Study On Nitrous Acid Chemistry In The Troposphere

Nitrous acid was first detected in the atmosphere by differential optical absorption spectroscopy (DOAS), since then many atmospheric measurements at various sites had been reported. During daylight hours, the rapid photolysis of HONO is a significant source of OH-radicals, which drive tropospheric chemistry and ozone-formation. Despite its importance, sources of HONO are still poorly understood. Direct emission of HONO or homogeneous chemical formation alone can not explain the high HONO-to-NO2 ratios often measured in the boundary layer. Today it is thus generally accepted that HONO is formed by heterogeneous hydrolysis of NO₂. However, large uncertainties about the importance of the ground or aerosol surfaces and the chemical conversion mechanism remain. The photolysis of HONO in the early morning hours is believed to be a significant source of OH- radical. Although the importance of this mechanism has been recognized for many years, no accurate quantification of this OH source is available, and the role of HONO photolysis is often underestimated. Up to now, there are not systemic researches on HONO in our country. This dissertation focuses on four parts as follows:1. The research on measurement method- differential optical absorption spectroscopy (DOAS). One of the major tasks was to develop a long path DOAS instrument with high quality. The work comprises mechanical construction of the system, improved retrieval approaches such as Fraunhofer structure research. Our DOAS measurements of NO₂, SO₂ and O₃ were compared with the conventional measurement instruments (API automatic monitoring instrument). The results showed that both systems exhibited strong compatibility with good correlation, therefore the DOAS system is able to provide reliable information on distribution patterns of major air pollutants.2. In this work, two field campaigns have been carried out in Tao Pu industrial park and Fudan University of Shanghai, which aimed at measuring of HONO by DOAS on different pollution levels. The first seasonal measurements of HONO using DOAS in China, to our knowledge, have been carried out. The results indicated that not only the HONO concentration but also HONO to NO₂ ratio has seasonal trend. The diurnal variation of HONO changed in the different season.3. The high relativity between PM10 concentration, aerosol density (retrieve from DOAS) and HONO concentration demonstrated that heterogeneous HONOproduction on the aerosol surface is the important source of atmospheric HONO. The HONO concentrations were found to scale with relative humidity in some ranges, and thus surface adsorbed water play an important role in the heterogeneous formations of HONO.4. The impact of HONO photolysis on the total OH budget in Shanghai were studied by modeling the photolysis frequencies of NO₂, HONO, O₃, HCHO. A comparison with the OH formation by photolysis of HONO, O₃ and HCHO shows that HONO photolysis contributed up to 14%(summer) - 34%(winter) of the total OH formed in a 24 hour period during our study. In the morning hours, HONO photolysis was by far the most important OH source.