Indoor and outdoor atmospheric ion mobility spectra, diurnal variation, and relationship with meteorological parameters

Abstract

We report measurements of small ion mobility spectra and concentrations made over a 5 month period using a Gerdien-type ion mobility spectrometer at one indoor site in central Bristol, UK, and one indoor and one outdoor site in a semirural area. We assess the diurnal variation in ion concentration and mobility, variability with meteorological parameters, and features within the ion mobility spectra. Ion concentration (typically in the range 100–1000 cm−3) was highest overnight and lowest in late morning and afternoon. Total conductivity also followed this trend suggesting that small ions contribute more to conductivity than larger charged particles, although at the semirural site, indoor activity produced charged aerosol intermittently, while infiltration of outdoor aerosol contributed at the urban site. Negative ion mobility (typically 1.6–1.9 cm2 V−1 s−1) was higher overnight than at other times, while positive ion mobility (typically 1.2–1.4 cm2 V−1 s−1) showed no diurnal variability. In general, temperature, pressure, wind speed, and water vapor pressure were inversely related to both concentration and mobility, except outdoors when (negative) mobility increased with increasing temperature and pressure, while increased relative humidity was associated with higher ion mobilities and concentrations. Analysis of high-resolution mobility spectra revealed several mobilities for each polarity at which peaks were identified across all sites, suggesting similarity in composition but with varying importance at each site. Intermediate ions (mobilities 0.2–0.3 cm2 V−1 s−1) were observed at the outdoor site, but individual nucleation events which may be associated with them were not identifiable.