STUDY ON HUMAN ACTIVITY INDUCED
I. Greenhouse Gases and Aerosols
WANG Mingxing and YANG Xin
State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry,
Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029,China
Extensive researches on the sources and sinks of greenhouse gases, carbon cycle modeling, and the characterization of atmospheric aerosols have been carried out in China during the last 5 years or so. This paper will present the major achievements in the fields of emissions of greenhouse gases from agricultural lands, carbon cycle modeling, the characterization of Asian mineral dust, source identification of the precursors of the tropospheric ozone, and the observations of the concentrations of atmospheric organic compounds. Special more detailed information on the emissions of methane from rice fields and the physical and chemical characteristics of the mineral aerosols are presented.
Key words: greenhouse gases, aerosol, dust, ozone
Among the many effects of human activities on climate change, the global scale changes of atmospheric composition due to emissions of industrial activities are most significant and obvious facts. Recent measurements show that the concentration of atmospheric CO2 has increased from 280 ppm before the industrial era up to 367 ppm in 1999; that of atmospheric methane has increased from 0.8 ppm before the industrial era up to 1.75 ppm in 1998; that of N2O has increased from 0.28 ppb before the industrial era up to 0.33 ppb in 1998. At least part of these observed changes may be attribute to the human activities (1). It is generally accepted that the observed changes of atmospheric concentrations of greenhouse gases and aerosols are important causes for the observed climate changes and may further modify the climate in the future. Therefore, study on the causes of these observed changes of atmospheric concentrations of greenhouse gases and aerosols and the prediction of their changing trend is of fundamental importance. During the last 5 years or so, China has carried out many researches in this fields. This paper will summarizes major results.
II. RESEARCHES ON GREENHOUSE GASES EMISSIONSl. Automatic Observational System of Greenhouse Gases Emission
An automatic sampling and analyzing system for methane emission fluxes, which was designed and assembled by our group on the basis of static sampling and a gas chromatography (GC)-flame ionization detector (FID) analyzing techlique[2,3], was employed for continuous measurement in rice field. Furthermore, we also developed automatic sampling and analyzing systems for N2O and NO emission. Using this system, the emission of CH4, N2O and NO from cropland can be simultaneously measured. This system can simultaneously measure 16-32 different fields emission data, 8-12 times per day. It is a usual tool to study the diurnal variation of methane emission in different fields. Since 1985, we have conducted continuous measurements of methane emission in the major rice culture regions in China and obtained lots of valuable methane emission data.
2. Mechanisms on the Production, Oxidation, Transportation of CH4 in Rice Fields
Methane emission rates from Chinese rice fields have been measured in all five major rice culture regions in China. Four types of diurnal variations of CH4 emission rates have been found. Seasonal variation patterns of CH4 emission differ slightly in different field locations, where climate system, cropping system and other factors are different. CH4 production mainly occurs in the reduced soil layer (2-20 cm). CH4 is oxidized mainly in the thin surface layer of paddy soi1, and the hizosphere of rice plants. Production and oxidation rates are affected by many factors: CH4 transport through rice plant, gas bubble and diffusion in flooded water. Relative importance of each routine is different in different stages during rice growing. The effects of various mineral fertilizers on CH4 emission are rather contradictory, while the amount of organic manure will enhance CH4 emission from rice fields, which has been also indicated by CH4 production rates. Application of fermented sludges from biogas generators and farmyard-stored manure instead of fresh organic manure seems to be promising.
3. Model Study of Methane Emission from Rice Paddies
Developing methane emission model is an effective approach to accurately estimate regional and global methane emissions. It is also a useful tool to develop mitigation methods of methane emission from rice fields. At present, the numerical modeling is still in a preliminary stage. The first model was developed in 1995. Based on the 13-year field experiments and studies on methane emission from rice fields, we developed the first version of methane emission model in China . This model was tested by our experimental data, and was proved to have the ability to simulate diurnal variations of CH4 emission and estimate the total emission of methane from rice fields. In recent years, the methane emission model has made some new progress. We are improving our model in order to correctly describe the effects of environmental factors, such as climate, soil properties, fertilizer application, water regime and rice cultivars on processes of methane production, transportation and oxidation and precisely predict the methane emission variations under ongoing climatic changes. Using this model, the mitigation options of methane emission from rice fields can be evaluated and the valuable mitigation methods can be provided to the government.
