Nitrate Mass Research Articles

Hygroscopic growth and activation changed submicron aerosol composition and properties in the North China Plain

Abstract. Aerosol hygroscopic growth and activation under high-relative-humidity (RH) conditions significantly influence the physicochemical properties of submicron aerosols (PM1). However, this process remains poorly characterized due to limited measurements. To address this gap, we deployed an advanced aerosol–fog sampling system that automatically switched between PM1, PM2.5 and total suspended particulate (TSP) inlets at a rural site in the North China Plain in the cold season. The results revealed that aerosol swelling due to water vapor uptake influenced aerosol sampling under high-RH conditions by shifting the cut-off size of impactors. At subsaturated high RH (> 90 %), over 25 % of aerosol mass with dry diameters below 1 µm resided in supermicron ranges, while in supersaturated foggy conditions, more than 70 % of submicron aerosol migrated to supermicron ranges. Hygroscopic growth and activation particularly affected highly hydrophilic inorganic salts, shifting a significant number of submicron sulfate and nitrate particles to supermicron ranges, with 27 %–33 % at 95 % ≤ RH ≤ 99 % and more than 78 % under supersaturated foggy conditions. Moreover, more than 10 % of submicron biomass burning organic aerosols grew beyond 2.5 µm during fog events, while fossil-fuel-related organic aerosol (FFOA) remained dominantly in submicron ranges, suggesting inefficient aqueous conversion of FFOA. The two secondary organic aerosol (SOA) factors (OOA1 and OOA2) behaved differently under supersaturated conditions, with OOA2 exhibiting a higher activated fraction despite a lower oxygen / carbon ratio. A substantial increase in organic nitrate and organosulfur mass concentrations in activated droplets during fog events suggested aqueous conversions and formations of brown carbon with potential radiative impacts. Overall, our study highlights remarkably different cloud and fog processing behaviors between primary and secondary aerosols, which would benefit a better understanding of aerosol–cloud interactions under distinct atmospheric conditions.

Study on Preparation of High-Performance Flake Silver Powder by Liquid Phase Reduction

Flake silver powder was prepared by liquid phase reduction method with silver nitrate as silver source, ferrous sulfate heptahydrate as reducing agent, citric acid and gum acacia as surfactants. The effects of reducing agent dosage, citric acid dosage, gum acacia dosage, silver nitrate dosage and reaction temperature on the properties of silver powder were studied. The results show that when the dosage of silver nitrate is 0.08 mol·L−1, the molar ratio of silver nitrate to ferrous sulfate heptahydrate is 1:2, the oxidizing solution is added to the reducing solution at the rate of 100 ml·min−1, the citric acid is added to the oxidizing solution at 7.5% of the mass of silver nitrate, the gum acacia is added equally dispersed to the oxidizing solution and the reducing solution, the addition amount is 20% of the mass of silver nitrate and the reaction temperature is 30 °C, the silver powder obtained is flake with good dispersion and narrow particle size distribution.

Coupled hydrogeological modeling and nitrate transport modeling in an anthropized valley, a case study of the lower Soummam valley (Bejaïa Northeast of Algeria)

Anthropogenic activities, including wastewater discharges and fertilizer use, were identified as the primary factors influencing nitrate concentrations in the lower Soummam Valley in the North-East of Algeria. This study aimed to investigate nitrogen pollution and simulate nitrate mass transport coupled with a numerical flow model. Hydrogeochemical findings revealed significant seasonal variations between the high-water period (May 2019 and 2021) and the low-water period (September 2019 to November 2020 and 2021). During the low-water period, both temperature and electrical conductivity increased. However, pH, dissolved oxygen, and nitrate levels decreased in the high-water period. Nitrites and ammonium exhibited irregular fluctuations. The numerical flow model was developed using PMWIN and coupled with a solute transport model simulated with the Model Transport 3 Dimensions (MT3D). Calibration was conducted under steady-state conditions from 2011 to 2021 and was validated by comparing piezometric heads and nitrate concentrations measured in May 2021. Three scenarios were devised to simulate transient conditions from 2011 to 2050, considering variations in both groundwater levels and nitrate concentrations. The simulation revealed that river and dependent head boundaries are the primary sources of groundwater recharge. These boundaries contributed to approximately 87% of the total inflow. The model also depicted the water exchange between the aquifer and the river. However, the constant head boundary represented the main outflow, accounting for 85.7% of the total. The nitrate model illustrated an increase in nitrate concentrations, reaching 3.8 mg/L in 27 out of the 29 simulated wells. Both the river and groundwater head boundaries were identified as the key factors governing the hydrodynamic aquifer system and nitrate transport in the alluvial aquifer of the lower Soummam Valley.

