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Gaseous air pollutants emitted primarily by anthropogenic sources form secondary products through photochemical reactions, complicating the regulatory analysis of anthropogenic emissions in the atmosphere. We used an environmental chassis dynamometer and a photochemical smog chamber to conduct a parameter sensitivity experiment to investigate the formation of secondary products from a gasoline passenger car. To simulate the mitigation of ammonia emissions from gasoline vehicle exhausts assuming future emission controls and to allow photochemical oxidation and aging of the vehicle exhaust, ammonia was selectively removed by a series of five denuders installed between the vehicle and photochemical smog chamber. Overall, there were no differences in the formation of secondary organic aerosols and ozone with or without ammonia mitigation. However, the potential for ammonium nitrate particle formation was significantly reduced with ammonia mitigation. In addition, ammonia mitigation resulted in increased aerosol acidity due to nitric acid in the gas phase not being neutralized by ammonia and condensing onto the liquid particle phase, indicating a potentially important secondary effect associated with ammonia mitigation. Thus, we provide new insights into the effects of ammonia mitigation on secondary emissions from gasoline vehicle exhaust and into a potentially useful experimental approach for determining primary and secondary emissions. [ABSTRACT FROM AUTHOR]

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

Particulate organic nitrates (pON) significantly contribute to the mass of organic aerosol and influence the nitrogen oxides cycle in the atmosphere, but their evolution and lifetime remain uncertain. This study performed simultaneous measurements on the anthropogenically affected surface site and the mountain site on top of the polluted planetary boundary layer (PBL). After aging in the PBL, organic nitrate was converted from primary sources (decreased from 8.7% to 4.3%) to secondary sources (increased from 6.3% to 36.1%), spanning from the surface to the mountain. The evaporation of more volatile inorganic nitrate and the production of secondary organic nitrate during aging in the PBL contributed to the enhanced pON fraction over the top of PBL. The contribution of light absorption by brown carbon increased by 57% at the top of PBL compared to the surface, consistent with the higher fraction of nitrogenous organic aerosols over the mountain. The results provide field evidence that the nitrogenous organic aerosols (OA) may be preserved by adding into secondary OA and significantly contribute to the enhanced importance of brown carbon after aging during vertical transport in the PBL. [ABSTRACT FROM AUTHOR]

Tropospheric ozone is a major air pollutant and greenhouse gas. It is also the primary precursor of OH, the main tropospheric oxidant. Global atmospheric chemistry models show large differences in their simulations of tropospheric ozone budgets. Here we implement the widely used GEOS-Chem atmospheric chemistry module as an alternative to CAM-chem within the Community Earth System Model version 2 (CESM2). We compare the resulting GEOS-Chem and CAM-chem simulations of tropospheric ozone and related species within CESM2 to observations from ozonesondes, surface sites, the ATom-1 aircraft campaign over the Pacific and Atlantic, and the KORUS-AQ aircraft campaign over the Seoul Metropolitan Area. We find that GEOS-Chem and CAM-chem within CESM2 have similar tropospheric ozone budgets and concentrations usually within 5 ppb but important differences in the underlying processes including (1) photolysis scheme (no aerosol effects in CAM-chem), (2) aerosol nitrate photolysis, (3) N2O5 cloud uptake, (4) tropospheric halogen chemistry, and (5) ozone deposition to the oceans. Global tropospheric OH concentrations are the same in both models, but there are large regional differences reflecting the above processes. Carbon monoxide is lower in CAM-chem (and lower than observations), at least in part because of higher OH concentrations in the Northern Hemisphere and insufficient production from isoprene oxidation in the Southern Hemisphere. CESM2 does not scavenge water-soluble gases in convective updrafts, leading to some upper-tropospheric biases. Comparison to KORUS-AQ observations shows an overestimate of ozone above 4 km altitude in both models, which at least in GEOS-Chem is due to inadequate scavenging of particulate nitrate in convective updrafts in CESM2, leading to excessive NO production from nitrate photolysis. The KORUS-AQ comparison also suggests insufficient boundary layer mixing in CESM2. This implementation and evaluation of GEOS-Chem in CESM2 contribute to the MUSICA vision of modularizing tropospheric chemistry in Earth system models. [ABSTRACT FROM AUTHOR]

• The effect of multiple factors on hygroscopicity of gluconic acid and nitrate aerosols was first studied. • The two components react and form gluconate, and the gluconate formation further reduces the hygroscopicity of gluconic acid-nitrate aerosols. • Salting-in effect promoted water-uptake ability for 1:2 gluconic acid/nitrate mixed particles. • CCN activity was estimated by hygroscopicity parameter κ. The aging process of atmospheric aerosols usually leads to a mixture of inorganic salts and organic compounds of anthropogenic origin. In organic compounds, polyhydroxy organic acids are important components, however, the study on composition and hygroscopic properties of the mixture containing inorganics and polyhydroxy organic acids is scanty. In this study, gluconic acid, the proxy of polyhydroxy organic acids, is mixed with the representative nitrate (Mg(NO 3) 2 , Ca(NO 3) 2) to form aerosols. ATR–FTIR and optical microscopy are employed to study the component changes and hygroscopicity as a function of relative humidity. As relative humidity fluctuates, the FTIR–ATR spectra display that the internal mixed gluconic acid (CH 2 (CH) 4 (OH) 5 COOH) and nitrate can react to release acidic gases, forming relevant gluconate and further affecting the hygroscopicity. The specific presentation is particles cannot be recovered to their original size after the dehydration–hydration process and there will be some disparities in GF for mixed particles. For the gluconic acid–Ca(NO 3) 2 /Mg(NO 3) 2 mixtures with molar ratios of 1:1, higher degree of reaction resulting in the production of large amounts of gluconate should be responsible to the lower hygroscopicity compared to ZSR model. For 1:2 gluconic acid–nitrate mixed systems (with higher nitrate content), the hygroscopicity of mixtures are higher than the ZSR prediction. A possible reason could be 'salt-promoting effect' on the organic fractions of the surplus inorganic salt in the mixture. These data can improve the chemical composition list evaluation, in turn hygroscopic properties and phase state of atmospheric aerosol, and then the climate effect. [Display omitted] [ABSTRACT FROM AUTHOR]

This study aims to explore the influence mechanism of ammonium nitrate produced by ozone denitrification on the crystallisation of ammonium sulfate, a by-product of ammonia desulfurisation. The laser method was used to study the influence of ammonium nitrate on the solubility and metastable zone width of ammonium sulfate. An experiment on the influence of ammonium nitrate on the particle size of ammonium sulfate was designed, and the influence mechanism was explored through scanning electron microscopy and X-ray diffraction. The findings showed that the addition of ammonium nitrate increased the size and aspect ratio of ammonium sulfate crystals. The addition of ammonium nitrate inhibited the dissolution of ammonium sulfate and widened its metastable zone. The addition of ammonium nitrate covered the active sites of crystal nucleus growth, which inhibited the formation of crystal nuclei to a certain extent, and crystal growth dominated the crystallisation process. Moreover, the addition of ammonium nitrate induced the preferred orientation of the specific crystal plane of ammonium sulfate, and the addition of a small concentration of ammonium nitrate decreased the crystallinity of ammonium sulfate. The research results can provide a reference for crystallisation optimisation and quality improvement of ammonium sulfate in the ammonia desulfurisation process. [ABSTRACT FROM AUTHOR]

