Nitrate Formation Research Articles

Hydrothermal liquefaction of sewage sludge and model compound: Heavy metals distribution and behaviors

Hydrothermal liquefaction (HTL) is an attractive thermochemical method for the conversion of wet sewage sludge (SS) into bio-oil. However, the bio-oil suffers from the contamination of heavy metals (HMs). In this study, the effects of reaction conditions on the HMs (Cu, Zn, Cd, Cr and Pb) migration during HTL of SS was investigated. Soybean protein was used as the typical model compound of SS to reveal the interactions between HMs and organic matters in the HTL process. Results showed that higher temperatures and longer reaction times promoted HMs enriched into the solid residues with more stable forms, while the proportion of HMs in the bio-oil was also increased. XPS analysis showed that the coordinate bond was formed between HMs and N during HTL of soybean protein at 180 °C. At higher temperature of 280 °C, metal oxides appeared in residues, while Cu and Cd led to the formation of nitrates. GC–MS analysis showed that Cu reduced the aromatic content by 52.8 %, while other HMs (Zn, Cd, Cr and Pb) reduced the oxygenated compounds content by 21.6 % at 180 ℃. In addition, the HMs had little effects on the bio-oil compositions at 280 °C due to the more stable forms of HMs at higher temperatures.

Evaluation of isoprene nitrate chemistry in detailed chemical mechanisms

Abstract. Isoprene nitrates are important chemical species in the atmosphere which contribute to the chemical cycles that form ozone and secondary organic aerosol (SOA) with implications for climate and air quality. Accurate chemical mechanisms are important for the prediction of the atmospheric chemistry of species such as isoprene nitrates in chemical models. In recent years, studies into the chemistry of isoprene nitrates have resulted in the development of a range of mechanisms available for use in the simulation of atmospheric isoprene oxidation. This work uses a 0-D chemical box model to assess the ability of three chemically detailed mechanisms to predict the observed diurnal profiles of four groups of isoprene-derived nitrates in the summertime in the Chinese megacity of Beijing. An analysis of modelled C5H9NO5 isomers, including isoprene hydroperoxy nitrate (IPN) species, highlights the significant contribution of non-IPN species to the C5H9NO5 measurement, including the potentially large contribution of nitrooxy hydroxyepoxide (INHE). The changing isomer distribution of isoprene hydroxy nitrates (IHNs) derived from OH-initiated and NO3-initiated chemistry is discussed, as is the importance of up-to-date alkoxy radical chemistry for the accurate prediction of isoprene carbonyl nitrate (ICN) formation. All mechanisms under-predicted C4H7NO5 as predominately formed from the major isoprene oxidation products, methyl vinyl ketone (MVK) and methacrolein (MACR). This work explores the current capability of existing chemical mechanisms to accurately represent isoprene nitrate chemistry in urban areas significantly impacted by anthropogenic and biogenic chemical interactions. It suggests considerations to be taken when investigating isoprene nitrates in ambient scenarios, investigates the potential impact of varying isomer distributions on iodide chemical ionisation mass spectrometry (I−-CIMS) calibrations, and makes some proposals for the future development of isoprene mechanisms.

Characterizing formation mechanisms of secondary aerosols on black carbon in a megacity in South China

Refractory black carbon (rBC) aerosols emitted from incomplete combustion are important climate forcers. Understanding the chemical characteristics and evolution of rBC-related components is particularly crucial to assess rBC environmental impacts. Here, we explored the chemical components of rBC in Shenzhen, China, using a soot-particle aerosol mass spectrometer (SP-AMS). The observations showed that the rBC coating was mainly composed of secondary aerosols with an average mass contribution of 84.7 %. Among them, secondary organic coating occupied ∼57.7 % of the total coating mass. Exploration of the relationship between secondary organic aerosol (SOA) coating and Ox (=NO2 + O3, an indicator of the extent of photochemical processing) showed that SOA coating was generated mainly through photochemical oxidation during the day. Similarly, sulfate coating, with a small mass fraction of 0.9 %, was also dominated by photochemical oxidation. In contrast, nitrate coating responded positively to ambient relative humidity, especially at night, indicating that it was driven by heterogeneous reactions. In addition, the increased ratio of nitrate on rBC to bulk nitrate at night suggested that black carbon surface could facilitate nocturnal nitrate formation.

