Vapor hydrogen and oxygen isotopes reflect water of combustion in the urban atmosphere

A passive method for sampling water in the soil-plant-atmosphere continuum for stable hydrogen and oxygen isotope analyses

RATIONALE

Hydrogen and oxygen isotopes in water molecules are powerful tools to constrain the dynamics of water cycling within the soil-plant-atmosphere continuum (SPAC). However, the recovery of water from the SPAC requires logistical arrangements and implementation of different time- and cost-consuming techniques in either the field or the laboratory.

METHODS

We developed a passive method to sample water from the three compartments of the SPAC by using a hygroscopic salt of a high water absorbance capacity (CaCl2 ). This method allows either H2 O(V) -H2 O(L) isotope equilibration in the case of infinite water reservoir (atmospheric water vapor (WV)) or quantitative absorption of water from a finite water reservoir (e.g. soil and plants). The water absorbed by CaCl2 was distilled first and subsequently processed for hydrogen and triple oxygen isotope mass spectrometry analyses. The distillation step can be bypassed when employing isotope analytical techniques that are based on equilibration.

RESULTS

Our experiments show that anhydrous CaCl2 absorbs WV of 210 ± 6% and 130 ± 6% of its dry weight from an infinite WV reservoir at relative humidity of 60% and 30%, respectively. Chemical and isotope equilibrations between WV and absorbed water were attained within 3 days at room temperature, enabling the back-calculation of the isotope composition of atmospheric WV. Preliminary experiments to extract water from plant and sand (i.e. finite WV reservoir) demonstrate a quasi-complete recovery of water in these matrices without significant isotope fractionation. The reproducibility of our method is better than 1.6‰, 0.32‰, 0.17‰ and 6‰ per meg for δ2 H, δ18 O, δ17 O and 17 O-excess.

CONCLUSIONS

The CaCl2 -H2 O absorption (passive) method requires very limited logistics in the field facilitating spatial and temporal water vapor/water sampling from atmosphere and soil at low resolution (i.e. average of 3-5 days). Moreover, it allows high sample throughput for the extraction of plant water in the laboratory. The reproducibility of this method is similar to the analytical uncertainty in mass spectrometry analyses.

Why is the air humid during wintertime heavy haze days in Beijing?

Atmospheric humidity has been shown to promote haze formation, but it remains unclear why the air is humid during heavy haze days in winter. Here we combine water vapor isotope measurements with WRF-Chem simulations to elucidate increasing humidity with aggravation of haze during wintertime in urban Beijing. The vapor isotopic analysis in Beijing shows that the combustion-derived water (CDW) constitutes 11.0± 6.2 % of the atmospheric moisture and its fraction in total moisture increases with aggravation of haze. Modeling results reveal that, in addition to the water vapor transported from south or east to Beijing with occurrence of haze, CDW has a considerable impact on the increasing humidity when haze becomes heavy or severe. Aerosol-radiation interactions generally decrease the water vapor content and only increase humidity with occurrence of severe haze with hourly PM_2.5 concentrations exceeding 250μg m^-3. Although CDW is insignificant in the global atmospheric vapor budget, it could play an important role in modifying the local weather during haze days.

In situ measurement of water vapor isotope ratios in air with a laser-based spectrometer

Simultaneous measurement of H₂¹⁷O/H₂¹⁶O, H₂¹⁸O/H₂¹⁶O, and HDO/H₂¹⁶O in air with a compact spectrometer based on a mid-infrared distributed feedback (DFB) laser was described. The obtained mixing ratios of H₂¹⁶O, H₂¹⁷O, and H₂¹⁸O agreed reasonably well with those measured by a hygrometer. The precision and repeatability of the spectrometer were analyzed. Indoor air tests demonstrated that its 220-s precision was 0.08 ‰, 0.06 ‰, and 0.14 ‰ for δ¹⁸O, δ¹⁷O, and δ²H respectively. The measured values of δ¹⁸O, δ¹⁷O, and δ²H in indoor air were highly correlated with the water vapor mixing ratios. The compact spectrometer provides in situ measurements of water vapor isotopes with high precision and fast time response, which opens new possibilities for its application in atmospheric and hydrological research in the future.

