[Evaluation of gap-filling methods for CH4 flux data based on eddy covariance method in the Lake Taihu, China]. / æ¶¡åº¦ç›¸å ³è§‚æµ‹çš„å¤ªæ¹–CH4通量数据插补方法评价.

Eddy covariance method has become a key technique to measure CH4 flux continuously in lakes. A large number of CH4 flux data was missing due to variable reasons. In order to reconstruct a complete time series of CH4 flux, it is necessary to find an appropriate gap-filling method to insert the CH4 flux data gap. Based on the routine meteorological data and CH4 flux data measured at Bifenggang site in the eastern part of the Taihu eddy flux network during 2014 to 2017, we analyzed the control factors of CH4 flux at the half-hour scale and daily scale. With those data, we tested that whether nonlinear regression method and two machine learning methods, random forest algorithm and error back propagation algorithm, could fill the CH4 flux gap at the half-hour scale and daily scale. The results showed that CH4 flux at the half-hour scale was mainly influenced by sediment temperature, friction velocity, air temperature, relative humidity, latent heat flux and water temperature at 20 cm in the growing season, and was mainly affected by relative humidity, latent heat flux, wind speed, sensible heat flux and sediment temperature in non-growing season. The CH4 flux at the daily scale was mainly affected by latent heat flux and relative humidity. Random forest model was the best in CH4 flux data gap filling at both time scales. The random forest model with the input variables of day of year, solar elevation angle, sediment temperature, friction velocity, air temperature, water temperature at 20 cm, relative humidity, air pressure, and wind speed was more suitable for filling the CH4 flux data gap at the half-hour scale. The random forest model with the input variables of day of year, sediment temperature, friction velocity, air temperature, water temperature at 20 cm, relative humidity, air pressure, wind speed, and downward shortwave radiation was more suitable for filling CH4 flux data gap at the day scale. The interpolation models could fill the data gap better at daily scale than that at the half-hour scale.

[Temporal and Spatial Variation Characteristics of Methane, Carbon Dioxide, and Nitrous Oxide Concentrations and the Influencing Factors in Small Aquaculture Pond].

As an important source of greenhouse gases, the changes in greenhouse gas concentrations of aquaculture ponds are not only the basis for accurate quantification of greenhouse gases emissions but are also important for identifying their influencing factors. The spatial and temporal variation characteristics of CH4, CO2, and N2O concentrations and the influencing factors in a typical small aquaculture pond in the Yangtze River Delta were analyzed based on the headspace equilibrium-gas chromatograph method. Except in spring, the concentrations of CH4, and N2O appeared high at noon or afternoon and were influenced by water temperature. Impacted by water temperature and aquatic plant photosynthesis, the concentrations of CO2 were high in the morning when photosynthesis was weak. The concentrations of CH4 and CO2 were the highest in autumn and the lowest in winter. The mean concentrations of CH4 in autumn and winter were 176.34 nmol·L-1 and 32.75 nmol·L-1, respectively, which were mainly affected by air temperature, water temperature, and dissolved oxygen. The average CO2 concentrations in autumn and winter were 134.37 µmol·L-1 and 23.10 µmol·L-1, respectively, and were mainly affected by aquatic vegetation photosynthesis and pH. N2O concentration was the highest in summer and the lowest in winter, with mean values of 97.05 nmol·L-1 and 19.41 nmol·L-1, respectively, which were mainly affected by air temperature and water temperature. In terms of the vertical spatial variations of the three greenhouse gases, the concentration of CH4decreased with water depth in summer, and the concentration differences between the surface layer and the bottom and middle layers were 71.28 nmol·L-1 and 42.80 nmol·L-1, respectively. The concentration of CH4 increased with water depth in autumn, and the concentration difference between the bottom layer and surface layer was 163.94 nmol·L-1. The CO2 concentration increased with water depth in summer and autumn. The concentration differences between the bottom and surface concentrations were 18.69 µmol·L-1 and 29.90 µmol·L-1, respectively. N2O concentration showed no obvious change in the vertical direction. For the horizontal variations, the concentrations of CH4, CO2, and N2O in the feeding area in summer and in chicken manure in spring were approximately 1.34-1.98 times and 1.95-2.42 times those in other areas, respectively, and the concentrations of N2O and CO2 in spring and summer were approximately 1.13-1.26 times and 1.39-1.74 times those in other areas.

