2. Measurements of Ozone (O3), Nitrogen Oxides (NOx), and Sulfur Dioxide (SO2) During the 1996-1998 IGAC/APARE/PEACAMPOT II Aircraft Observations
Shiro Hatakeyama (National Institute for Environmental Studies)
Hiroshi Bandow (Department of Technology, Osaka Prefecture University)
Ozone in the background of the troposphere is an important compound that affects the oxidizing capacity of the atmosphere. It is also important as a greenhouse gas which contributes to global warming. Gaining particular attention are recent reports from Europe and the United States that ozone in the troposphere in the northern hemisphere is increasing1),2). It has been pointed out that NO2 is the only precursor of ozone in the troposphere, and that the increasing human-initiated emissions of NOx gases, including NO, are causing this increase in ozone concentrations. Thus, ozone and NOx are, from various perspectives, important factors that govern the chemistry in the troposphere, and the identification of their concentration distribution in three dimensions is indispensable in understanding the changes in the troposphere.
At the same time, SO2 plays an important role as a precursor of sulfuric acid that causes acid rain. Although it is presumed that there is advection of a great quantity of SO2 from East Asia, including China and Korea3)-6), there have hitherto been few cases in which this was actually observed continuously over the Sea of Japan and the East China Sea.
In our previous work from 1991 to 1995 we carried out experiments over the Sea of Japan, the Yellow Sea, and the East China Sea. Observation data for those aerial experiments are reported in papers7)-10) and in a CD-ROM11).
The purpose of our aircraft observations in fiscal year 1996-1998 was to clarify the state of the atmospheric pollutants in the skies over the northern East China Sea from the offshore area of the Fukue Island, Japan to the offshore area of Cheju Island, Korea. The air mass transported from eastern central China was the main target, since there are a number of large-scale sources of atmospheric pollutants.
(2) Observation Method
The measuring instruments used in the aircraft observations and their dimensions are the same as those used in previous years, and are listed below:
Ozone: The ozone monitor was based on the UV absorption method (Model 49, Thermo Electron) with a single illuminant and the double beam dual cell method. This monitor was modified into a 4-second switchover high-speed response type. Both pressure and temperature were automatically corrected. Detection limit was 2 ppb. The ozone monitor was calibrated using an ozone calibrator (Model 49PS, Thermo Electron) owned by NIES as the standard.
Nitrogen Oxides: A nitrogen oxide analyzer based on the NO2 chemiluminescence method, which itself is based on the reaction of NO + O3 -> NO2*, was modified for use in aircraft observations. The main modifications were the introduction of the sample air intake method (800 SCCM) using a mass flowmeter and a method to generate ozone for reaction using pure oxygen, as well as reduction of the pressures of the reactor (absolute pressure of 43-39 Torr) to improve the efficacy of chemiluminescence. A metallic molybdenum reducing agent (with a reaction temperature of 320° C) supported on a carrier was used to convert NOx to NO. Contamination by background emissions was eliminated by an automatic switching of the zero emission mode that passes through the front end reactor (the measurement time was 30 seconds for each cycle, which consisted of NO, NOx and zero measurements). Integration was conducted for two minutes to average the data. Under these conditions, the detection limit of nitrogen oxides was 25 pptv (S/N = 1). It must be noted that the values obtained during the first hour after the commencement of each flight were smaller than the other values by about 5% - 15% due to the insufficient cooling of the photomultiplier (based on the thermoelectric cooling method). Accordingly, the data obtained during this hour had errors of ± 15%.
The analyzer was calibrated by using a commercial NO standard gas diluted by nitrogen. The measurement error during the observations was about 5% or less except during the one hour immediately after the commencement of each flight.
Sulfur Dioxide: Analysis was conducted continuously using an automatic analyzer (Model 43S, produced by Thermo Electron) which is based on the UV pulse fluorescence analysis method. Nevertheless, in this fiscal year, a problem occurred with the control of the sample flow velocity of the measurement device, and the sample flow velocity varied depending on altitude (air pressure). Since this reduced the reliability of the absolute values of the measured data, the flow velocity was corrected by checking the pressure and the sample flow velocity during the observation flight. In addition, using the data of SO2 concentrations measured based on the diffusion scrubber method by a group from Keio University, correction was conducted to average the observation data obtained by the continuous measurement device mentioned above. During this time, a diffusion scrubber conducted sampling to coincide with the SO2 concentrations obtained by the diffusion scrubber method.
The analog output (0 - 1 V) data from three measuring instruments were recorded by a small data recorder (DR-F3, TEAC), and were read by a reader program created by our laboratory. The raw data were recorded at 0.1 Hz. Tables 1-8 show the mean values per minute, while Figures1-8 show the data of NOx, SO2, and ozone plotted out along with altitudinal profiles.
1) J.K. Angell and J. Korshover, J. Climate Appl. Meteorol., 22, 1611 (1983).
2) J.A. Logan, J. Geophys. Res., 90, 10463 (1985).
3) H. Rodhe, Ambio, 18, 155-160 (1989).
4) J.N. Galloway, Ambio, 18, 161-166 (1989).
5) N. Kato and H. Akimoto, Atmos, Environ., 26A, 2997-3017 (1992).
6) H. Akimoto and H. Narita, Atmos. Environ., 28A, 213-225 (1994).
7) S. Hatakeyama, K. Murano, H. Bandow, F. Sakamaki, M. Yamato, S. Tanaka, and H. Akimoto, J. Geophys. Res., 100, 23143-23151 (1995).
8) S. Hatakeyama, K. Murano, H. Bandow, H. Mukai, and H. Akimoto, Terre. Atmos. Oceanic Sci., 6, 403-408 (1995).
9) S. Hatakeyama, K. Murano, H. Mukai, F. Sakamaki, H. Bandow, I. Watanabe, M. Yamato, S. Tanaka, and H. Akimoto, J. Aerosol Res. Jpn., 12, 91-95 (1997).
10) I. Watanabe, M. Nakanishi, J. Tomita, S. Hatakeyama, K. Murano, H. Mukai, and H. Bandow, Environmental Pollution, 102, S1,.253-261 (1998).11) S. Hatakeyama, Ed. "Data of IGAC/APARE/PEACAMPOT Aircraft and Ground-based Observations '91-'95 Collective Volume", CGER-D014(CD)-'98, Center for Global Environmental Research, National Institute for Environmental Studies, Japan, 1998.