Passive Diffusion Samplers Research Articles

A New Look at Diffusion in Vapor Intrusion Assessments; Passive Adsorptive Diffusion Samplers

AbstractVapor intrusion of toxic volatile organic compounds (VOCs) from subsurface soil vapor, through a building slab/floor, and into the indoor air is an important environmental contaminant transport mechanism. It is widely believed that advective flow, driven by the pressure differential between the subslab and indoor air, is the primary mechanism of subslab soil vapor entry into buildings. This paper explores the hypothesis that molecular diffusion through the slab may potentially play a larger role in vapor intrusion than previously believed and may even be the predominant vapor intrusion mechanism when the subslab vapor source strength is sufficiently high or the pressure differential is relatively low. A novel sampling device, referred to as a Passive Adsorptive Diffusion Sampler (PADS), is presented for the purpose of directly measuring the diffusion of VOCs through a building slab. A vacant warehouse was identified as a case study site where historical sampling had determined that vapor intrusion of trichloroethene (TCE) was adversely impacting the indoor air. Calculations using Fick's First Law of Diffusion are presented which demonstrate that diffusion alone can theoretically account for all the TCE observed in the indoor air at this building based on an effective diffusion coefficient for concrete that was calculated from the Johnson and Ettinger Model. Two groups of nine replicate PADS were deployed at two areas on the slab and used to measure the flux and effective diffusion coefficient at each of the 18 total points, which showed an order of magnitude variability within each area and over two orders of magnitude variability overall. These results indicate that diffusion through concrete is inherently variable when measured at a sub‐meter scale. However, when combined over both areas, the overall average approached that calculated from the Johnson and Ettinger Model. An additional 12 PADS were deployed across the building slab (for a total of 30) to quantify the overall building‐wide diffusive flux. This area‐weighted average diffusive flux was consistent with the predicted diffusive flux as calculated from Fick's First Law and the vapor intrusion mass input required to achieve the observed indoor air TCE concentration. The results of this study show that PADS provides a simple way to measure diffusive flux directly without having to drill through the slab. However, significant variability in the measured flux should be expected and will need to be accounted for by the inclusion of a relatively large number of samples including replicates. When using PADs at a new site, the collection of traditional subslab vapors at a select number of locations is recommended for the verification of a building‐specific effective diffusion coefficient, which may not necessarily be the same as for this building.

Calibration of a passive sampling device for the determination of nitrogen dioxide in ambient air

Nitrogen dioxide (NO2), a common air pollutant, has been widely admitted to be harmful to both the environment and human health, demanding its well-control procedure and corresponding quantification. In this study, NO2 in ambient air was collected by a passive sampling method using the Willems badge diffusive sampler, followed by a derivatization step with the Griess-Saltzman solution, and analyzed by ultraviolet-visible (UV–vis) spectroscopy at 543 nm. The device can be utilized for 168 h of continuous field sampling. The experimental sampling rate (Ke) of (4.02 ± 0.29) × 10−3 m3h−1 with a relative standard deviation (% RSD) of 9.6 % was determined by conducting parallel experiments between an active sampling method (ISO 6768:1998) and the Willems samplers. After exposure time, samplers could be stored for two weeks in a refrigerator at 4 °C before analyzing. The studied passive diffusive sampler was simple, low-cost, easy to reuse; permitted determining the average concentration of NO2 in ambient air. The average NO2 concentrations for 2-hour to 4-hour sampling periods at different studied sampling sites in Ho Chi Minh city (Vietnam) were ranged from 11.5 to 189 μg m−3.

Evaluation of Passive Vapor Diffusion Samplers to Quantify Acetylene, Ethene, and Ethane in Groundwater