[Effects of chamber characteristics on CO2 and CH4 flux at the water-air interface measured by the chamber method]. / 箱体特征对箱式法观测水-气界面CO2和CH4通量的影响.

The chamber method is widely used to measure CO2 and CH4 flux in inland water. However, the designs of chamber used in various studies are different and lack unified standards, which would affect the observation results. To clarify the impacts of chamber characteristics, including light transmittance, air pressure difference inside and outside the chamber, and gas mixing degree in the chamber, on CO2 and CH4 flux measurements at the water-air interface, we compared the effects of transparent/opaque chamber, the chamber with/without air pressure equalizing device and fan on CO2 and CH4 flux measurements in the aquaculture pond, based on the multi-channel closed dynamic chamber system. The results showed that, during the daytime in summer, compared with the transparent chamber which could measure the actual CO2 flux, when CO2 was emitted from the pond, the opaque chamber overestimated the CO2 flux by 90%; when CO2 was absorbed by the pond, the opaque chamber underestimated the CO2 flux by 50%. The CH4 diffusion flux measured by the opaque chamber was 40% lower than that measured by the transparent chamber. There was no significant difference between CO2 and CH4 flux measured by the chamber with and without air pressure equalizing device. CO2 flux observed by the chamber without fan had poor representativeness, being 20% higher than that observed by the chamber with fan. Moreover, CH4 flux emitted through different pathways could not be distinguished using the chamber without fan. Therefore, when the chamber method was used to observe the CO2 and CH4 flux at the water-air interface, the chamber shall be transparent and be installed with fan.

[Effects of Water Vapor Source and Local Evaporation on the Stable Hydrogen and Oxygen Isotopic Compositions of Precipitation].

Stable hydrogen and oxygen isotopic compositions in precipitation are good tracers and can provide unique information about the water cycle. Precipitation samples were collected at the Nanjing, Liyang, Yixing, and Dongshan sites in 2016, and the HDO and H218O compositions of precipitation were measured. The temporal variability of HDO and H218O compositions and deuterium-excess of precipitation were analyzed, and the influence of the water vapor source and local evaporation on stable isotopic composition of precipitation were discussed. The results indicated that:① Seasonal variations in the HDO composition, H218O composition, and deuterium-excess of precipitation occurred due to different water vapor sources during the summer and winter monsoon seasons. The HDO and H218O compositions were depleted during the summer monsoon season and enriched during the winter monsoon season. The deuterium-excess during the summer monsoon season was lower compared to the winter monsoon season. ② During the summer monsoon, the evaporation of Lake Taihu made the deuterium-excess of downwind precipitation and the downwind intercept of the local meteoric water line higher. During the winter monsoon season, local evaporation had little influence on HDO and H218O components in precipitation. ③ Both of the intercepts and slopes of the local meteoric water line were higher than those of the global meteoric water line, due to moisture recycling during the winter monsoon season and different water vapor sources between the summer and winter monsoon seasons.

[Characteristics and Source Analysis of Carbonaceous Components of PM2.5 During Autumn in the Northern Suburb of Nanjing].

In this study, PM2.5 samples were collected from October to November of 2015 in the northern suburb of Nanjing. The mass concentrations of organic carbon (OC), elemental carbon (EC), and levoglucosan in the samples were analyzed by thermal optical transmittance (TOT) and ion chromatography. The average concentrations of OC and EC were respectively (11.3±4.9) µg·m-3 and (1.1±0.9) µg·m-3. The average total carbon (TC) was 22.9%, and the OC/EC was 7.4. The quality concentrations of PM2.5, OC, EC, and SOC all reflected daytime features, and the correlation between OC and EC was better during the day than at night (correlation coefficients of 0.86 for day and 0.7 for night). By analyzing the mass concentrations of PM2.5, levoglucosan, and SOC, as well as the data of backward trajectories and fire point data, it was determined that the northern suburb of Nanjing is affected by the long-distance transportation of biomass from Hebei and other places from October 13-16. The correlations between levoglucosan and OC, EC, or SOC were significant (correlation coefficients of 0.78, 0.79, and 0.65, respectively), and the contribution of biomass combustion during sampling to OC was 21.9%.

