Total OH reactivity measurement by Laser Flash Pump and Probe Technique in Urban and Sub-urban Area in Tokyo

OH reactivity in the atmosphere provides useful information as an index for evaluation of air quality and atmospheric chemistry processes. There are several hundreds or more of VOCs in the atmosphere which react with OH radicals and contribute to either oxidants or aerosols formation. We developed an instrument to measure the lifetime of OH radical in the ambient air by laser induced pump and probe technique. OH radicals are generated in the photolysis of ozone by 266 nm laser-light. Decay of resultant OH is monitored by novel LIF-FAGE technique using 308 nm laser-light. The air quality in Tokyo is a great concern because oxidants are found to have a positive trend with ca.1% per year in spite of the great reductions of precursor species such as NOx and NMHCs. There are a couple of possible reasons which explain the positive trend of ozone in Tokyo, (i) Increase of back ground ozone due to long range transport from Eurasia, (ii) Effective ozone production under lower NOx concentration, (iii) Suppression of titration of NOx due to decreased NOx level, (iv) Contribution of un-measured VOCs which might have a positive trend, (v) Warming of urban temperature due to heat island phenomena. In order to know the mechanism of oxidant formation in Tokyo, we measured OH reactivity in urban and sub-urban Tokyo as well as simultaneous measurements of VOCs by GC technique and proton transfer mass spectrometry. We compared OH reactivities determined by pump- probe technique and VOC analysis. In the case of VOC analysis we considered more than 70 species. Systematically greater OH reactivity was observed in spring and summer periods compared to that predicted by the sum of each reactive species of OH. The average of difference is found to be more than 30 % of OH reactivity. On the contrary fairly good agreement was obtained in winter time. The missing OH reactivity represents the existence of unknown OH reactive species. Neither OH up-take on the aerosols surface nor the uncertainty of OH+NO2 reaction rate constant can explain the missing sink. Either secondary generated oxidants such as OVOCs or unknown VOCs emitted from plants or both seem to be responsible. In case of urban air there is still some missing sink observed but the nature of the air is somewhat different from semi- urban air. Since more fresh pollutants could be expected in urban atmosphere, the missing sink could have a contribution from primary emission rather than secondary oxidation products expected in sub-urban air.