AbstractAcetylene, ethene, and ethane are products from the degradation of trichloroethene (TCE) in contaminated aquifers. However, the volatility of these gases makes it challenging to avoid excessive losses during sampling. The objective of this study was to compare the quantification of acetylene, ethene, and ethane using passive vapor diffusion (PVD) samplers vs. conventional low‐flow groundwater collection. Samples were obtained from 8 to 13 monitoring wells at three sites that show potential for biotic and abiotic degradation of TCE in fractured rock aquifers. Method reporting limits (MRLs) for the PVD samplers were 0.25 μg/L for acetylene (0.0094 μM) and 0.28 μg/L for ethene and ethane (0.0099 and 0.0092 μM, respectively); the MRLs for conventional low‐flow groundwater samples were ~40% higher. For two of the sites, the maximum concentrations of acetylene, ethene, and ethane obtained with the PVD samplers were comparable to the conventional low‐flow samples. The frequency of detection for these gases with the PVD samplers was also comparable to conventional low‐flow groundwater sampling. For one of the sites with higher levels of acetylene (maximum of 13 μg/L), the concentrations from the PVD samplers were approximately twofold higher than those with conventional low‐flow groundwater sampling. Based on robust detection of acetylene, ethene, and/or ethane, it appears likely that degradation of TCE is occurring at the three sites that were sampled. The use of PVD samplers can reduce the possibility of false negative results to provide another line of evidence in support of natural attenuation.

Outdoor trends and indoor investigations of volatile organic compounds in two high schools of southern Italy

Volatile organic compounds (VOCs) are a class of ubiquitous substances that are present in outdoor and indoor air. They are emitted by a wide range of sources and can penetrate and accumulate specifically in indoor environments. Concern is growing among the scientific community regarding the potential health impacts of exposure to a high concentration of VOCs in indoor spaces. Due to their still-developing respiratory and immune systems, children may be the most fragile subjects in this regard, and therefore, the study of indoor air quality in schools is of the utmost importance. In this work, the concentrations of total volatile organic compounds (TVOCs) and of 20 specific compounds belonging to this class were determined in a school in Squinzano, a town in the province of Lecce (Apulia region, southern Italy). Sampling was carried out in indoor (classrooms and bathrooms) and outdoor (terrace) areas using passive diffusive samplers for VOCs and photoionization detectors for TVOCs. Average concentrations of both TVOC (303 ± 47 µg m−3) and individual VOCs (< 0.5 µg m−3) were below levels of concern; however, specific indoor sources such as cleaning activities and student occupancy were responsible for peaks in TVOC concentrations above harmless levels for children and school staff. The data were then compared to the ones obtained in a similar study conducted in a school in Galatina, another town of the Apulia region, highlighting the impact of the surrounding outdoor environment on the indoor profile of VOCs.

Spatial Distribution of Ammonia Concentrations and Modeled Dry Deposition in an Intensive Dairy Production Region.

Agriculture generates ~83% of total US ammonia (NH3) emissions, potentially adversely impacting sensitive ecosystems through wet and dry deposition. Regions with intense livestock production, such as the dairy region of south-central Idaho, generate hotspots of NH3 emissions. Our objective was to measure the spatial and temporal variability of NH3 across this region and estimate its dry deposition. Ambient NH3 was measured using diffusive passive samplers at 8 sites in two transects across the region from 2018-2020. NH3 fluxes were estimated using the Surface Tiled Aerosol and Gaseous Exchange (STAGE) model. Peak NH3 concentrations were 4-5 times greater at a high-density dairy site compared to mixed agriculture/dairy or agricultural sites, and 26 times greater than non-agricultural sites with prominent seasonal trends driven by temperature. Annual estimated dry deposition rates in areas of intensive dairy production can approach 45 kg N ha-1 y-1, compared to <1 kg N ha-1 y-1 in natural landscapes. Our results suggest that the natural sagebrush steppe landscapes interspersed within and surrounding agricultural areas in southern Idaho receive NH3 dry deposition rates within and above the range of nitrogen critical loads for North American deserts. Finally, our results highlight a need for improved understanding of the role of soil processes in NH3 dry deposition to arid and sparsely vegetated natural ecosystems across the western US.

Application of Nanoconfinement Technology for Highly Effective Monitoring of Chemical Exposure During Military Service.

There is myriad of volatile compounds to which military personnel are exposed that can potentially have negative effects on their health. Military service occurs in a broad array of environments so it is difficult to predict the hazardous compounds to which the personnel might be exposed. XploSafe is developing passive diffusive samplers to facilitate the sampling and quantification of a wide range of chemical vapor exposures that personnel may be exposed to in the workplace. Passive diffusive samplers were constructed by filling porous Teflon tubes with OSU-6, a nanoporous silica sorbent, to produce sampler tokens. Three of these tokens were placed within a badge to fabricate passive samplers. Absorption experiments were performed to determine linear exposure regimes, sampling rates, and limits of quantification for 11 compounds, representing 8 chemical classes. The sampling rates were determined for 11 compounds representing 8 chemical classes. The measured linear ranges for the studied compounds are sufficiently large to allow effective sampling for 8 hours or longer. Accurate dosimetry is possible even with exposure times of days or weeks. The samplers were able to detect the presence of five airborne compounds in a paint booth of a military contractor located in Bristow, Oklahoma, and determine their average exposure concentrations. OSU-6 based sampler badges were able to detect the presence and quantify the average exposures of five airborne compounds in a paint booth of a military contractor located in Bristow, Oklahoma. Experiments show that these samplers can adsorb and quantify a broad array of different volatile organic compounds whose high sampling rates coupled with high capacity provide both sensitivity and the ability to quantify over a large range of exposures. This technology can meet the requirements for personal samplers to create Individual Longitudinal Exposure Record for each military person.

