Re-analysis And Evaluation Of Atmospheric Carbon Dioxide (CO₂), Methane (CH₄) And Carbon Monoxide (CO) At Mount Waliguan, China

Mixing ratios of atmospheric carbon dioxide (CO₂), methane (CH₄) and carbon monoxide (CO) observed at Mount Waliguan (WLG), a global background station in remote western China, were re-analyzed and evaluated. The data periods are 1995²008, 2002²006 and July 2004^June 2007 for CO₂, CH₄ and CO respectively. Corrections for drift in reference gases were also included in the data revision according to the history calibrations and intercomparison experiments. The measurement data of CO₂, CH₄ and CO at WLG have been updated to latest X2007, NOAA04 and WMO2000 scale respectively. Data treatment and quality control procedure has also been established to improve scientific use and application of the data. A mathematical procedure based on robust local regression was applied to distinguish background and non-background data, as is actually to classify the impact of regional emissions or influence of polluted air parcel. From 1995 to 2008, approximately 72%±5%, 17%±4% and 11%±2% of all observed CO₂ data have been selected as background, polluted and sink data, respectively. The percent for polluted CO₂ data increased from 14-15% in 1995-2000 to 18-19%, reflecting enhanced impact from human activities (e.g. fossil fuel emissions) in recent years; the percent of data representing CO₂ sinks didn't change much (10-14%) indicating a relatively constant uptakes from terrestrial ecosystems in the region. However, the more polluted (elevated) than the uptakes CO₂ suggested an imbalance between emissions and sinks for atmospheric CO₂ there during the past 15 years at WLG. By this method, about 58% and 52% of all data were selected as CH₄ and CO background data, respectively, indicating significant influences from regional emissions/sources.By analyzing 5d back-trajectory (500hPa) of 1997²008 at WLG, it showed air parcels arriving at WLG were predominately from the west, except in summer when advection from the east and southeast prevailed. >90% of trajectories from the east typically brought polluted air to the site, having passed over populated urban areas upwind. The case study combined with trajectory analysis showed the episode of high CO₂, CH₄ and CO mixing ratios were associated with advection from the heavily populated regions east or southeast (e.g. Xining and Lanzhou) of WLG and northwest of Qinghai via Ge'ermu urban area where growing industrial activities as well as crops residue burning provide large sources of CO suggesting a large source area due to human activities; whereas, the low values were observed most frequently when air masses originated from the sparsely populated Tibet and south of Qinghai and Xinjiang Uygur Autonomous Region (XUAR). By combining the observed data with trajectory-based statistics and cluster analysis, it has been demonstrated that air masses originating from the east to southeast of WLG have the strongest impact on CO and CH₄, however, only in summer transport from this direction significant. In the other seasons, air parcels arriving via the northwest are more common. These exhibit both background and polluted characteristics. Air from the central XUAR and the Ge'ermu urban area have show enhanced CO and CO₂ levels due to the growing economy in west China as well as biomass burning in the region. Air parcels coming from the sparsely populated Tibet contribute least to enhance all CO₂, CH₄ and CO values observed at WLG. It should be noticed that mixing ratios of atmospheric CH₄ would be enhanced when air masses originated from Ningxia and northwest of Gansu agriculture areas (mainly growing rice) along the Yellow river region, especially in summer. The probability that air parcels pass over regions of clean or polluted regions were further identified using potential source contribution function (PSCF) analysis, and all the CO₂, CH₄ and CO displayed similar source region distribution as above described. Intercorrelation of atmospheric CO₂,CH₄ and CO above the background showed a very well correlation (r2>0.7, p<0.01) in winter probably in account of their common sources being mainly from fossil fuel and biomass burning. Hence, CO₂ and CH₄ emissions from fossil fuel have been estimated by using CO ratio method. Emissions for CO₂ in China were 6214.7 Mt in 2005 and 6320.6Mt in 2006 respectively, which is similar to the results by statistical method. Emissions