Since the distinct thermostability difference of sulfate ammonium and nitrate ammonium aerosols, their distributions, evolutions and sources could be unpredictable on a long‐range transport condition. Here, we highlighted the 3‐D structures and sources of SO42−, NO3− and NH4+ (SNA) during two cold front episodes in east China. Cold fronts effectively uplift and transport PM2.5 and its precursors from upstream sources to the Yangtze River Delta (YRD). Specifically, in the YRD, surface SO42− is mostly imported from the upstreams, accounting for ∼48%, significantly higher than the contribution from the YRD itself (∼29%). In contrast, NH4NO3 is thermally unstable and more easily lost in the warmer and lower boundary layer (BL) ahead of cold front. Consequently, only 20% of the total NO3− originates from upstreams, while the YRD contributes 28%. In the upper BL, the contribution of SO42− from upstreams remain high (∼49%), with only 18% originating from the YRD. However, due to the intense thermostability of NH4NO3 in colder and wetter air, the YRD's contribution to NO3− is 27%, and upstreams contribute 20%. The physical processes exert relatively consistent effects on variations of PM2.5 and SNA concentrations. The aerosol chemical process (AERO) of (NH4)2SO4 consistently contributes positively throughout the entire BL. Conversely, the temperature‐sensitive NH4NO3 undergoes repeated dissociation/condensation and deposition, causing positive AERO contributions in upper BL and negative contributions in lower BL. Results indicate that one difference in physicochemical property of species could induce their distinct distributions and sources in large scale, and should be considered in regional air pollution control. Plain Language Summary: Long‐range transport of PM2.5 by cold frontal passage is an important haze formation mechanism. Previous field and modeling studies have revealed that the vertical distribution of particle sulfate and particle nitrate concentrations varied significantly. However, few studies have explored the 3‐D structures and sources of particle sulfate and nitrate under a long‐range transport conditions. In this study, we used combined observations and model simulations during cold front events to address the unrevealed inconsistent 3‐D structures and sources of sulfate and nitrate resulting from temperature stability differences. Cold front episodes redistribute SNA and precursors in three dimensions, driven by drastic meteorology changes and long range transport. Consequently, the distributions, variations and sources of SNA may exhibit complexity and unpredictablility during long‐range transport. Specificlly, (NH4)2SO4 is mostly imported from the upstream in the whole boundary layer (BL), but NH4NO3 is dominant by local source. Moreover, uplifting by cold front, temperature‐sensitive NH4NO3 form in the upper BL and dissociation in the lower BL, whereas (NH4)2SO4 forms uniformly across the whole BL. These findings suggest that one difference in physicochemical property of species can induce distinct distributions and sources in large scale, highlighting the importance of considering such differences in regional air pollution control. Key Points: Uplifted by cold fronts, the temperature‐sensitive NH4NO3 forms in the upper BL and dissociates in the lower BL(NH4)2SO4 forms in the whole BL because it is highly thermally stable and impervious to dissociationThe differences in thermostability and lifetime result in local sources being dominant for NH4NO3, while upstream sources dominate for (NH4)2SO4 [ABSTRACT FROM AUTHOR]

Aggregation and sharp reactivity decrease are the key problems of using nano zero-valent iron (nZVI) as a potential reaction medium for a permeable reactive barrier (PRB). In this study, nZVI particles encapsulated within an acrylonitrile–butadiene–styrene (ABS) matrix (nZVI/(ABS + EC)) was fabricated, which for the first time successfully simultaneously solved the above problems via accurately regulating the distribution of nZVI particles in the ABS matrix and regulating the contact between nZVI particles and the contaminated aqueous environment. In addition, the size and number of the pores throughout the ABS matrix were first regulated by ethyl cellulose (EC) for the purpose of controlling the contact between nZVI particles and the nitrate contaminant, affording apparent rate constants (kobs) for denitrification performance in the range of 0.0423 to 0.0820 min−1. The remediation of simulated nitrate-contaminated solution by nZVI/(ABS + EC) was suitably described by the first-order kinetics model, with kobs ranging from 0.0423 to 0.2036 min−1, and functional relationship models of kobs with the dosages of EC (dEC) and nZVI (dFe) during encapsulation were developed for the quantitative regulation of a sustainable denitrification performance. Results revealed that encapsulation prevents the aggregation of nZVI, rendering a sustainable denitrification performance of the material; the denitrification performance was demonstrated to be affected and quantitatively regulated by the encapsulation and application conditions. Using nZVI/(ABS + EC) as the reaction medium for PRB, the pore blocking of PRB can be avoided, and the sustainable remediation performance can be quantitatively regulated and predicted. [ABSTRACT FROM AUTHOR]

Tropospheric ozone is an air pollutant and a greenhouse gas whose anthropogenic production is limited principally by the supply of nitrogen oxides (NOx) from combustion. Tropospheric ozone in the northern hemisphere has been rising despite the flattening of NOx emissions in recent decades. Here we propose that this sustained increase could result from the photolysis of nitrate particles (pNO3−) to regenerate NOx. Including pNO3− photolysis in the GEOS‐Chem atmospheric chemistry model improves the consistency with ozone observations. Our simulations show that pNO3− concentrations have increased since the 1960s because of rising ammonia and falling SO2 emissions, augmenting the increase in ozone in the northern extratropics by about 50% to better match the observed ozone trend. pNO3− will likely continue to increase through 2050, which would drive a continued increase in ozone even as NOx emissions decrease. More work is needed to better understand the mechanism and rates of pNO3− photolysis. Plain Language Summary: In the troposphere, ozone is an air pollutant and a greenhouse gas. Tropospheric ozone forms from reactions involving carbon monoxide and volatile organic compounds in the presence of nitrogen oxides. Global emissions of nitrogen oxides have been leveling off in the past few decades, yet tropospheric ozone levels have kept on rising. We propose that this rise in ozone could be driven by a growing source of nitrogen oxides from the photolysis of nitrate particles, which have become more abundant due to falling sulfur dioxide and rising ammonia emissions. We find that including nitrate particle photolysis in an atmospheric chemistry model improves its consistency with the observed ozone distribution and trends. Our results point to the importance of considering nitrate particle photolysis for future projections of climate forcing from tropospheric ozone, and the need for further work to reduce the uncertainty in the mechanism and rates of the process. Key Points: Particulate nitrate photolysis improves the consistency of tropospheric ozone in the GEOS‐Chem model with observationsIncrease in particulate nitrate due to falling SO2 and rising NH3 emissions could augment the long‐term increase in tropospheric ozoneBetter characterization of the mechanism and rates of particulate nitrate photolysis is needed [ABSTRACT FROM AUTHOR]

The influence of continental emissions on the origin and formation mechanisms of atmospheric particulate nitrate (ρ‐NO3−) aerosols in the marine boundary layer remains unclear. Here, synchronous observations of nitrogen isotope ratios (δ15N–NO3−) and oxygen isotope anomaly (Δ17O–NO3−) in ρ‐NO3− were conducted across the Yellow Sea and Bohai Sea in Eastern China. Nitrate concentrations, δ15N–NO3− and Δ17O–NO3− exhibited a pronounced north‐to‐south latitudinal gradient. Combined with backward air mass trajectory analysis, the high nitrate concentration and isotopic characteristics in the northern sea area were found to be affected by the continental outflow near China while the low values in the southern sea area were more related to the oceanic inflow. Stable isotope analysis in R (SIAR) indicated that near the northern sea area, the nitrate radicals (NO3) reacted with hydrocarbons (HC) or dimethyl sulfides (DMS) pathway (NO3 + HC/DMS) played a leading role in nitrate production, whereas the NO2 + OH pathway dominated near the southern sea area. Nitrate in the northern seas originated mainly from nitrogen oxides (NOx = NO + NO2, the gaseous precursor of nitrate) emitted from continental sources, especially coal combustion and biomass burning. While closer to the southern seas, the proportion of NOx generated in the marine environment (from ship and biogenic emissions) increased. Overall, the differential relative contributions of continental and marine atmospheric chemistry and NOx sources lead to the spatial distribution characteristics of atmospheric nitrate concentrations and isotopic values over the Yellow and Bohai Seas. Plain Language Summary: This study investigated the impact of emissions from land sources on the formation of nitrate particles in the air above the marine boundary layer. The researchers conducted observations of nitrogen and oxygen isotopes in nitrate particles across the Yellow Sea and Bohai Sea in Eastern China. They found that nitrate concentrations and isotopic ratios showed a clear gradient from north to south. By analyzing the trajectory of air masses, they determined that high nitrate concentrations in the northern sea area were influenced by emissions from the nearby continent, while lower values in the southern sea area were more related to the ocean. The analysis also revealed that different chemical pathways were responsible for nitrate production in these regions. In the northern seas, nitrate production was primarily driven by the reaction of nitrate radicals with hydrocarbons or dimethyl sulfides, while in the southern seas, the dominant pathway involved the reaction of nitrogen dioxide with hydroxyl radicals. The study further showed that nitrate in the northern seas mainly originated from nitrogen oxides emitted by continental sources such as coal combustion and biomass burning, whereas closer to the southern seas, a larger proportion of nitrogen oxides were generated within the marine environment. Key Points: Continental outflow significantly affects the sources and formation mechanisms of atmospheric nitrate in nearshore marine areasAs continental outflow diminishes, local chemistry and marine‐generated NOx become pivotal in producing atmospheric NO3− in the open ocean. In polluted marine boundary layers, NO3 radical‐related chemistry may play a significant role in the formation of nitrate [ABSTRACT FROM AUTHOR]