Industrial Particulate Pollution and Historical Land Use Contribute Metals of Concern to Dust Deposited in Neighborhoods Along the Wasatch Front, UT, USA

The Salt Lake Valley, UT, USA, is proximal to the desiccating Great Salt Lake (GSL). Prior work has found that this lakebed/playa contributes metals-laden dust to snow in the Wasatch and Uinta Mountains. Dust and industrial particulate pollution are also delivered to communities along the Wasatch Front, but their sources, compositions, and fluxes are poorly characterized. In this study, we analyzed the dust deposited in 18 passive samplers positioned near the GSL, in cities in and near the Salt Lake Valley for total dust flux, the <63 µm dust fraction, ⁸⁷Sr/⁸⁶Sr, and trace element geochemistry. We compared spatial patterns in metal flux and abundance with community-level socioeconomic metrics. We observed the highest dust fluxes at sites near the GSL playa. Within the urban corridor, ⁸⁷Sr/⁸⁶Sr and trace element relative abundances suggest that most of the dust to which people are regularly exposed may be fugitive dust from local soil materials. The trace metal content of dust deposited along the Wasatch Front exceeded Environmental Protection Agency screening levels and exhibited enrichment relative to both the upper continental crust and the dust collected adjacent to GSL. Sources of metals to dust deposited along the Wasatch Front may include industrial activities like mining, oil refining, as well as past historical pesticide and herbicide applications. Arsenic and vanadium indicated a statistically significant positive correlation with income, whereas lead, thallium, and nickel exhibited higher concentrations in the least wealthy and least white neighborhoods.

Temporal variations of the contribution of combustion-derived water vapor to urban humidity during winter in Xi'an, China

Combustion-derived water vapor (CDV) has significant impacts on urban climate and environment. However, temporal variations of contribution of CDV (CCDV) to urban humidity are unclear due to lack of observations. This study examined the temporal variations of CCDV in Xi'an during winter from 2016 to 2019. We found that the diurnal variation of CCDV is mainly controlled by atmospheric stability, but the peak of CCDV at 9 am is due to the increasing water vapor emission by motor vehicles during the morning rush hour. In addition, the monthly variation of CCDV is related to fossil fuel consumption, but the low values of CCDV in late January and early February is due to substantial decrease of energy utility because of the massive outflow of population during the Spring Festival. Our findings may be helpful for urban pollution control because CDV can play an important role in the secondary conversion of pollutants.

Role of Vehicular Catalytic Converter Temperature in Emission of Pollutants: An Assessment Based on Isotopic Analysis of CO2 and N2O

Vehicular catalytic converters are used to regulate, reduce, and convert toxic and environmentally unfriendly compounds in exhaust gases into relatively inert and less harmful chemical species. The efficiency, however, is largely affected by the operating temperature of the converter which is set by the hot exhaust gas released from the combustion chamber. A major gas released during combustion is CO₂, and its multiply substituted isotopocule, namely, ¹³C¹⁶O¹⁸O, provides a window of opportunity to probe directly the effective temperature of the converter in operation. Here, we report multiple isotopic measurements in exhaust CO₂ (δ¹³C, δ¹⁷O, δ¹⁸O, and Δ₄₇) of diesel (trucks and buses) and gasoline (sedans, trucks, and two-wheel motorcycles)-powered vehicles. For investigating the efficiency of a converter in reducing toxic compounds, we studied NO_x processes through isotopic analysis of the exhaust N₂O. We found that the degree of N₂O reduction to N₂ in gasoline-powered vehicles is high when the temperature is above 200 °C (inferred by Δ₄₇). In contrast, diesel-powered vehicles produce N₂O in abundance, probably a consequence of selective catalytic reduction of NO_x, and the reduction efficiency depends on the converter temperature. In other words, the catalytic converters act as sinks and sources of N₂O to the atmosphere in gasoline- and diesel-operated vehicles, respectively. We also report a new set of field data by measuring the isotopic compositions of CO₂ and N₂O in the Hsuehshan tunnel, a ∼13 km long highway tunnel in Taiwan. Elevated N₂O concentrations inside the tunnel indicate that the emission of N₂O by heavy-duty diesel vehicles is much higher compared to the reduction by gasoline-operated passenger cars, making the vehicular exhausts a net source of N₂O to the atmosphere. The combined study of clumped isotopes and N₂O concentration in exhaust gases suggests that it is useful to probe the operational temperature of catalytic converters and monitor the pollution level in operation, thus providing an opportunity for manufacturers to optimize the catalytic efficiency to reduce the level of toxic pollutants to the environment.

Vapor isotopic evidence for the worsening of winter air quality by anthropogenic combustion-derived water

Water vapor emitted from anthropogenic combustion for winter heating in northern China may exacerbate air pollution. This hypothesis is of considerable scientific and environmental interest. We conducted a multiyear sampling campaign of air vapor isotope compositions and associated atmospheric data from the city of Xi’an, located in an enclosed basin in northwestern China. We found that the fraction of combustion-derived water vapor increases with increasing relative humidity and with the concentration of particulate matter with an aerodynamic diameter less than 2.5 μm in polluted conditions based on field observation, isotopic analysis, and numerical simulation. Our results demonstrated that combustion-derived water is nontrivial when considering energy policy for improving air quality.