The Impacts of Dust Storms With Different Transport Pathways on Aerosol Chemical Compositions and Optical Hygroscopicity of Fine Particles in the Yangtze River Delta

AbstractAerosol physicochemical properties during two dust storms (DS1: March 30–31, and DS2: May 7–8) are measured in 2021 in Nanjing, aiming to investigate the impacts of dust storms on aerosol chemical compositions and optical hygroscopicity in the Yangtze River Delta (YRD). During DS1, the dust air masses are transported in the lower atmosphere and pass through the inlands and sea areas before reaching the YRD region. During DS2, the dust air masses are transported in the upper atmosphere and pass through the inlands only. Both of them are accompanied by an increase in black carbon (BC) mass fraction and a decrease in nitrate mass fraction in fine particles (PM2.5, particles with diameters less than 2.5 μm) near the surface. However, the impacts on the mass fractions of organics or sulfate are adverse between DS1 and DS2. During dust‐influence periods the enlarged mass ratios of sulfate to nitrate (greater than 4) promote the occurrence of deliquescent behavior of ambient aerosols although dust aerosols can significantly suppress aerosol optical hygroscopicity. To improve model simulations of aerosol optical hygroscopicity, it is necessary to use a segment parameterization to describe aerosol light scattering enhancement factor during dust‐influence periods. The closure study of optical hygroscopicity parameters with two different methods reveals the impact of aerosol scattering Ångström exponent on the estimation of optical hygroscopicity parameter. The results highlight that the impact of dust storms on aerosol chemical compositions and optical hygroscopicity are different for DS events with different transport pathways.

Aerosol Thermodynamics: Nitrate Loss from Regulatory PM2.5 Filters in California.

Fine particulate matter (PM2.5) mass concentrations reported by regulatory networks are declining across the United States. It is well established that ammonium nitrate contributes substantially to the PM2.5 mass in the western United States, and that Teflon filters commonly used by regulatory monitors are subject to negative mass artifacts due to ammonium nitrate volatilization. This study focuses on the San Joaquin Valley (SJV), an environmental justice (EJ) and agricultural region with persistently poor air quality. The SJV is a serious nonattainment area of PM2.5 National Ambient Air Quality Standards (NAAQS) with substantial nitrate mass concentrations. We explicitly model the chemical thermodynamic equilibrium of the ammonium nitrate-nitric acid systems and quantify volatilization across California as a function of the deliquescence point relative humidity (%DRH). Nitrate loss is estimated at all federal reference method (FRM) and federal equivalent method (FEM) monitors from 2001 to 2021. Nearly 20% of PM2.5 mass is lost from filters in the SJV area, especially during winter and fall when particulate nitrate mass is most abundant. All decadal PM2.5 trends calculated from reported measurements in Kern, Tulare, and Fresno counties in the SJV show greater decline in PM2.5 mass when nitrate loss is accounted for, up to a factor of 20 in Kern county. This suggests PM2.5 mass concentrations reported in regulatory networks are biased low relative to the actual atmospheric burden, notably in an EJ area that lags behind most of the country's air quality improvements.