To quantify the volatility of organic aerosols (OA), a comprehensive campaign was conducted in the Chinese megacity. Volatility distributions of OA and particle‐phase organic nitrate (pON) were estimated based on five methods: (a) empirical method and (b) kinetic model based on the measurement of a thermodenuder (TD) coupled with an aerosol mass spectrometer; (c) Formula‐based SIMPOL model‐driven method; (d) Element‐based estimations using molecular formula measurements of OA; and (e) gas/particle partitioning. Our results demonstrate that the ambient OA volatility distribution shows good agreement between the two heating methods and the formula‐based method when assuming ambient OA was mainly composed of organic nitrate (pON), organic sulfate and acid groups using the SIMPOL model. However, the element‐based method tends to overestimate the volatility of OA compared to the above three methods, suggesting large uncertainties in the parameterizations or in the representativeness of the molecular measurements that need further refinement. The volatility of ambient OA is generally lower than that of the laboratory‐derived secondary OA, emphasizing the impact of aging. A large fraction at the higher and lower volatility ranges (approximately log C* ≤ −9 and ≥2 μg m−3) was found for pON, implying the importance of both extremely low volatile and semi‐volatile species. Overall, this study evaluates different methods for volatility estimation and gives new insight into the volatility of OA and pON in urban areas. Plain Language Summary: Volatility, which controls the gas/particle partitioning of organic compounds, is one of most the important physiochemistry properties of organic aerosols (OA). Multiple methods have been used to estimate OA volatility, whereas the accuracy of each method is still unclear. The purpose of this study is to verify the volatility of ambient OA and its key component (i.e., particulate organic nitrate, pON) in urban areas by comparing the estimated results using different methods. With the help of state‐of‐art mass spectrometers, the estimation methods, including two heating methods, one formula‐based method, one element‐based method, and one gas/particle partitioning method, were achieved. In general, we found the heating methods show good agreement with the formula‐based method with a reasonable assumption of functional groups contained in OA and pON. In general, we observed a large faction of extremely low volatile compounds exist in OA (30%–40%), and both extremely low volatile and semi‐volatile species exist in pON, suggesting the complex evolution of atmospheric particles. The results provide a better understanding of the volatility of OA and pON in urban areas and benefit accurate volatility estimation that facilitates the model simulations of OA. Key Points: The volatility of ambient organic aerosols (OA) and organic nitrate (pON) estimated from five methods were systematically comparedThe OA volatility from SIMPOL‐driven model under organic sulfate, pON and acid assumption are consistent with those from heating methodThe volatility distribution of pON indicates both extremely low volatility compounds as oligomers and semi‐volatile species exist in pON [ABSTRACT FROM AUTHOR]

The "2+26" cities, suffering the most severe winter haze pollution, have been the key region for air quality improvement in China. Increasing prominent nitrate pollution is one of the most challenging environmental issues in this region, necessitating development of an effective control strategy. Herein, we use observations, and state-of-the-art model simulations with scenario analysis and process analysis to quantify the effectiveness of the future SO 2 -NO X -VOC-NH 3 emission control on nitrate pollution mitigation in "2+26" cities. Focusing on a serious winter haze episode, we find that limited NO X emission reduction alone in the short-term period is a less effective choice than VOC or NH 3 emission reduction alone to decrease nitrate concentrations, due to the accelerated NO X -HNO 3 conversion by atmospheric oxidants and the enhanced HNO 3 to NO 3 − partition by ammonia, although deep NO X emission reduction is essential in the long-term period. The synergistic NH 3 and VOC emission control is strongly recommended, which can counteract the adverse effects of nonlinear photochemistry and aerosol chemical feedback to decrease nitrate more. Such extra benefits will be reduced if the synergistic NH 3 and VOC reduction is delayed, and thus reducing emission of multiple precursors is urgently required for the effective control of increasingly severe winter nitrate pollution in "2+26" cities. [ABSTRACT FROM AUTHOR]

Nitrate has been recognized as a key aerosol component in regional haze formation in China. However, reducing nitrate aerosol concentration remains a major challenge. Generally, the formation of particulate nitrate (NO3-) is mainly affected by two processes: oxidation (to generate gaseous HNO3 or particulate NO3-) and gas-particle partitioning (HNO3-NO3- partition). Here, we proposed a new method to explore the contributions of above two processes ( C Oxi obs (%) and C G / P obs (%)) to nitrate formation based on field observation, and combined theoretical calculation and modeling to verify it. Quantitative results showed that gas-particle partitioning process (average C G / P obs (%) was 64.90%) always contributed more than oxidation process (average C Oxi obs (%) was 35.10%) for particulate nitrate formation under different pollution scenarios in the ambient environment. We argued that this phenomenon was mainly caused by high aerosol pH (>4.5). Nevertheless, as pollution level rose, the C Oxi obs (%) will also increase (contributing to 32%, 38%, 40% and 41% under clean, light, medium and heavy pollution levels) which may be attributed to the increased HNO3 production rate and relatively enhanced heterogeneous reaction pathway. The results indicate future strategies for prevention and control of nitrate pollution should both consider reducing precursors emission and regulating aerosol acidity, in order to increase the effectiveness of reducing nitrate dominated pollution. Copyright © 2023 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Fine particulate matter (PM 2.5) samples were collected between November 2020 and October 2021 at the Minzu University of China in Beijing, and the n -alkyl nitrate concentrations in the PM 2.5 samples were determined to investigate n -alkyl nitrate pollution and formation mechanisms. C 9 –C 16 n -alkyl nitrate standards were synthesized, and the n -alkyl nitrate concentrations in PM 2.5 were determined by gas chromatography triple quadrupole mass spectrometry. Temporal trends in and correlations between particulate-bound n -alkyl nitrate, ozone, PM 2.5 , and nitrogen dioxide concentrations were investigated to assess the relationships between particulate-bound n -alkyl nitrate concentrations and gas-phase homogeneous reactions in the photochemical process and speculate the particulate-bound n -alkyl nitrates' formation mechanisms. The n -alkyl nitrate concentrations in the PM 2.5 samples were 9.67–2730 pg m -3 , and the mean was 578 pg m -3. The n -alkyl nitrate homologue group concentrations increased as the carbon chain length increased; i.e., long-chain n -alkyl nitrates contributed more than short-chain n -alkyl nitrates to the total n -alkyl nitrate concentrations in PM 2.5. The n -alkyl nitrate concentrations clearly varied seasonally and diurnally, the concentrations decreasing in the order winter > spring > autumn > summer and the mean concentrations being higher at night than in the day. The particulate-bound n -alkyl nitrate and ozone concentrations significantly negatively correlated despite gas-phase alkyl nitrate and ozone concentrations previously being found to positively correlate. This indicated that long-chain alkyl nitrates may not be produced during gas-phase homogeneous reactions. The particulate-bound n -alkyl nitrate concentrations followed the same trends as and significantly positively correlated with the PM 2.5 and nitrogen dioxide concentrations. Nitrogen dioxide is an important contributor of nitrates in particulate matter. This indicated that particulate-bound n -alkyl nitrates may form through non-homogeneous reactions between alkanes and nitrates on particulate matter surfaces. As secondary pollutants, particulate-bound alkyl nitrates are important components of PM 2.5 during haze events and strongly affect haze pollution and atmospheric visibility. [ABSTRACT FROM AUTHOR]

The heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) is an important pathway in nitrate formation; however, its formation rate and relative contribution to total particulate nitrate (pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$) are highly variable. Here we report that nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is dependent on the regime defined by the ratio of NO2 to O3. Nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is suppressed in an O3‐limited regime but enhanced in a NO2‐limited regime. The results have crucial implications for effective control of nitrate pollution in the future. An exclusive decrease in NO2 will decrease nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation in a NO2‐limited regime but may be less effective or even increase nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation in an O3‐limited regime. Plain Language Summary: Our observations show that nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via dinitrogen pentoxide (N2O5) hydrolysis in the residual layer over megacity Beijing is more efficient than at ground level. Further investigations suggest nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is dependent on the regime defined by the ratio of NO2 to O3. Nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is suppressed in an O3‐limited regime but enhanced in a NO2‐limited regime. As a result, an exclusive decrease in NO2 will decrease nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation in a NO2‐limited regime but may be less effective or even increase nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation in an O3‐limited regime. The above result is also substantiated by observations during the COVID‐19. Key Points: Nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is dependent on the regime defined by the ratio of NO2 to O3Nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis in the residual layer over megacity Beijing is more efficient than at ground levelNocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is suppressed in an O3‐limited regime but enhanced in a NO2‐limited regime [ABSTRACT FROM AUTHOR]

