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Nitrous acid (HONO) is a typical secondary pollutant in the atmospheric environment, as amajor source of OH radicals, it makes significant contribution to the atmospheric oxidizingcapacity. In addition, it’s also a kind of indoor pollutants, which can react with organic aminerapidly to produce cancerogenic substance-nitrosamine which threatens human health.In this subject, a new cheap, sensitive and continuously working HONO monitor wasdeveloped. We performed a filed observation with the instrument and analyzed the diurnalvariation tendency of HONO and its source mechanism preliminarily. The conclusions obtainedwere showed below:The LOPAP was developed successfully. Analyzing the potential interferences and obtaineda consult that there was HONO signals only with the NO2existence. The real interference wasabout0.02%. The instrument adopts the two channel systems to exclude the interference of NO2and other gases as more as possible. The design of long path absorption improves the sensitivity.By changing different lengths of LWCC, the detection ranges can from9×10-3mg/m3to30mg/m3, it can meet not only the needs of the field observations but also laboratory studies. Theinstrument’s time resolution3-5min, with an accuracy of±10%. The long path absorptionphotometer had a good linearity and can be operated steadily for a long time. Except detectingHONO, it can also detect the NO2and O3in the atmosphere, the concentration of nitrite in thewater and the detection limit is0.25μg/L.The continuous field campaign was successfully performed with the LOPAP. By theobservation of HONO, NO2, the particle size distribution, the relative humidity, we studied theHONO concentration during one day and analyzed the HONO source mechanism preliminarily,especially the chemical reaction during night. And found that:(1) HONO had an obvious diurnalvariation tendency, it accumulated at night and reached the maximum values before sunrise;After the sunrise, its concentration reduced continually with the photolysis, reached theminimum values when the sunlight is strongest.(2) The contribution of direct emission toHONO was almost identical during day and night, however, the heterogeneous reaction of NO2to produce HONO mainly performed at night; during night, the heterogeneous reaction of NO2 mainly occurred on soot, when RH <65%, HONO/NO2and humidity had a positively correlatedrelationship，RH>65%, the relationship was negative.
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