Enhanced Nitrate Fraction: Enabling Urban Aerosol Particles to Remain in a Liquid State at Reduced Relative Humidity

AbstractNitrate has become the primary inorganic compound in urban aerosol particles, but its effects on particle phase state, which is crucial in multiphase chemistry, remains largely unknown. Herein, particle rebound measurements were conducted to explore the relationship between the liquid–phase–transition threshold relative humidity (RHthreshold) and the inorganic compounds mass fraction in dry particles (Finorg). Results revealed negative correlations between RHthreshold and Finorg, with more nitrate leading to lower RHthreshold. Even with RH < 20%, particles with ∼50% nitrate mass fraction remained in non‐solid state. Taking Beijing as an example, decreases were observed in RHthreshold from 64% in 2015 to below 53% nowadays during the moderate‐pollution periods (PM2.5 = 35–75 μg/m3) due to an enhanced nitrate fraction. This allowed urban aerosol particles to exist in liquid state at lower RH, and consequently, kinetic limitation by bulk diffusion in nitrate‐dominated particles might be negligible, making them key seeds for secondary aerosol formation through multiphase reactions.

Photothermal Superhydrophobic Cotton Fabric Based on Silver Nanoparticles Cross-Linked by Polydopamine and Polyethylenimide.

Photothermal materials that can convert solar energy into heat energy through photothermal conversion have attracted extensive attention, but these materials are easily polluted by the environment. Here, we propose a simple and effective strategy for constructing photothermal superhydrophobic cotton fabrics with self-cleaning ability. The PDA@PEI@GA@Ag@PDMS-coated cotton fabric can achieve good superhydrophobicity (water contact angle: 159.6°) by a simple dipping method and mussel-inspired dopamine surface modification, which is regulated by the mass of dopamine, the mass of silver nitrate, and the concentration of polydimethylsiloxane (PDMS). The coated cotton fabric has good physical and chemical stability. Meanwhile, the coated cotton fabric has excellent self-cleaning and antifouling properties. The superhydrophobic PDA@PEI@GA@Ag@PDMS fabric exhibits excellent and stable photothermal properties, with the surface temperature reaching 70.4 °C under simulated sunlight with a current of 20 A. This photothermal superhydrophobic fabric with self-cleaning properties is expected to be applied in the field of photothermal conversion.

Effect of mixed physical barrier on seawater intrusion and nitrate accumulation in coastal unconfined aquifers.

Physical barrier has been proven to be one of the most effective measures to prevent and control seawater intrusion (SWI) in coastal areas. Mixed physical barrier (MPB), a new type of physical barrier, has been shown to have higher efficiency in SWI control. As with conventional subsurface dam and cutoff wall, the construction of MPB may lead to the accumulation of nitrate contaminants in coastal aquifers. We investigated the SWI control capacity and nitrate accumulation in the MPB using a numerical model of variable density flow coupling with reactive transport, and performed sensitivity analysis on the subsurface dam height, cutoff wall depth and opening spacing in the MPB. The differences in SWI control and nitrate accumulation between MPB and conventional subsurface dam and cutoff wall were compared to assess the applicability of different physical barrier. The numerical results show that the construction of MPB will increase the nitrate concentration and contaminated area in the aquifer. The prevention and control efficiency of MPB against SWI is positively correlated with the depth of the cutoff wall, reaching the highest efficiency at the minimum effective dam height, and the retreat distance of the saltwater wedge is positively correlated with the opening spacing. We found a non-monotonic relationship between the change in subsurface dam height and the extent of nitrate accumulation, with total nitrate mass and contaminated area increasing and then decreasing as the height of the subsurface dam increased. The degree of nitrate accumulation increased linearly with increasing the height of the cutoff wall and the opening spacing. Under certain conditions, MPB is 46-53% and 16-57% more efficient in preventing and controlling SWI than conventional subsurface dam and cutoff wall, respectively. However, MPB caused 14-27% and 2-12% more nitrate accumulation than subsurface dam and cutoff wall, respectively. The findings of this study are of great value for the protection of coastal groundwater resources and will help decision makers to select appropriate engineering measures and designs to reduce the accumulation of nitrate pollutants while improving the efficiency of SWI control.