Particulate nitrate is a major component of fine particulate matter (PM2.5). Its formation may be varyingly sensitive to emissions of ammonia (NH3), nitrogen oxides (NOx ≡ NO + NO2), and volatile organic compounds (VOCs), depending on local conditions. Diagnosing these sensitivities is critical for successful air quality management. Here, we show that satellite measurements of tropospheric NH3 and NO2 columns can be used as a quick indicator of the dominant sensitivity regime through the NH3/NO2 column ratio together with the NO2 column. We demonstrate the effectiveness of this indicator with the GEOS‐Chem chemical transport model and define thresholds to separate the different sensitivity regimes. Applying the method to wintertime IASI and OMI observations in East Asia reveals that surface nitrate is dominantly VOC‐sensitive in the southern North China Plain (NCP), NOx‐sensitive in most of the East China Plain, and NH3‐sensitive in the northern NCP, southern China, and Korea. Plain Language Summary: We present a novel application of satellite remote sensing to better understand the causes of particulate nitrate pollution. Particulate nitrate is a major air pollutant throughout the urbanized world. It is produced by atmospheric oxidation of emitted nitrogen oxides (NOx) but may be more sensitive to emissions of ammonia (NH3) or volatile organic compounds (VOCs). Understanding which of NH3, NOx, or VOC emissions is most important in driving nitrate formation is critical for air quality management. We show that satellite measurements of the NH3/NO2 column ratio along with NO2 columns is an effective indicator to determine the dominant sensitivity regime (NH3−,NOx−,orVOC−sensitive ${\text{NH}}_{3}-,{\text{NO}}_{\mathrm{x}}-,\text{or}\,\text{VOC}-\text{sensitive}$). We develop this approach using an atmospheric chemistry model and apply it to wintertime satellite observations in East Asia. The approach should be applicable to other continents, seasons, and a broader range of satellite instruments, providing valuable insights for particulate nitrate reduction strategies. Key Points: Reducing particulate nitrate pollution requires understanding its local sensitivities to NH3, NOx, and volatile organic compound emissionsSatellite observation of the NH3/NO2 column ratio is an effective indicator for diagnosing these sensitivitiesIASI NH3 and OMI NO2 observations reveal varying regimes of nitrate sensitivity across wintertime East Asia [ABSTRACT FROM AUTHOR]

Aerosol physicochemical properties during two dust storms (DS1: March 30–31, and DS2: May 7–8) are measured in 2021 in Nanjing, aiming to investigate the impacts of dust storms on aerosol chemical compositions and optical hygroscopicity in the Yangtze River Delta (YRD). During DS1, the dust air masses are transported in the lower atmosphere and pass through the inlands and sea areas before reaching the YRD region. During DS2, the dust air masses are transported in the upper atmosphere and pass through the inlands only. Both of them are accompanied by an increase in black carbon (BC) mass fraction and a decrease in nitrate mass fraction in fine particles (PM2.5, particles with diameters less than 2.5 μm) near the surface. However, the impacts on the mass fractions of organics or sulfate are adverse between DS1 and DS2. During dust‐influence periods the enlarged mass ratios of sulfate to nitrate (greater than 4) promote the occurrence of deliquescent behavior of ambient aerosols although dust aerosols can significantly suppress aerosol optical hygroscopicity. To improve model simulations of aerosol optical hygroscopicity, it is necessary to use a segment parameterization to describe aerosol light scattering enhancement factor during dust‐influence periods. The closure study of optical hygroscopicity parameters with two different methods reveals the impact of aerosol scattering Ångström exponent on the estimation of optical hygroscopicity parameter. The results highlight that the impact of dust storms on aerosol chemical compositions and optical hygroscopicity are different for DS events with different transport pathways. Plain Language Summary: Dust storm (DS) is one of the severest natural disasters. Studying the change of aerosol properties during DS is important to understand the role of dust in the earth‐atmosphere system. This study chooses two DS events (DS1 and DS2) in March and May 2021 to investigate the impacts of DS on aerosol chemical compositions and optical hygroscopicity in the YRD region. It is found that DS1 transporting in the lower atmosphere makes a larger increase of coarse‐mode particles near the surface than DS2 transporting in the upper atmosphere. The weak impact of DS2 on the surface aerosols leads to a small change in aerosol chemical compositions of fine particles near the surface. The enlarged mass ratio of sulfate to nitrate (greater than 4) during dust periods can promote ambient aerosol deliquescence, which is important to quantify aerosol optical properties. To provide a reference for improving the model simulation of aerosol optical hygroscopicity, different parameterization schemes are derived to fit the change of aerosol light scattering enhancement factor with RH during dust and no‐dust periods. The closure study of aerosol optical hygroscopicity parameters suggests that aerosol scattering Ångström exponent plays an important role in the overestimation of optical hygroscopicity parameters. Key Points: The dust storms influencing the YRD region have different transport pathways and impacting atmospheric layersThe enlarged mass ratio of sulfate to nitrate in fine particles makes the deliquescence phenomenon appear more in dust‐influence periodsAerosol scattering Ångström exponent is an important factor to estimate optical hygroscopicity parameters [ABSTRACT FROM AUTHOR]

Nitrous acid (HONO), a vital precursor of atmospheric hydroxyl radicals (OH), has been extensively investigated to understand its characteristics and formation mechanisms. However, discerning fundamental mechanisms across diverse environments remains challenging. This study utilizes measurements from Mount Lao, a coastal mountain in eastern China, and an observation-based chemical box model (OBM) to examine HONO budgets and their subsequent impacts on atmospheric oxidizing capacity. The model incorporates additional HONO sources, including direct emissions, heterogeneous conversions of NO 2 on aerosol and ground surfaces, and particulate nitrate photolysis. The observed mean HONO concentration was 0.46 ± 0.37 ppbv. The updated model reproduced daytime HONO concentrations well during dust and photochemical pollution events. During dust events, daytime HONO formation was dominated by photo-enhanced heterogeneous reactions of NO 2 on aerosol surfaces (> 50 %), whereas particulate nitrate photolysis (34 %) prevailed during photochemical pollution events. Nevertheless, the model uncovers a significant unidentified marine HONO source in a "sea case", with its HONO production rate reaching up to 0.70 ppbv h -1 at noon. Without considering this unidentified source, an extraordinarily high photolysis coefficient of nitrate and/or a heterogeneous uptake coefficient of NO 2 would be required to match observed HONO concentrations. This missing marine HONO source affected the peak O 3 production rate and OH radical concentration by 36 % and 28 %, respectively, at the observation site. Given the limited HONO observation data in coastal and marine settings, the unidentified HONO source may cause an underestimation of the atmosphere's oxidizing capacity. This study highlights the necessity for further investigation of the role of HONO in atmospheric chemistry in coastal and marine environments. [ABSTRACT FROM AUTHOR]

Nanoscale zero-valent iron (nZVI) has been widely used to remove pollutants from water. However, nZVI particles are easily exposed to air during preparation and consequently undergo oxidative aggregation, which affects the reactivity. In this study, nZVI particles were prepared for nitrate removal from wastewater using different polyhydroxy alcohols (glycerol, butanetetrol, and pentaerythritol) as the liquid phase reduction medium. The NO3- removal ability and anti-aggregation ability of the prepared nZVI particles were evaluated in different pH environments. The specific reaction pathways that occurred during the NO3- removal process for three materials were determined at different dosages. In addition, SEM, BET, FTIR, and XRD were used to characterize the nZVI particles and analyze their nitrate removal mechanism. The experimental results showed that the nZVI prepared with polyhydroxy alcohols exhibited overall higher nitrate removal efficiency than the pure aqueous medium. The nitrate removal rate reached 94.73% at the initial solution pH=2 and the dosage of 3.0 g/L. Polyhydroxy alcohols can replace the iron oxide layer and form a shell of alcohol hydroxyl groups during the core–shell structure formation of the nZVI reduction process, and this shell improves the dispersion performance and reactivity of nZVI particles in the nitrate reduction process. [ABSTRACT FROM AUTHOR]

