Isoprene nitrates: preparation, separation, identification, yields, and atmospheric chemistry

Isoprene is an important atmospheric volatile organic compound involved in ozone production and NO<sub>x</sub> (NO+NO<sub>2</sub>) sequestration and transport. Isoprene reaction with OH in the presence of NO can form either isoprene hydroxy nitrates ("isoprene nitrates") or convert NO to NO<sub>2</sub> which can photolyze to form ozone. While it has been shown that isoprene nitrate production can represent an important sink for NO<sub>x</sub> in forest impacted environments, there is little experimental knowledge of the relative importance of the individual isoprene nitrate isomers, each of which has a different fate and reactivity. In this work, we have identified the 8 individual isomers and determined their total and individual production yields. The overall yield of isoprene nitrates at atmospheric pressure and 295 K was found to be 0.070(+0.025/−0.015). Three isomers, representing nitrates resulting from OH addition to a terminal carbon, represent 90% of the total IN yield. We also determined the ozone rate constants for three of the isomers, and have calculated their atmospheric lifetimes, which range from ~1–2 h, making their oxidation products likely more important as atmospheric organic nitrates and sinks for nitrogen

Measurements of ambient HONO concentrations and vertical HONO flux above a northern Michigan forest canopy

Systems have been developed and deployed at a North Michigan forested site to measure ambient HONO and vertical HONO flux. The modified HONO measurement technique is based on aqueous scrubbing of HONO using a coil sampler, followed by azo dye derivatization and detection using a long-path absorption photometer (LPAP). A Na<sub>2</sub>CO<sub>3</sub>-coated denuder is used to generate "zero HONO" air for background correction. The lower detection limit of the method, defined by 3 times of the standard deviation of the signal, is 1 pptv for 1-min averages, with an overall uncertainty of ±(1 + 0.05 [HONO]) pptv. The HONO flux measurement technique has been developed based on the relaxed eddy accumulation approach, deploying a 3-D sonic anemometer and two HONO measurement systems. The overall uncertainty is estimated to be within ±(8 × 10<sup>−8</sup> + 0.15 <i>F</i><sub>HONO</sub>) mol m<sup>−2</sup> h<sup>−1</sup>, with a 20-min averaged data point per 30 min. Ambient HONO and vertical HONO flux were measured simultaneously at the PROPHET site from 17 July to 7 August 2008. The forest canopy was found to be a net HONO source, with a mean upward flux of 0.37 × 10<sup>−6</sup> moles m<sup>−2</sup> h<sup>−1</sup>. The HONO flux reached a maximal mean of ~0.7 × 10<sup>−6</sup> moles m<sup>−2</sup> h<sup>−1</sup> around solar noon, contributing a major fraction to the HONO source strength required to sustain the observed ambient concentration of ~70 pptv. There were no significant correlations between [NO<sub>x</sub>] and daytime HONO flux and between <i>J</i><sub>NO<sub>2</sub></sub> × [NO<sub>2</sub>] and HONO flux, suggesting that NO<sub>x</sub> was not an important precursor responsible for HONO daytime production on the forest canopy surface in this low-NO<sub>x</sub> rural environment. Evidence supports the hypothesis that photolysis of HNO<sub>3</sub> deposited on the forest canopy surface is a major daytime HONO source

Environmentally Persistent Free Radicals (EPFRs). 3. Free versus Bound Hydroxyl Radicals in EPFR Aqueous Solutions

Additional experimental evidence is presented for in vitro generation of hydroxyl radicals because of redox cycling of environmentally persistent free radicals (EPFRs) produced after adsorption of 2-monochlorophenol at 230 °C (2-MCP-230) on copper oxide supported by silica, 5% Cu(II)O/silica (3.9% Cu). A chemical spin trapping agent, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), in conjunction with electron paramagnetic resonance (EPR) spectroscopy was employed. Experiments in spiked O17 water have shown that ∼15% of hydroxyl radicals formed as a result of redox cycling. This amount of hydroxyl radicals arises from an exogenous Fenton reaction and may stay either partially trapped on the surface of particulate matter (physisorbed or chemisorbed) or transferred into solution as free OH. Computational work confirms the highly stable nature of the DMPO–OH adduct, as an intermediate produced by interaction of DMPO with physisorbed/chemisorbed OH (at the interface of solid catalyst/solution). All reaction pathways have been supported by ab initio calculations

