Atmospheric CO2 in the megacity Hangzhou, China: Urban-suburban differences, sources and impact factors.

Limited observation sites and insufficient monitoring of atmospheric CO2 in urban areas restrict our comprehension of urban-suburban disparities. This research endeavored to shed light on the urban-suburban differences of atmospheric CO2 in levels, diurnal and seasonal variations as well as the potential sources and impact factors in the megacity of Hangzhou, China, where the economically most developed region in China is. The observations derived from the existing Hangzhou Atmospheric Composition Monitoring Center Station (HZ) and Lin'an Regional Atmospheric Background Station (LAN) and the newly established high-altitude Daming Mountain Atmospheric Observation Station (DMS), were utilized. From November 2020 to October 2021, the annual averages of HZ, LAN and DMS were 446.52 ± 17.01 ppm, 441.56 ± 15.42 ppm, and 422.02 ± 10.67 ppm. The difference in atmospheric CO2 mole fraction between HZ and LAN was lower compared to the urban-suburban differences observed in other major cities in China, such as Shanghai, Nanjing, and Beijing. Simultaneous CO2 enhancements were observed at HZ and LAN, when using DMS observations as background references. The seasonal variations of CO2 at LAN and DMS exhibited a high negative correlation with the normalized difference vegetation index (NDVI) values, indicating the strong regulatory of vegetation canopy. The variations in boundary layer height had a larger influence on the low-altitude HZ and LAN stations than DMS. Compared to HZ and LAN, the atmospheric CO2 at DMS was influenced by emissions and transmissions over a wider range. The potential source area of DMS in autumn covered most areas of the urban agglomeration in eastern China. DMS measurements could provide a reliable representation of the background level of CO2 emissions in the Yangtze River Delta and a broader region. Conventional understanding of regional CO2 level in the Yangtze River Delta through LAN measurements may overestimate background concentration by approximately 10.92 ppm.

[Characteristics and Influencing Factors of Greenhouse Fluxes from Urban Lawn].

As an important component of urban green spaces, greenhouse gas uptake or emissions from urban lawns cannot be ignored. However, studies of greenhouse gas fluxes from subtropical urban lawns are relatively sparse. The static chamber-gas chromatography method was applied to monitor the ground-air exchange fluxes of various greenhouse gases(CO2, CH4, N2O, and CO) in typical urban lawns of Hangzhou City. Our results showed that the average fluxes had significant seasonal cycles but ambiguous diurnal variations. The grassland and the soil(naked soil without vegetation coverage) acted as sources of atmospheric N2O, with the average fluxes of (0.66±0.17) and (0.58±0.20) µg·(m2·min)-1 for N2O, respectively; however, they were also sinks of CH4 and CO, with the average fluxes of (-0.21±0.078) and (-0.26±0.10) µg·(m2·min)-1 for CH4 and (-6.36±1.28) and (-6.55±1.69) µg·(m2·min)-1 for CO, respectively. The average CO2emission fluxes of urban grassland and soil were(5.28±0.75) and (4.83±0.91) mg·(m2·min)-1, respectively. The correlation analysis indicated that the CO2 and N2O fluxes of grassland and soil were negatively correlated with precipitation, whereas the CH4 and CO fluxes were positively correlated with it. There was no significant correlation between grassland CH4 fluxes and soil temperature, and N2O fluxes had a significant negative correlation with soil temperature; the other greenhouse gas fluxes showed a significant positive correlation with soil temperature. In addition, the seasonal variation in CO2 (R2=0.371 and 0.314) and N2O(R2=0.371 and 0.284) fluxes from both grassland and soil was affected by precipitation, whereas CO fluxes (R2=0.290 and 0.234) were mainly driven by soil temperature compared with the other greenhouse gases.

Contrasting effects of temperature and biological processes on aragonite saturation state at the A4HDYD station in the North Yellow Sea.

