Abiotic mechanism for the formation of atmospheric nitrous oxide from ammonium nitrate

Effects of SO2 on NH4NO3 Photolysis: The Role of Reducibility and Acidic Products

Nitrate photolysis is a vital process in secondary NOx release into the atmosphere. The heterogeneous oxidation of SO₂ due to nitrate photolysis has been widely reported, while the influence of SO₂ on nitrate photolysis has rarely been investigated. In this study, the photolysis of nitrate on different substrates was investigated in the absence and presence of SO₂. In the photolysis of NH₄NO₃ on the membrane without mineral oxides, NO, NO₂, HONO, and NH₃ decreased by 17.1, 6.0, 12.6, and 57.1% due to the presence of SO₂, respectively. In the photolysis of NH₄NO₃ on the surface of mineral oxides, SO₂ also exhibited an inhibitory effect on the production of NOx, HONO, and NH₃ due to its reducibility and acidic products, while the increase in surface acidity due to the accumulation of abundant sulfate on TiO₂ and MgO promoted the release of HONO. On the photoactive oxide TiO₂, HSO₃^-, generated by the uptake of SO₂, could compete for holes with nitrate to block nitrate photolysis. This study highlights the interaction between the heterogeneous oxidation of SO₂ and nitrate photolysis and provides a new perspective on how SO₂ affects the photolysis of nitrate absorbed on the photoactive oxides.

Converting rice husk to biochar reduces bamboo soil N2O emissions under different forms and rates of nitrogen additions

The effects of biochar application combined with different forms and rates of inorganic nitrogen (N) addition on nitrous oxide (N2O) emissions from forest soils have not been well documented. A microcosm experiment was conducted to study the effects of rice husk and its biochar in combination with the addition of N fertilizers in different forms (ammonium [NH4+] and nitrate [NO3-]) and rates (equivalent to 150 and 300 kg N ha-1 yr-1) on N2O emissions from Lei bamboo (Phyllostachys praecox) soils. The application of rice husk significantly increased cumulative N2O emissions under the addition of both NO3--N and NH4+-N. Biochar significantly reduced cumulative N2O emissions by 15.2 and 5.8 μg N kg-1 when co-applied with the low and high rates of NO3--N, respectively, compared with the respective NO3--N addition rate without biochar. There was no significant difference in soil N2O emissions between the two NH4+-N addition rates, and cumulative N2O emission decreased with increasing soil NH4+-N concentration, mainly due to the toxic effect caused by the excessive NH4+-N on soil N2O production from the nitrification process. Cumulative N2O emissions recorded 18.74 and 14.04 μg N kg-1 under low and high rates of NO3--N addition, respectively, which were higher than those produced by NH4+-N addition. Our study demonstrated that the conversion of rice husk to biochar could reduce N2O emissions under the addition of different N forms and rates. Moreover, rice husk or its biochar in combination with NH4+-N fertilizer produced less N2O in Lei bamboo soil, compared with NO3--N fertilizer.

Exoplanet Biosignatures: A Framework for Their Assessment

Finding life on exoplanets from telescopic observations is an ultimate goal of exoplanet science. Life produces gases and other substances, such as pigments, which can have distinct spectral or photometric signatures. Whether or not life is found with future data must be expressed with probabilities, requiring a framework of biosignature assessment. We present a framework in which we advocate using biogeochemical "Exo-Earth System" models to simulate potential biosignatures in spectra or photometry. Given actual observations, simulations are used to find the Bayesian likelihoods of those data occurring for scenarios with and without life. The latter includes "false positives" wherein abiotic sources mimic biosignatures. Prior knowledge of factors influencing planetary inhabitation, including previous observations, is combined with the likelihoods to give the Bayesian posterior probability of life existing on a given exoplanet. Four components of observation and analysis are necessary. (1) Characterization of stellar (e.g., age and spectrum) and exoplanetary system properties, including "external" exoplanet parameters (e.g., mass and radius), to determine an exoplanet's suitability for life. (2) Characterization of "internal" exoplanet parameters (e.g., climate) to evaluate habitability. (3) Assessment of potential biosignatures within the environmental context (components 1-2), including corroborating evidence. (4) Exclusion of false positives. We propose that resulting posterior Bayesian probabilities of life's existence map to five confidence levels, ranging from "very likely" (90-100%) to "very unlikely" (<10%) inhabited. Key Words: Bayesian statistics-Biosignatures-Drake equation-Exoplanets-Habitability-Planetary science. Astrobiology 18, 709-738.

