HONO: A Study To Its Sources And Impacts From Field Measurements At The Sub-urban Areas Of PRD Region

After the first photochemical smog event in Lanzhou at the end of 1970s, high oxidant concentrations have been recognized in many developed areas of China. Nowadays, O3 has become one of the most severe air pollution problems in China. Understanding of the characteristics and mechanisms of photochemical pollution in China is hence of great importance not only to the atmospheric chemistry itself but also to the air pollution control. As one of the reactive nitrogen-containing compounds in the atmosphere, HONO (nitrou acid) influences the photochemistry through its impact on OH radicals. Though the importance of HONO as an OH precursor has been recognized for years, its chemical formation pathway is still not well understood. Current mechanisms used in air quality models can not re-produce the observed HONO, especially during the daytime. The Pearl River Delta (PRD) is one of the most polluted areas suffering from severe O3 pollution. As reveald by remote sensing, emission investigations and ground-base measureents, the PRD was characterized by high NO2 concentrations which implied a high HONO level. During the 2004 and 2006 field campaigns, Program of Regional Integrated Experiments of Air Quality over Pear River Delta (PRIDE-PRD), measurements of HONO were carried out at two sub-urban sites (Xinken and Backgarden) by Wet Denuder/Ion Chromatography system (WD/IC). High HONO concentrations were observed at Xinken and Backgarden with daytime concentration around 1 ppbv and 350 pptv respectively. The high daytime HONO led to large net OH production rates POH(HONO)net (2.5*107cm-3s-1 for Xinken and 0.9*107cm-3s-1 for Backgarden), which were 4.2 times (at Xinken) and 59% (at Backgarden) of the OH production rates by O3 photolysis. The facts that POH(HONO)net was large during the whole daytime is different from previous concepts that HONO was only important to OH in the morning. Our results were also observed at other places afterwards and were graduately accepted in peer reviews. To explore the potential formation mechanisms, the budget equation of HONO was solved. An unknown source RUnknown was derived and was much larger than ROH+NO , another source of HONO by the gaseous reactions of NO with OH. At Xinken and Backgarden sites, RUnknown are 3.0*107molecule cm-3s-1 and 7.5*106molecule cm-3s-1 respectively, which is 3-5 times faster than ROH+NO. The existence of RUnknown has increased the local ozone formation potential by 140% (Xinken) and 20% (Backgarden). Poor correlations (R =0.08 for Xinken and R=0.27 for Backgarden) were found between RUnknown and NO2, the precursor of HONO. The correlations were significantly improved by introducing photolysis rate values into account. The correlation coefficients between the unknown source RUnknown and J(NO2)×NO2 increased to 0.56 and 0.40 implying a photo-enhanced process. The photo-enhanced process was then proved in laboratory studies. For the nighttime, the influences of physical process to the HONO varation were recognized and an improved method was proposed in the calculation of HONO formation rate. This research shows the existence of high HONO and its importance in the atmospheric photochemistry of the PRD sub-urban areas. The analysis of POH(HONO)net and RUnknown has triggered a deep discussion of HONO chemistry and will finally contribute to the decision making process about O3 contronl strategy .