To explore the effects of seawater temperature and biological processes on aragonite saturation state (Ωarag), eight field surveys were conducted from December 2014 to November 2015 at A4HDYD station in the North Yellow Sea (NYS). Low regional yearly initial values of seawater Ωarag (1.26-1.76) were obtained in winter due to low seawater temperature. During spring to early autumn, surface-water Ωarag increased from 2.02 to 3.47 for the increasing seawater temperature and biological production, whereas bottom-water Ωarag remained as low as 1.44-1.79 due to low seawater temperature, net community respiration and the influence of the North Yellow Sea Cold Water Mass (NYSCWM). Following the collapse of stratification in late autumn, the seawater was well-mixed with an average Ωarag of 2.17. The annual cycle of seawater temperature dominated the seasonal variations of Ωarag, counteracted by biological production increasing Ωarag of 0.49 in surface water in March and strengthened by net community respiration decreasing Ωarag of 0.11-0.41 below the pycnocline during summer and autumn. It is, therefore, mid-to-high latitude coastal areas impacted by human activities are likely subject to ocean acidification due to the combined effects of low seawater temperature and net community respiration.

Hydroxyl radicals in natural waters: Light/dark mechanisms, changes and scavenging effects.

Hydroxyl radicals (•OH) are the most active, aggressive and oxidative reactive oxygen species. In the natural aquatic environment, •OH plays an important role in the biogeochemistry cycle, biotransformation, and pollution removal. This paper reviewed the distribution and formation mechanism of •OH in aquatic environments, including natural waters, colloidal substances, sediments, and organisms. Furthermore, factors affecting the formation and consumption of •OH were thoroughly discussed, and the mechanisms of •OH generation and scavenging were summarized. In particular, the effects of climate change and artificial work on •OH in the largest natural aquatic environment, i.e., marine environment was analyzed with the help of bibliometrics. Moreover, Fenton reactions make the •OH variation more complicated and should not be neglected, especially in those areas with suspended particles and sediments. Regarding the •OH variation in the natural aquatic environment, more attention should be given to global change and human activities.

Natural iron minerals in an electrocatalytic oxidation system and in situ pollutant removal in groundwater: Applications, mechanisms, and challenges.

Natural iron-bearing minerals are widely distributed in the environment and show prominent catalytic performance in pollutant removal. This work provides an overview of groundwater restoration technologies utilizing heterogeneous electro-Fenton (HEF) techniques with the aid of different iron forms as catalysts. In particular, applications of natural iron-bearing minerals in groundwater in the HEF system have been thoroughly summarized from either the view of organic pollutant removal or degradation. Based on the analysis of the catalytic mechanism in the HEF process by pyrite (FeS2), goethite (α-FeOOH), and magnetite (Fe3O4) and the geochemistry analysis of these natural iron-bearing minerals in groundwater, the feasibility and challenges of HEF for organic degradation by using typical iron minerals in groundwater have been discussed, and natural factors affecting the HEF process have been analyzed so that appropriate in situ remedial measures can be applied to contaminated groundwater.

[Monitoring of atmospheric CH4, CO, CO2, N2O and SF6 using three-channel gas chromatography].

