Filter sampling was performed on board the Meteorological Research Flight (MRF) Hercules C-130 of the U.K. Met Office. The aircraft has a maximum endurance of 12 h (with fuel tank reserves). The minimum operational flight altitude is 17 m over water and 35 m over land and the maximum operating altitude is ~ 11,000 m.
The aerosol sampling system used onboard the C-130 is described in detail in Andreae et al. [2000]. 
Two two-stage stacked-filter units (SFUs) were mounted in parallel. The first stage of each SFU consisted of a Nuclepore filter (nominal pore size 8.0 ?m) with 50% cut-off diameter around 0.9–1 µm. 
One SFU was used for measuring water-soluble ions and major, minor, and trace elements using a PTFE Teflon filter (nominal pore size 1.0 ?m). 
The second SFU was used for measuring carbonaceous aerosols. In this case the second stage consisted of two Pallflex quartz filters arranged in a tandem configuration.