for CH₄ in China were 16.44 Mt in 2005 and 20.11 Mt in 2006.Regarding diurnal variation of atmospheric CO₂ mixing ratios based on observed data from 1995 to 2008, it has been found that atmospheric CO₂ showed higher during daytime and lower in early morning and nighttime in spring and winter, the highest mixing ratios most frequently occurred at 11:00¹5:00 when residents'activities (i.g. grazing, heating) increased, but in the cold season photosynthesis is very weak and make no significant impact on CO₂ levels. Whereas mixing ratios of CO₂ exhibited daytime minimum and nighttime maximum in summer and autumn, the lowest CO₂ mixing ratios presented at 16:00¹8:00, as mainly due to the enhanced uptake of CO₂ by vegetations in summer in north hemisphere. In addition, the meteorological conditions of strong convection in summer are also advantageous to pollution diffusion and will decrease CO₂ mixing ratios during daytime. Meanwhile, effects of temperature inversion during evening or early morning at high altitude site like WLG also would accumulate CO₂ mixing ratios. Averaged diurnal variation of CO₂ displayed largest in summer with amplitude of 2.3ppm (part per million) but only with ⁰.7ppm both in spring and summer, suggesting differences in carbon exchange between terrestrial ecosystems and atmosphere attribute to plant growing cycle over one year. Diurnal variation of atmospheric CH₄ and CO based on data during 2002²006 and during July 2004^June 2007 respectively showed a similar pattern with CO₂ in all seasons, but with different causes of the lower mixing ratios for CO and CH₄ during daytime in summer. As we known, it's because of photosynthesis for lower CO₂ mixing ratios in the afternoon; however, the mainly impelling force is photochemical reaction with OH radicals which would decrease mixing ratios of CH₄ and CO during daytime in summer.The averaged CO₂ seasonal cycle from 1995 to 2008 at WLG displayed April maximum and August minimum, declining rapidly in late spring and early summer (from May to June) and increasing in autumn (from September to November) mainly caused by plant and soil respiration as well as plant photosynthesis. The overall characteristic is similar to that of observed at Mauna Loa, Schneefernerhaus and Niwot Ridge, but exhibiting certain phase delay mainly influenced by the differences in interaction of regional sources/sinks as well as the underlying surfaces. The averaged CH₄ seasonal cycle during 2002²006 showed minimum in spring and winter, maximum in summer (from June to August), as is totally opposite to that of observed at Mauna Loa and Niwot Ridge. The high levels of CH₄ at WLG should be attributed to enhancing regional/local sources (i.g. herd) around the site as well as the dominant polluted air flow from southeast region in summer. Another possible reason is, compared to Mauna Loa and Niwot Ridge, the photochemical effect at WLG (located at higher latitude) is weaker, as means CH₄ sink is smaller than that of at these sites with lower latitude in summer. Seasonal cycle of atmospheric CO mixing ratios from July 2004 to June 2007 exhibited maximum in spring (March and April) and minimum in autumn (September and October). This is in good agreement with the variability observed at Jungfraujoch, Niwot Ridge and Mauna Loa. Averaged seasonal amplitude of CO₂, CH₄ and CO during the observed periods was 9.0ppm, 11ppb (part per billion) and 26ppb, respectively. Mixing ratios of atmospheric CO₂ increased exponentially, as is well agree with the trend observed at Mauna Loa. The growth rate of has accelerated since measurements began at WLG in 1991 where CO₂ increased from nearly 1.3 part per million per year (ppm yr-1) in 1991-1995 (the data before 1994 obtained from flask measurement at WLG) to 2.4ppm yr-1 in the recent years, but with large year to year variations, as is closely associated with the climate events (e.g. El Nino).Preliminary results of isotope effect on measuring CO₂ mixing ratios by using different techniques or systems showed that the differences are within±1.0ppm for the samples with natural abundances del13C(-7.7^-9.0‰) of CO₂; The differences could increase to 10ppm when del13C decrease to about -25.0‰. And also we found that both high CO₂ concentration and low del13C can lead to large measurement bias by different techniques. However it is still need further studies to demonstrate this conclusion and to calibrate the bias due to the istope effect qutitativly.