Assessment of annual effective dose and health risk due to radon exposure in nurseries in the city of Poznań, Poland

The results of the scientific work presented in the World Health Organisation (WHO) report indicate that exposure to radon increases the risk of lung cancer in humans. In the past, radon concentrations of 400 Bq/m3 were considered dangerous, but today the level that poses a threat has been lowered to 100 Bq/m3. Children are particularly vulnerable to the effects of ionising radiation. In the article, the exposure of children to the effects of radon was evaluated on the basis of measurements in nurseries taken in the course of several months. A pulsed ionisation chamber with a passive diffusion sampling method was used for the measurements. The effective dose was assumed as the evaluation criterion. The research was carried out in selected rooms of two nurseries located in the city of Poznań in Poland. Similar profiles of variability in radon concentration (r = 0.88, p < 0.001) were observed in two different nurseries. The higher values of radon concentration in the rooms were found to be influenced by the low value of the wind speed. The experimental studies were supplemented with questionnaire studies, which showed that the average person's awareness of radioactivity is at a low level. The results of the effective dose level can be used to optimise future radon testing in buildings intended for various purposes.

Preliminary Air Quality and Microclimatic Conditions Study in the Santuario della Beata Vergine dei Miracoli in Saronno (VA).

In the present work, the microclimatic conditions (temperature (T), relative humidity (RH), and illuminance (I)), together with the air quality (both aerosol particulate matter (PM) and gaseous pollutants), were monitored to evaluate the environmental conditions inside the Santuario della Beata Vergine dei Miracoli in Saronno (VA), a masterpiece of the Italian Renaissance. For this purpose, dataloggers were used to carry out the T, RH, and I measurements, whereas an optical particle counter (OPC) was employed to perform the particle count and determine the concentration of the aerosol PM. Finally, diffusive passive samplers were used to determine the concentration of nitrogen dioxide (NO2) and BTEX (benzene, toluene, ethylbenzene, and xylenes). To identify possible spatial variations, the studies were conducted at different sites and different heights in the Sanctuary. Particular focus was given to the Easter week during which liturgical services attracting large numbers of people were carried out. Additionally, a comparison with the outdoor values was performed to highlight the accumulation phenomena and other variations in the concentrations of the species. Despite the indoor concentrations of pollutants and variations in the thermohygrometric parameters being generally lower compared to the outdoors (e.g., 5.2-15.0 µg m-3 versus 17.7-45.3 µg m-3 for NO2), the microclimatic conditions were often not in line with the Italian legislation and technical standards.

What are the limitations for the use of Carbopack X as passive or active sampling adsorbent for determination of 1,3-butadiene in ambient environment?

Two widely used sampling methods for measurement of 1,3 butadiene in ambient air include active sampling and radial passive sampling techniques using sorbent tubes filled with Carbopack X adsorbent. The laboratory study was designed to evaluate the suitability of these sampling methods for long-term monitoring of 1,3-butadiene in ambient air. The results point against the use of passive radial diffusive sampling for determination of 1,3-butadiene due to the dependence of its uptake rate on experimental conditions. Inability to correctly reflect the occurrence of episodes of high concentration levels using radial diffusive passive sampling was proven experimentally. As for active sampling technique, caution must be taken as to not overreach the safe sampling limit which considerably decreases as a function of temperature and relative humidity. At 10 ml min −1 flow-rate, the breakthrough volume per gram of Carbopack X material is established at 81.6–84.0 L at 50% RH, 20 °C; 61.2–64.8 L at 80% RH, 20 °C; and 24.0–28.8 L at 80% RH, 30 °C. Relatively low sampling times associated with these sampling volumes point to the need to develop more adequate supports for active sampling to enable long-term ambient monitoring. • Limitations of current passive sampling techniques for measurement of 1,3-butadiene. • Limitations of current active sampling techniques for measurement of 1,3-butadiene. • Dependence of uptake rate of 1,3-butadiene using Radiello on environmental conditions. • Decrease of safe sampling limit as a function of environmental conditions.