Understanding Aerobic Nitrogen Photooxidation on Titania through In Situ Time‐Resolved Spectroscopy

AbstractNitrate is an important raw material for chemical fertilizers, but it is industrially manufactured in multiple steps at high temperature and pressure, urgently motivating the design of a green and sustainable strategy for nitrate production. We report the photosynthesis of nitrate from N2and O2on commercial TiO2in a flow reactor under ambient conditions. The TiO2photocatalyst offered a high nitrate yield of 1.85 μmol h−1as well as a solar‐to‐nitrate energy conversion efficiency up to 0.13 %. We combined reactivity and in situ Fourier transform infrared spectroscopy to elucidate the mechanism of nitrate formation and unveil the special role of O2in N≡N bond dissociation. The mechanistic insight into charge‐involved N2oxidation was further demonstrated by in situ transient absorption spectroscopy and electron paramagnetic resonance. This work exhibits the mechanistic origin of N2photooxidation and initiates a potential method for triggering inert catalytic reactions.

Isotopic components and source analysis of inorganic nitrogen in coastal aerosols of the Yellow Sea

The coastal atmospheric environment is one of the most complex environments on earth. It is shaped by terrestrial, marine, and atmospheric processes and acts as an external nutrient source for coastal waters. At present, there are few observations of inorganic nitrogen isotopes of China coastal aerosols, let alone the Yellow Sea. In this study, a weekly collection of total suspended particulate aerosols was conducted on the Qianliyan Island in 2018 for the measurements of inorganic nitrogen species (NO3− and NH4+) and their isotopic ratios (δ15N-NO3−, δ18O-NO3−, and δ15N-NH4+). At the Qianliyan Island, the average NO3− and NH4+ concentrations were 2.49 ± 2.12 and 3.33 ± 2.68 μg·m−3, respectively; the average δ15N-NO3−, δ18O-NO3−, and δ15N-NH4+ were 2.4‰ ± 5.7‰, 78.7‰ ± 8.0‰, and −2.6‰ ± 6.3‰, respectively. The major nitrate formation pathways were •OH oxidation and N2O5 hydrolysis paths, and the dominant sources of inorganic nitrogen aerosols were coal combustion (29% ± 7%), marine (19% ± 15%), and fertilizer (16% ± 13%). Aerosol δ15N-NO3− and δ18O-NO3− were obviously higher in winter and lower in summer; conversely, aerosol δ15N-NH4+ was slightly higher in summer and slightly lower in winter. The difference in nitrogen sources was considered to be the best explanation for the aerosol δ15N-NO3− and δ15N-NH4+ differences between summer and winter, of which coal combustion contributed the most. The seasonal difference in nitrate formation paths was considered to be the best explanation for the difference of Qianliyan aerosol nitrate δ18O-NO3− between summer and winter. Aerosol inorganic nitrogen deposition flux was estimated to be 3.4 nmol N·m−2·s−1, which induced less than 1% to marine primary production, and aerosol inorganic nitrogen deposition, compared with N2 fixation, contributed some 80% of δ15N-NO3− depression of the summer Yellow Sea thermocline.

Electronic Structure Investigation of NO2 and NO Binding on Vanadium Oxides.

NO2 and NO, which are generated in combustion processes, binding to vanadium oxide clusters including TiO2-supported catalyst models in the selective catalytic reduction (SCR) of NO has been studied by density functional theory and coupled cluster methods. NOx binding on vanadium oxides is predicted to depend on several factors, including the excitation energy of the oxide, ionization energies of both the unbound oxide and the deoxygenated reduced oxide, and the strength of the molecular V-O bonds. NO2 chemisorption occurs either through covalent bond formation in a HONO-like pattern or through abstraction of a metal oxide oxygen leading to the formation of NO3-. Nitrate formation is more favorable than what was predicted for group IVB or group VIB oxides [except (CrO3)n] and is either the lowest energy binding mode or within a few kcal/mol of the lowest mode in all clusters, likely due to the stability of V in the +4 oxidation state. Physisorption on V oxides is very weak. V with 2 oxo groups have a lower excitation energy and a more sterically open geometry which results in strong chemisorption as predicted for group IVB oxides. Tetrahedrally coordinated vanadia with a single oxo group and 3 V-O single bonds are predicted to have significantly higher excitation energies and behave like group VIB oxides such that chemisorption is unlikely and weak physisorption dominates the interaction. In larger clusters, including SCR catalyst models, only tetrahedrally coordinated vanadia are present and NO2 binding is not expected to occur. NO adsorption is weaker overall than NO2 binding and occurs either as physisorption or as chemisorption through the formation of NO2- analogous to nitrate formation in NO2 binding. The ability of NO to bind reflects the patterns predicted for NO2, such that NO is strongly bound vanadia with two V═O groups and only weakly physisorbed when there is a single V═O or none at all.

