Thermodynamic Modeling Suggests Declines in Water Uptake and Acidity of Inorganic Aerosols in Beijing Winter Haze Events during 2014/2015-2018/2019

During recent years, aggressive air pollution mitigation measures in northern China have resulted in considerable changes in gas and aerosol chemical composition. But it is unclear whether aerosol water content and acidity respond to these changes. The two parameters have been shown to affect heterogeneous production of winter haze aerosols. Here, we performed thermodynamic equilibrium modeling using chemical and meteorological data observed in urban Beijing for four recent winter seasons and quantified the changes in the mass growth factor and pH of inorganic aerosols. We focused on high relative humidity (>60%) conditions when submicron particles have been shown to be in the liquid state. From 2014/2015 to 2018/2019, the modeled mass growth factor decreased by about 9%-17% due to changes in aerosol compositions (more nitrate and less sulfate and chloride), and the modeled pH increased by about 0.3-0.4 unit mainly due to rising ammonia. A buffer equation is derived from semivolatile ammonia partitioning, which helps understand the sensitivity of pH to meteorological and chemical variables. The findings provide implications for evaluating the potential chemical feedback in secondary aerosol production and the effectiveness of ammonia control as a measure to alleviate winter haze.

Self-Assembled Liquid-Gated Zinc Oxide Nanowire Transistors

Vivek Pachauri

This thesis aims at the site-specific realization of self-assembled field-effect transistors (FETs) based on semiconducting Zinc oxide NWs and their application towards chemical and bio-sensing in liquid medium. At first, a solution based growth method for hierarchical ZnO nanostructures was devised in order to achieve synthesis of high quality ZnO NWs. This solution based growth method was then deployed for the growth of NWs from preferred sites on a substrate. In order to make a transistor, microelectrode pairs prewritten on a Si/SiO2 chip (4mm◊4mm) using standard photolithography procedure served as growth sites and also formed source and drain terminals. The NWs bridging these "source" and "drain" electrodes formed the transistor channel. The procedure here yields ZnO NWs in up to 100 percent of the positions available for growth. The site-specific self-assembled fabrication of ZnO NW FETs was evaluated in terms of scalability and applications. The procedure was extended from 4mm◊4mm silicon chips to large area silicon wafers (80mm◊80mm) and flexible substrates of Kapton polyimide of the same size. On the large area substrates this procedure yields ZnO NWs in up to 80 percent of the positions available for growth, which can be bettered. Furthermore, the method was also employed to fabricate ZnO NW FETs in situ in a microfluidic channel. For this purpose the precursor solution was let to flow on to the microelectrode pairs with the help of microfluidic channels assembled on top of the substrate. The substrate was heated locally under the microelectrode pairs to stimulate the NW growth. Thus a solution-based self-assembled fabrication of NW FETs was achieved locally inside a microfluidic channel. Microfluidic channels were made of silicon nitride (Si3N4) on top of Si/SiO2 chips and facilitated the characterization of FETs in liquid medium. Alternatively, microfluidic channels made of polydimethylsiloxane (PDMS) were used for characterization of transistor devices in liquids. In order to deploy FETs in liquids, the back-gate is replaced by a reference electrode (Ag/AgCl) and the potential is applied through a liquid surrounding the transistor channel (ZnO NW). The liquid surrounding the ZnO NW creates an electrochemical double layer (EDL) on the NW surface which in turn gives rise to the gate capacitance. The resulting gating effect can be used to modulate the NW conductance depending on the voltage applied through the liquid. The ZnO NW transistors showed a current modulation of up to 6 orders of magnitude, high field-effect mobilities (around 1.85 cm2/Vs) and sub-threshold slopes as low as 105 mV/decade. This is the first demonstration of liquid-gated FETs using ZnO NWs. The liquid-gated FETs are used as a basic device for further sensing trials in liquids. For this purpose, the FETs were functionalized with receptor molecules. These so called ion-selective FETs (ISFETs) were demonstrated as functional pH sensors for liquids with pH values from 6 to 10. The NWs functionalized with analyte-sensitive molecules on their surface are influenced electrically by the presence of analytes in the surrounding liquid. Any change in the amount of analyte present in the surrounding liquid is thus reflected in the electrical transport characteristics of the FET. 3-aminopropyltriethoxysilane (3-APTES) molecules were used to functionalize the ZnO NWs in order to construct the pH sensors. Subsequently, the realization of a biosensor based on ZnO NW transistors is demonstrated. Label-free direct detection of urea molecules was carried out from a solution of urea in buffer. ZnO NWs were electrochemically functionalized with the enzyme urease incorporated into an electro-polymerized polypyrrole matrix. Urease molecules act as specific receptors for urea molecules and catalyze an enzymatic reaction. This reaction causes a pH change in the vicinity of NWs, which is reflected in the field-effect characteristics of transistor. The performance of liquid-gated ZnO NW FETs used as chemical and biological sensor can be further improved by employing a metal gate in liquid medium. This was established by fabricating liquid gated metal-semiconductor FET (MESFET) based on ZnO NWs. ZnO NWs were decorated with metal nanoparticles (NPs) by using an electrochemical deposition method. The NPs were then used as metal gate and devices were characterized by applying a gate voltage on the NPs through the liquid. The realization of metal-semiconductor gate in liquids considerably improved the field-effect characteristics of liquid-gated FETs based on ZnO NWs. The realization of high performance liquid-gated transistors based on ZnO NWs thus constitutes a suitable platform for label-free detection of biomolecules and shows promise for future applications in chemical analysis and medical diagnostics.

