An experimental and modeling study of an ambient RO* (HO + H02 + RO + R02) detector Is presented. As described previously, the detector utilizes chemical amplification of the radical concentration through a chain reaction Involving NO and CO to produce N02. Modifications reported here overcome a PAN and PNA Interference to produce a detector that Is a factor of ~25 less sensitive to these Interferences than the conventional design, has Improved rejection of artifact signals, and can have variable-length Inlets without requiring a controlled nitrogen flow. A model of the chemical amplifier chemistry, which Includes chemical and wall loss of radicals, showed that simplified calculations greatly overestimate the chain length. The variation of the chain length with wall loss rates, radical concentration, reaction time, and radical type has been Investigated. The wide variation In reported chain lengths has been attributed to a chain length dependence on radical concentration and Inadequacies associated with one of the calibration techniques. Absolute radical calibration of the Instrument was performed by using the thermal decomposition of PAN as a source of known concentrations of peroxyacetyl radicals. Ambient measurements over a 4-day period show a diurnal variation of RO* radical concentrations and that daytime maximum concentrations of 3 pptv are readily discernible.
Measurements of peroxy radicals (RO2* = HO2 + ∑RO2, where HO2 is the hydroperoxyl radical and R is an organic group) were made using the chemical amplifier technique (PERCA) during the Air chemistry and Lidar studies of tropospheric and stratospheric species on the Atlantic Ocean (ALBATROSS) campaign on board the German research vessel Polarstern (cruise ANT XIV/1, 1996). The data obtained are compared to previous results from an earlier cruise in 1991 (ANT X/1). Reasonable agreement between the two data sets was observed, indicating the reliability of these measurements. Both data sets take into account the sensitivity of the radical amplifier to the presence of ambient water vapor. Maximum RO2* mixing ratios around noon between 40 and 80 pptv were measured. Nighttime signals were observed on many days. Air masses in different latitude regions in the North and South Atlantic could be characterized using back trajectories. In spite of the fact that the RO2* is relatively short lived, its mixing ratio appears to be influenced by the path traveled by the air mass somewhat higher levels being associated with sources of pollution. A box model based on CH4 and CO oxidation chemistry describes RO2* reasonably well but could not explain the persistent nighttime signals and the HCHO observed. An additional source of HCHO is required, indicating the importance of nonmethane hydrocarbon (NMHC) chemistry in the remote Atlantic boundary layer. Both back trajectories and variations of trace gas concentrations indicate that biomass burning, ship, and natural emissions are likely responsible for the observed deviations from the assumed chemistry.
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