Yield of winter barley with different systems of mineral fertilizer and use of urease inhibitor

The use of urease inhibitors with nitrogen fertilizers is gaining relevance. There was a need to substantiate the expediency of combining these technology elements when growing winter barley. We conducted field experiments at the Lviv National Environmental University in the Banded Forest of the Western Forest Steppe conditions in 2020–2022. Our research goal is to describe the interaction of mineral fertilizers with the N-LokTM brand inhibitor, which restrains the formation of nitrates but improves the agrochemical parameters of the soil. This had a positive effect on the yield of winter barley. The soil is a dark gray forest gilded light loam with low humus. We used traditional methods of field research and standardized methods of laboratory analyses. The nitrogen fertilization system N97(NH4NO3) during vegetation restoration + N-LokTM (before sowing or during vegetation restoration) on the background of N23P60K60 ensured the initial content of easily hydrolyzable nitrogen in the arable layer of 132–136 mg/kg of soil. Applying N97(NH4NO3) + N-LokTM to winter barley during the restoration of vegetation on the background of N23P60K60 under plowing provided the highest yield in the experiment of 7.69 t/ha on average for 2020–2022. The average grain increase relative to the N97(NH4NO3) control (vegetation restoration) on the N23P60K60 background was 1.08 t/ha. On the other hand, applying N97(NH4NO3) during the restoration of vegetation + N-LokTM in autumn before sowing on the same background contributed to a more significant yield increase – by 1.26 t/ha only in 2021. The absence of a phosphorus-potassium background in the winter barley fertilization system reduced the crop yield by 0.11–0.44 t/ha. The significant importance of mineral fertilizers for obtaining high grain harvests, especially in less good years, is proven by the established high correlation coefficients of the parameters of available compounds of nitrogen, phosphorus, and potassium at the start of the growing season with the harvest in 2020–2021. The regression 3D model of winter barley grain yield under the influence of the synergistic effect of nitrogen fertilization rates and the resulting enrichment of the soil with easily hydrolyzable nitrogen illustrates the importance of fertilizers for increasing the fertility of the dark-gray podzolized light loam soil in Pasmovy Pobuzhzhia of the Western Forest Steppe.

Promoting effect of basic metal additives on DeNOx reactions over Pt-based three-way catalysts

Pt has been economically favored over Rh and Pd. However, Pt-based three-way catalysts (TWCs) have been studied less than Pd- and Rh-based TWCs; therefore, their properties have not yet been sufficiently evaluated, especially under conditions relevant to modern three-way catalysis operation, wherein deviations from stoichiometric conditions are often encountered. In this study, we examined the promotional effect of basic metal additives in Pt/M/Al2O3 (M = Ba, La, Sr) on the efficiency of three-way catalytic reactions. The catalytic activities of the Pt/M/Al2O3 catalysts for NO, CO, and total hydrocarbons were superior to those of Pt/Al2O3; moreover, Ba enhanced the catalytic activity of Pt-based catalysts more efficiently than La and Sr. The best-performing catalyst, Pt/Ba/Al2O3, was subjected to kinetic studies and various in situ/operando spectroscopic experiments, namely, X-ray absorption spectroscopy, infrared spectroscopy, and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), to investigate the role of Ba in the NO reduction reaction. The Pt0 species loaded on Ba/Al2O3 were more electron-rich than those loaded on Al2O3, thereby promoting NO dissociation into N and O atoms, as revealed by the AP-XPS results. Moreover, the efficient formation of intermediate surface NOx species, including nitrites and nitrates, and their reactivities toward reductant gases, such as H2 or CO, were critical for promoting the effect of Ba on the NO reduction reaction.