Modification of a dual‐chamber denitrification bioreactor with a surface water pumping system

AbstractDenitrification bioreactors are an edge‐of‐field conservation practice being implemented to reduce nonpoint nitrogen pollution to downstream waterbodies. In the Midwestern region of the United States, these bioreactors are commonly used for the treatment of nitrate‐laden subsurface drainage systems. Innovative strategies will be needed to reach the nutrient reduction goals established; here, a typical denitrification bioreactor was retrofitted with a supplemental surface water pumping system to enhance the bioreactor use and performance. Potential benefits of the pumped bioreactor system include extended treatment beyond the typical drainage season, increased nitrate mass removal, extended bioreactor lifespan, and extended applications of the bioreactor such as treatment of surface waters. Current challenges associated with pumped bioreactors exist with the timing of the pumping and water source identification. Considerations include the water availability and the potential need to obtain a permit for the water extraction, and nitrate concentration, temperature, and carbon content of the source to be pumped from. Conditions that would promote complete nitrate removal should be avoided. Additional potential applications for these pumped bioreactors have been identified and include, but are not limited to, the treatment of additional surface water sources, irrigation waters, drainage ditches, and groundwater.

Boosting Nitrate to Ammonia Electroconversion through Hydrogen Gas Evolution over Cu-foam@mesh Catalysts.

The hydrogen evolution reaction (HER) is often considered parasitic to numerous cathodic electro-transformations of high technological interest, including but not limited to metal plating (e.g., for semiconductor processing), the CO2 reduction reaction (CO2RR), the dinitrogen → ammonia conversion (N2RR), and the nitrate reduction reaction (NO3-RR). Herein, we introduce a porous Cu foam material electrodeposited onto a mesh support through the dynamic hydrogen bubble template method as an efficient catalyst for electrochemical nitrate → ammonia conversion. To take advantage of the intrinsically high surface area of this spongy foam material, effective mass transport of the nitrate reactants from the bulk electrolyte solution into its three-dimensional porous structure is critical. At high reaction rates, NO3-RR becomes, however, readily mass transport limited because of the slow nitrate diffusion into the three-dimensional porous catalyst. Herein, we demonstrate that the gas-evolving HER can mitigate the depletion of reactants inside the 3D foam catalyst through opening an additional convective nitrate mass transport pathway provided the NO3-RR becomes already mass transport limited prior to the HER onset. This pathway is achieved through the formation and release of hydrogen bubbles facilitating electrolyte replenishment inside the foam during water/nitrate co-electrolysis. This HER-mediated transport effect "boosts" the effective limiting current of nitrate reduction, as evidenced by potentiostatic electrolyses combined with an operando video inspection of the Cu-foam@mesh catalysts under operating NO3-RR conditions. Depending on the solution pH and the nitrate concentration, NO3-RR partial current densities beyond 1 A cm-2 were achieved.

Extended pilot test of a cross-injection in situ denitrification system for pre-emptive treatment of municipal well water

Elevated groundwater nitrate concentrations have been linked to deleterious health and environmental effects. A significant source of the nitrate is nitrogen fertilizers applied to agricultural landscapes. Beneficial Management Practices (BMPs), including the optimization of fertilizer use and selective crop rotations, have proven to be effective in some cases. The city of Woodstock in southern Ontario relies on public wells for all of its municipal supply. Several of the wells have experienced chronic increases in nitrate concentrations exceeding the maximum allowable limit of 10 mg/L N-NO3−. While BMPs are established, an interim reduction plan based on enhanced in situ denitrification (Cross Injection System, CIS) in a 15 m thick zone of high nitrate mass flux within the aquifer zone was evaluated. Based in the results of preliminary acetate injection experiments, a C:N ratio of 2.35, (approximately 260 mg acetate/L), was selected to optimize the denitrification reaction. Injections were performed for six hours a day every day for a period of approximately two months. Dissolved oxygen (DO) and nitrate concentrations recorded over time indicated that reduction of both commenced within a few days of the beginning of the acetate injections and reduced levels were maintained for the remainder of the two-month injection period. Denitrification occurred throughout the profile although nitrate reduction was the highest in the lower groundwater velocity zones. An overall reduction of nitrate of 50% was achieved through the treated section of the aquifer. It is estimated that an upscaled treatment system utilizing a treatment width of only 70 m would be sufficient to reduce the nitrate concentrations to below the drinking water limit demonstrating the potential for the CIS method to functions as an interim groundwater nitrate reduction strategy.

