A new instrument to measure gaseous nitrous acid (HONO) in the atmosphere

A new in situ instrument (LOPAP: long path absorption photometer) to measur e gaseous nitrous acid (HONO) using wet chemical sampling and photometric d etection has been developed. This instrument is aimed to overcome the known problems with current HONO measurement techniques and was designed to be a cheap, sensitive, compact,and continuously working HONO monitor for ambien t air measurements in the troposphere or for measurements of higher concent rations e.g. in smog chambers, in exhaust gases, and in indoor environments . Laboratory investigations were carried out to characterize the instrument components with respect to collection efficiency, optimum dye formation, o ptimum detection, and interfering species. Detection limits ranging from ap proximately 3 to 50 pptV have been obtained with response times from 4 to 1 .5 min, respectively, using different instrument parameters. The accuracy o f the measurements is in the range between +/- (10-15)%. The validation of the instrument was performed in the laboratory for HONO concentrations of 3 and 30 ppbV using ion chromatography and with a DOAS (differential optical absorption spectrometer) instrument in a large outdoor smog chamber in the range from 0.1 to 20 ppbV. The deviations were well within the errors of t he measurements; however, when comparing the data with the DOAS instrument systematically higher values were found with the LOPAP instrument.