The Pacific Rim region of East Asia is characterized by high and rapidly growing anthropogenic emissions resulting from the high population and rapidly growing energy consumption. The main objective of IGAC's East Asian-North Pacific Regional Experiment (APARE) is to study the impact of human activity in this peculiar region on the marine air over the Pacific. One of the targets of APARE is tropospheric ozone and this report focuses on this aspect among other themes.
Increase of tropospheric ozone as observed in various regions of the Northern Hemisphere is of particular concern in regard to East Asia. Firstly, still rapidly growing emissions of ozone precursors (nitrogen oxides, carbon monoxide and non-methane hydrocarbons (NMHC)) from this region would be a major contributor to the expected ozone increase on a hemispheric scale in the coming decades. Because it is a greenhouse gas, the resulting ozone could affect the global climate. Secondly, increase of near-surface ozone, together with acid deposition, could be a potential threat to terrestrial ecosystems and to agricultural productivity, which may affect food supply adversely.
In order to study chemical processes and long range transport of trace species in the Western Pacific, the NASA PEM-West (Pacific Exploratory Mission-West), the Japanese PEACAMPOT (Perturbation by East Asian Continental Air Mass to Pacific Oceanic Troposphere), CATS (Climate and Air Quality Taiwan Station) in Taiwan, and the Hong Kong Monitoring Station (Phase B) were coordinated under APARE. The major scientific objective was to evaluate the budget of ozone in the western Pacific and to characterize long range transport of ozone in the East Asian Pacific Rim region. The first phase of these missions, Phase A, was conducted during September-October, 1991, when the region was under the influence of relatively clean Pacific air. The second phase, B, was conducted during February-March, 1994, when the influence of continental outflow was near its annual maximum. The PEM-West mission included intensive airborne measurements of trace species from the NASA DC-8 aircraft (see Figure 1) coordinated with flights of the Japanese Cessna-404 aircraft and ground based stations (see Figure 2) operated in coordination with the above three campaigns. This article reports some major findings of Phase A.
Figure 1. Flight tracks for the DC-8 aircraft during the PEM-West A mission.
The PEM-West A data set is best described in terms of two geographical domains: the Western North Pacific Rim (WNPR) and the Western Tropical North Pacific (WTNP) when examined in terms of photochemical ozone precursors such as NOx (=nitric oxide (NO) + nitrogen dioxide) distributions. The WNPR region is one that was influenced by both natural and anthropogenic continental sources. High-altitude outflow from Asia as well as from other Northern Hemisphere continents appears to have been involved. By contrast, the WTNP regime can be viewed as a region whose chemical fingerprint reflected either relatively clean tropical/equatorial Pacific air masses or aged, well-processed continental air. In all cases the photochemical destruction of ozone, D(ozone), was found to decrease more rapidly with altitude than photochemical formation, F(ozone). Thus the ozone tendency, P(ozone) as defined by F(ozone)-D(ozone), typically was negative at low altitudes (e.g., < 6 km) but positive for altitudes >6 to 8 km. The most important chemical factor controlling the altitude trend in D(ozone) was the water vapor mixing ratio. The trend in F(ozone) with altitude showed very modest decreases, reflecting the fact that decreases in HOx (=hydroxyl + hydroperoxyl) radical levels with altitude were substantially offset by increases in the mixing ratio of NO. For altitudes <4 km the two most important ozone formation processes were identified as reactions of NO with hydroperoxyl and methylperoxyl radicals; whereas for altitudes >4 km reaction of NO with hydroperoxyl was the dominant process. This observation indicates that NMHC emissions were typically of minor importance as ozone precursor species during the time period of PEM-West A.
Figure 2. Locations of collaborating ground-based sites during the Pacific Exploratory Mission-West A (PEM-West A).
Diurnal-averaged, column-integrated photochemical formation and destruction fluxes for the WNPR region were shown to exceed those for NH dry deposition and NH stratospheric injection by a factor of nearly 6. For this same region a near balance was found between photochemical ozone production and destruction, suggesting that this region was near steady state. Ozone column lifetime arguments, together with small seasonal changes in total column ozone, suggest that the WTNP should also have been near steady state. In fact, the column-integrated fluxes show that photochemical destruction exceeded production by nearly 80%. Two hypotheses were put forward in an effort to explain this deficit. The first involves the possibility that ozone-rich air could have been transported from mid-latitudes into the tropics; the second proposes that the unsampled atmospheric column from 10 to 17 km might have provided the additionally needed photochemical F(ozone). The latter hypothesis requires relatively high levels of NO (e.g., 150 pptv); however, these do not appear to be totally out of line with those estimated to be produced by tropical lightning. In this context, results from the present study indicate that NOx would have an extended lifetime of 3 to 9 days at altitudes of 8-12 km and even longer for still higher altitudes. This suggests that for some seasons of the year, NOx produced from the deep convection over regions of Asia and Malaysia/Indonesia could lead to significant enhancements in high-altitude ozone formation that might extend well out into the North Pacific.
A synoptic analysis of the PEM-West A database by several different investigating groups resulted in five different air mass classification schemes. These were examined in terms of their respective values of P(ozone). The general trend that emerged showed that the largest positive values occurred for continental boundary layer air, within 2 days of mainland Asia or Japan and for high-altitude air parcels (e.g., >7 km) influenced by deep convection/lightning. Significant negative values of P(ozone) were found when encountering clean marine boundary layer air or relatively clean lower free-tropospheric air parcels.
The ground based observations at selected remote sites near the East Asian Pacific Rim, including Oki Island (Japan), Okinawa (Japan) and Kenting (Taiwan), revealed that surface ozone can be characterized essentially by four types of air masses. One is a continental clean air mass (CCAM) coming down directly through the area with relatively low anthropogenic emission intensity over far eastern Siberia. The CCAM is thought to be of representative of air unperturbed by strong anthropogenic emissions in East Asia. The second is a continental polluted air mass (CPAM) which passes through high anthropogenic emission areas of either coastal China, the southern part of the Korean Peninsula, and/or a part of Japan. The average mixing ratio of ozone in the CPAM was higher than that in the CCAM by about 7 ppbv during the campaign period. The difference is thought to be due to ozone buildup in the boundary layer air after passing through the area of strong anthropogenic emissions, which is in accord with data based on aircraft measurements described above. The third is a marine Pacific air mass (MPAM) which contained the lowest concentration of ozone, typically about 10 ppbv but often less than a few ppbv. The fourth is a marine South China Sea air mass (MSAM) which contained typically about 20 ppbv of ozone during the observed period. The CCAM was sampled only at the northern-most station, Oki Island, while CPAM often covered all the sites from Japan to Taiwan in this season. The MPAM was a common air mass at Okinawa and Taiwan and the MSAM reached only to the southern-most station at Kenting.
In summary, photochemistry and the ozone budget over the western North Pacific were studied in detail for the first time during the APARE campaigns. The findings will provide a scientific base for evaluating the influence of future increase of anthropogenic emissions on the ozone budget and its concentrations from the upper troposphere to the boundary layer in this region. The scientific papers and the names of science team members who made the measurements and developed the findings summarized in this article can be found in a special section of the January 1996 issue of the Journal of Geophysical Research - Atmospheres (Vol. 101, No. D1).
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