Airborne, continuous measurement of carbon monoxide in the lower troposphere

Continuous measurements of CO on-board a fast moving research aircraft in the lower troposphere were made using a system operating on the principle of reduction of HgO and subsequent detection of Hg vapor by flameless atomic absorption (AA). The AA detector is a commercial unit with a modified HgO reaction bed. The system is operated at constant pressure to stabilize reaction conditions, system response, and background signal. The key analytical parameters are as follows: detection limit 4 ppb, precision +/-2.4 ppb, and 90% response time 110 s. The system was successfully deployed in a large number of research flights over central Europe. The response time is long compared to some other instruments (such as the NOy analyzer). However, the CO measurements can be interpreted in terms of differences between the lower free troposphere (LFT) and planetary boundary layer (PBL), in terms of quantitative analyses of vertical profiles within the PBL, and with respect to relative enhancement ratios of primary pollutants in plumes. During our 1992 and 1993 research flights, we found CO mixing ratios between 90 and 160 ppb to be typical for the LFT over Germany. Within the PBL, typical mixing ratios were between 150 and 320 ppb, by factors between 1.3 and 3 higher than those in the LFT. Higher mixing ratios were found over or downwind of urban agglomerations, where area sources, mainly traffic, lead to peak mixing ratios of several hundred ppb up to altitudes of about 150 m above ground.