[Characteristics of Methane Emission from Urban Traffic in Nanjing].

Urban traffic is an important source of greenhouse gases such as CH4. The observations on CH4 are the basis for quantitative analysis of urban carbon emissions. Taken into consideration the weekly and daily changing characteristics of urban traffic, we conducted experiments to analyze the features of traffic CH4emission and its influential factors. The experiments were conducted on 3 main roads in Nanjing on Oct. 17, 18, 20, 23, 2014 with 5 periods of observation per day, and in Nanjing Yangtze River tunnel in the morning and at night of Sep.11 2014. The results showed that:① The average concentration of CH4 on the urban main road of Nanjing city was greater than that of the background atmosphere. Affected by traffic conditions, the spatial difference of ΔCH4 concentration was significant on three typical main roads. ΔCH4 concentration's diurnal variation showed inverted "W" type, and its peak appeared in the morning and evening rush hours. ② Due to the "piston wind" in the tunnel, the CH4 concentration in Nanjing Yangtze River tunnel gradually increased from the inlet to the outlet and the difference of concentration between the inlet and the outlet was 0.21×10-6-0.38×10-6. ③ There was a good linear correlation between CH4 concentration and CO2 concentration. The atmospheric ΔCH4:ΔCO2 value of urban main road in Nanjing was 0.0091 and the atmospheric ΔCH4:ΔCO2 value of Nanjing Yangtze River Tunnel was 0.00047-0.0014. ④ Traffic volume and the proportion of natural gas vehicles were the main factors influencing atmospheric ΔCH4 concentration and ΔCH4:ΔCO2.

[Greenhouse Gas Fluxes at Water-Air Interface in Small Pond Using Flux-Gradient Method Based on Spectrum Analyzer].

As an important part of inland waters,small pond is a neglected source of greenhouse gas.The main objective of the study was to quantify greenhouse gas fluxes (CO2 and CH4) from small pond in the Yangtze Delta using flux-gradient method.The results showed that:① zero-gradient test indicated that the flux measurement precision for water vapor,CO2,and CH4 was 7.525 W·m-2,0.022 mg·(m2·s)-1,and 0.054 µg·(m2·s)-1,respectively.During the test period,84%,80%,and 94% of half-hourly flux data for H2O,CO2,and CH4 were higher than the zero-gradient measurement precision.② Based on the measurement,the small pond was the source of CO2 and CH4 for the atmosphere in summer,the mean emission flux of CO2 and CH4 was 0.038 mg·(m2·s)-1 and 0.889 µg·(m2·s)-1,respectively.The CH4 emission fluxes from the small pond were more higher than the median value of emission for global lakes.The results indicated that greenhouse gas emission from small pond was an important part for estimating inland water greenhouse gas emissions,especially for CH4 emission.These results can provide scientific reference for making emission inventory of regional greenhouse gas.

[Correlation Analysis of the Urban Heat Island Effect and Its Impact Factors in China].

Urban heat islands (UHIs), which are urban areas with higher surface or air temperatures than surrounding rural areas, can further enhance the heat stress already exacerbated by global warming. This poses great challenges to human health and sustainable development. China has been burdened with heavy air pollution in recent years, and the effect of haze pollution on UHIs is still far less well understood. This study investigated the spatial variations of daytime, nighttime, and seasonal surface UHI effects in China during 2003-2013, based on MODIS land surface temperature data, and analyzed the correlations between the UHI effect and its impact factors from both biophysical and biochemical perspectives. Our results show that MODIS-derived annual nighttime UHI effect (3.4 K±0.2 K, mean±1 s.e.) is higher than the annual mean daytime UHI effect (2.1 K±0.3 K). The daytime UHI effect is strongest in summer and weakest in winter but contrasts with the seasonal variation characteristics of nighttime UHI effect. During daytime, UHI effects in humid and semi-humid regions are much more obvious than those in semi-arid/arid regions; during nighttime, the UHI effect in semi-humid and semi-arid/arid regions is much stronger than that in humid regions. A Daytime Urban Cool Island effect exists in semi-arid/arid regions during spring, autumn, and winter seasons. Population, vegetation activity, and irrigation are three factors controlling annual mean daytime UHI effect. The nighttime UHI is significantly influenced by latitude, albedo, precipitation, and aerosol concentrations. We provide evidence for a long-held hypothesis that the biogeochemical effect of urban aerosols is an important contributor to the UHI effect. The important role played by rural background environment in calculating the UHI effect is further discussed in terms of surface aerodynamic roughness and the contrast in albedo between urban and rural areas. Mitigation of haze pollution has a co-benefit of reducing the UHI effect and heat stress for urban dwellers. It is also conducive to reducing negative impacts of regional and global climate change.