Impact of high PM2.5 nitrate on visibility in a medium size city of Pearl River Delta

Daily PM2.5 were collected at urban Zhongshan, a medium size city in the Pearl River Delta (PRD), and its major chemical components were determined. The contribution of NO3− to PM2.5 concentration (17%) during the campaign in Zhongshan was higher than that of other PRD cities on a seasonal basis. Among the inorganic ions, NO3− elevated dramatically with PM increase. Among the inorganic ions, NO3− elevated dramatically with PM increase. The NO3− was the most abundant species in PM2.5 (19%) during the polluted cases, followed by SO42− (13%) and NH4+ (9%). All haze events were associated with high nitrate and ammonium rich (AR) aerosols; the average [NH4+]/[SO42−] was equal to 3.4 and ranged from 2.4 to 4.8. The nitrogen oxidation rate (NOR) during the haze events was significantly higher than during the non-haze. Potential source contribution function (PSCF) and back trajectory analysis indicated that trans-regional transmission of NO2 and NH3 from the northeast and east of the PRD might contribute to local nitrate formation. The reduced visibility was associated with a higher percentage of NH4NO3. The average contribution of NH4NO3 enhanced markedly from 19% (>20 km) to 54% (<5 km). NH4NO3 became the most important contributor of extinction coefficients (bext) than (NH4)2SO4 when the visibility was less than 10 km, and the estimated b'ext in the dry condition was about 30% lower than that in ambient conditions.

PM10 aerosol enhancement in the anticyclonic anomalies caused by the East Asian spring warming of 2021

This study analyzed the PM10 aerosol enhancement caused by East Asian atmospheric warming during transboundary transport in March of 2021. It was found to be cooled near the north polar area and to have shifted the polar vortices northward over the East Asian region in the mid-troposphere. The Siberian high was weakened and the East Asian region was characterized by a positive air temperature anomaly and negative zonal wind speeds. The increasing rate of air temperature anomalies at 2 m was higher in the high latitude areas over 40°N in Mongolia and northeastern China, causing a decrease in zonal wind speeds at 10 m over eastern China and Korea. The anomalous variations of the MODIS-induced aerosol optical depth (AOD) were found to be positive with the highest value of +0.93 in the area ranging from northeastern China to Korea, despite the recent decreasing trends. The positive anomalous region of the MODIS-induced AOD was correlated with positive air temperatures and negative zonal wind speeds, causing enhanced formations of secondary aerosols in the atmosphere. As cold northerly winds were suppressed in the East Asian region, anomalous high pressures (anticyclonic anomalies) developed frequently in eastern China and moved to Korea. The Weather Research and Forecasting model to which meteorological and climate data were applied coupled with Chemistry (WRF-Chem) simulations on increases in the PM10 enhancement ratio, indicating enhanced formation amounts, during transboundary transport for the period of March 13–15, 2021. An air mass near the high terrain surface over eastern China under the backward side of an anticyclone warmed and moved toward the relatively low terrain of the Yellow Sea and Korea in keeping with the height. Air temperature inversion developed in the anomalous warmer center in the mid-troposphere, reaching the surface of the Yellow Sea and Korea. High PM10 concentrations were constrained near the shallow inversion layer. Furthermore, the PM10 enhancement ratio showed a positive level over the Yellow Sea with nearly zero anthropogenic emissions on the surface, gradually increasing at a rate of approximately 60% in Korea. The hourly contribution by the warming-induced PM10 enhancement (PM10 CbWIE) was calculated and found to be at the highest level of 70% in Seoul, Korea. Among PM10 secondary inorganic aerosols, nitrate formations were enhanced during transboundary transport in the warming-induced anticyclone with reactions rich in ammonium. While the sulfate enhancement ratio reached a positive level over the Yellow Sea, the gradual decrease during transboundary transport may be reflected by the decreased emissions of sulfur dioxide in eastern China.

Single-atom dispersed Zn-N3 active sites bridging the interlayer of g-C3N4 to tune NO oxidation pathway for the inhibition of toxic by-product generation

Photocatalysis is regarded as one of the most promising methods for removing low concentrations of nitrogen oxides (NOx) discharged from mobile sources, but its low activity and secondary contamination with harmful by-products restrict its practical implementation. Herein, we introduced single Zn atoms into the interlayer of g-C3N4 prepared through a simple pyrolysis method with an ultrahigh loading amount of 37.0 wt% (10.2 at%). Zn-N3 bridging structure is created by single atomic scattered Zn atoms connecting with three N atoms of two layers of g-C3N4, which enhance visible light absorption and electron-hole pairs’ separation. The introduced single Zn atom induced the adsorbed O2 and NO to fabricate the Zn-O2-NO structure, which facilitated the dissociation of O2 and the direct formation of nitrate from NO, tuning NO oxidation pathway and resulting in increased NO oxidization (49 %) but low toxic NO2 generation (2.6 %). This current work introduces a novel approach to fabricating single-atom catalysts for highly effective environmental remediation and secondary pollution control.

