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 ce ll type multiple reflection system. The detectable fractional absorban ce of 1x10(-5) corresponds to a sensitivity of 50 pptv and 200 pptv NO 2 for measurements performed at 612.7 nm or 616.0 nm, respectively. Th e averaging time is at maximum one minute. Since the system can be ope rated simultaneously in the VIS and the UV (second harmonic) it is cap able to measure NO2 as well as OH, SO2, CH2O and C10H8 at the same tim e. The MOAS (Multipass Optical Absorption Spectroscopy) instrument is interesting for in situ measurements of atmospheric NO2 With high spat ial and temporal resolution and sensitivity. It can be used as an exte rnal standard to calibrate other NO2 devices under field conditions. I ts capability to measure OH and NO2 simultaneously within the same com partment offers a unique possibility to perform field tests of model c alculations of fast tropospheric photochemistry.