Nitrate has become the primary inorganic compound in urban aerosol particles, but its effects on particle phase state, which is crucial in multiphase chemistry, remains largely unknown. Herein, particle rebound measurements were conducted to explore the relationship between the liquid–phase–transition threshold relative humidity (RHthreshold) and the inorganic compounds mass fraction in dry particles (Finorg). Results revealed negative correlations between RHthreshold and Finorg, with more nitrate leading to lower RHthreshold. Even with RH < 20%, particles with ∼50% nitrate mass fraction remained in non‐solid state. Taking Beijing as an example, decreases were observed in RHthreshold from 64% in 2015 to below 53% nowadays during the moderate‐pollution periods (PM2.5 = 35–75 μg/m3) due to an enhanced nitrate fraction. This allowed urban aerosol particles to exist in liquid state at lower RH, and consequently, kinetic limitation by bulk diffusion in nitrate‐dominated particles might be negligible, making them key seeds for secondary aerosol formation through multiphase reactions. Plain Language Summary: The phase state of aerosol particles plays a crucial role in the mass‐transfer processes between gas and particles, which is essential for determining particle mass concentration. Investigating the characteristics of the aerosol phase state is crucial for comprehending the mechanisms behind secondary particle formation and improving air quality. Currently, the chemical composition of urban aerosol particles has shown notable changes. The mass fraction of inorganic components has increased, with nitrate emerging as the dominant inorganic component. However, the impacts of these changes on the phase state of urban aerosol particles remain largely unknown. This study demonstrates that particles with a higher mass fraction of inorganic compounds, particularly nitrate, tend to exist in a liquid state. Consequently, an increased nitrate fraction allows urban aerosol particles to exist in a liquid state at lower relative humidity levels. These findings suggest that changes in the phase state of particles due to changes in chemical properties in urban aerosol particles should be considered to accurately gauge the mass‐transfer processes and promote the air quality improvements in urban cities. Key Points: Particles with more inorganic compounds, particularly nitrate, can exist in a liquid state at lower relative humidity levelsParticles with normalized aerosol liquid water content larger than 0.3 exist in a liquid stateNitrate‐dominated particles can serve as key seeds where secondary aerosol formation can occur via multiphase reactions [ABSTRACT FROM AUTHOR]

Giant clams are reef-dwelling bivalves that reach unusual sizes through a partnership with photosymbiotic algae. To date, no shell-based biogeochemical proxy has been found which directly records the photosymbiotic development and health of these animals. We present new results showing a size-related decline in nitrogen isotopic values of shell-bound organic matter from the hinge layers of giant clams from the Northern Red Sea. In three of four tested shells, δ15N values decline from >+4‰ at the juvenile stage to between 0 and −2.5‰ at maturity. These trends are consistent with a transition from heterotrophic nutrition early in the bivalve's life to receiving most of their nutrition from photosynthetic symbionts and external dissolved inorganic nitrogen at maturity. We find more muted declines or no change within the outer shell layer, with more inter-individual variability, which is likely related to the greater influence of the symbionts in the adjacent siphonal mantle of the animals. We use a von Bertalanffy-linked trophic model that uses δ15N of nitrate and particulate organic matter to corroborate and explain the trophic transition in the ontogeny of the clams, and propose that high-resolution δ15N measurements in bivalves could be used as a proxy for photosymbiosis and reef paleoenvironmental conditions in the fossil record. [ABSTRACT FROM AUTHOR]

Daytime HONO photolysis is an important source of atmospheric hydroxyl radicals (OH). Knowledge of HONO formation chemistry under typical haze conditions, however, is still limited. In the Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain in 2018, we investigated the wintertime HONO formation and its atmospheric implications at a rural site Gucheng. Three different episodes based on atmospheric aerosol loading levels were classified: clean periods (CPs), moderately polluted periods (MPPs) and severely polluted periods (SPPs). Correlation analysis revealed that HONO formation via heterogeneous conversion of N O 2 was more efficient on aerosol surfaces than on ground, highlighting the important role of aerosols in promoting HONO formation. Daytime HONO budget analysis indicated a large missing source (with an average production rate of 0.66 ± 0.26, 0.97 ± 0.47 and 1.45 ± 0.55 ppbV/hr for CPs, MPPs and SPPs, respectively), which strongly correlated with photo-enhanced reactions (N O 2 heterogeneous reaction and particulate nitrate photolysis). Average OH formation derived from HONO photolysis reached up to (0.92 ± 0.71), (1.75 ± 1.26) and (1.82 ± 1.47) ppbV/hr in CPs, MPPs and SPPs respectively, much higher than that from O 3 photolysis (i.e., (0.004 ± 0.004), (0.006 ± 0.007) and (0.0035 ± 0.0034) ppbV/hr). Such high OH production rates could markedly regulate the atmospheric oxidation capacity and hence promote the formation of secondary aerosols and pollutants. [ABSTRACT FROM AUTHOR]

The acidity of aerosols is closely related to various atmospheric activities such as aerosol formation, gas-particle partitioning, deposition flux, and nutrient transport. In this study, continuous one-year monitoring of PM2.5, PM2.5-10, and their components was conducted in Handan City, Hebei Province from March 2015 to February 2016 using ACSA-14. The study revealed that the annual mean concentration of PM2.5 in Handan was 142.2 µg/m³, exceeding that of PM2.5-10, which measured at 77.0 µg/m³. PM2.5 constituted 64.9% of PM10. Additionally, the proportions of sulfate, nitrate, and soluble organic compounds in PM2.5 were higher than those in PM2.5-10, with annual mean values of 15.5 µg/m³, 17.5 µg/m³, and 9.8 µg/m³, respectively. Both PM2.5 and PM2.5-10 exhibited moderate acidity, with respective pH values of 4.5 and 4.3, and they were more acidic during the spring and winter. The activity coefficients of nitrate in PM2.5 were lower, making nitrate more readily partition into the particulate phase. Increasing the pH facilitated the transfer of nitrate into the particulate phase, and the effect of pH on the activity coefficients of sulfate, nitrate, and ammonium displayed significant seasonal variations. During winter, the activity coefficients of sulfate, nitrate, and ammonium showed a slight increase, with nitrate and ammonium displaying stronger alkaline values in the winter months. This stronger alkalinity facilitated the rapid transition of sulfate into the liquid phase, allowing it to engage in chemical reactions within the liquid phase. These findings hold significant implications for understanding the characteristics and formation mechanisms of atmospheric particulate matter in Handan. [ABSTRACT FROM AUTHOR]

Particulate secondary organic nitrates play a key role in understanding secondary organic aerosol production, ozone formation, and the atmospheric nitrogen cycle. However, the formation of particulate secondary organic nitrates in ambient air remains poorly understood. In this study, the nighttime formation processes of particulate secondary organic nitrates were investigated based on size‐resolved aerosols measured in urban air of China with a soot particle long‐time‐of‐flight aerosol mass spectrometer. The results show a bimodal size distribution of particulate secondary organic nitrates, peaking at ∼350 nm in condensation mode (100–400 nm) and ∼750 nm in droplet mode (400–2,500 nm), respectively. The nighttime formation processes of particulate secondary organic nitrates in the two size modes were respectively governed by temperature‐dependent condensation and aqueous‐phase processing. In particular, the mass concentration of particulate secondary organic nitrates in condensation mode was positively correlated with nitrate radical production and negatively correlated with temperature, suggesting that the formation processes were associated with the gas‐particle conversion of nitrate radical oxidation products. In contrast, the enhanced particulate secondary organic nitrates in droplet mode were predominantly contributed by aqueous‐phase processing, as indicated by the strong positive correlation with aerosol liquid water content and typical fragment ions from aqueous processing products (r = 0.52–0.59, P < 0.01). Our results highlight the potential of a size‐dependent mechanism to elucidate the formation processes of particulate secondary organic nitrates. Plain Language Summary: Particulate organic nitrates are an important nitrogen‐containing compound in the ambient atmosphere and have significant effects on air quality. However, the formation processes of particulate organic nitrates are complex in ambient air, leading to difficulties in understanding of their atmospheric processes and impacts. Here, nighttime formation of particulate secondary organic nitrates in different size particles is investigated in urban air. The results show that the nighttime formation pathways of particulate secondary organic nitrates in different size particles are distinct. Furthermore, aqueous processing plays an important role in the nighttime formation of particulate secondary organic nitrates in large size particles, especially in fog‐rain days. This information provides for a better understanding of the formation of particulate secondary organic nitrates in ambient air and is useful for future design of laboratory and atmospheric chemistry model studies. Key Points: Particulate secondary organic nitrates concentrations were significantly enhanced during fog‐rain days compared to non‐fog‐rain daysSize distributions of secondary organic nitrates were characterized by bimodal structure, peaking at ∼350 and ∼750 nm, respectivelyThe nighttime formation of organic nitrates in two size modes was respectively dominated by gas‐particle partitioning and aqueous processing [ABSTRACT FROM AUTHOR]

