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Nitrous acid (HONO) is an important trace gas in tropospheric photochemistry, which plays a significant role in polluted urban atmosphere. Based on the measuring principle, a long path absorption photometer (LOPAP) for the detection of nitrous acid was built in-house, which uses wet chemical sampling and photometric detection. This instrument aims 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 ambient air measurements in the troposphere, or for measurements of higher concentration, e.g., in a smog chamber and in exhaust gases. The system includes two channels, by subtracting the signal of the second channel from the first channel the occurrence of possible unknown interferences can be minimized. The instrument has a detection limit of 9×10-12 mol•mol-1 and an accuracy of 10% at 5 min resolution, with a good linear relationship exhibited. In addition, we tested probable interferences from NO, NO2, O3, NO2+O3, NO2+SO2, and O3+HONO. It was found that there were interference signals only when NO2 existed (value about 0.02%). With the instrument, 6 days of field observation was performed at the Institute of Chemistry, Chinese Academy 1408 CHEN Lin et al.: Development of a Home-Made LOPAP for the Sensitive Detection of Nitrous Acid No.8

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