An improved method of measuring tropospheric NO2 and RO2 by matrix isolation and electron spin resonance

A cryogenic system for the airborne and ground based sampling of ambient radicals by matrix isolation is described. The trapped radicals, e.g., NO2 and RO2, are analyzed by ESR. The technique has been improved, mainly by addition of water vapor to the sampled air, to yield a collection efficiency of (90 + or - 10) percent and a lower detection limit of about 20 ppt, but is still does not distinguish between the different RO2. Careful calibration reduced the measurement error (1 sigma) to + or - 10 percent for NO2 and + or - 15 percent for HO2. Two diurnal variations of RO2 and NO2 at ground level and vertical profiles in the lower troposphere are presented.