4. Methane Emissions from China's and Global Rice Fields
Based on the 13-yearsfield experiment in all the five typical rice field and model simulation results, China's rice fields contribute 9.67-12.66 Tg/a to the atmosphere [6,10], much less than the estimates of 30-50 Tg/a made by other studies before . If we extrapolate the measured data in China with consideration of measured data in other countries, the total global emission of CH4 from rice is estimated to be 20-40 Tg/a[6,10]. This value has been gradually accepted and cited. In the new report of Intergovernmental Panel on Climate Change (IPCC), the methane emission from global rice fields has changed from1990s estimated value of 110 Tg/a to 60 Tg/a.
5. Mitigation Technologies of Methane Emission from Rice Fields
Based on the 13-year field experiments, we have found that agriculture management is one of the most important factors in determining methane emission from rice fields. Appropriate and scientific readjustment of agriculture management can reduce methane emission without lowering rice yield. Particularly, intermittently flooded irrigation and usage of biogas residues instead of fresh organic fertilizer are the most worthy techniques in reducing methane emission [14-16].
6. Emission Characters of N2O and NO in Croplands
Based on a four-year in situ measurement of soil moisture and nitrous oxide (N2O) emission from a rice-wheat rotation ecosystem of Southeast China and simulated experiments in laboratory, the impact of soil moisture on N2O emission was investigated. Soil moisture is the most sensitive factor to regulate N2O emission from croplands. Optimum emission of N2O from the rice-based ago-ecosystem was found to happen at the soil moisture of 99% water-filled pore space (WFPS), which is different from the 75% WFPS values of soil moisture in grassland and forest . By analyzing the experimental data, two typical patterns of NO diurnal emission from wheat fields were discovered. They are the day-peak pattern and night-peak pattern. For the former, the maximum emission takes place in the early aftenoon and the emission at night keeps a relatively low but stable level. This day-peak pattern is closely related to the temperature variation. For the night-peak pattern，the maximum emission occurs during the period of 18－24 o'clock while the minimum emission occurs in the early afternoon. The intensive up-take of available N (NH4+-N) via wheat plant reduces the NH4+-N supply for nitrification microbes, as a result, less NO is emitted. This principle of competition for available N between plant roots and soil microbes determines the diurnal pattern of NO emission.
III. GLOBAL CARBON CYCLINGAt present, the unbalanced carbon budget is a hotspot in carbon research, which means that there is a large “missing carbon sink”. Analysis results for 1980s indicate that the “missing sink” is about 1.3 PgC/a[l8,19] (see following equation, unit: PgC/a, 1Pg=1 Gt=1015g).
Fossil + Land-use =Atmospheric increase + Northern forests + Oceans + Residual sink
55(士0.5)+1.6(士0.7)=3.2(士0.2) +0.6(±0.6) +2.0(士0.8)+13(士1.1)
Balancing of the carbon sources and sinks becomes the critical problem in the fields of climate and biogeochemistry, and is an important prerequisite in predicting the future climate. Although there are many“proofs”suggesting that the terrestrial ecosystem absorb more carbon from atmosphere [20,21], direct measurement data are still scarce and dispersed. Many researches indicated that ocean plays an important role . So, ocean and land ecosystems are two key factors needing more investigation.
l. The Relationship between Terrestrial Carbon Fluxes and Climate
Increasing in atmospheric CO2 concentration is the most important factor influencing climatic change which is mainly due to human activities. It is believed that climate changes, carbon and nitrogen fertilization effects are the three main factors affecting net carbon fluxes of the land ecosystem. At present, more attention has focused on the climate effect, as warming would induce more CO2 release from some soil types and this would enlarge the “missing sink” to some extent. The observed data in the past decades show that the natural atmospheric CO2 concentration is positive correlated with the global land temperature and negatively correlated with the global land precipitation on decadal scale . By contrast, based on the analysis of observed data, we have found a significant positive correlation between the inter annual variability of CO2 growth rate and the year-to-year changes in the global land precipitation . Then, a new relationship between atmospheric CO2 and climatic variables is proposed.
This positive relationship indicates that an increase in current year rainfall does not cause net increases in the absorbed carbon by the vegetation. However, the increases in total cloudiness will reduce the uptake of carbon by plants. An additional analysis shows that the low latitude region (especially the Eastern Asia monsoon region) plays a main role in causing this phenomenon. The cloud is the main limited factor for vegetation growth in the low latitude region. The increases in cloudiness in low latitude areas would certainly reduce the solar radiation reaching plant canopy, and then lower the photosynthesis and cause a positive anomaly in CO2 emission from the terrestrial biosphere. This mechanisn can provide a better explanation to the observed fact firstly reported by Bacastow in 1976 that the CO2 flux anomalies lead changes in the Southern Oscillation Index (SOI) by about half year. Although further investigation is needed, this new finding has important meaning for global carbon cycling, especially for the inter annual CO2 variations. Further more, this phenomenon closely relates the climate changes with the global atmospheric CO2 concentration. So, the effects of clouds (concurrent with precipitation) on solar radiation and then on photosynthesis should be considered in the future ecosystem models in order to give a precise description of ecosystem carbon cycle.
