| Popis: |
Results from the laboratory characterisation of a new design of hydroxyl radical (OH) flow reactor are presented. Further to this details of its coupling with a gas chromatograph time of flight mass spectrometer (GC-TOF-MS) system, to form a new instrument for the identification of ‘missing’ OH reactivity is discussed. Preliminary measurements in ambient air demonstrated the potential value of this system which could be used to chemically identify ‘missing’ OH sinks in various environments, and also to determine bimolecular rate coefficients for their reaction with OH. Observations of OH reactivity and formaldehyde (HCHO) are presented from an urban background site in York in the summer of 2014. OH reactivity was measured using laser flash photolysis coupled with laser induced fluorescence spectroscopy (LFP-LIF). The average ‘missing’ OH reactivity was ~27 % when measurements were compared with values predicted by a calculation that utilised measured concentrations of a very detailed suite of volatile organic compounds (VOCs). It is concluded that a combination of unidentified VOCs and products of VOC photo-oxidation account for this discrepancy. HCHO was measured using a new fast response (1 s time resolution) laser induced fluorescence (LIF) spectroscopy instrument and some evidence of daytime diurnal behaviour suggested that the dominant HCHO source was photo-chemical (~1.3 ppb diurnal peak). Observations of OH reactivity and HCHO are also presented from a coastal site in Weybourne, Norfolk, using the same instrumentation during the summer of 2015. The average ‘missing’ OH reactivity was ~44 %. It is concluded that much of the ‘missing’ reactivity was likely due to unmeasured VOCs and their photo-oxidation products. Strong diurnal behaviour of HCHO was observed and is consistent with an atmosphere where the dominant source is photo-chemical (~1.1 ppb diurnal peak). Unusual behaviour was observed for HCHO during a thunderstorm where sharp fluctuations in concentration were observed. It has so far not been possible to conclude the exact cause of this, however, it is suggested that the source was marine. Finally, preliminary results from three experiments of an intercomparison of OH reactivity instrumentation are presented. The results demonstrate the reliability of the Leeds LFP-LIF instrument for the measurement of ambient OH reactivity. |