Preparation of Spherical Ultrafine Silver Particles Using Y-Type Microjet Reactor.

Herein, micron-sized silver particles were prepared using the chemical reduction method by employing a Y-type microjet reactor, silver nitrate as the precursor, ascorbic acid as the reducing agent, and gelatin as the dispersion at room temperature (23 °C ± 2°C). Using a microjet reactor, the two reaction solutions collide and combine outside the reactor, thereby avoiding microchannel obstruction issues and facilitating a quicker and more convenient synthesis process. This study examined the effect of the jet flow rate and dispersion addition on the morphology and size of silver powder particles. Based on the results of this study, spherical and dendritic silver particles with a rough surface can be prepared by adjusting the flow rate of the reaction solution and gelatin concentration. The microjet flow rate of 75 mL/min and the injected gelatin amount of 1% of the silver nitrate mass produced spherical ultrafine silver particles with a size of 4.84 μm and a tap density of 5.22 g/cm3.

Aspect ratio and baffles impact hydraulic performance of full-size denitrifying bioreactors

Denitrifying woodchip bioreactors treating subsurface drainage in the US have high aspect ratios (i.e., length: width ratios; approximately 4:1) to encourage plug flow dynamics. Improved understanding of bioreactor hydraulics across aspect ratios would help assess possible increased flexibility for this practice to capture greater hydraulic loading or provide greater nitrate mass removal. The objective of this study was to assess the hydraulic impacts of aspect ratio and baffles using conservative tracer testing at full-scale denitrifying woodchip bioreactors. Fourteen tracer tests were performed at six bioreactors, spanning three design styles: (1) “conventional” bioreactors with high aspect ratios of ≥2.6:1; (2) a “wide” bioreactor with a low aspect ratio of 0.3:1; and (3) a relatively wide “advanced” bioreactor with baffles placed to route flow sinuously perpendicular (sideways) to the hydraulic gradient. The wide bioreactor had the most dispersion, the most short circuiting, and was the most well mixed based on the tanks-in-series model. The advanced design with baffles had higher volumetric efficiencies than the conventional and wide designs (2.9, 2.2, and 2.1, respectively) and trended toward the highest nitrate removals. The concept of baffles at relatively wide bioreactors merits additional field-scale assessment to increase hydraulic loading while maintaining hydraulic efficiency. The Morill Dispersion and Short Circuiting Indices were strongly and significantly correlated (Pearson's r: −0.88) across the fourteen tests as were the volumetric and hydraulic efficiency metrics (r: 0.87). While aspect ratio and baffles have been well studied in wetlands and other reactor types, this work is the first to establish these concepts using tracer testing at three woodchip bioreactor design styles.

