| Summary: | 碩士 === 國立成功大學 === 環境工程學系碩博士班 === 90 === Gaseous nitrous acid (HONO) is one of odd nitrogen species (NOy) in the atmosphere. Nitrous acid is considered to play an important role in atmospheric chemistry, mainly due to its ability to produce OH-radicals and NO through direct photolysis in the early morning. Therefore, The OH-radicals are the main initiators of the chain-reactions which lead to photochemical smog. The main formation mechanism for HONO is the heterogeneous production on the surfaces of ground, buldings and soot.
The purpose of this study is to understand the interferences of HONO measurements during denuder sampling. At first, we want to ensure the quality of HONO artifact measurement. First, below half of the artifact nitrite forms before sampling denuder. Second, the concentration of organic soot as a sudstract for the NO2 heterogeneous reaction is below detection limit. On the other hand, the formation of HONO artifact cannot be inhibited by replacing soureces of soot in the sampling system. So the effect of soot can reasonably be negligible at gas sources. Therefore, it ensured that a small proportion of the HONO interferent is present at the concentrations encountered. Then we discuss the formation mechanism for HONO artifact. First, the exist of NO does not contribute to the formation of HONO artifact. So the formation mechanism for HONO artifact is not NO + NO2 + H2O → 2HONO. Second, Our experiment show that HONO which originates from NO2 heterogeneous reaction in the denuder is much more than HNO3. So 2NO2 + H2O → HONO + HNO3 is not the main formation mechanism for HONO artifact, too. In conclution, the main formation mechanism is the NO2 reducing reaction directly in the annular denuder. The concentration of artifact nitrite shows relative NO2 dependence with saturation reached at 40-50 ppb NO2. However, HONO artifact is independent of components of coating solution. If two denuders are placed in series, the amount of nitrite found in the first denuder is more than the second one. Moreover, HONO artifact kept constant with increasing sampling time. Finallly, Ion chromatographic analysis of samples showed that artifact nitrite in direct proportion to the concentration of organic oxidized formate. In conclusion, the reducing agent that can lead to the formation of artifact nitrite from NO2 is the organic material which exists in productive gas. At last, we discuss about the situation of atmosphere. The artifact nitrite correlates directly with relative humidity and SO2 concentration. In addition, during the experiment, it was shown the conversion of nitrite to nitrate in the presence of ozone. It is worth noting that artifact nitrite decreased with increasing flow rate. The artifact nitrate increased correlatively.
In the field experiment, field measurements had to be modified from laboratory studies. It can be seen that HONO diurnal cycles are similar whether the concentrations are modified or not. There appears to be a maximum concentration of HONO artifact at night in the polluted sites. But the highest effect of NO2 interference is observed around noon. It should be pointed out that the conversion of nitrite to nitrate in the presence of ozone does not proceed at an appreciable extent during field experiments. In addition, SO2 might have caused a positive artifact in our field experiments. In conclusion, a poor accuracy is achieved because the ratio of interference is beyond half of measurement. It is necessary to separate the species of interest from NO2 during sampling step and evaluate individual contribution to the total nitrite. Then, an accurate determination of nitrous acid can be performed.
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