Connecting Energetic Nitropyrazole and Nitrobenzene Moieties with C−C Bonds using Suzuki Cross‐Coupling Reaction: A Novel Route to Thermally Stable Energetic Materials

AbstractThis work describes a new route to thermally stable energetic materials by connecting different types of energetic moieties (nitropyrazole and polynitrobenzene) with C−C bonds, where the first step involves a Suzuki cross‐coupling reaction between 4‐bromo‐3,5‐dinitro‐1H‐pyrazole and 4‐chlorophenylboronic acid. Further nitration, amination, oxidation, and salt formation reactions on the resulting C−C coupled framework resulted in various thermally stable energetic materials. All the compounds were fully characterized using IR, NMR [1H, 13C{1H}], differential scanning calorimetry (DSC), elemental analysis, and HRMS studies. Compounds 7 and 9 were further characterized by 15N NMR, and compounds 3 and 6 were characterized by single‐crystal X‐ray diffraction studies. Theoretical heats of formation and energetic performance for all the energetic compounds were calculated using Gaussian 09 and EXPLO5 v6.06 programs, respectively. This research introduces a novel method for synthesizing C−C coupled energetic materials that are not accessible by any other routes.

Peroxy Radical Autoxidation and Sequential Oxidation in Organic Nitrate Formation during Limonene Nighttime Oxidation.

Limonene is an abundant monoterpene released into the atmosphere via biogenic emissions and biomass burning. However, the atmospheric oxidation and secondary organic aerosol (SOA) formation mechanisms of limonene, especially during nighttime, remain largely understudied. In this work, limonene was oxidized synergistically by ozone (O3) and nitrate radicals (NO3) in a flow tube reactor and a continuous flow stirred tank reactor. Upon oxidation, many highly oxidized organic nitrates and nitrooxy peroxy radicals (RO2) were observed in the gas phase within 1 min. Combining quantum chemical calculations with kinetic simulations, we found that the primary nitrooxy RO2 (C10H16NO5) through NO3 addition at the more substituted endocyclic double bond and at the exocyclic double bond (previously considered as minor pathways) can undergo autoxidation with rate constants of around 0.02 and 20 s-1 at 298 K, respectively. These pathways could explain a major portion of the observed highly oxidized organic nitrates. In the SOA, highly oxidized mono- and dinitrates (e.g., C10H17NO7-8 and C10H16,18N2O8-10) make up a significant contribution, highlighting nitrooxy RO2 autoxidation and sequential NO3 oxidation of limonene. The same organic nitrates are also observed in ambient aerosol during biomass burning and nighttime in the southeastern United States. Therefore, the present work provides new insights into the nighttime oxidation of limonene and SOA formation in the atmosphere.

New Reactions for the Formation of Organic Nitrate in the Atmosphere.

Organic nitrates make an important contribution to the formation of secondary organic aerosols, but the formation mechanisms of organic nitrates are not fully understood at the molecular level. In the present work, we explore a new route for the formation of organic nitrates in the reaction of formaldehyde (HCHO) with nitric acid (HNO3) catalyzed by water (H2O), ammonia (NH3), and dimethylamine ((CH3)2NH) using theoretical methods. The present results using CCSD(T)-F12a/cc-pVTZ-F12//M06-2X/MG3S unravel that dimethylamine has a stronger catalytic ability in the reaction of HCHO with HNO3, reducing the barrier by 21.97 kcal/mol, while water and ammonia only decrease the energy barrier by 7.35 and 13.56 kcal/mol, respectively. In addition, the calculated kinetics combined with the corresponding concentrations of these species show that the HCHO + HNO3 + (CH3)2NH reaction can compete well with the naked HCHO + HNO3 reaction at 200-240 K, which may make certain contributions to the formation of organic nitrates under some atmospheric conditions.