Catalytic oxidation of methylene blue by using Ni-Fe bimetallic catalyst/NaClO system: Performance, kinetics, mechanism, and DFT calculations

Nickel-iron bimetallic catalysts (Ni-Fe/Al2O3) with activated alumina as the carrier were prepared by impregnation pyrolysis, and the structure and morphology of the catalysts were analyzed by characterization via X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, programmed temperature rise reduction, and X-ray photoelectron spectroscopy. The prepared Ni-Fe/Al2O3 was used to catalyze sodium hypochlorite (NaClO) for oxidative degradation of methylene blue (MB). The effects of the dosage of iron nitrate in the preparation process and the catalyst dosage, effective chlorine concentration, pH, temperature, initial MB concentration, and coexisting ions on the MB removal effect in the oxidative degradation experiment were investigated. Then, the oxidative degradation mechanism and practical application prospects of the system were examined. Results showed that the Ni and Fe nitrate mass ratio of 5:1 was favorable for the dispersion of Ni elements in the catalyst preparation process. When the temperature was 45 °C, the initial concentration of MB was 50 mg/L, the dosage of Ni-Fe/Al2O3 was 20 g/L, the initial effective chlorine concentration was 1.76%, and the initial pH was 4.0. The removal rate of MB could reach 98.98% within 40 min, and the degradation process was in accordance with quasi primary kinetics, with reaction rate constant of 0.0873 min−1. The experimental result of free radical quenching and electron paramagnetic resonance indicated that the addition of Ni-Fe/Al2O3 induced the decomposition of NaClO to produce a large amount of singlet oxygen (1O2), 1O2 and HOCl worked together to oxidatively degrade MB. Ni3+/Ni2+ redox cycles play an important role in activating HOCl to produce active species. The common inorganic anions (Cl−, NO3−, and SO42−) did not inhibit the removal of MB significantly, whereas humic acid showed a more significant inhibitory effect only at concentrations greater than 20 mg/L. In the five recycling experiments, the catalysts showed low metal leaching rate and high MB removal rate. Ni-Fe/Al2O3 was reusable, and it showed excellent application potential.

Experimental study on phase change heat storage of valley electricity and economic evaluation of commercial building heating

• A composite PCM of Mg (NO 3 ) 2 6H 2 O/NaNO 3 for an electric boiler was developed. • A modular PCHS unit was constructed and tested, showing stable heating performance. • The heating tests in commercial buildings show 53% savings in operating costs. • The valley power PCHS heating technology shows good application prospects. The application of valley power phase change heat storage (PCHS) in commercial building heating has practical significance for the city's sustainable development. In this study, the experimental study on valley power PCHS is carried out, focusing on the winter heating of a commercial building. An inorganic hydrated salt phase change material (PCM) of magnesium nitrate hexahydrate and sodium nitrate (mass ratio, 92:8) with a phase change temperature of 80–83 °C for an electric boiler was developed. A modular PCHS unit was constructed and tested for 2.5 months from 2018 to 2019 and 4.5 months from 2020 to 2021. The results show that the PCM is stable at about 81 °C after 240 cycles. Moreover, the heat storage and the exothermic power of the PCHS unit reached 10.95 kW and 10.15 kW in the 9-hour test, respectively. The 12-day trial of an office shows that the indoor temperature was maintained at 22–24 °C stably during the daytime working hours (7:00–17:00). Furthermore, the two-year test of the building heating shows that the valley power PCHS can effectively adapt to the outdoor environment changes. The power consumption of the PCHS unit has a negative linear correlation with the ambient temperature. In addition, the operating cost of the PCHS unit system is only 3.5 RMB/month/m 2 , which is less than half of the municipal central heating system cost of 7.5 RMB/month/m 2 , reflecting good economy, and the advantages of energy-saving, emission reduction, and environmental benefits are obvious. The findings of this study provide a valuable supplement for the heating mode and low-carbon energy utilization of urban buildings.

Abundant Nitrate and Nitric Acid Aerosol in the Upper Troposphere and Lower Stratosphere

AbstractThe tropospheric and stratospheric nitrate aerosol is simulated by a sectional aerosol model coupled to the Community Earth System Model. The simulated nitrate mass fractional contribution to aerosols is significantly higher in the upper troposphere and lower stratosphere (UTLS) than that at the surface. Both in situ measurements and simulations show that nitrate aerosol accounts for about 30%–40% of the aerosol mass at the tropopause of the Asian summer monsoon (ASM) region. Furthermore, simulated condensed nitric acid particles account for ∼20% of the annual mean aerosol mass at the tropical tropopause, and over 95% in the UTLS at the South Pole in June‐July‐August. Our study suggests that the extremely cold ambient conditions in the UTLS of the tropics, ASM and polar regions thermodynamically favor the condensation of ammonia and nitric acid. The widely distributed nitrate aerosol in the global UTLS may be overlooked by climate models.