China is approaching a critical period of carbon peak and carbon neutrality. To assess the impact of carbon peak and carbon neutrality measures, an accurate understanding of the variations of the spatial and temporal distribution of greenhouse gases is crucial. Gas chromatography, a classical approach for greenhouse gas observation, can be employed for the high-precision analysis of partial greenhouse gases. In this research, a new greenhouse gas analytical system capable of measuring five gases (CH4, CO, CO2, N2O and SF6) on a single instrument was developed based on the traditional gas chromatography approach. The following are the chromatographic operation conditions. The carrier gases were high purity N2(99.999%) and argon-methane (5% methane in argon, 99.9999%), and a stainless steel switching valve triggered the injection. Compressed CH4, CO, CO2, N2O and SF6 mixed standard gases were stored in a 0.029 m3 aluminum alloy steel cylinder for this experiment. After numerous rounds of calibration by Greenhouse Gas Laboratory of Atmospheric Sounding Center of China Meteorological Administration, the gas scale met the primary standard of World Meteorological Organization (WMO). The main performance of the system, including the measurement precision, accuracy and linear response, was tested. The results showed that the detection performance of the system met the quality standards of WMO/Global Atmospheric Watch (GAW). Precision test results indicated that the relative standard deviations (RSDs) of the mole fractions of CH4, CO, CO2, N2O and SF6 were 0.08%, 1.90%, 0.05%, 0.08%, and 0.66%, respectively. For the linear and accuracy test, the C1-C5 tested standard gases were employed and the deviations of five gases (CH4, CO, CO2, N2O and SF6) between the calculated mole fractions of the regression equation and calibrated mole fractions were 0.15×10-9, 0.20×10-9, 0.37×10-6, 0.35×10-9 and 0.02×10-12, respectively. For CH4, CO, CO2, N2O and SF6, the linear regression coefficients (R2) between the peak areas or heights and calibrated mole fractions were 0.9999. The linear regression residual and accuracy could roughly meet the expanded target of WMO/GAW quality control. The atmospheric greenhouse gases in the Hangzhou urban area were continuously measured from May 2021 to July 2021 using the developed system. The results revealed that atmospheric CH4, CO, CO2 and N2O have visible diurnal variation characteristics that were primarily affected by anthropogenic emissions. The target standard gases were measured every 2 h to monitor the stability of the system operation, and the gas mole fractions of the system response were routinely computed and compared with the assigned calibrated values. The results demonstrated that the system had good stability during the observation period and could meet the requirements of high-precision monitoring. The comprehensive test and trial operation results showed that the developed system had good precision, accuracy, linearity and stability.

Methane emission via sediment and water interface in the Bohai Sea, China.

Sediment is recognized as the largest reservoir and source of methane (CH4) in the ocean, especially in the shallow coastal areas. To date, few data of CH4 concentration in sediment have been reported in the China shelf seas. In this study, we measured CH4 concentration in sediment and overlying seawater columns, and conducted an incubation experiment in the Bohai Sea in May 2017. CH4 concentration was found to be ranged from 3.075 to 1.795 µmol/L in sediment, which was 2 to 3 orders of magnitude higher than that in overlying seawater columns. The surface sediment was an important source of CH4, while bottom seawater acted as its sink. Furthermore, the net emission rate via sediment water interface (SWI) was calculated as 2.45 µmol/(m2∙day) based on the incubation experiment at station 73, and the earthquake may enhance CH4 release from sediment to seawater column in the eastern Bohai Sea.

Methane emissions from oil and gas platforms in the Bohai Sea, China.

Although oil and gas explorations contribute to atmospheric methane (CH4) emissions, their impact and influence along the shelf seas of China remain poorly understood. From 2012 to 2017, we conducted four ship-based surveys of CH4 in the seawater column and boundary layer of the Bohai Sea, China, and further measured CO2 and several meteorological parameters. The average observed CH4 mixing ratios in the boundary layer and its concentrations in seawater column were 1950 ± 46 ppb in November 2012 (dissolved CH4 was not observed in this survey), 2222 ± 109 ppb and 13.0 ± 5.9 nmol/L in August 2014, 2014 ± 20 ppb and 5.4 ± 1.4 nmol/L in February 2017, and 1958 ± 25 ppb and 5.3 ± 3.8 nmol/L in May 2017, respectively. The results demonstrated that the CH4 emissions from the oil and gas platforms accounted for approximately 72.5 ± 27.0% of the increase in the background atmospheric CH4 in the local area. The remaining emissions were attributed to land-sea air mass transportation. Conversely, the influence of the air-sea exchange was negligible, measuring within the 10-3 ppb range. For carbon balance calibration, the mean flaring efficiency of the oil-associated gas based on the enhancement of CO2 (ΔCO2) and enhancement sum of CO2 and CH4 (ΔCO2 + ΔCH4) was 98.5 ± 0.5%. Furthermore, the CH4 emission rate from the oil and gas platforms was 0.026 ± 0.017 Tg/year, which was approximately 7.2 times greater than the sea-to-air CH4 flux over the entire Bohai Sea area. Thus, oil and gas platforms must be recognized as important artificial hotspot sources of atmospheric CH4 in the Bohai Sea.

Multiple factors dominate the distribution of methane and its sea-to-air flux in the Bohai Sea in summer and autumn of 2014.