[Temporal Dynamics of Stable Isotopic Composition in Lake Taihu and Controlling Factors].

The composition of hydrogen and oxygen stable isotopes in lake water is important to the researches in hydrology, meteorology and paleoclimatology. In this study, long-term and continuous measurement on the compositions of HDO and H218O in lake water (δDL and δ18OL) was conducted over Lake Taihu, the deuterium excess (dL) was calculated, and the temporal variability and controlling factors were analyzed. The results indicated that ① the variation of isotopic enrichment in lake water was significant, ranging from -59.8‰ to -24.2‰ for δDL, from -8.6‰ to -2.6‰ for δ18OL, and from -7.9‰ to 12.9‰ for dL, respectively. In comparison to cold season, δDL and δ18OL were higher and dL was lower during warm season. ② On monthly time-scale, lake evaporation and the ratio of total water inputs lost by evaporation controlled the isotopic enrichment in lake water. When lake evaporation or the ratio increased, δDL and δ18OL increased, but dL decreased. ③ Over Lake Taihu, the isotopic composition in precipitation and water temperature did not control the isotopic enrichment. The results provide scientific reference for isotope hydrology and the researches related to the isotopic enrichment in lake water in meteorology and paleoclimate.

[13 C-based Sources Partitioning of Atmospheric CO2 During Youth Olympic Games, Nanjing].

Observations of urban atmospheric CO2 molar fraction and its 13 C isotope composition (δ13 C) is of great importance to interpret the effect of anthropogenic and biologic sources on local or regional carbon cycle. High-frequency in-situ observation on atmospheric in urban airsheds was performed during Youth Olympic Games (YOG) in Nanjing. The hourly, diurnal and daily differences of CO2 concentration and its δ13 C between the period with and without temporary CO2 emission controls were compared. The results showed that short-term emission reduction measures could cause 21×10-6 decrease in atmospheric CO2 concentration in a regional and short-term scale. The reduction of coal combustion during YOG in YRD was about 5%. The overall isotopic signature of local surface sources δ13 CS in Yangtze River Delta (YRD) was determined by Miller-Tans, and the isotopic signatures of anthropogenic and natural sources in YRD were also determined based on literature investigation. According to the above results, the surface net CO2 flux, plant flux and anthropogenic flux in YRD were quantified using mass-balance equation. The CO2 emission from cement production (non-energy industrial process) was the key human factor of high atmospheric δ13 C of CO2 in YRD during summer (2.36‰). The plant effect could offset 23% to 39% anthropogenic CO2 emission in YRD during summer. In this study, we tried to provide new solution to partition carbon sources in urban areas by combining top-down atmospheric observation and traditional IPCC's emission inventory.

[Direct Observation on the Temporal and Spatial Patterns of the CO2 Concentration in the Atmospheric of Nanjing Urban Canyon in Summer].