A comprehensive study about the in-cloud processing of nitrate through coupled measurements of individual cloud residuals and cloud water

Abstract. While the formation and evolution of nitrate in airborne particles are extensively investigated, little is known about the processing of nitrate in clouds. Here we present a detailed investigation on the in-cloud formation of nitrate, based on the size-resolved mixing state of nitrate in the individual cloud residual and cloud-free particles obtained by single particle mass spectrometry, and also the mass concentrations of nitrate in the cloud water and PM2.5 at a mountain site (1690 m a.s.l. – above sea level) in southern China. The results show a significant enhancement of nitrate mass fraction and relative intensity of nitrate in the cloud water and the cloud residual particles, respectively, reflecting a critical role of in-cloud processing in the formation of nitrate. We first exclude the gas-phase scavenging of HNO3 and the facilitated activation of nitrate-containing particles as the major contribution for the enhanced nitrate, according to the size distribution of nitrate in individual particles. Based on regression analysis and theoretical calculations, we then highlight the role of N2O5 hydrolysis in the in-cloud formation of nitrate, even during the daytime, attributed to the diminished light in clouds. Nitrate is highly related (R2= ∼ 0.6) to the variations in [NOx][O3], temperature, and droplet surface area in clouds. Accounting for droplet surface area greatly enhances the predictability of the observed nitrate, compared with using [NOx][O3] and temperature. The substantial contribution of N2O5 hydrolysis to nitrate in clouds with diminished light during the daytime can be reproduced by a multiphase chemical box model. Assuming a photolysis rate at 30 % of the default setting, the overall contribution of N2O5 hydrolysis pathway to nitrate formation increases by ∼ 20 % in clouds. Given that N2O5 hydrolysis acts as a major sink of NOx in the atmosphere, further model updates would improve our understanding about the processes contributing to nitrate production in cloud and the cycling of odd nitrogen.

Paired denitrifying bioreactors with wide orientation for increased drainage flow capacity

Denitrifying bioreactors are a conservation drainage practice for reducing nitrate loads in subsurface agricultural drainage. Bioreactor hydraulic capacity is limited by cross-sectional area perpendicular to flow through the woodchip bed, with excess bypass flow untreated. Paired bioreactors with wide orientations were built in 2017 in Illinois, USA, to treat drainage from a relatively large 29 ha field. The paired design consisted of: a larger, Main bioreactor (LWD: 6.1 × 18.3 × 0.9 m) for treating base flow, and 2) a smaller, Booster bioreactor (7.8 × 13.1 × 0.9 m) receiving bypass flow from the Main bioreactor during periods of high flow. Over three years of monitoring, the paired bioreactor captured 84–92% of the annual drainage discharge which demonstrated an expanded cross-sectional area could improve bioreactor flow capture, even for a large drainage area. However, the paired bioreactors removed 6–28% of the annual N load leaving the field (1.8–5.6 kg N ha−1 removed; 52–161 kg N), which was not a notable improvement compared to bioreactors treating smaller drainage areas. The design operated as intended at low annual flow-weighted hydraulic retention times (HRTs) of usually ≤2 h, but these short HRTs ultimately limited bioreactor nitrate removal efficiency. Daily HRTs of <2 h often resulted in nitrate flushing. The Main bioreactor had higher hydraulic loading as intended and was responsible for the majority of flow captured in each year although not always the most nitrate mass removal. The Booster bioreactor provided better nitrate removal than the Main at HRTs of 3.0–11.9 h, possibly due to its drying cycles which may have liberated more available carbon. This new design approach tested at the field-scale illustrated tradeoffs between greater flow capacity (via increased bioreactor width) and longer HRT (via increased length), given a consistent bioreactor surface footprint.