Widespread production of nonmicrobial greenhouse gases in soils

Carbon dioxide (CO₂ ), methane (CH₄ ), and nitrous oxide (N₂ O) are the three most important greenhouse gases (GHGs), and all show large uncertainties in their atmospheric budgets. Soils of natural and managed ecosystems play an extremely important role in modulating their atmospheric abundance. Mechanisms underlying the exchange of these GHGs at the soil-atmosphere interface are often assumed to be exclusively microbe-mediated (M-GHGs). We argue that it is a widespread phenomenon for soil systems to produce GHGs through nonmicrobial pathways (NM-GHGs) based on a review of the available evidence accumulated over the past half century. We find that five categories of mechanistic process, including photodegradation, thermal degradation, reactive oxidative species (ROS) oxidation, extracellular oxidative metabolism (EXOMET), and inorganic chemical reactions, can be identified as accounting for their production. These pathways are intricately coupled among themselves and with M-GHGs production and are subject to strong influences from regional and global change agents including, among others, climate warming, solar radiation, and alterations of atmospheric components. Preliminary estimates have suggested that NM-GHGs could play key roles in contributing to budgets of GHGs in the arid regions, whereas their global importance would be enhanced with accelerated global environmental changes. Therefore, more research should be undertaken, with a differentiation between NM-GHGs and M-GHGs, to further elucidate the underlying mechanisms, to investigate the impacts of various global change agents, and to quantify their contributions to regional and global GHGs budgets. These efforts will contribute to a more complete understanding of global carbon and nitrogen cycling and a reduction in the uncertainty of carbon-climate feedbacks in the Earth system.

Reflections on O2 as a Biosignature in Exoplanetary Atmospheres

Oxygenic photosynthesis is Earth's dominant metabolism, having evolved to harvest the largest expected energy source at the surface of most terrestrial habitable zone planets. Using CO₂ and H₂O-molecules that are expected to be abundant and widespread on habitable terrestrial planets-oxygenic photosynthesis is plausible as a significant planetary process with a global impact. Photosynthetic O₂ has long been considered particularly robust as a sign of life on a habitable exoplanet, due to the lack of known "false positives"-geological or photochemical processes that could also produce large quantities of stable O₂. O₂ has other advantages as a biosignature, including its high abundance and uniform distribution throughout the atmospheric column and its distinct, strong absorption in the visible and near-infrared. However, recent modeling work has shown that false positives for abundant oxygen or ozone could be produced by abiotic mechanisms, including photochemistry and atmospheric escape. Environmental factors for abiotic O₂ have been identified and will improve our ability to choose optimal targets and measurements to guard against false positives. Most of these false-positive mechanisms are dependent on properties of the host star and are often strongest for planets orbiting M dwarfs. In particular, selecting planets found within the conservative habitable zone and those orbiting host stars more massive than 0.4 M_⊙ (M3V and earlier) may help avoid planets with abundant abiotic O₂ generated by water loss. Searching for O₄ or CO in the planetary spectrum, or the lack of H₂O or CH₄, could help discriminate between abiotic and biological sources of O₂ or O₃. In advance of the next generation of telescopes, thorough evaluation of potential biosignatures-including likely environmental context and factors that could produce false positives-ultimately works to increase our confidence in life detection. Key Words: Biosignatures-Exoplanets-Oxygen-Photosynthesis-Planetary spectra. Astrobiology 17, 1022-1052.