Exposure to Air Pollution from Road Traffic and Incidence of Respiratory Diseases in the City of Meknes, Morocco

For monitoring spatio-temporal variations of nitrogen dioxide (NO2) content, passive diffusive samplers have been deployed in 14 near-road and residential sites for 14 days. In parallel with the winter campaign to measure the NO2 tracer, road traffic counting sessions were carried out on the city’s main roads. The coupling of the results of the measurement campaigns and the counting sessions under Arcgis 9.3 made it possible to determine the areas most affected by automobile pollution and to carry out a high spatial resolution mapping of the pollutant prospected. The results of this study show that atmospheric NO2 concentrations reach maximum values in the city center and decrease towards its periphery. The analysis of the epidemiological situation of the principal diseases related to air pollution in the city of Meknes during the study period (2010–2014) showed that among subjects aged five years and older, acute respiratory diseases occurred more in women than men. The most affected age group was between 15 and 49 years, while asthma attacks were noted mainly among women aged 50 years and older. Acute respiratory illness and asthma attacks were prevalent in the winter and fall. Among children under five years of age, the age group most affected by pneumonia was those under 11 months. Our integrative approach combined spatialized GIS-based health indicators of these diseases, the location of stationary and mobile sources of air pollution, and measured NO2 levels. This combination has made it possible to detect that residents in areas with heavy road traffic are likely to be more affected than those in areas near industrial activity. The habitat type also contributes significantly to the development and exacerbation of the pathologies studied, especially in the districts of the old Medina.

Assessment of air pollution in the informal settlements of the Western Cape, South Africa

&#x0D; &#x0D; &#x0D; &#x0D; Introduction: South Africa has the highest ambient air pollution exposure associated with morbidity in the Sub-Saharan region, accounting for a total number of 14356 confirmed cases of mortality on an annual basis. The study assessed air pollution exposure levels of Khayelitsha and Marconi-Beam neighbourhoods that are not monitored by the fixed-site monitoring station provided by the government.&#x0D; Materials and methods: Weekly ambient air pollution measurements of Particulate Matter less than 2.5 μm (PM2.5) and Nitrogen Dioxide (NO2) were collected at households in neighbourhoods of Khayelitsha and Marconi-Beam during the summer and winter seasons. PM2.5 measurements were collected using Mesa Labs GK2.05 (KTL) cyclone attached to a GilAir Plus air sampling pump. Gases of NO2 were measured using passive diffusive samplers. Data were recorded on a Microsoft Excel 2016 spreadsheet. Statistical analyses were performed using IBM SPSS Statistics 28.0. The outcome of the seasonal exposure levels will be compared with the South African ambient air quality standards to ascertain the risk of potential exposure to significant levels of PM2.5 and NO2 that are hazardous to human health.&#x0D; Results: The results suggest that summer NO2 concentrations in the Khayelitsha neighbourhood ranged between (0 and 28 μg/m3), while in winter NO2 concentrations nearly doubled. A similar trend was observed regarding PM2.5 behaviour with summer overall exposure level of 7 μg/m3 and 13 μg/m3 during winter.&#x0D; Conclusion: Whilst there are no legislative guidelines to compare the measured weekly average results, the concentrations were still lower than the World Health Organization (WHO), and South African air quality standards values for PM2.5 and NO2 pollutants.&#x0D; &#x0D; &#x0D; &#x0D;

Vertical stratification of microbial communities and isotope geochemistry tie groundwater denitrification to sampling location within a nitrate-contaminated aquifer