Nitration Poisoning of Silver on Al2O3 for the Catalytic Oxidation of Soot in NOx-Containing Atmospheres

Silver-based catalysts are promising candidates for soot oxidation because of their good catalytic performances and low costs compared with platinum catalysts. NO2 acts as a strong agent for the oxidation of soot over Pt/Al2O3. However, an abnormal inhibition effect of NOx was observed over the Ag/Al2O3 catalyst during the processes of soot temperature-programmed oxidation and transient reaction analysis. The formation of silver nitrates was confirmed by temperature-programmed decomposition of NOx, ultraviolet–visible spectroscopy, and temperature-programmed reduction with H2. The intrinsic reactivity of silver nitrate was found to be much lower than that of metallic silver in ″loose″ and ″tight″ contacts between soot and catalyst. Moreover, the formed silver nitrate was relatively thermally stable, with the decomposition completed at around 450 °C. At higher temperatures, the catalyst recovered and the nitrate poisoning effect no longer worked.

Self-supported Mo-doped TiO2 electrode for ambient electrocatalytic nitrogen oxidation

Electrocatalytic nitrogen oxidation reaction (NOR) provides an eco-friendly pathway for ambient N2 fixation and nitrate/nitric acid synthesis but is plagued by sluggish kinetics due to the weak adsorption of N2 molecules and cleavage of NN triple bonds. Herein, a self-supported Mo-doped TiO2 electrode prepared by one-step hydrothermal method is reported for efficient nitrogen oxidation. Electrochemical measurements and spectroscopy investigations reveal that the inert TiO2 is effectively triggered by Mo substitution in terms of enhanced N2 adsorption and activation. As further confirmed by density function theory calculations, the incorporation of Mo reduces the barrier of the rate-determining step and provides an energetically favorable NOR pathway compared to pristine rutile TiO2. Electrochemical results show that Mo-doped TiO2 electrode achieves a nitrate yield of 5.44 μg h−1 mgcat−1 (or 8.77 ± 0.39 nmol h−1 cm−2) with a Faradaic efficiency of 2.88%. The formation of nitrate from N2 under positive bias is further demonstrated by in situ ultraviolet-visible spectrophotometry. This work provides experimental and theoretical evidences that Mo doping can stimulate the stable yet electrocatalytically inert Ti-based oxide toward NOR via promoting active sites formation and lowering activation energy barriers.

In Operando Investigation of the Concentration Dependent NO2 Sensing Mechanism of Bi2S3 Nanorods at Low Temperatures and the Interference of O3.

The demand for gas sensors that can detect gases selectively at low temperatures has increased steadily over recent years. Most devices use semiconducting metal oxides as sensing materials which often require high operation temperatures and suffer from a lack of selectivity. Semiconducting metal sulfides were found to be a reasonable alternative for the application in sensing devices at low temperatures. Since metal sulfides are a relatively new class of materials applied in gas sensors, there is little work on sensing mechanisms and overall sensing characteristics of these materials. In this work, the authors investigated the sensing performance of Bi2S3 nanorods operated at 50 °C in the presence of several target gases and found a selective response to oxidizing gases. With the help of DC resistance measurements, diffuse reflectance infrared Fourier transform spectroscopy and work function measurements in a Kelvin Probe setup, the NO2 and O3 sensing mechanisms of Bi2S3 nanorods were revealed. While initially sulfur vacancies were the predominant reaction sites, the formation of nitrates became the key reaction in higher NO2 concentrations. Additionally, it was found that the reaction with O3 healed sulfur vacancies effectively inhibiting the reaction with NO2.

Nitrate dynamics and source identification of rainwater in Beijing during rainy season: Insight from dual isotopes and Bayesian model