Photochemistry of Thin Solid Films of the Neonicotinoid Imidacloprid on Surfaces

Imidacloprid (IMD) is the most widely used neonicotinoid insecticide found on environmental surfaces and in water. Analysis of surface-bound IMD photolysis products was performed using attenuated total reflectance Fourier transfer infrared (ATR-FTIR) analysis, electrospray ionization (ESI-MS), direct analysis in real time mass spectrometry (DART-MS), and transmission FTIR for gas-phase products. Photolysis quantum yields (ϕ) for loss of IMD were determined to be (1.6 ± 0.6) × 10^-3 (1s) at 305 nm and (8.5 ± 2.1) × 10^-3 (1s) at 254 nm. The major product is the imidacloprid urea derivative (IMD-UR, 84% yield), with smaller amounts of the desnitro-imidacloprid (DN-IMD, 16% yield) product, and gaseous nitrous oxide (N₂O). Theoretical calculations show that the first step of the main mechanism is the photodissociation of NO₂, which then recombines with the ground electronic state of IMD radical to form IMD-UR and N₂O in a thermally driven process. The photolytic lifetime of IMD at a solar zenith angle of 35° is calculated to be 16 h, indicating the significant reaction of IMD over the course of a day. Desnitro-imidacloprid has been identified by others as having increased binding to target receptors compared to IMD, suggesting that photolysis on environmental surfaces increases toxicity.

Hybrid Nitrous Oxide Production from a Partial Nitrifying Bioreactor: Hydroxylamine Interactions with Nitrite

The goal of this study was to elucidate the mechanisms of nitrous oxide (N₂O) production from a bioreactor for partial nitrification (PN). Ammonia-oxidizing bacteria (AOB) enriched from a sequencing batch reactor (SBR) were subjected to N₂O production pathway tests. The N₂O pathway test was initiated by supplying an inorganic medium to ensure an initial NH₄⁺-N concentration of 160 mg-N/L, followed by ¹⁵NO₂^- (20 mg-N/L) and dual ¹⁵NH₂OH (each 17 mg-N/L) spikings to quantify isotopologs of gaseous N₂O (⁴⁴N₂O, ⁴⁵N₂O, and ⁴⁶N₂O). N₂O production was boosted by ¹⁵NH₂OH spiking, causing exponential increases in mRNA transcription levels of AOB functional genes encoding hydroxylamine oxidoreductase (haoA), nitrite reductase (nirK), and nitric oxide reductase (norB) genes. Predominant production of ⁴⁵N₂O among N₂O isotopologs (46% of total produced N₂O) indicated that coupling of ¹⁵NH₂OH with ¹⁴NO₂^- produced N₂O via N-nitrosation hybrid reaction as a predominant pathway. Abiotic hybrid N₂O production was also observed in the absence of the AOB-enriched biomass, indicating multiple pathways for N₂O production in a PN bioreactor. The additional N₂O pathway test, where ¹⁵NH₄⁺ was spiked into 400 mg-N/L of NO₂^- concentration, confirmed that the hybrid N₂O production was a dominant pathway, accounting for approximately 51% of the total N₂O production.

A survey of photogeochemistry

The participation of sunlight in the natural chemistry of the earth is presented as a unique field of study, from historical observations to prospects for future inquiry. A compilation of known reactions shows the extent of light-driven interactions between naturally occurring components of land, air, and water, and provides the backdrop for an outline of the mechanisms of these phenomena. Catalyzed reactions, uncatalyzed reactions, direct processes, and indirect processes all operate in natural photochemical transformations, many of which are analogous to well-known biological reactions. By overlaying photochemistry and surface geochemistry, complementary approaches can be adopted to identify natural photochemical reactions and discern their significance in the environment.

Introductory lecture: atmospheric chemistry in the Anthropocene

The term “Anthropocene” was coined by Professor Paul Crutzen in 2000 to describe an unprecedented era in which anthropogenic activities are impacting planet Earth on a global scale. Greatly increased emissions into the atmosphere, reflecting the advent of the Industrial Revolution, have caused significant changes in both the lower and upper atmosphere. Atmospheric reactions of the anthropogenic emissions and of those with biogenic compounds have significant impacts on human health, visibility, climate and weather. Two activities that have had particularly large impacts on the troposphere are fossil fuel combustion and agriculture, both associated with a burgeoning population. Emissions are also changing due to alterations in land use. This paper describes some of the tropospheric chemistry associated with the Anthropocene, with emphasis on areas having large uncertainties. These include heterogeneous chemistry such as those of oxides of nitrogen and the neonicotinoid pesticides, reactions at liquid interfaces, organic oxidations and particle formation, the role of sulfur compounds in the Anthropocene and biogenic–anthropogenic interactions. A clear and quantitative understanding of the connections between emissions, reactions, deposition and atmospheric composition is central to developing appropriate cost-effective strategies for minimizing the impacts of anthropogenic activities. The evolving nature of emissions in the Anthropocene places atmospheric chemistry at the fulcrum of determining human health and welfare in the future.