2. Model Study on Ocean Carbon Cycle
The oceans contain about 40000 GtC in dissolved, particulate, and living forms. By contrast, land biota, soils and detritus contain a total of about 2200 GtC. Living and dead biogenic matter in the ocean contains at least 700 GtC, almost equal to the amount of CO2 in the atmosphere (about 750 GtC). So, ocean carbon cycle has an important influence on the global carbon cycle. Marine biota plays an important role in marine biogeochemical processes and in determining the marine carbon cycle. Due to the difficulties in directly collecting marine data, models are useful tools to study marine biogeochemical processes[26,27].
A two-dimensional atmospheric CO2-ocean carbon cycle model has been developed and used to simulate the surface distributions, vertical distributions and the isogram distributions along the meridian of various chemical species in Atlantic, Indian Ocean . Based on a two-dimensional ocean thermohaline circulation carbon cycle model, we studied carbon cycling in Pacific Ocean [30,31].
Our models have overcome the shortcoming of the box model and include the processes such as the exchange of carbon dioxide between the atmosphere and ocean, photosynthesis, decomposition of organic matter, calcium carbonate production and dissolution, and sinking of suspended particles. In particular, the effect of the ocean biota on carbon cycle is coupled into the model.
The horizontal and vertical distributions of various chemical species in Indian Ocean have been studied . It is found that the distributions of total carbon both in ocean and in atmosphere at a steady state depended on the various chemical, physical processes and also on boundary conditions. The distributions of chemical compounds are sensitive to the horizontal diffusion coefficient and photochemical action constant rate. Simulated distributions of the total dissolved inorganic carbon, alkalinity, nutrient, dissolved oxygen and isotopes 14C are close to the observed data. It is also found that the 14C export to deep ocean mainly through a key region in South Indian Ocean (10-30oS) indicating that anthropogenic CO2 will be transported from ocean surface to deep ocean though that region.
Dust is a main component of atmospheric aerosols. It is estimated that 1000—3000 Megatons of mineral dust are injected into atmosphere per year accounting for half of the total tropospheric aerosol. The mineral dust particles originate mainly from the great desert areas in northern Africa，southwestern America and Asia. Mineral aerosols play key roles in atmospheric chemistry, ecology and the earth's radiative balance. The interactions of dust with earth's radiation field are more complicated than those of other atmospheric aerosols because mineral particles can scatter and absorb so1ar radiation leading to either cooling or heating of the climate system under various conditions. Currently, there are large uncertainties about direct radiative forcing from dust, even larger uncertainties about its indirect effects[32,33].
Nowadays, extensive researches have been focused on dust in Sahara, but few on Asia dust. Therefore comprehensive study should be carried out on Asian-dust characters, influences and transport mechanisms etc. In recent years, Chinese scientists began to study dust storms by both experiments and models. Due to the lack of observation data, the parameters in the models are too simple and mostly based on the results obtained in Sahara Desert, so it can not satisfactorily describe the process of Asia-dust formation and transportation.
In spring 2000, the dust storm weather occurred frequent1y in northern China. The analysis on the chemical composition of super dust storm on April Sixth shows that the pollution caused by dust storm is very serious . In the dust storm period, the total concentration of 20 elements reached 1536 μg/m3 which is 31.4 times as that in 1999 spring. Even after the dust storm, the concentration of 20 elements kept 338.7 μg/m3, which is 7 times as that in 1999 spring. It is found that the higher peaks of concentration of most elements of particles appear at a diameter value above 16 μm, which is not observed in other time. The number concentrations of coarse particles (D>2 μm) are more than 20 times as that after the dust storm, while the number concentrations of fine particles (D<2 μm) are only 7 times as that after the dust storm.