Modeling hydrological impacts of afforestation on intermittent streams

Although the majority of river networks across the world are intermittent or ephemeral, afforestation management of these catchments is mostly founded on studies in perennial catchments. The hydrological model CATHY (CATchment HYdrology) was used here to simulate the effects that different degrees of progressive conversion from pasture to plantation have on the streamflow generation in intermittent streams. The model was applied to two rural catchments with different size and topographic features in southwest Victoria, Australia. Simulated scenarios included different levels of plantation establishment in pasture areas planting gradually from downslope to upslope and vice versa. Different models for root water uptake were compared to account for water stress, oxygen stress, and root water compensation. A function of root growth over time was also explored to see how it affected model results. The model results show that complex interactions between topographic features and afforestation patterns are crucial in controlling catchments hydrological behavior. In particular, results show that planting in the prone-saturation areas has the largest effects on streamflow. Oxygen stress has a more significant impact than root water compensation on streamflow changes. A time dependent root growth results in smaller streamflow reduction on average, although with different impacts on the two catchments, also due to the interplay between topography and plantation patterns. Overall, our results show that there are multiple factors affecting the water balance when a catchment is partially or completely afforested and those must be taken into account when implementing forestry management strategies

A review of the numerical modelling of salt mobilization from groundwater-surface water interactions

Salinization of land and water is a significant challenge in most continents and particularly in arid and semi arid regions. The need to accurately forecast surface and groundwater interactions has promoted the use of physically based numerical modelling approaches in many studies. In this regard, two issues can be considered as the main research challenges. First, in contrast with surface water, there is generally less observed level and salinity data available for groundwater systems. These data are critical in the validation and verification of numerical models. The second challenge is to develop an integrated surface groundwater numerical model that is capable of salt mobilization modelling but which can be validated and verified against accurate observed data. This paper reviews the current state of understanding of groundwater and surface water interactions with particular respect to the numerical modelling of salt mobilization. 3D physically based fully coupled surface subsurface numerical model with the capability of modelling density dependent, saturated unsaturated solute transport is an ideal tool for groundwater surface water interaction studies. It is concluded that there is a clear need to develop modelling capabilities for the movement of salt to, from, and within wetlands to provide temporal predictions of wetland salinity which can be used to assess ecosystem outcomes.

Prediction of surface flow by forcing of climate forecast system reanalysis data

Meteorological data are key variables for hydrologists to simulate the rainfall-runoff process using hydrological models. The collection of meteorological variables is sophisticated, especially in arid and semi-arid climates where observed time series are often scarce. Climate Forecast System Reanalysis (CFSR) Data have been used to validate and evaluate hydrological modeling throughout the world. This paper presents a comprehensive application of the Soil and Water Assessment Tool (SWAT) hydrologic simulator, incorporating CFSR daily rainfall-runoff data at the Roodan study site in southern Iran. The developed SWAT model including CFSR data (CFSR model) was calibrated using the Sequential Uncertainty Fitting 2 algorithm (SUFI-2). To validate the model, the calibrated SWAT model (CFSR model) was compared with the observed daily rainfall-runoff data. To have a better assessment, terrestrial meteorological gauge stations were incorporated with the SWAT model (Terrestrial model). Visualization of the simulated flows showed that both CFSR and terrestrial models have satisfactory correlations with the observed data. However, the CFSR model generated better estimates regarding the simulation of low flows (near zero). The results of the uncertainty analysis showed that the CFSR model predicted the validation period more efficiently. This might be related with better prediction of low flows and closer distribution to observed flows. The Nash-Sutcliffe (NS) coefficient provided good- and fair-quality modeling for calibration and validation periods for both models. Overall, it can be concluded that CFSR data might be promising for use in the development of hydrological simulations in arid climates, such as southern Iran, where there are shortages of data and a lack of accessibility to the data