Nitrate pollution is a major threat to groundwater quality in agricultural areas. Natural attenuation of nitrate in contaminated aquifers is mediated by denitrifying microbial populations in anoxic environments. Vertical distribution of denitrifying microbial communities in aquifers is greatly influenced by groundwater redox conditions, local hydrogeological parameters, and seasonal variability in groundwater flow and recharge. In this study, we investigated groundwater geochemistry and the composition of bacterial and archaeal communities with increasing depth in a shallow nitrate-contaminated aquifer in British Columbia, Canada. High-resolution passive diffusion sampling was conducted to collect groundwater at 10-cm intervals from 4 to 20 m below ground surface (mbgs) in the aquifer. Geochemical analyses of major ions indicated a general shift in the groundwater chemistry below 16 mbgs including decreasing chloride concentrations that suggest two-end member mixing of shallow and deep groundwater with different chemistries. A redoxcline was further observed within a 2 m transition zone at 18–20 mbgs characterized by sharp declines in nitrate concentrations and increases in sulfate and total inorganic carbon. Excursions in δ15N-NO3− and δ18O-NO3− in the same depth interval are consistent with denitrification, and a concomitant decrease in δ34S-SO42− suggested that denitrification was coupled to sulfide or sulfur oxidation. Microbial communities within this depth interval were significantly dissimilar to those above and below, featuring putative lithotrophic denitrifying bacteria belonging to the genera Sulfurifustis, Sulfuritalea and Sulfuricella. These lineages were detected in greatest abundance at 19 mbgs while the abundances of putative heterotrophic sulfate-reducing bacteria belonging to the genus Desulfosporosinus were greatest at 20 mbgs. In addition to help distinguish denitrification from mixing-induced changes in groundwater chemistry, the above observed vertical stratification of the microbial key players connects nitrate removal to the locations of the aquifer sampled.

Diffusive uptake rates for passive air sampling: Application to volatile organic compound exposure during FIREX-AQ campaign

Passive (diffusive) sampling using sorbents is an economical and versatile method of measuring pollutants in air, including volatile organic compounds (VOCs). Diffusive uptake rates (UTRs) are needed for each analyte to obtain average concentrations during a specific passive sampling time duration. Here, a simultaneous active/diffusive ambient air sampling technique on Tenax®TA was employed to measure 24-hours, 7, 14 and 28-days UTRs of up to 27 VOCs, including benzene, toluene, ethylbenzene, xylenes (BTEX), C6–C12 hydrocarbons, benzenes derivatives, tetrachloroethylene, pinenes and limonene. Samples were analyzed via thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS) for desired analytes. Seven-day UTR values ranged from 0.17 to 0.59 mL/min and many compounds exhibited a linear relationship with UTR and time duration up to 14 or 28 days. This may be the most comprehensive UTR tabulation of VOCs on Tenax®TA for time periods of 24 hours –28 days available. These rates were applied to VOC data measured during the 2019 NASA/NOAA Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign, with goals to determine the chemical composition of western US wildfire smoke and to assess human exposure to air toxics. Summer 2019 exposure levels of BTEX at five Northwestern cities were low and the cancer risk due to benzene was assessed during FIREX-AQ to be background or 1 × 10−6. The UTRs derived here can be useful in applications of diffusive sampling, including estimation of sub-chronic to chronic human exposure risk of air toxics and wildfire smoke.

A case study on monitoring the ambient ammonia concentration in paddy soil using a passive ammonia diffusive sampler

A case study on monitoring the ambient ammonia concentration in paddy soil using a passive ammonia diffusive sampler

Ground-level Particulate Sulphate and Gaseous Sulphur Dioxide Downwind of an Aluminium Smelter

ABSTRACT Particulate sulphate (pSO42–) is an atmospheric pollutant known to affect human/environmental health and global radiative-forcing. The Rio Tinto (RT) aluminium smelting facility in Kitimat, British Columbia, is the primary source of sulphur dioxide (SO2) emissions to the surrounding Kitimat Valley, a relatively isolated and unpolluted region. A network of active two-stage filter-packs and passive-diffusive samplers was established between June 2017 to October 2018 with the objective to evaluate the spatiotemporal variation and relative contribution of pSO42– to total anthropogenic atmospheric oxidized sulphur (SOx = SO2 + pSO42–). Average pSO42– across all sites (n = 9) was 0.41 µg m–3 (24–48 hour exposures) and ranged from 0.03 to 2.03 µg m–3. In contrast, average filter-pack SO2 ranged from 0.11 to 8.9 µg m–3 (during the same exposure periods). The filter-pack pSO42–/SOx concentration ratio (Fs) increased downwind of the smelter, indicating that the relative concentration of pSO42– increased with distance from the smelter. Furthermore, the increasing pSO42–/vanadium (V) ratio (used as a tracer of smelter emissions) relative to distance confirmed particulate formation was occurring within the emission plume during the sampling period. Irrespective of in-plume aerosol formation, pSO42– contributed a relatively minor fraction of total atmospheric SOx within the emission plume (field campaign averages Fs 1.0 µg S m–3).

