Background Aerosol

The aerosol in the mid and upper troposphere, the so-called free troposphere, is often termed background aerosol. It is well-aged aerosol with a composition and size distribution reflecting the simultaneous effects of gas-to-particle conversion, longrange transport, and removal processes. The number concentration of background aerosol is in the range of 300/cm3 (Raes et al., 2000), and its size distribution is nearly monodisperse with peak diameters in the 0.2 to 0.5 pm range (Leaitch and Isaac, 1991). Regions with the lowest mass concentrations generally exhibit the highest number concentrations (Clarke, 1993), suggesting that nucleation may be a major source of particle number. Volatility measurements of the free tropospheric aerosol suggest a composition dominated by sulfates (Clarke, 1993; Hofmann, 1993)

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