Nitrous oxide fluxes in estuarine environments: response to global change

Nitrous oxide is a powerful, long-lived greenhouse gas, but we know little about the role of estuarine areas in the global N2 O budget. This review summarizes 56 studies of N2 O fluxes and associated biogeochemical controlling factors in estuarine open waters, salt marshes, mangroves, and intertidal sediments. The majority of in situ N2 O production occurs as a result of sediment denitrification, although the water column contributes N2 O through nitrification in suspended particles. The most important factors controlling N2 O fluxes seem to be dissolved inorganic nitrogen (DIN) and oxygen availability, which in turn are affected by tidal cycles, groundwater inputs, and macrophyte density. The heterogeneity of coastal environments leads to a high variability in observations, but on average estuarine open water, intertidal and vegetated environments are sites of a small positive N2 O flux to the atmosphere (range 0.15-0.91; median 0.31; Tg N2 O-N yr(-1) ). Global changes in macrophyte distribution and anthropogenic nitrogen loading are expected to increase N2 O emissions from estuaries. We estimate that a doubling of current median NO3 (-) concentrations would increase the global estuary water-air N2 O flux by about 0.45 Tg N2 O-N yr(-1) or about 190%. A loss of 50% of mangrove habitat, being converted to unvegetated intertidal area, would result in a net decrease in N2 O emissions of 0.002 Tg N2 O-N yr(-1) . In contrast, conversion of 50% of salt marsh to unvegetated area would result in a net increase of 0.001 Tg N2 O-N yr(-1) . Decreased oxygen concentrations may inhibit production of N2 O by nitrification; however, sediment denitrification and the associated ratio of N2 O:N2 is expected to increase.

Identification of nitrous oxide emissions from green areas of Chihuahua City irrigated with treated wastewater

Since 2000, the city of Chihuahua had a distribution system of treated wastewater for irrigation of green areas and has replaced this water for processes that do not require the consumption of drinking water. This replacement was necessary in order to meet the growing demand for potable water which has exceeded the current supply of 700 L (184,88 gallons) per second. Nowadays it is necessary to identify and assess the risks to public health and the environment due to the substitution of drinking water by treated wastewater in the last 10yr. Treated wastewater contains compounds whose effects have not been evaluated when used for irrigation in public green areas. Therefore, it is not known whether there is a danger to the health of park visitors due to exposure and/or inhalation of the emitted gases, accidental ingestion of water, or impact to the environment. The purpose of the research, using an experimental prototype, is to identify the changes from nitrogen present in the treated wastewater to nitrous oxide. The research objective is the generation of data to simulate a regional scale at this stage, which will be analyzed and statistically validated using Minitab and Origin software. The experiment was performed using three different samples to compare water quality: drinking water treated wastewater, and water with nitrogen-based fertilizer (urea). Prototypes were filled with two types of soil: sand and clay Each type of water was sprinkled on the prototype, grass was planted in it, and the prototype was equipped with samplers to capture the gas in the root zone. The authors found high emissions of nitrous oxide in the clay-filled lysimeters, and climate and growing conditions of vegetation were the most important factors for producing nitrous oxide.

IMPLICATIONS

Major problems in the ecosystem arise from solutions that are not based on environmental public policy research or experimentation. For example, before application a specific policy or regulation, research should be performed to evaluate long term effects to the ecosystem and this can be done through close monitoring. This study raises awareness about the public policy of substituting potable water with treated wastewater for irrigation of green areas in Chihuahua City. The present study was performed during a sufficient period of time in order to assess the impact to the environment.