The equilibrium between nitric acid gas (HNO3(g)) and ammonium nitrate aerosol (NH4NO3(p)) in ambient air was studied based on the monitoring data obtained using a five-stage filter-pack system, in which the fine aerosol and the coarse aerosol were separately collected; this made it possible to evaluate the actual situation of the equilibrium more accurately. The partition between HNO3(g) and coarse particulate nitrate (c-NO3-(p)), as well as that between HNO3(g) and fine particulate nitrate (f-NO3-(p)), could be evaluated individually thanks to the classification separation of the aerosol by size. The c-particle proportion c-NO3-(p)/(c-NO3-(p) + HNO3(g)) between HNO3(g) and c-NO3-(p) had a weak negative correlation (r = -0.46, p<0.001) with air temperature; in contrast, the f-particle proportion f-NO3-(p)/(f-NO3-(p) + HNO3(g)) between HNO3(g) and f-NO3-(p) had a moderate negative correlation (r = -0.80, p<0.001) with air temperature in total; furthermore, the f-particle proportion had an interesting and discriminative dependence on air temperature which could be divided into two regions by an air temperature around 15°C. The condition of high air temperature accompanied by high relative humidity frequently resulted in the deliquescent state of NH4NO3(p), providing the disconnect from the theoretical prediction for the products of [NH3(g)] and [HNO3(g)] ([NH3(g)][HNO3(g)]) by Seinfeld and Pandis (1998). [ABSTRACT FROM AUTHOR]

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

Particulate free amino acids (FAAs) are essential components of organonitrogen that have critical climate impacts, and they are usually considered stable end-products from protein degradation. In this work, we investigated the decay of glycine (GC) as a model FAA under the photolysis of different particulate nitrate salts using an in situ Micro-Raman system. Upon cycling the relative humidity (RH) between 3 % and 80 % RH, ammonium nitrate (AN) and GC mixed particles did not exhibit any phase change, whereas sodium nitrate (SN) and GC mixed particles crystallized at 60 % and deliquesced at 82 % RH. Under light illumination at 80 % RH, AN + GC particles showed almost no spectral changes, while rapid decays of glycine and nitrate were observed in SN + GC particles. The interactions between nitrate and glycine in AN + GC particles suppressed crystallization but also hindered nitrate photolysis and glycine decay. On the other hand, glycine may form a complex with Na+ in deliquescent SN + GC particles and allow unbonded nitrate to undergo photolysis and trigger glycine decay, though nitrate photolysis was greatly hindered upon particle crystallization. Our work provides insights into how FAAs may interact with different nitrate salts under irradiation and lead to distinct decay rates, which facilitates their atmospheric lifetime estimation. [ABSTRACT FROM AUTHOR]

Stream degradation increases with high anthropogenic activity and climate variability, while declines occur in biodiversity. However, few studies have been undertaken on tropical waterways, a major impediment to biodiversity conservation. The present study was conducted on 95 relatively pristine small streams in Eastern Thailand with 10 reasonably uncommon species of balitorid fishes. Measurements were made of 21 physical and chemical factors and the substrate particle size. Stepwise regression identified the direct importance of substrate particle size and nitrate on the species' richness of balitorids, whereas its abundance was negatively related with iron concentrations. A Canonical Correspondence Analysis identified three fish groups: the 1st group was negatively correlated with ammonia and positively correlated with dissolved silica, the 2nd group was positively correlated with substrate particle size and negatively correlated with stream ambient temperature and ammonia concentration, and the 3rd group was negatively correlated with low dissolved silica, respectively. The results of this study may indicate the vulnerability of balitorids under climate warming and anthropogenic pressure that alter the water physicochemical factors and river degradation including the substrate type. Thus, a conservation framework should be provided regarding the limits for water temperature, ammonia, and iron in Thailand's Water Quality Criteria to better protect its freshwater ecosystem. Balitorid is a potential bioindicator for evaluating the river temperature effect in combination with ammonia nutrient stressors as long as the way-of-life habits of the species are taken into account. [ABSTRACT FROM AUTHOR]

Tropospheric ozone (O3) concentration continued to increase over past years, thereby becoming a growing environmental concern in China. Most studies have focused on the analysis of daily maximum 8‐hr O3 concentration, while there is still a dearth of investigations of nocturnal O3. Here, by analyzing the data of 1,313 sites from the China National Environmental Monitoring Center, we show a remarkable increase in the nocturnal O3 concentration during summertime of 2015–2019 in most regions of China, revealing an aggravation of nocturnal O3 pollution. Combining with a GeoDetector model and statistical analysis, we clarify that the aggravation of nocturnal O3 pollution is mainly caused by reduction in both ambient nitrogen dioxide (NO2) concentration and wet scavenging in recent years. We further reveal that the increasing O3 may have enhanced the nocturnal particulate nitrate (NO3− ${{\mathrm{N}\mathrm{O}}_{3}}^{-}$) formation through N2O5 heterogeneous hydrolysis, and thereby driving the variation and long‐term trend of nocturnal NO3− ${{\mathrm{N}\mathrm{O}}_{3}}^{-}$. Plain Language Summary: The high concentrations of nocturnal ozone exposure pose potential threats to the physiological characteristics of vegetation and crops. Here, we reveal the aggravation of nocturnal O3 pollution during summertime of 2015–2019 in most regions of China. The ambient NO2 reduction and weakened wet scavenging effect in recent years is further clarified as the dominant factor to cause the intensified nocturnal O3 pollution. We also show that the nocturnal O3 plays an important role in the nocturnal NO3− ${{\mathrm{N}\mathrm{O}}_{3}}^{-}$ formation through N2O5 heterogeneous hydrolysis, revealing that the interannual changes in nocturnal O3 likely has driven the variation and long‐term trend of nocturnal NO3− ${{\mathrm{N}\mathrm{O}}_{3}}^{-}$. Key Points: Nocturnal O3 pollution was intensified in summer of 2015–2019 in ChinaReduction in both NO2 and wet scavenging leads to the worsening of nocturnal O3 pollutionThe aggravated nocturnal O3 pollution drives the nighttime NO3− ${{\mathrm{N}\mathrm{O}}_{3}}^{-}$ interannual variations [ABSTRACT FROM AUTHOR]

Nitrous acid (HONO), a vital precursor of atmospheric hydroxyl radicals (OH), has been extensively investigated to understand its characteristics and formation mechanisms. However, discerning fundamental mechanisms across diverse environments remains challenging. This study utilizes measurements from Mount Lao, a coastal mountain in eastern China, and an observation-based chemical box model to examine HONO budgets and their subsequent impacts on atmospheric oxidizing capacity. The model incorporates additional HONO sources, including direct emissions, heterogeneous conversions of NO2 on aerosol and ground surfaces, and particulate nitrate photolysis. The observed mean HONO concentration was 0.46 ± 0.37 ppbv. The updated model well reproduced daytime HONO concentrations during dust and photochemical pollution events. During dust events, daytime HONO formation was dominated by photo-enhanced heterogeneous reactions of NO2 on aerosol surfaces (>50 %), whereas particulate nitrate photolysis (34 %) prevails during photochemical pollution events. Nevertheless, the model uncovers a significant unidentified marine HONO source in the "sea case", with its HONO production rate reaching up to 0.70 ppbv h–1 at noon. Without considering this unidentified source, an extraordinarily high photolysis coefficient of nitrate and/or heterogeneous uptake coefficient of NO2 would be required to match observed HONO concentrations. This missing HONO source affected the peak O3 production rate and OH radical concentration by 36 % and 28 %, respectively. Given the limited HONO observations data in coastal and marine settings, the unidentified HONO source may cause an underestimation of the atmosphere's oxidizing capacity. This study highlights the necessity for further investigation of the role of HONO in atmospheric chemistry in coastal and marine environments. [ABSTRACT FROM AUTHOR]

