LOCAL MONITORING OF ABSOLUTE NO2 CONCENTRATIONS IN AMBIENT AIR BY MULTIPASS ABSORPTION-SPECTROSCOPY

A method for monitoring tropospheric NO2 is described which makes use of differential absorption spectroscopy in the red at 600-620 nm. The absorption path-length of up to 2.7 km is folded inside a 6 m White cell type multiple reflection system. The detectable fractional absorbance of 1x10(-5) corresponds to a sensitivity of 50 pptv and 200 pptv NO2 for measurements performed at 612.7 nm or 616.0 nm, respectively. The averaging time is at maximum one minute. Since the system can be operated simultaneously in the VIS and the UV (second harmonic) it is capable to measure NO2 as well as OH, SO2, CH2O and C10H8 at the same time. The MOAS (Multipass Optical Absorption Spectroscopy) instrument is interesting for in situ measurements of atmospheric NO2 With high spatial and temporal resolution and sensitivity. It can be used as an external standard to calibrate other NO2 devices under field conditions. Its capability to measure OH and NO2 simultaneously within the same compartment offers a unique possibility to perform field tests of model calculations of fast tropospheric photochemistry.