Nitrous acid (HONO) plays a key role in tropospheric photochemistry, primarily due to its role as a source of hydroxyl (OH) radicals via its rapid photolysis. OH radicals are involved in photooxidation processes, such as the formation of tropospheric 03 and other secondary atmospheric pollutants (peroxyacetyl nitrate/PAN] and secondary particles). Recent field and modeling studies have postulated the occurrence of a strong and unknown daytime HONO source, but there are still many significant uncertainties concerning the identification and formation mechanisms of these unknown sources. Up to now, five HONO formation pathways are known: direct emission, homogeneous gas-phase reactions, heterogeneous reactions, surface photolysis; and biological processes. In this review paper the HONO sources proposed to explain the observed HONO budget, especially during daytime, are discussed, highlighting the knowledge gaps that need further investigation. In this framework it is crucial to have available accurate and reliable measurements of atmospheric HONO concentrations; thus, a short description ofHONO measurement techniques currently available is also reported. The techniquesare divided into three basic categories: spectroscopic techniques, wet chemical techniques, and off-line methods.

Abstract. The atmospheric concentrations of gaseous ammonia have been measured during two field campaigns in the winter and in the summer of 2007 at Beijing (China). These measurements were carried out by means of diffusion annular denuders coated with phosphorous acid. The results were discussed from the standpoint of temporal and diurnal variations and meteorological effects. The daily average NH 3 concentrations were in the range of 0.20-44.38 µg/m 3 and showed regular temporal variations with higher concentrations during summer and with lower during winter. The temporal trends seemed to be largely affected by air temperature because of agricultural sources. No diurnal variability was observed for gaseous NH 3 levels in both winter and summer seasons. The highest ammonia value of 105.67 µg/m 3 was measured in the early morning during the summer period when stable atmospheric conditions occurred. The diurnal winter and summer trends of ammonia showed a weak dependence on the air temperature and they were affected nearly by wind direction suggesting regional and local source influences. Ammonia was also correlated with the atmospheric mixing in the boundary layer, and, with NO x , CO and PM 2.5 air concentrations supporting the hypothesis that the traffic may be also an important source of ammonia in Beijing.

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Abstract. The atmospheric concentrations of gaseous HNO 3 , HCl and NH 3 and their relative salts have been measured during two field campaigns in the winter and in the summer of 2007 at Beijing (China), as part of CARE-BEIJING (Campaigns of Air Quality Research in Beijing and Surrounding Region). In this study, annular denuder technique used with integration times of 2 and 24h to collect inorganic and soluble PM 2.5 without interferences from gas-particle and particle-particle interactions. The results were discussed from the standpoint of temporal and diurnal variations and meteorological effects. Fine particulate Cl − , NH 4 were higher during summer (12.30 µg m −3 and 18.24 µg m −3 , respectively) than during winter (6.51 µg m −3 and 7.50 µg m −3 , respectively). Daily mean concentrations of fine particulate Cl − were higher during winter (2.94 µg m −3 ) than during summer (0.79 µg m −3 ), while fine particulate NO − 3 showed similar both in winter (8.38 µg m −3 ) and in summer (9.62 µg m −3 ) periods. The presence of large amounts of fine particulate NO − 3 even in summer are due to higher local and regional concentrations of NH 3 in the atmosphere available to neutralize H 2 SO 4 and HNO 3 , which is consistent with the observation that the measured particulate species were neutralized. The composition of fine particulate matter indicated the domination of (NH 4 ) 2 SO 4 during winter and summer periods. In addition, the high relative humidity conditions in summer periodCorrespondence to: A. Ianniello (ianniello@iia.cnr.it) seemed to dissolve a significant fraction of HNO 3 and NH 3 enhancing fine particulate NO − 3 and NH + 4 in the atmosphere. All measured particulate species showed diurnal similar patterns during the winter and summer periods with higher peaks in the early morning, especially in summer, when humid and stable atmospheric conditions occurred. These diurnal variations were affected by wind direction suggesting regional and local source influences. The fine particulate species were correlated with NO x and PM 2.5 , supporting the hypothesis that traffic may be also an important source of secondary particles.

Abstract. The atmospheric concentrations of gaseous HNO3, HCl and NH3 and their relative salts have been measured during two field campaigns in the winter and in the summer of 2007 at Beijing (China), as part of CAREBEIJING (Campaigns of Air Quality Research in Beijing and Surrounding Region). In this study, annular denuder technique was used with integration times of 2 and 24 h to collect inorganic and soluble PM2.5 without interferences from gas-particle and particle-particle interactions. The results were discussed from the standpoint of temporal and diurnal variations and meteorological effects. Fine particulate Cl−, NH4+ and SO42− exhibited distinct temporal variations, while fine particulate NO3− did not show much variation with respect to season. Daily mean concentrations of fine particulate NH4+ and SO42− were higher during summer (12.30 μg m−3 and 18.24 μg m−3, respectively) than during winter (6.51 μg m−3 and 7.50 μg m−3, respectively). Instead, daily mean concentrations of fine particulate Cl− were higher during winter (2.94 μg m−3) than during summer (0.79 μg m−3), while fine particulate NO3− showed similar variations both in winter (8.38 μg m−3) and in summer (9.62 μg m−3) periods. However, the presence of large amounts of fine particulate NO3− even in summer are due to higher local and regional concentrations of NH3 in the atmosphere available to neutralize H2SO4 and HNO3, which is consistent with the observation that the measured particulate species were neutralized. Indeed, the composition of fine particulate matter indicated the domination of (NH4)2SO4 during winter and summer periods. In addition, the high relative humidity conditions in summer period seemed to dissolve a significant fraction of HNO3 and NH3 enhancing fine particulate NO3− and NH4+ in the atmosphere. All measured particulate species showed diurnal similar patterns during the winter and summer periods with higher peaks in the early morning, especially in summer, when humid and stable atmospheric conditions occurred. These diurnal variations were affected nearly by wind direction suggesting regional and local source influences. Indeed, the fine particulate species were also correlated with NOx and PM2.5, supporting the hypothesis that the traffic may be also an important source of secondary particles.