937An instrument for the continuous detection of NO, in the sub-part-per-billion range Is descrlbed. The Instrument Is based upon the chemllumlnescent reactlon between NO, In air and luminol (5-amlno-2,3-dlhydro-l,6phthalazinedione) In alkallne solutlon. The present detector exhlblts a 2-Hr response speed to changes of f20 ppb and a field detectlon limlt of 30 pptr (parts per trllilon). The instrumental technlque has been expanded to measure NO by the catalytlc oxidation of NO to NO, using CrO, on silica gel as the oxldlring agent; however, at low amblent NO concentratlons some drlft In the NO zero Is observed. Interference from ambient 0, Is elimhated by modlflcatlon of the inlet system and iumlnol solution.The nitrogen oxides, especially NO and NOz (NO,), are very important species in atmospheric chemistry. It has been shown that NO, is an important source of ozone in the troposphere (1) and that NO, is a reactant in tropospheric odd hydrogen chemistry (2). The NO, reactions produce a variety of inorganic and organic nitrates (3). Of these products, nitric acid and peroxyacetyl nitrate (PAN) are of prime interest with H N 0 3 playing an important role in acid rain chemistry (4).To fully understand the atmospheric chemistry of NO,, a detector is needed which will measure NO, a t the low parts-per-billion level or less for clean air studies (5).Currently a number of methods are being used to monitor NO, concentrations in ambient air. By far the most common technique is based upon the chemiluminescent reaction between NO and O3 (6, 7). While this reaction is specific for NO (8,9), the technique can be expanded to measure NOz by first reducing NO2 to NO (10) and then measuring total NO, as NO. Unfortunately, the reduction of NOz to NO is not specific for NOz, so that other nitrogen species (particularly HN03 and PAN) are also reduced to NO and act as interferences in the NOz measurements (11). Hence, NOz measurements are normally subject to some question. There exist two more direct means of NOz analysis for which field data have been reported. The first of these involves the photodissociation of NOz to form NO followed by chemiluminescent reaction with O3 (12). This method is similar to those already mentioned but does not suffer from HN03 and PAN interference. The second direct NOz method uses the absorption of infrared radiation as its basis for detection. A tunable diode laser is used as the light source (13).Recently, Maeda et al. described the development of an NO2 detector based )upon the chemiluminescent reaction of NOz with luminol(5-mino-2,3-dihydro-1,4-phthalazinedione) (14). Luminol has been under study since 1928 (15) and is known to chemiluminesce with a number of oxidizing agents in alkaline solution when metal catalysts are provided (16). This system has been used to measure H20z as well as other oxi-' Address correspondence to author at The University of Michigan, Space Physics Research Bldg., Room 1206, 2245 Hayward St., Ann Arbor, MI 48109. dizers produced by biological systems (1 7,18) and to me...