Anthropogenic combustion-derived water (CDW) may accumulate in an airshed due to stagnant air, which may further enhance the formation of secondary aerosols and worsen air quality. Here we collected three-winter-season, hourly resolution, water-vapor stable H and O isotope compositions together with atmospheric physical and chemical data from the city of Xi’an, located in the Guanzhong Basin (GZB) in northwestern China, to elucidate the role of CDW in particulate pollution. Based on our experimentally determined water vapor isotope composition of the CDW for individual and weighted fuels in the basin, we found that CDW constitutes 6.2% of the atmospheric moisture on average and its fraction is positively correlated with [PM_2.5] (concentration of particulate matter with an aerodynamic diameter less than 2.5 μm) as well as relative humidity during the periods of rising [PM_2.5]. Our modeling results showed that CDW added additional average 4.6 μg m⁻³ PM_2.5 during severely polluted conditions in the GZB, which corresponded to an average 5.1% of local anthropogenic [PM_2.5] (average at ∼91.0 μg m⁻³). Our result is consistent with the proposed positive feedback between the relative humidity and a moisture sensitive air-pollution condition, alerting to the nontrivial role of CDW when considering change of energy structure such as a massive coal-to-gas switch in household heating in winter.

Recycled moisture in an enclosed basin, Guanzhong Basin of Northern China, in the summer: Contribution to precipitation based on a stable isotope approach

Recycled moisture, mainly originated from evapotranspiration (surface evaporation and transpiration), is the main sources of precipitation. Influenced on the different regional/local environments, the contributions of recycled moisture to precipitation present as different proportions. Recycled moisture has an important impact on the hydrological cycle, further occurred a series of environmental effect for regional/local. Aimed to estimate the contribution of recycled moisture to precipitation in an enclosed basin, Guanzhong Basin of northern China, precipitation and lake/reservoir samples were collected. The isotope ratio analysis was done for the summer season, and a three-component mixing model based on the stable hydrogen and oxygen isotopes was applied. The results indicated that the averaged contribution of recycled moisture to precipitation was 17.44% in Guanzhong Basin of northern China, while the mean proportions of surface evaporation moisture and transpiration moisture were found to be 0.38% and 16.97%, respectively. Comparatively, most of the recycled moisture mainly comes from transpiration moisture rather than evaporation moisture, suggesting that transpiration moisture from cropland, vegetation, and plants instead of evaporation is dominant in moisture recycling of the Guanzhong Basin.

Anthropogenic Effects on Hydrogen and Oxygen Isotopes of River Water in Cities

Stable hydrogen and oxygen isotopes are important indicators for studying water cycles. The isotopes are not only affected by climate, but are also disturbed by human activities. Urban construction has changed the natural attributes and underlying surface characteristics of river basins, thus affecting the isotopic composition of river water. We collected urban river water isotope data from the Global Network for Isotopes in Rivers (GNIR) database and the literature, and collected river water samples from the Naqu basin and Huangshui River basin on the Tibetan Plateau to measure hydrogen and oxygen isotopes. Based on 13 pairs of urban area and non-urban area water samples from these data, the relationship between the isotopic values of river water and the artificial surface area of cities around rivers was analyzed. The results have shown that the hydrogen and oxygen isotope (δD and δ¹⁸O) values of river water in urban areas were significantly higher than those in non-urban areas. The isotopic variability of urban and non-urban water was positively correlated with the artificial surface area around the rivers. In addition, based on the analysis of isotope data from 21 rivers, we found that the cumulative effects of cities on hydrogen and oxygen isotopes have led to differences in surface water line equations for cities with different levels of development. The combined effects of climate and human factors were the important reasons for the variation of isotope characteristics in river water in cities. Stable isotopes can not only be used to study the effects of climate on water cycles, but also serve as an important indicator for studying the degree of river development and utilization.

Characteristics of Stable Hydrogen and Oxygen Isotopes of Soil Moisture under Different Land Use in Dry Hot Valley of Yuanmou