Indoor and outdoor air quality: A university cafeteria as a case study

A short but exhaustive air sampling campaign was conducted in a university cafeteria, an occupational environmental not yet studied. Carbonyls and volatile organic compounds were collected by passive diffusion samplers. Temperature, relative humidity, CO2, CO and particulate matter were continuously monitored indoors and outdoors. Simultaneous PM10 sampling with high and low volume instruments, equipped with quartz and Teflon filters, respectively, was performed during working hours and at night. The quartz filters were analysed for their carbonaceous content by a thermo-optical technique and organic constituents by GC-MS. Water-soluble ions and elements were analysed in the Teflon filters by ion chromatography and PIXE, respectively. Low air change rates (0.31–1.5 h−1) and infiltration factors of 0.14, for both PM2.5 and PM10, indicate poor ventilation conditions. Concentrations of both gaseous pollutants and particulate matter were much higher in the cafeteria than outdoors, showing strong variations throughout the day depending on occupancy and activities. The average concentration of indoor-generated PM10 was estimated to be 32 μg m−3. Organic compounds in PM10 included alkanes, PAHs, saccharides, phenolics, alcohols, acids, alkyl esters, triterpenoids, sterols, among others. The complex particle composition reveals the multiplicity of sources, formation reactions and removal processes, not yet fully known, and suggests the contribution from dust resuspension, abrasion and off-gassing of building materials, cooking emissions, tobacco smoke, and several consumer products. Many compounds are in the list of ingredients of personal care products, pesticides, plasticisers, flame retardants and psychoactive drugs. The inhalation cancer risks of metals and PAHs were found to be negligible.

Personal exposure to NO2 and benzene in the Cape Town region of South Africa is associated with shorter leukocyte telomere length in women

Air pollution exposure is a major global health concern and has been associated with molecular aging. Unfortunately, the situation has not received much attention in the African region. The aim of this study was to investigate whether current personal ambient NO2 and benzene, toluene, ethyl-benzene and xylenes (ortho (o)-, meta (m)- and para (p)-xylene (BTEX) exposure is associated with leukocyte telomere length (LTL), a marker of molecular ageing, in apparently healthy women (mean ± SD age: 42.5 ± 13.4 years) residing in the Cape Town region of South Africa. The repeated measures study collected data from 61 women. Seven-day median (interquartile range (IQR)) personal NO2 and BTEX exposure levels were determined via compact passive diffusion samplers carried on the person prior to baseline (NO2: 14.2 (9.4–17.2) μg/m³; Benzene: 3.1 (2.1–5.3) μg/m³) and 6-month follow-up (NO2: 10.6 (6.6–13.6) μg/m³; Benzene: 2.2 (1.3–4.9) μg/m³) visits. LTL was measured at baseline and follow-up using a real-time PCR method. Multiple linear mixed model analyses (adjusting for age, body mass index, smoking, employment status, level of education and assessment visit) showed that each IQR increment increase in NO2 (7.0 μg/m³) and benzene (3.3 μg/m³) was associated with −7.30% (95% CI: −10.98 to −3.46%; p < 0.001) and −6.78% (95% CI: −11.88 to −1.39%; p = 0.015) difference in LTL, respectively. The magnitude of these effects of NO2 and benzene corresponds to the effect of an increase of 10.3- and 6.0-year in chronological age on LTL. Our study shows that personal exposures to NO2 and benzene are associated with molecular ageing as indicated by LTL in healthy women residing in the Cape Town region.

Impact of urban expansion on the air pollution landscape: A case study of Hanoi, Vietnam

The rapid urban expansion of Hanoi over the last few decades has transformed a lot of agricultural land into urban land uses accompanying pollution by traffic, industrial, and residential emission sources. In this work, the impact of urban expansion on the air pollution landscape has been assessed using the NO2 and SO2 ambient concentrations measured by passive diffusion samplers at 176 sites across the nine urban and the five peri-urban districts of Hanoi spanning an area of 921 km2. The NO2 values ranged from 5.5 to 70 µg m−3 with an arithmetic mean of 34.3 µg m−3. The corresponding figures for SO2 are 1, 51, and 14.5 µg m−3. The pollutant concentrations decrease from the city center outward, reflecting the history of urban expansion with the city fringe being urbanized in the 1980s and the peri-urban area having undergone development from the early 1990s. The pollution landscapes show elevated concentration levels in the recently built-up areas at 6, 10, and 21 km from the city center. The NO2 and SO2 digital maps generated from monitoring data by the kriging technique show numerous traffic and industrial emission hot spots in the peri-urban area. As revealed by the land use regression models, the factors driving the spatial variations of pollutant concentrations across the city include the population density, the road density, and the distances of the monitoring site to the urban center and the nearest roadway.