The dry deposition flux of particles smaller than 2.5 µm was investigated considering agricultural and urban regions. The concentrations, size distributions, and chemical characterization of particulate matter (PM) were evaluated and related to emission sources. The concentrations of fine PM were associated to the highest number of wildfire outbreaks. The results of the PM samples combined with the analyses of the air mass trajectories evidenced that the transport of this pollutant may have come from regions with industries that use biomass as fuel. The predominant water‐soluble ions in coarse and fine particulate were nitrate, potassium, calcium, magnesium, potassium, sulfate, and nitrate, respectively. The deposition fluxes of potassium, nitrate, and sulfate ions were higher during the sugarcane harvest than during the off‐season. [ABSTRACT FROM AUTHOR]

SO 2 and NO 2 are the critical precursors in forming sulfate and nitrate in ambient particles. We studied the mechanism of sulfate and nitrate formation during the co-uptake of NO 2 and SO 2 into NaCl droplets at different RHs under irradiation and dark conditions. A significant formation of nitrate attributable to NO 2 hydrolysis was observed during the NO 2 uptake under all conditions, and its formation rate increases with decreasing RH. The averaged NO 2 uptake coefficient, γNO2 , from the unary uptake of NO 2 into NaCl droplets under dark conditions is 1.6 × 10 -5 , 1.9 × 10 -5 , and 3.0 × 10 -5 at 80 %, 70 %, and 60 % RH, respectively. Chloride photolysis and nitrate photolysis play a crucial role in sulfate formation during the co-uptake. Nitrate photolysis generates reactive species (e.g., OH radicals, NO 2 , and N(III)) that directly react with S(IV) to produce sulfate. The OH radicals generated from nitrate photolysis can also react with chloride ions to form reactive chlorine species and then sulfate. To parameterize the role of nitrate photolysis and chloride photolysis in forming sulfate, the SO 2 uptake coefficient, γSO2 , as a function of the nitrate photolysis rate, PNO3- (jNO3- × [NO 3- ]), and chloride photolysis rate, PCl- (jCl- × [Cl - ]), was derived as γSO2 = 0.41 × PNO3- + 0.34 × PCl-. Our findings open up new perspectives on the formation of secondary aerosol from the combined effect of nitrate photolysis and chlorine chemistry. [ABSTRACT FROM AUTHOR]

Fine particulate matter (PM2.5) samples were collected between November 2020 and October 2021 at the Minzu University of China in Beijing and the n -alkyl nitrate concentrations in the PM2.5 samples were determined to investigate n -alkyl nitrate pollution and formation mechanisms. C9–C16 n -alkyl nitrate standards were synthesized and the n -alkyl nitrate concentrations in PM2.5 were determined by gas chromatography triple quadrupole mass spectrometry. Temporal trends in and correlations between particulate-bound n -alkyl nitrate, ozone, PM2.5, and nitrogen dioxide concentrations were investigated to assess the relationships between particulate-bound n -alkyl nitrate concentrations and photochemical reactions and identify the n -alkyl nitrate formation mechanisms. The n -alkyl nitrate concentrations in the PM2.5 samples were 9.67–2730 pg/m3, and the mean was 578 pg/m3. The n -alkyl nitrate homologue group concentrations increased as the carbon chain length increased, i.e, long-chain n -alkyl nitrates contributed more than short-chain n -alkyl nitrates to the total n -alkyl nitrate concentrations in PM2.5. The n -alkyl nitrate concentrations clearly varied seasonally and diurnally, the concentrations decreasing in the order winter > spring > autumn > summer and the mean concentrations being higher at night than in the day. The particulate-bound n -alkyl nitrate and ozone concentrations significantly negatively correlated despite gas-phase alkyl nitrate and ozone concentrations previously being found to positively correlate. This indicated that long-chain alkyl nitrates may not be produced during photochemical reactions. The particulate-bound n -alkyl nitrate concentrations followed the same trends as and significantly positively correlated with the PM2.5 and nitrogen dioxide concentrations. Nitrogen dioxide is an important contributor of nitrates in particulate matter. This indicated that particulate-bound n -alkyl nitrates may form through reactions between alkanes and nitrates on particulate matter surfaces. Particulate-bound n -alkyl nitrates are important components of PM2.5 during haze events and strongly affect atmospheric visibility. Particulate-bound n -alkyl nitrates are secondary pollutants that strongly influence haze pollution. [ABSTRACT FROM AUTHOR]

Emission controls have substantially brought down aerosol pollution in China; however, aerosol mass reductions have slowed down in recent years in the Pearl River Delta (PRD) region, where secondary organic aerosol (SOA) formation poses a major challenge for air quality improvement. In this study, we characterized the roles of SOA in haze formation in urban Guangzhou in the PRD using year-long aerosol mass spectrometer measurements for the first time and discussed possible pathways of SOA formations. On average, organic aerosols (OA) contribute dominantly (50 %) to nonrefractory submicron aerosol mass (NR-PM 1). The average mass concentration of SOA (including by less- and more-oxidized OA, LOOA and MOOA) contributed most to NR-PM 1 , reaching about 1.7 times that of primary organic aerosols (POA, including hydrocarbon-like and cooking-related OA) and accounting for 32 % of NR-PM 1 , even more than sulfate (22 %) and nitrate (16 %). Seasonal variations of NR-PM 1 revealed that haze-formation mechanisms differed much among distinct seasons. Sulfate mattered more than nitrate in fall, while nitrate was more important than sulfate in spring and winter, with SOA contributing significantly to haze formations in all the seasons. Daytime SOA formation was weak in winter under low oxidant levels and air relative humidity, whereas prominent daytime SOA formation was observed in fall, spring and summer almost on a daily basis, suggesting important roles of photochemistry in SOA formations. Further analysis showed that the coordination of gas-phase photochemistry and subsequent aqueous-phase reactions likely played significant roles in quick daytime SOA formations. Obvious nighttime SOA formations were also frequently observed in spring, fall and winter, and it was found that daytime and nighttime SOA formations together had resulted in the highest SOA concentrations in these seasons and contributed substantially to severe haze formations. Simultaneous increases in nitrate with SOA after sunset suggested the important roles of NO 3 radical chemistry in nighttime SOA formations, and this was further confirmed by a continuous increase in the NO + / NO 2+ fragment ratio that related to measured particulate nitrate after sunset. The findings of this study have promoted our understanding of haze pollution characteristics of the PRD and laid down future directions for investigations of SOA-formation mechanisms in urban areas of southern China that share similar emission sources and meteorological conditions. [ABSTRACT FROM AUTHOR]

To study the effect of interface behaviour on the mechanical properties and damage evolution of PBX under combined tensioneshear loading, the present work establishes the numerical model of a PBX three-phase hybrid system, which introduces a nonlinear plastic damage cohesion model to study the mechanical response and damage process. The parameters in the model were fitted and calibrated. Taking the crack growth rate as the feature, the damage state in each stage was determined, and the damage instability criterion was given. The effects of interfacial tensile strength and shear strength on the damage process of PBX were studied. On this basis, serrated and hemispherical structures interface of PBX has been developed, which affects the damage process and instability during the loading process. The results indicate that damage state response of PBX experiences the process of stable load bearing, unstable propagation, and complete failure. At the critical moment of instability, the overall equivalent effective strain of material reaches 3024 me and instability loading displacement reaches 0.39 mm. The increase of interfacial tensile strength and shear strength significantly inhibits the damage of PBX. The effect of interfacial shear strength on critical instability of PBX is approximately 1.7 times that of the interfacial tensile strength. Further, interface opening along the normal direction is the main damage form at the interface. Serrated and hemispherical rough interfaces can significantly inhibit propagation of cracks, and the load bearing capacity is improved by 22% and 9.7%, respectively. Appropriate improvement of the roughness of the interface structure can effectively improve the mechanical properties. It is significantly important to have a better understanding of deformation, damage and failure mechanisms of PBX and to improve our predictive ability. [ABSTRACT FROM AUTHOR]