Measurements of ambient HONO concentrations and vertical HONO flux above a northern Michigan forest canopy

Systems have been developed and deployed at a North Michigan forested site to measure ambient HONO and vertical HONO flux. The modified HONO measurement technique is based on aqueous scrubbing of HONO using a coil sampler, followed by azo dye derivatization and detection using a long-path absorption photometer (LPAP). A Na<sub>2</sub>CO<sub>3</sub>-coated denuder is used to generate "zero HONO" air for background correction. The lower detection limit of the method, defined by 3 times of the standard deviation of the signal, is 1 pptv for 1-min averages, with an overall uncertainty of ±(1 + 0.05 [HONO]) pptv. The HONO flux measurement technique has been developed based on the relaxed eddy accumulation approach, deploying a 3-D sonic anemometer and two HONO measurement systems. The overall uncertainty is estimated to be within ±(8 × 10<sup>−8</sup> + 0.15 <i>F</i><sub>HONO</sub>) mol m<sup>−2</sup> h<sup>−1</sup>, with a 20-min averaged data point per 30 min. Ambient HONO and vertical HONO flux were measured simultaneously at the PROPHET site from 17 July to 7 August 2008. The forest canopy was found to be a net HONO source, with a mean upward flux of 0.37 × 10<sup>−6</sup> moles m<sup>−2</sup> h<sup>−1</sup>. The HONO flux reached a maximal mean of ~0.7 × 10<sup>−6</sup> moles m<sup>−2</sup> h<sup>−1</sup> around solar noon, contributing a major fraction to the HONO source strength required to sustain the observed ambient concentration of ~70 pptv. There were no significant correlations between [NO<sub>x</sub>] and daytime HONO flux and between <i>J</i><sub>NO<sub>2</sub></sub> × [NO<sub>2</sub>] and HONO flux, suggesting that NO<sub>x</sub> was not an important precursor responsible for HONO daytime production on the forest canopy surface in this low-NO<sub>x</sub> rural environment. Evidence supports the hypothesis that photolysis of HNO<sub>3</sub> deposited on the forest canopy surface is a major daytime HONO source

Reduce blue water scarcity and increase nutritional and economic water productivity through changing the cropping pattern in a catchment

Water-stressed countries need to plan their food security and reduce the pressure on their limited water resources. Agriculture, the largest water-using sector, has a major role in addressing water scarcity and food security challenges. While there has been quite some attention to water management solutions like soil mulching and improved irrigation, less attention has been paid to adapting the cropping pattern to save water. Here, we investigate how a change in which crops are grown where and when can influence the green and blue water footprint (WF) of crop production, save blue water, reduce blue water scarcity and increase both food and cash crop production, using FAO's AquaCrop model. The performance of two potential solutions, first a strategy of mulching plus drip irrigation, and second a strategy with changing the cropping pattern in addition to mulching and drip irrigation, were compared in one of the most water-stressed catchments in the world, the Upper Litani Basin in Lebanon. Our results show a substantial potential for more efficient use of green water resources for food production while saving scarce blue water resources. Whereas mulching and drip irrigation together decrease the blue WF in the basin by 4.5%, changing the cropping pattern as well can decrease it by 20.3%. Food and cash production could increase by 3% and 50% by changing the cropping pattern, compared to 1.5% and 2.1% by mulching and drip irrigation. Changing the cropping pattern could thus significantly reduce water scarcity and enlarge food and cash production in the basin. © 2020Authors are thankful for the support from the UN-FAO headquarter and FAO-Lebanon. The ground data used for the simulation and modelling were collected during the field visit of the Litani Basin funded by the FAO-WaPOR project (FRAME consortium