Soil moisture, stable hydrogen, and oxygen isotopes were sampled and determined in a demonstration area of soil and moisture conservation at the Laocheng Town of Yuanmou County in Chuxiong Prefecture, Yunnan of three land use types: Leucaena Benth artificial forest, Heteropogon contortus grass field, and farmland. The characteristics of stable hydrogen and oxygen isotopes of soil moisture in these different land use types at different soil depths were analyzed to investigate the regularities in the quantitative formation of soil moisture balance. In terms of forest land, we found that the variable coefficient of hydrogen isotopes in the 0-20 cm soil layer was the smallest, but decreased with depth under 20 cm. While in grassland, the variable coefficient in 80-100 cm was the largest, and decreased with depth above 80 cm. As for farmland, the variable coefficient in the top 20 cm was the largest, followed by 40-60 cm, and the medium 20-40 cm was the smallest. The soil moisture hydrogen isotope values of three land use type were different at surface layer, but prone to be consistent in each type. Along the soil depth in forest land, the hydrogen isotope increased first and then decreased, while increased in the end, and the maximum appeared in 80-100 cm. In grassland, the hydrogen isotope increased initially as the forest land but then decreased continuously, so the maximum was found at 20-40 cm. And in grassland, the hydrogen isotope of all depths were higher than which of forest land and farmland. In same land use type, the hydrogen isotope of soil moisture changed significantly at the surface, and the variation of hydrogen isotopes was obviously decreased along the depth. Our findings could provide reference data which would contribute to the assessment of regional groundwater resources in the dry-hot valley of Yuanmou in this study.

Effects of Styrene-Acrylic Sizing on the Mechanical Properties of Carbon Fiber Thermoplastic Towpregs and Their Composites

Thermoplastic towpregs are convenient and scalable raw materials for the fabrication of continuous fiber-reinforced thermoplastic matrix composites. In this paper, the potential to employ epoxy and styrene-acrylic sizing agents was evaluated for the making of carbon fiber thermoplastic towpregs via a powder-coating method. The protective effects and thermal stability of these sizing agents were investigated by single fiber tensile test and differential scanning calorimetry (DSC) measurement. The results indicate that the epoxy sizing agent provides better protection to carbon fibers, but it cannot be used for thermoplastic towpreg processing due to its poor chemical stability at high temperature. The bending rigidity of the tows and towpregs with two styrene-acrylic sizing agents was measured by cantilever and Kawabata methods. The styrene-acrylic sized towpregs show low torque values, and are suitable for further processing, such as weaving, preforming, and winding. Finally, composite panels were fabricated directly from the towpregs by hot compression molding. Both of the composite panels show superior flexural strength (>400 MPa), flexural modulus (>63 GPa), and interlaminar shear strength (>27 MPa), indicating the applicability of these two styrene-acrylic sizing agents for carbon fiber thermoplastic towpregs.

Continuous measurements of stable isotopes of carbon dioxide and water vapour in an urban atmosphere: isotopic variations associated with meteorological conditions

Isotope ratios of carbon dioxide and water vapour in the near-surface air were continuously measured for one month in an urban area of the city of Nagoya in central Japan in September 2010 using laser spectroscopic techniques. During the passages of a typhoon and a stationary front in the observation period, remarkable changes in the isotope ratios of CO₂ and water vapour were observed. The isotope ratios of both CO₂ and water vapour decreased during the typhoon passage. The decreases can be attributed to the air coming from an industrial area and the rainout effects of the typhoon, respectively. During the passage of the stationary front, δ¹³C-CO₂ and δ¹⁸O-CO₂ increased, while δ²H-H₂Ov and δ¹⁸O-H₂Ov decreased. These changes can be attributed to the air coming from rural areas and the air surrounding the observational site changing from a subtropical air mass to a subpolar air mass during the passage of the stationary front. A clear relationship was observed between the isotopic CO₂ and water vapour and the meteorological phenomena. Therefore, isotopic information of CO₂ and H₂Ov could be used as a tracer of meteorological information.

Urban Emissions of Water Vapor in Winter

Elevated water vapor (H2Ov) mole fractions were occassionally observed downwind of Indianapolis, IN, and the Washington, D.C.-Baltimore, MD, area during airborne mass balance experiments conducted during winter months between 2012 and 2015. On days when an urban H2Ov excess signal was observed, H2Ov emissions estimates range between 1.6 × 104 and 1.7 × 105 kg s-1, and account for up to 8.4% of the total (background + urban excess) advected flow of atmospheric boundary layer H2Ov from the urban study sites. Estimates of H2Ov emissions from combustion sources and electricity generation facility cooling towers are 1-2 orders of magnitude smaller than the urban H2Ov emission rates estimated from observations. Instances of urban H2Ov enhancement could be a result of differences in snowmelt and evaporation rates within the urban area, due in part to larger wintertime anthropogenic heat flux and land cover differences, relative to surrounding rural areas. More study is needed to understand why the urban H2Ov excess signal is observed on some days, and not others. Radiative transfer modeling indicates that the observed urban enhancements in H2Ov and other greenhouse gas mole fractions contribute only 0.1°C day-1 to the urban heat island at the surface. This integrated warming through the boundary layer is offset by longwave cooling by H2Ov at the top of the boundary layer. While the radiative impacts of urban H2Ov emissions do not meaningfully influence urban heat island intensity, urban H2Ov emissions may have the potential to alter downwind aerosol and cloud properties.