The Bohai Sea is well-known as a source of atmospheric methane (CH4). However, the main regulate factors of the spatiotemporal distribution of CH4 and its sea-to-air flux remain largely unknown. In this study, the observed CH4 concentration ranged from 4.8 to 32.7 nmol/L and 3.1 to 15.2 nmol/L in August and November of 2014, respectively. The main factors that influence the distribution of CH4 and its sea-to-air flux were stratification, solubility, and current structure for the mid-west depression basins, the permanent well-mixed seawater column and CH4 source strength for the centre shallow ridge zone, and the upwelling for the east depression basin, respectively. Meanwhile, wind also plays an important role in sea-to-air CH4 flux in the study area except the centre shallow ridge zone. Upwelling made the east depression basin the most intensive source of CH4, with a flux of 2 to 4 times higher than the other sub-regions.

Impact of air-sea exchange on the spatial distribution of atmospheric methane in the Dalian Bay and adjacent coastal area, China.

To date, the impact of air-sea exchange on spatial distribution of atmospheric methane (CH4 hereafter) remains less understood in the coastal areas of China. Here we measured the dissolved and atmospheric CH4 in the Dalian Bay and adjacent area in July and August 2014, respectively. Results showed that the study area was a net significant source of atmospheric CH4, with a mean sea-to-air CH4 flux of 170.6 ± 149.5 µmol/(m2·day). We optimized a method to accurately quantify the elevated atmospheric CH4 mole fraction (△CH4 hereafter) caused by air-sea exchange. The calculated △CH4 in the study area ranged from 15.4 to 102.1 nmol/mol, 1.5-10.2 nmol/mol, and 0.03-0.22 nmol/mol at the mixing height of 1, 10, and 471 m, respectively. The △CH4 mole fractions caused by air-sea exchange were positive with sea-to-air CH4 flux and in situ observed atmospheric CH4 mole fraction, while negative with altitude. Under the standard conditions, we defined 50.8 µmol/(m2·day) as the criteria value of sea-to-air CH4 flux which could result in a detectable elevation atmospheric CH4 mole fraction at a height of 10 m.

The abundance and inter-relationship of atmospheric peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), O3, and NOy during the wintertime in Beijing, China.

Although atmospheric peroxyacetyl nitrate (PAN) and O3 have been extensively measured in Beijing during the summertime, the abundances of PAN, peroxypropionyl nitrate (PPN) and the total odd-reactive nitrogen budget (NOy) and their inter-relationship have been studied comparatively less in the winter. Here we measured atmospheric PAN, PPN, O3, NOx, and NOy in Beijing from Nov. 2012 to Jan. 2013. Compared with our previous results in the summertime, much lower levels were observed in the winter, with the mean and maximum values of 311.8 and 1465 pptv for PAN, 52.8 and 850.6 pptv for PPN, and 11.6 and 36.7 ppbv for O3. In contrast, high levels were found as 94.2 and 374.9 ppbv for NOy, with a major constituent of NOx (75.9%). The source to the west and northwest made the significant contribution to the relatively high O3 concentrations during nighttime. PAN concentrations were highly related with the PAN-rich air mass transported from the southeast during the nighttime, whereas predominated by local photochemical production during the daylight. The distributions of NOx and NOy were dominated by local emission and photochemical production during daylight but also influenced by air masses transported from south direction during nighttime. Significant positive correlation (R2 = 0.9, p < 0.0001) between PAN and PPN with a slope (∆PPN/∆PAN) of 0.17 indicated that anthropogenic volatile organic compounds (AVOCs) dominated the photochemical formation of PANs in Beijing, and the independent relationship between the PPN/PAN ratio and PAN (>500 pptv) implied a steady state between PAN and PPN achieving rapidly in the polluted air masses. Negative correlation and slopes between PAN and O3 likely resulted from their weak photochemical productions in the winter, coupled with the large NO sources which acted as a local sink for O3, but much less so for PAN due to its enhanced thermal stability under low temperature.

Aragonite saturation state variation and control in the river-dominated marginal BoHai and Yellow seas of China during summer.

