cm<jats:sup>2</jats:sup> V<jats:sup>−1</jats:sup> s<jats:sup>−1</jats:sup> in the positive spectrum, and a tendency of positive ions to evolve to have lower mobilities with increasing ion age is observed. Four additional peaks are sometimes resolved at around 0.9 cm<jats:sup>2</jats:sup> V<jats:sup>−1</jats:sup> s<jats:sup>−1</jats:sup>, indicating formation of heavy positive ions (probably over 400 amu). The negative ion mobility spectrum obtained is characterized by two prominent peaks at 1.9 and 1.7 cm<jats:sup>2</jats:sup> V<jats:sup>−1</jats:sup> s<jats:sup>−1</jats:sup>. The relative intensity of the two peaks varies with ion age, and the ions with a mobility of 1.9 cm<jats:sup>2</jats:sup> V<jats:sup>−1</jats:sup> s<jats:sup>−1</jats:sup> dominate after about 1 s. Further conversions of the negative ions to those with lower mobilities can be expected from a comparison with the natural ion spectra obtained so far by the Gerdien type aspiration condenser.</jats:p>

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