Direct observation of urban atmospheric CO2 concentration is vital for the research in the contribution of anthropogenic activity to the atmospheric abundance since cities are important CO2 sources. The observations of the atmospheric CO2 concentration at multiple sites/heights can help us learn more about the temporal and spatial patterns and influencing mechanisms. In this study, the CO2 concentration was observed at 5 sites (east, west, south, north and middle) in the main city area of Nanjing from July 18 to 25, 2014, and the vertical profile of atmospheric CO2 concentration was measured in the middle site at 3 heights (30 m, 65 m and 110 m). The results indicated that: (1) An obvious vertical CO2 gradient was found, with higher CO2 concentration [molar fraction of 427. 3 x 10(-6) (±18. 2 x 10(-6))] in the lower layer due to the strong influences of anthropogenic emissions, and lower CO2 concentration in the upper layers [411. 8 x 10(-6) (±15. 0 x 10(-6)) and 410. 9 x 10(-6) (±14. 6 x 10(-6)) at 65 and 110 m respectively] for the well-mixed condition. The CO2 concentration was higher and the vertical gradient was larger when the atmosphere was stable. (2) The spatial distribution pattern of CO2 concentration was dominated by wind and atmospheric stability. During the observation, the CO2 concentration in the southwest was higher than that in the northeast region with the CO2 concentration difference of 7. 8 x 10(-6), because the northwest wind was prevalent. And the CO2 concentration difference reduced with increasing wind speed since stronger wind diluted CO2 more efficiently. The more stable the atmosphere was, the higher the CO2 concentration was. (3) An obvious diurnal variation of CO2 concentration was shown in the 5 sites. A peak value occurred during the morning rush hours, the valley value occurred around 17:00 (Local time) and another high value occurred around 19:00 because of evening rush hour sometimes.

[Temporal and Spatial Characteristics of Lake Taihu Surface Albedo and Its Impact Factors].

Lake surface albedo determines energy balance of water-atmospheric interface and water physical environment. Solar elevation angle, cloudiness, wind speed, water quality and other factors can affect lake surface albedo. Using solar radiation, wind speed, and water quality data (turbidity and chlorophyll-a concentration) which were observed in four eddy covariance sites (Meiliangwan, Dapukou, Bifenggang and Xiaoleishan i. e. MLW, DPK, BFG and XLS) in Lake Taihu and clearness index (k(t)), the influence of these factors on Lake Taihu surface albedo and the reasons that led to its spatial difference were investigated. The results showed that solar elevation angle played a leading role in the diurnal and seasonal change of lake surface albedo; lake surface albedo reached two peaks in 0 < k(t) < 0.1 and 0.4 < k(t) < 0.6 respectively, when solar elevation angle was below 35 degrees. The surface albedo increased with the increasing wind speed, turbidity and chlorophyll-a concentration. However, wind could indirectly affect surface albedo through leading to the changes in sediment resuspension and chlorophyll-a distribution. The sequence of albedo in the four sites was XLS > BFG > DPK > MLW. XLS and BFG belonged to the higher albedo group, while DPK and MLW belonged to the lower albedo group. The different biological environments caused by aquatic macrophytes and algae resulting in the spatial variation of Lake Taihu surface albedo. The relationship between albedo and chlorophyll-a concentration was not a very sensitive factor for indicating the outbreak of algae. This study can provide theoretical reference for lake albedo parameterization.

[Effects of antiseptic on the analysis of greenhouse gases concentrations in lake water].

To gain insight into antiseptic effects on the concentrations of CO2, CH4, and N2O in lake water, antisepetic (CuSO4 and HgCl2) were added into water sample, and concentrations of greenhouse gases were measured by the gas chromatography based on water equilibrium method. Experiments were conducted as following: the control group without antisepetic (CK), the treatment group with 1 mL CuSO4 solution (T1), the treatment group with 5 mL CuSO4 solution (T2), and the treatment group with 0.5 mL HgCl2 solution (T3). All groups were divided into two batches: immediately analysis (I), and after 2 days analysis (II). Results showed that CuSO4 and HgCl2 significantly increased CO2 concentration, the mean CO2 concentration (Mco2) of CK (I) and CK (II) were (11.5 +/- 1.47) micromol x L(-1) and (14.38 +/- 1.59) micromol x L(-1), respectively; the Mco2 of T1 (I) and T1 (II) were (376 +/- 70) micromol x L(-1) and (448 +/- 246.83) micromol x L(-1), respectively; the Mco2 of T2 (I) and T2 (II) were (885 +/- 51.53) micromol x L(-1) and (988.83 +/- 101.96) micromol x L(-1), respectively; the Mco2 of T3 (I) and T3 (II) were (287.19 +/- 30.01) micromol x L(-1) and (331.33 +/- 22.06) micromol x L(-1), respectively. The results also showed that there was no difference in CH4 and N2O concentrations among treatments. Water samples should be analyzed as soon as possible after pretreatment. Our findings suggest that adding antiseptic may lead an increase in CO2 concentration.