Quantification of Groundwater Nitrate Loading after Raspberry Field Renovation Using High‐Resolution Passive Diffusion Sampling

The Abbotsford‐Sumas Aquifer is a permeable, unconfined aquifer in British Columbia, Canada, where raspberry (Rubus idaeus L.) production is an important source of groundwater NO3 contamination. Renovation of raspberry fields (i.e., canes chopped, soil tilled and fumigated, and spring manure application prior to replanting), which typically occurs every 6 to 10 yr in response to decreased crop vigor, has been suggested as a possible cause of significant interannual variation in groundwater NO3 concentrations. This study used high‐resolution passive diffusion sampling to quantify the magnitude and timing of NO3 loading to shallow groundwater from a commercial raspberry field during a 6‐yr (2009–2015) monitoring period after crop renovation. After renovation, the annual NO3 loading increased from ∼95 kg N ha−1 in Year 1 to ∼245 kg N ha−1 in Year 2 and decreased to ∼85 kg N ha−1 in Year 3. The average annual NO3 loading from Years 4 to 6 (72 kg N ha−1) was assumed to reflect annual loading without a renovation effect, and the increased loading during Years 1 to 3 was attributed to renovation. Renovation contributed an estimated 33 to 23% of total groundwater NO3 from this field for a 6‐ to 10‐yr renovation cycle. Most of the NO3 loading associated with renovation occurred in Year 2 and was attributed to the manure application. The increased NO3 loading after renovation likely contributes to the spatial and temporally varying NO3 patterns observed in the aquifer. Reducing manure applications during renovation and decreasing renovation frequency have the potential to decrease the groundwater NO3 concentration.Core Ideas We estimated NO3 flux using vertically high‐resolution groundwater sampling and Darcy flux. Loading was estimated under raspberry field for 6 yr after stand renovation. Renovation frequency is an important control on long‐term NO3 concentration. An estimated one‐third to one‐quarter of groundwater NO3 was from renovation in a 6‐ to 10‐yr cycle. Manure was potentially the primary source of additional NO3 loading during renovation.

Crab burrows as preferential flow conduits for groundwater flow and transport in salt marshes: A modeling study

Crab burrows can act as preferential flow conduits for pore water-surface water interactions in salt marshes, but the effect of preferential flow on subsurface transport in these tidally-influenced systems is not fully understood. We used numerical models based on salt marshes of North Inlet, South Carolina, to investigate the impacts of crab burrows on porewater salinity. This modeling effort was inspired by field results from North Inlet, where prior field studies that used a combination of tension samplers and passive diffusion samplers measure salinity in crab burrows and in the adjacent sediment matrix found that the minimum salinity (28 PSU) reported by the tension samplers was larger than the maximum salinity (26 PSU) reported by passive diffusion samplers. Two kinds of numerical models were developed to investigate the effect of crab burrows on tidally-driven groundwater flow and salt transport. In the equivalent-continuum model (ECM), crab burrows were included via a shallow surface layer with hydraulic properties representing a bulk average of sediment matrix and crab burrow properties. In the preferential flow model (PFM), an independent high-permeability material was embedded in the surface muddy layer to explicitly simulate preferential flow conduits. The simulated results showed that both models can depict the effect of crab burrow on soil saturation and salt transport in salt marshes. The presence of crab burrows can greatly increase tidally-driven water exchange, improve the intensity and duration of soil aeration and enhance salt transport in salt marshes. The effect of crab burrows on groundwater flow and salt transport varied spatially from the creek bank to marsh interior. PFM models demonstrated that salinity is likely to differ between crab burrows and the sediment matrix, which supports observed differences in results between tension samplers and passive diffusion samplers. These findings may have important implications for practical pore water sampling and hydrochemical investigation in coastal wetlands.