Aerosol liquid water content of PM2.5 and its influencing factors in Beijing, China

Aerosol liquid water content (ALWC) has important influences on atmospheric radiation and aerosol chemical processes. In this work, the changes in ALWC of PM2.5 were investigated over four seasons based on hourly monitoring of inorganic water-soluble ions and their gaseous precursors using the thermodynamic model ISORROPIA II. The results showed that the ALWC concentrations exhibited pronounced seasonal (autumn > summer > spring > winter) and diurnal variation characteristics. The sensitivity tests indicated that ALWC depended strongly on TSO4 (total sulfate (gas and aerosols) expressed as equivalent H2SO4), followed by TNO3 (total nitrate (gas and aerosols) expressed as equivalent HNO3). The relatively low concentration of TCl (total chloride (gas and aerosols) expressed as equivalent HCl) limit its importance in the atmosphere. ALWC showed exponential growth features as a function RH in all four seasons. RH became the most influential factor on the variation of ALWC when RH exceeded 80% in all seasons. The seasonal average data showed that the ALWC increased from 2.92 μg·m−3 to 75.83 μg·m−3 when ambient RH increased from 30% to 90%, the associated sulfate, nitrate, and ammonium (abbreviated as SNA) mass fraction in PM2.5 rose from 0.39 to 0.58 in the atmosphere. The ALWC facilitated the formation of SNA through gas-particle conversion and partitioning. The self-amplifying processes between ALWC and SNA enhanced aerosol formation. By modeling ALWC under different seasonal atmospheric scenarios, it was found that reductions in chemical species could reduce ALWC concentrations in different degrees. Based on the current emission conditions, controlling excess NH3 emission could effectively reduce ALWC to a maximum of 45.71% in summer, indicating that NH3 control was crucial for reducing ALWC and PM2.5 concentrations under high levels of SO42− and NO3−.

Influence of layered heterogeneity on nitrate enrichment induced by cut-off walls in coastal aquifers

Cut-off walls have proven to be an effective countermeasure for mitigating seawater intrusion (SI). However, construction of cut-off walls may lead to upstream accumulation of nitrate contaminants in coastal aquifers. While stratified geological heterogeneities are widely present in coastal areas, how they may influence nitrate enrichment induced by cut-off walls remains poorly understood. In the present study, we employ variable-density coupling with reactive transport numerical models to understand and quantify the dynamic changes of salinity and NO3− in layered heterogeneous coastal aquifers in the presence of cut-off walls. Our comprehensive simulation results indicate that aquifer heterogeneities are a primary factor in regulating nitrate accumulation. For example, the presence of a middle low-permeability layer between two high-permeability layers reduces the influence of cut-off wall height on SI if the bottom of the wall is placed inside the low-permeability layer. While the thickness of the middle low-permeability layer has a minor impact on SI, a thicker middle low-permeability layer leads significantly aggravated nitrate contamination. Moreover, our simulations reveal that the degree of nitrate accumulation changes non-monotonically with changes in the hydraulic conductivity of low-permeability layer—the total nitrate mass (TNM) first increases and then decreases as the hydraulic conductivity of the low-permeability layer increases. We also found that the impact of heterogeneities is a function of the location and depth of the cut-off wall. With the increases of the distance between wall and the ocean, the nitrate enrichment degree of nitrate pollution in the upstream aquifer decreases while seawater intrudes further in layered aquifer (i.e. case HLH). TNM increases as the wall depth increases, but the growth rates of TNM becomes lower when the bottom of the wall is placed inside the low-permeability layer. These quantitative analyses have critical implications for the planning and design of cut-off walls to effectively control SI while at the same mitigating nitrate pollution.