Anthropogenic reactive nitrogen emissions have a significant impact on atmospheric chemical composition and earth surface ecosystem. As one of the most important sinks of atmospheric nitrogen, the wet deposition of nitrate (rainwater NO3−) has been widely concerned. Yet, the sources and transformation processes of wet deposited NO3− were not well revealed in megacity during rainy season in the context of global climate change. Here, we investigated the concentrations of nitrogen components and dual isotopes of rainwater nitrate collected in Beijing during July to August 2021 (rainy season). The main findings showed that the concentrations of NH4+-N, NO3−-N, and NO2−-N ranged 0.5– 6.7 mg L−1, 0.3– 4.5 mg L−1, and 0.05– 0.18 mg L−1, respectively, with the average relative percentages of 69 %, 29 %, and 2 %. The stoichiometry analysis of characteristic ion ratios indicated that the contribution of municipal wastes and agricultural sources to rainwater NH4+-N is relatively significant, while traffics were the major contributor of NO3−-N instead of the fixed emission. Rainwater δ15N-NO3− and δ18O-NO3− presented slightly 15N-depleted characteristic compared to previous studies with the average values of −3.9 ± 3.1 ‰ and 58.7 ± 12.6 ‰. These isotope compositions suggesting an origin of rainwater NO3− from the mixing of multi-sources and was mainly generated via the pathway of OH radical oxidization. Further source apportionment of rainwater NO3− by Bayesian mixing model evaluated that traffic (30.3 %) and soil (30.3 %) emissions contributed mostly to NO3−, while the contribution of biomass burning (18.8 %) and coal combustion (20.6 %) were relatively lower. This study highlighted the important role of dual isotopes in rainwater nitrate source identification and formation processes in megacity.

Effects of Nitrogen Oxides on the Production of Reactive Oxygen Species and Environmentally Persistent Free Radicals from α-Pinene and Naphthalene Secondary Organic Aerosols.

Reactive oxygen species (ROS) and environmentally persistent free radicals (EPFR) play an important role in chemical transformation of atmospheric aerosols and adverse aerosol health effects. This study investigated the effects of nitrogen oxides (NOx) during photooxidation of α-pinene and naphthalene on the EPFR content and ROS formation from secondary organic aerosols (SOA). Electron paramagnetic resonance (EPR) spectroscopy was applied to quantify EPFR content and ROS formation. While no EPFR were detected in α-pinene SOA, we found that naphthalene SOA contained about 0.7 pmol μg–1 of EPFR, and NOx has little influence on EPFR concentrations and oxidative potential. α-Pinene and naphthalene SOA generated under low NOx conditions form OH radicals and superoxide in the aqueous phase, which was lowered substantially by 50–80% for SOA generated under high NOx conditions. High-resolution mass spectrometry analysis showed the substantial formation of nitroaromatics and organic nitrates in a high NOx environment. The modeling results using the GECKO-A model that simulates explicit gas-phase chemistry and the radical 2D-VBS model that treats autoxidation predicted reduced formation of hydroperoxides and enhanced formation of organic nitrates under high NOx due to the reactions of peroxy radicals with NOx instead of their reactions with HO2. Consistently, the presence of NOx resulted in the decrease of peroxide contents and oxidative potential of α-pinene SOA.

Contrasting mass absorption efficiency of carbonaceous aerosols between PM1 and PM2.5 in urban Beijing

Carbonaceous aerosol, mainly comprising elemental carbon (EC) and organic carbon (OC), plays an important role in aerosol–climate interaction owing to its light-absorbing feature. The radiative forcing of carbonaceous aerosol is largely determined by its mass absorption efficiency (MAE). To depict the MAE of carbonaceous aerosol and its variations with aerosol size and chemical composition, light absorption coefficients and the main chemical composition of PM1 and PM2.5 were comparably measured in a winter field campaign in urban Beijing. On average, aerosol absorption by PM1 at 880 nm wavelength contributes to approximately half of aerosol absorption by PM2.5, mainly because aerosol absorption at this wavelength was dominated by EC and nearly half of the total EC mass in PM2.5 existed in PM1. The average MAE of EC at 880 nm (MAEEC,880nm) was 3.9 ± 0.7 m2 g−1 for PM2.5, lower than that for PM1 (4.4 ± 0.8 m2 g−1) likely because of the larger EC cores and lower coating degrees of EC particles in PM1–2.5. Variation in MAEEC,880nm was related to the bulk mass fractions of nitrate in PM1 and PM2.5, implying the important impact of secondary nitrate formation on the modification of EC mixing states and enhancing EC absorption efficiency in Beijing. Absorption by OC took up about 40% of the absorption coefficients for PM1 and PM2.5 at 370 nm. The mean MAE of OC at 370 nm (MAEOC,370nm) was 2.4 ± 0.9 m2 g−1 for PM1, higher than that of for PM2.5 (1.9 ± 0.6 m2 g−1). The high value of MAEOC,370nm might be associated with regionally transported aerosols during clean and polluted periods. Enlarged particle sizes might have considerably weakened MAEOC,370nm for PM2.5 while exerting negligible impact on MAEOC,370nm for PM1.