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 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⁻, 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). 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 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 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. 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 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.

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 NH₃ concentrations were in the range of 0.20–44.38 μg/m³ 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 NH₃ levels in both winter and summer seasons. The highest ammonia value of 105.67 μg/m³ 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.

Snow, through its trace constituents, can have a major impact on lower tropospheric chemistry, as evidenced by ozone depletion events (ODEs) in oceanic polar areas. These ODEs are caused by the chemistry of bromine compounds that originate from sea salt bromide. Bromide may be supplied to the snow surface by upward migration from sea ice, by frost flowers being wind-blown to the snow surface, or by wind-transported aerosol generated by sea spray. We investigate here the relative importance of these processes by analyzing ions in snow near Alert and Ny-Ålesund (Canadian and European high Arctic) in winter and spring. Vertical ionic profiles in the snowpack on sea ice are measured to test upward migration of sea salt ions and to seek evidence for ion fractionation processes. Time series of the ionic composition of surface snow layers are investigated to quantify wind-transported ions. Upward migration of unfractionated sea salt to heights of at least 17cm was observed in winter snow, leading to Cl- concentration of several hundred µM. Upward migration thus has the potential to supply ions to surface snow layers. Time series show that wind can deposit aerosols to the top few cm of the snow, leading also to Cl- concentrations of several hundred µM, so that both diffusion from sea ice and wind transport can significantly contribute ions to snow. At Ny-Ålesund, sea salt transported by wind was unfractionated, implying that it comes from sea spray rather than frost flowers. Estimations based on our results suggest that the marine snowpack contains about 10 times more Na+ than the frost flowers, so that both the marine snowpack and frost flowers need to be considered as sea salt sources. Our data suggest that ozone depletion chemistry can significantly enhance the Br- content of snow. We speculate that this can also take place in coastal regions and contribute to propagate ODEs inland. Finally, we stress the need to measure snow physical parameters such as permeability and specific surface area to understand quantitatively changes in snow chemistry.

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 O₃ 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 of HONO measurement techniques currently available is also reported. The techniques are divided into three basic categories: spectroscopic techniques, wet chemical techniques, and off-line methods.

Measurements of atmospheric and snow mixing ratios of nitrates and nitrites and their fluxes above the snow surface were made during two intensive campaigns during spring time 2001 at Ny-Ålesund, Svalbard as part of the EU project "`The NItrogen Cycle and Effects on the oxidation of atmospheric trace species at high latitudes" (NICE). At this coastal site close to the unseasonably unfrozen fjord, of the measured nitrogen species, only HNO₃ showed a significant flux on to the snow surface; a mean deposition of -8.7 nmol h^-1 m^-2 was observed in late April / early May 2001. These fluxes may be due to the reaction of HNO₃ with sea salt, and especially NaCl, or may be simply uptake of HNO₃ by ice, which is alkaline because of the sea salt in our marine environment. During snowfall periods dry deposition of HNO₃ may contribute up to 10% of the N budget in the snow; however, the main source for N is wet deposition in falling snow. The surface snow at Ny-Ålesund showed very complex stratigraphy; the NO₃^- mixing ratio in snow varied between 65 and 520 ng g^-1, the total NO₃^- content of the snowpack was on the order of 2700 ng cm^-2. In comparison the atmospheric boundary layer column showed a NO₃^- content of only 8 ng cm^-2. The limited exchange, however, between the snow and the atmosphere was attributed to low mobility of NO₃^- in the observed snow. Contrary to other Arctic sites (i.e. Alert, Nunavut or Summit, Greenland) deposition of sea salt and crustal aerosols in this marine environment made the surface snow alkaline; snow NO₃^- was associated with heavier cations and was not readily available for physical exchange or photochemical reactions.