Based on a survey conducted from June to July 2013, aragonite saturation state variation and control in the river-dominated marginal BoHai and Yellow seas were investigated. Surface water Ωarag ranged from 2.0-3.8, whereas subsurface water Ωarag was generally lower than 2.0. Temperature changes had a strong influence on Ωarag through induced CO2 solubility changes in seawater. Riverine freshwater input decreased Ωarag in the Changjiang and Yalu river estuaries, but induced higher Ωarag in the Yellow River estuary. Biological processes had opposite effects on Ωarag, whereby elevated biological production led to the highest Ωarag in the South Yellow Sea surface water, whereas net community respiration/remineralization induced low Ωarag in subsurface water. Stratification affected the level and scale of low Ωarag in subsurface water. By the year 2100, surface water with Ωarag > 2.0 will disappear except for the Yellow River estuary, and most of the subsurface water will develop substantial aragonite undersaturation.

Ratios of greenhouse gas emissions observed over the Yellow Sea and the East China Sea.

During a cruise of the survey vessel Dongfanghong II on the Yellow Sea and the East China Sea in the spring of 2017 we performed accurate measurements of the mole fractions of carbon dioxide (CO2), methane (CH4), carbon monoxide (CO) and nitrous oxide (N2O) using two types of Cavity Ring-Down Spectrometers (CRDS). The spatial variations of the mole fraction of the four trace gases were very similar. The emission sources of these gases were divided into several regions by using the NOAA HYSPLIT model. Then we analyzed the variations of the ratios of the mole fraction enhancements between every pair of trace gases downwind of these source areas. The ratios showed that the distributions of these trace gases over the Yellow Sea and the East China Sea in the spring were mainly caused by the emissions from Eastern China. The much higher enhancement ratio of ΔCO/ΔCO2 and the lower ratio of ΔCH4/ΔCO observed in the air parcels from big cities like Beijing and Shanghai indicated high CO emission from the cities during our time of observation. Compared with the values of NOAA's Marine Boundary Layer (MBL), the ratios of the averages in the air coming from the Northern sector (Russia) were on average closer to the MBL, and the air that stayed over the Yellow Sea and the East China Sea was a mixture of emissions from wide regional areas. The highly variable N2O data of the air from Qingdao and Shanghai showed much more fluctuation.

Aragonite saturation state and dynamic mechanism in the southern Yellow Sea, China.

Based upon surveys conducted in November 2012 and June 2013, the distribution and dynamics of aragonite saturation state (Ωarag) were investigated in the southern Yellow Sea (SYS) of China. In summer, surface water Ωarag ranged from 2.1-3.8 and enhanced biological production fueled by Changjiang River freshwater input increased Ωarag to 3.8 in the southern SYS. However, subsurface water Ωarag was <2.0 in the central SYS. During autumn, surface water Ωarag was 2.0-2.9, lower than that in summer due to ventilation between surface and low Ωarag (1.0-1.4) subsurface waters in the central SYS. Community respiration and/or aerobic remineralization dominated low Ωarag in subsurface waters, while water stratification influenced the level and scale of acidity accumulation. By the end of this century, waters with Ωarag>2.0 could disappear from the SYS with increasing atmospheric CO2, while bottom waters Ωarag may become undersaturated due to the impact of eutrophication.

Enhanced methane emissions from oil and gas exploration areas to the atmosphere--the central Bohai Sea.

The distributions of dissolved methane in the central Bohai Sea were investigated in November 2011, May 2012, July 2012, and August 2012. Methane concentration in surface seawater, determined using an underway measurement system combined with wavelength-scanned cavity ring-down spectroscopy, showed marked spatiotemporal variations with saturation ratio from 107% to 1193%. The central Bohai Sea was thus a source of atmospheric methane during the survey periods. Several episodic oil and gas spill events increased surface methane concentration by up to 4.7 times and raised the local methane outgassing rate by up to 14.6 times. This study demonstrated a method to detect seafloor CH4 leakages at the sea surface, which may have applicability in many shallow sea areas with oil and gas exploration activities around the world.