V. OZONE VARIATIONS IN SURFACE LAYER
The change of tropospheric ozone is also a factor having effect on climate. One of the primary sources of tropospheric ozone is from photochemical reaction of anthropogenic pollutants [35,36]. Most of anthropogenic and natural po11utants, such as NOx NMHC (non-methane hydrocarbon),CO etc., are directly emitted to low troposphere, and are increasing year after year. Most researches have focused on the urban region, but few on rural areas. So the mechanism of ozone variation in clean areas are not clear by now. On the basis of meteorological fields provided by a mesoscale nonhydrostatic model (MM5), a three-dimensional regional chemical model (RADM) is applied to China to address the photochemical reaction mechanisms of surface ozone [38,39].In polluted areas, surface ozone is primarily dependent on the photochemical reaction. This is due to the higher concentration of ozone precursors such as NOx, NMHC, CO, etc. Physical factors such as transport and diffusion are secondary in determining surface ozone. But in clean areas the photochemical reaction is not the controlling factor. Surface ozone on the Tibetan Plateau is sensitive to ozone perturbation in upper layers. So it is suggested that the higher surface ozone concentration in summer time on Tibetan Plateau is mainly due to the high background ozone level above the planetary boundary layer that can be carried to surface layer by vertical diffusion and transport.
OH and HO2 radicals are the primary atmospheric oxidant and determine the lifetime of most substance in atmosphere. They play key roles in tropospheric photochemistry. At the same time, the variation of ozone precursors such as NOx, NMHC,and CO, which are largely influenced by human activities, may directly or indirectly affect the concentration of OH and HO2. Based on theory and model analysis, we find that the feedback effect of ozone variation on OH and HO2 radicals is very important.
There is a complicated nonlinear relationship between surface ozone and NOx. This nonlinear relationship not only influences the horizontal distribution of surface ozone but also its vertical profile, especially in the heavy polluted areas. Where there is high NOx pollution, there may be higher ozone concentrations in upper layer . During summer midday time, when the steady state is achieved, the ratio of O3 to NO2/NO has a constant value of 15:1, which can be deduced by the photochemistry equilibrium theory of O3, NO2 and NO.
VI. LONG TIME OBSERVATION OF ATMOSPHERIC CO2 AND CH4
During 1985-1988, atmospheric CO2 and CH4 in Minqin, Gansu Province had been continuously observed. Data show that CO2 concentration has an increasing trend of 0.3% per year; CH4 concentration has evident seasonal variation with an averaged increasing trend of 1.7% per year. A project to monitor the concentration of atmospheric CH4 and its long-term changes has been carried out in Beijing, China since 1985. Data show that atmospheric CH4 in Beijing is still increasing, although its increasing rate has significantly decreased from the averaged value of 1.76% per year during 1985-1989 to 0.50% during 1990-1997. The seasonal variation of CH4 has shown a double-peak pattern, one peak appearing in winter and the other in summer. After 1993, the annual seasonal increasing rate of CH4 in summer is negative while the increasing rate in winter (due to the emission from non-biogenic sources, such as fossil fuel combustion)is positive and about 25 ppbv per year. As a result, the increase of CH4 emission from non-biogenic sources in winter is the major cause of the annually seasonal increasing rate from 1993 to 1997.
VII. ANALYSES OF TRACE ORGANIC COMPONENTS IN THE ATMOSPHERE
The concentrations of trace organic components in atmosphere, such as NMHC, CFCs, HCFCs etc, are very low, but they have important effects on climate. The automatic system, to monitor trace chlorofluorocarbon (CFCs) and benzene, toluene, xylene, ethylbenzene (BTXEs) and NMHC in Xinglong, Minqin, Dingshan and Beijing, consists of Ion Trap-GC/MS and an intelligent interface of cryo-concentrating and sampling trace organic components of the atmosphere (ICCS) developed by us. We, for the first time, get the concentrations of 36 organic pollutants in the atmosphere baseline in China . The automatic system, to monitor NMHC and the other volatile organic pollutants of the urban boundary layer of the atmosphere, consists of Quadrupole-GC/MS and the ICCS, too. ICCS can concentrate 500—2000 cm3 air sample in an analysis process. The instrument provides bulk composition measurements with a detection limit of 10-12v/v (pptv). In other words, the lowest detective limit of the commercial instrument GUMS is lowered from l0-6 to 10-12 with ICCS. ICCS-GC/MS can monitor all of the organic compounds if its concentration is over 10-12v/v. One of the systems is used to monitor the trace volatile organic components of the atmosphere in Beijing. About 25 species of CFCs, 30 species of BTXEs and 50 species of other NMHCs can be monitored by the automatic system, routinely.
VIII. EMISSION OF HFCs, PFCs AND SF6 FROM CHINA IN 1995Based on the investigation of industry production data and consumption data, the emissions of hydrofluorocarbons (HFCs) perfluorocarbon (PFCs) and sulfurhexafluoride (SF6) from China in 1995 are estimated primarily as 2244, 2581 and 215 tons, which account for 0.9%, 6.5% and 3.7% of the world total emission, respectively .
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