Since NO₃ is present at significant levels during the night, NO₃ reacts with various VOCs, such as monoterpens, generating peroxy radicals and nitric acid. In addition, N₂O₅, formed through a reaction between NO₃ and NO₂, reacts heterogeneously with water vapor on aerosol surfaces yielding nitric acid. These are the main loss processes of NO_x at night. Thus, measurement of NO₃ and N₂O₅ is essential for an understanding of nocturnal chemistry. We have developed a new instrument to measure NO₃ which is fast, sensitive, and simple in technique, laser-induced-fluorescence (LIF). This instrument can detect not only NO₃ but also N₂O₅ by heating air samples just before the sample inlet. The limits of detection (LOD) for NO₃ and N₂O₅ are determined to be 5 and 7 pptv, respectively for 10 minute intervals.
Ambient N₂O₅ at Tokyo Metropolitan University in December 2003 was successfully measured with this instrument for the first time. N₂O₅ was observed at significant concentration even when NO was large. Assuming that the heterogenenous reaction of N₂O₅ is predominant in the NO_x removal reaction in winter, the amount of NO_x removal through N₂O₅ is estimated as 5.4 ppbv/night. It is comparable to the daytime NO_x removal by the OH radical. Thus, the N₂O₅ reaction is demonstrated to be significant as a source of nitric acid.

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An estimation of the annual deposition of sulfur oxides was made using disclosed data of the Acid Deposition Monitoring Network in East Asia in 2001. In order to estimate dry deposition, deposition velocities of sulfur dioxide and non-sea-salt sulfate were calculated at each site using world meteorological data and land use data from the United States Geological Survey. Dry and wet depositions were compared with anthropogenic sulfur emissions at each site to evaluate its budget. Deposition velocities that were estimated in each land use type were within the parameters of previous studies. The deposition of sulfur oxides was higher at Chinese sites than at the other sites. At an urban site in Mongolia, two inland sites in China and two urban sites in Southeast Asia, the sulfur emissions were clearly higher than the depositions; on the other hand, at a coastal site in China, the sulfur emissions were clearly lower than the depositions. The former indicates outflow from the areas; the later indicates inflow to the area. When comparing remote areas in Mongolia, Japan and Malaysia, an inflow of the sulfur oxides from other areas was found in the Japanese area.

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Measurements of dry deposition for acidic substances were conducted by an aerodynamically designed disktype surrogate surface with a 300mm diameter mounted with three filter holders. A 6% K₂CO₃ impregnated quartz fi lter for SO₂, a Nylon filter for HNO₃, and a Teflon filter for SPM were used to measure dry deposition and atmospheric concentration of these substances. Simultaneous observation of meteorological parameters was also conducted to estimate aerodynamic resistance ra and boundary layer resistance rb near the ground surface.
The results of the observations showed that the dry deposition of SO₂ increased in the cold season and decreased in the warm season, and the dry deposition of HNO₃ increased in the warm season and decreased in the cold season. These seasonal variations coincided with the concentrations of SO₂ and HNO₃ in the lower atmosphere. On the basis of the simultaneous observation of deposition flux F and concentration C, deposition velocities vs of SO₂ and HNO₃ were estimated, as F/C. Estimated deposition velocities were about 0.6cm s^-1 for SO₂ and 1.1cm s^-1 for HNO₃; these values agreed with recent reported values obtained by various methods. On the basis of weather observation, surface resistance rc was estimated as 1/v_s-(r_a+r_b). Estimated surface resistance was about 0.7s cm^-1 for SO₂, and nearly equal to zero for HNO₃. The validity of using the surrogate method to study the transport and effects of acidic substances was also discussed.

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Measurements of the chemical composition of fog water on Mt. Tateyama were performed during the autumn season of 2003. The ranges in pH at Bijodaira, mid-slope of Mt. Tateyama, and at Murododaira, near the summit of Mt. Tateyama, were from 4.1 to 5.0 and from 3.3 to 5.1, respectively. The ratio of sea-salt components was high at Bijodaira because Mt. Tateyama is located near the coast of the Japan Sea. The raio of NO₃^-/nss-SO₄^2- (N/S) was higher at Bijodaira than at Murododaira. Strong acidic fogs (pH< 4) were sometimes observed at Murododaira. A high concentration of sulfate was detected in the acidic the fog water. The lowest pH of fog water was observed on 19 September. The backward trajectory analysis showed that the air sampled at Murododaira originated from the pollutant regions of the Asian continent.

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monitoring program for PAHs was conducted for three periods January, April, and July & October (these represent, respectively, the heating period for homes from November to March, the spring-sandstorm period from March to May and the non-heating period for homes from June to October in Yinchuan). The monitoring program for PAHs started in January 1989 and was carried out in 7 specific areas including urban residential, industrial, commercial, cultural & educational, public parks, suburban, and heavy traffic roads in Yinchuan, the capital city of the Ningxia Hui Autonomous Region. The aim of the study was to determine temporal variations of PAHs and their distribution in specific areas and the concentration of particle size-segregated PAHs. A total of 210 samples were collected and 6 PAHs species were identified, they are Phenanthrene (PHEN), Pyrene (PYR), Chrysene (CHR), Perylene (PER), Benzo [a] pyrene (Bap) and Benzfluorence (BFLU).
It is documented that the concentration of PAHs is the highest in January, the PAHs mainly come from coal burning for heating and vehicle emissions. The concentration of PAHs of the 7 specific areas are arranged in decreasing order: suburbs, heavy traffic roads, urban residential areas, commercial centers, public parks, cultural & educational areas and industrial areas. Bap concentration is ranged from 6.05-29.55 ng/m³ in January, 3.23-13.70ng/m³ in April, which is the spring-sandstorm period, and 0.30-8.70ng/m³ from July to October, which is the non-heating period. Bap has been detected in the specific areas, except for the suburbs in the July through October period. The PHEN, PYR, BFLU and PER are higher in January and have been detected in each specific area. PHEN, PYR and CHR were not detected in the cultural & educational areas nor in the parks in the July & October Period. More than 50% of the PAHs are absorbed in particles less than 2.5μm in diameter with few exceptions.
Concentration of PAHs is higher in Yinchuan than that in other places concerned in this paper, so great efforts should be made to improve air quality in this region.

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