EPFL2011

BIOPHYSICAL CONTROLS ON EVAPOTRANSPIRATION: NUMERICAL MODELS AND EXPERIMENTAL OBSERVATIONS

Federico Croci

Water and food are basics of human life, unfortunately climate changes and water pollution have reduced the amount of water available for human uses. In this context the study of the hydrologic cycle is becoming of growing importance in order to optimize the anthropogenic exploitation of water resource. Evapotranspiration is the term used to describe the part of the water cycle which removes liquid water from an area with vegetation into the atmosphere by the processes of both transpiration and evaporation. Evapotranspiration is an important part of the hydrologic cycle because it represents a considerable percentage of water losses in watersheds. This process is also one of the main consumers of solar energy at the Earth's surface. The estimation of evapotranspiration it is of great importance for agriculture and water resources management, for example, by knowing the rate of water loss from a region, farmers will be better placed to efficiently manage the irrigation strategies. The rate of evapotranspiration at any location on the Earth's surface is controlled by several factors such as energy availability, humidity gradient, wind speed immediately above the surface, water availability, type of vegetation, stomatal resistance and soil characteristics. The problem of estimating evapotranspiration has been widely studied. During the past several decades, different approaches have been proposed and many formulas are available in the literature to fulfill this task. The complexity of the phenomena and its great variability all over the world make difficult to find a universal and robust method for the evapotranspiration prediction and estimation. Evapotranspiration can be obtained by many estimation methods. Some of these methods need many weather parameters as inputs while others need fewer. Factors such as data availability, the intended use, and the time scale required by the problem must be considered when choosing the evapotranspiration calculation technique. The Penman-Monteith equation was derived from the classic Penman equation adding to the terms already considered in the Penman formulation (aerodynamic resistance and energy balance) the canopy resistance. This approach requires many meteorological data, such as maximum and minimum air temperatures, relative humidity, solar radiation, and wind speed. If some of these data are not available, alternative techniques have been developed to overcome this drawback. However, this estimations of meteorological data affect the quality of the result, alternatively, in such cases other simpler methods may be used. The Penman-Monteitu equation is assumed to be the most reliable because is based on physical principles and because it considers all the climatic factors, which affect evapotranspiration.

2013

Particle water and pH in the eastern Mediterranean: Source variability and implications for nutrient availability

Athanasios Nenes

Particle water (liquid water content, LWC) and aerosol pH are important parameters of the aerosol phase, affecting heterogeneous chemistry and bioavailability of nutrients that profoundly impact cloud formation, atmospheric composition, and atmospheric fluxes of nutrients to ecosystems. Few measurements of in situ LWC and pH, however, exist in the published literature. Using concurrent measurements of aerosol chemical composition, cloud condensation nuclei activity, and tandem light scattering coefficients, the particle water mass concentrations associated with the aerosol inorganic (Winorg) and organic (Worg) components are determined for measurements conducted at the Finokalia atmospheric observation station in the eastern Mediterranean between June and November 2012. These data are interpreted using the ISORROPIA-II thermodynamic model to predict the pH of aerosols originating from the various sources that influence air quality in the region. On average, closure between predicted aerosol water and that determined by comparison of ambient with dry light scattering coefficients was achieved to within 8ĝ€% (slopeĝ€ Combining double low line ĝ€0.92, R2ĝ€ Combining double low line ĝ€0.8, nĝ€ Combining double low line ĝ€5201 points). Based on the scattering measurements, a parameterization is also derived, capable of reproducing the hygroscopic growth factor (f(RH)) within 15ĝ€% of the measured values. The highest aerosol water concentrations are observed during nighttime, when relative humidity is highest and the collapse of the boundary layer increases the aerosol concentration. A significant diurnal variability is found for Worg with morning and afternoon average mass concentrations being 10-15 times lower than nighttime concentrations, thus rendering Winorg the main form of particle water during daytime. The average value of total aerosol water was 2.19ĝ€±ĝ€1.75ĝ€μgĝ€mĝ'3, contributing on average up to 33ĝ€% of the total submicron mass concentration. Average aerosol water associated with organics, Worg, was equal to 0.56ĝ€±ĝ€0.37ĝ€μgĝ€mĝ'3; thus, organics contributed about 27.5ĝ€% to the total aerosol water, mostly during early morning, late evening, and nighttime hours.The aerosol was found to be highly acidic with calculated aerosol pH varying from 0.5 to 2.8 throughout the study period. Biomass burning aerosol presented the highest values of pH in the submicron fraction and the lowest values in total water mass concentration. The low pH values observed in the submicron mode and independently of air mass origin could increase nutrient availability and especially P solubility, which is the nutrient limiting sea water productivity of the eastern Mediterranean. © Author(s) 2016.

Copernicus GmbH2016