Air quality network data in China and South Korea show very high year-round mass concentrations of coarse particulate matter (PM), as inferred by the difference between PM 10 and PM 2.5. Coarse PM concentrations in 2015 averaged 52 µ g m -3 in the North China Plain (NCP) and 23 µ g m -3 in the Seoul Metropolitan Area (SMA), contributing nearly half of PM 10. Strong daily correlations between coarse PM and carbon monoxide imply a dominant source from anthropogenic fugitive dust. Coarse PM concentrations in the NCP and the SMA decreased by 21 % from 2015 to 2019 and further dropped abruptly in 2020 due to COVID-19 reductions in construction and vehicle traffic. Anthropogenic coarse PM is generally not included in air quality models but scavenges nitric acid to suppress the formation of fine particulate nitrate, a major contributor to PM 2.5 pollution. GEOS-Chem model simulation of surface and aircraft observations from the Korea–United States Air Quality (KORUS-AQ) campaign over the SMA in May–June 2016 shows that consideration of anthropogenic coarse PM largely resolves the previous model overestimate of fine particulate nitrate. The effect is smaller in the NCP which has a larger excess of ammonia. Model sensitivity simulations for 2015–2019 show that decreasing anthropogenic coarse PM directly increases PM 2.5 nitrate in summer, offsetting 80 % the effect of nitrogen oxide and ammonia emission controls, while in winter the presence of coarse PM increases the sensitivity of PM 2.5 nitrate to ammonia and sulfur dioxide emissions. Decreasing coarse PM helps to explain the lack of decrease in wintertime PM 2.5 nitrate observed in the NCP and the SMA over the 2015–2021 period despite decreases in nitrogen oxide and ammonia emissions. Continuing decrease of fugitive dust pollution means that more stringent nitrogen oxide and ammonia emission controls will be required to successfully decrease PM 2.5 nitrate. [ABSTRACT FROM AUTHOR]

Fine particulate matter (PM 2.5) pollution is a severe problem in China. Research on the sources of Chinese PM 2.5 pollution has focused on the contributions of China's domestic emissions. However, the impact of foreign anthropogenic emissions has typically been simplified or neglected, partly due to the perception that the short lifetime of PM 2.5 (a few days) does not allow long-distance transport. Here we explore the role of foreign anthropogenic emissions in Chinese PM 2.5 pollution in 2015 using the GEOS-Chem chemical transport model. We validate the model simulations with a comprehensive set of observations of PM 2.5 and its composition, including sulfate, nitrate, ammonium, black carbon, and primary organic aerosols, over China and its surrounding regions. We find that 8 % of PM 2.5 (5 µ g m -3) and 19 % of nitrate (2.6 µ g m -3) over eastern China in 2015 was contributed by foreign anthropogenic emissions. The contributions were the highest in January (6.9 µ g m -3 PM 2.5 , with 68 % nitrate) and the lowest in July (2.7 µ g m -3 PM 2.5 , with 11 % nitrate). Yet, only 30 % of such foreign contributions in January were through direct atmospheric transport. The majority (70 %) were instead through chemical interactions between foreign-transported aerosol precursors and China's domestic emissions of pollutants. Specifically, the transport of non-methane volatile organic compounds (NMVOCs) from foreign countries enhanced the atmospheric oxidizing capacity and facilitated the oxidation of Chinese nitrogen oxides (NO x) to form nitric acid (HNO 3) over eastern China. The abundance of Chinese ammonia (NH 3) further partitioned nearly all HNO 3 gas to particulate nitrate, leading to considerable foreign contributions of nitrate and PM 2.5 to eastern China. Over southwestern China, foreign anthropogenic emissions contributed 4.9 µ g m -3 PM 2.5 concentrations (18 % of total PM 2.5 mass) to Yunnan Province, with 37 % as organics and 27 % as sulfate. Our findings suggest that foreign anthropogenic emissions play an important role in Chinese PM 2.5 pollution because of direct aerosol transport and, more importantly, chemical interactions between transported pollutants and China's local emissions. Thus, foreign emission reductions will be very beneficial for improving Chinese air quality. [ABSTRACT FROM AUTHOR]

The northeastern US represents a mostly urban corridor impacted by high population and fossil fuel combustion emission density. This has led to historically degraded air quality and acid rain that has been a focus of regulatory-driven emissions reductions. Detailing the chemistry of atmospheric nitrate formation is critical for improving the model representation of atmospheric chemistry and air quality. The oxygen isotopic compositions of atmospheric nitrate are useful indicators in tracking nitrate formation pathways. Here, we measured oxygen isotope deltas (Δ (17 O) and δ (18 O)) for nitric acid (HNO 3) and particulate nitrate (pNO 3) from three US EPA Clean Air Status and Trends Network (CASTNET) sites in the northeastern US from December 2016 to 2018. The Δ (17 O, HNO 3) and δ (18 O, HNO 3) values ranged from 12.9 ‰ to 30.9 ‰ and from 46.9 ‰ to 82.1 ‰, and the Δ (17 O, pNO 3) and δ (18 O, pNO 3) ranged from 16.6 ‰ to 33.7 ‰ and from 43.6 ‰ to 85.3 ‰, respectively. There was distinct seasonality of δ (18 O) and Δ (17 O), with higher values observed during winter compared to during summer, suggesting a shift in O 3 to HO x radical chemistry, as expected. Unexpectedly, there was a statistical difference in Δ (17 O) between HNO 3 and pNO 3 , with higher values observed for pNO 3 (27.1 ± 3.8) ‰ relative to HNO 3 (22.7 ± 3.6) ‰, and significant differences in the relationship between δ (18 O) and Δ (17 O). This difference suggests atmospheric nitrate phase-dependent oxidation chemistry that is not predicted in models. Based on the output from GEOS-Chem and both the δ (18 O) and Δ (17 O) observations, we quantify the production pathways of atmospheric nitrate. The model significantly overestimated the heterogeneous N 2 O 5 hydrolysis production for both HNO 3 and pNO 3 , a finding consistent with observed seasonal changes in δ (18 O) and Δ (17 O) of HNO 3 and pNO 3 , though large uncertainties remain in the quantitative transfer of δ (18 O) from major atmospheric oxidants. This comparison provides important insight into the role of oxidation chemistry in reconciling a commonly observed positive bias for modeled atmospheric nitrate concentrations in the northeastern US. [ABSTRACT FROM AUTHOR]

Despite significant precursor emission reductions in the US over recent decades, atmospheric nitrate deposition remains an important terrestrial stressor. Here, we utilized statistical air mass back trajectory analysis and nitrogen stable isotope deltas (δ (15 N)) to investigate atmospheric nitrate spatiotemporal trends in the northeastern US from samples collected at three US EPA Clean Air Status and Trends Network (CASTNET) sites from December 2016–2018. For the considered sites, similar seasonal patterns in nitric acid (HNO 3) and particulate nitrate (pNO 3) concentrations were observed with spatial differences attributed to nitrogen oxide (NO x) emission densities in source contributing regions that were typically ≤ 1000 km. Significant spatiotemporal δ (15 N) variabilities in HNO 3 and pNO 3 were observed with higher values during winter relative to summer, like previous reports from CASTNET samples collected in the early 2000s for our study region. In the early 2000s, δ (15 N) of atmospheric nitrate in the northeast US had been suggested to be driven by NO x emissions; however, we did not find significant spatiotemporal changes in the modeled NO x emissions by sector and fuel type or δ (15 N, NO x) for the source regions of the CASTNET sites. Instead, the seasonal and spatial differences in the observed δ (15 N) of atmospheric nitrate were driven by nitrate formation pathways (i.e., homogeneous reactions of NO 2 oxidation via hydroxyl radical or heterogeneous reactions of dinitrogen pentoxide on wetted aerosol surfaces) and their associated δ (15 N) fractionation. Under the field conditions of low NO x relative to O 3 concentrations and when δ (15 N, NO x) emission sources do not have significant variability, we demonstrate that δ (15 N) of atmospheric nitrate can be a robust tracer for diagnosing nitrate formation. [ABSTRACT FROM AUTHOR]

Inorganic nitrate (NO3−) is a crucial component of fine particulate matter (PM2.5) in haze events in China. Understanding the formation mechanisms of nitrate and the sources of NOx was critical to control the air pollution. In this study, measurements of multiple isotope compositions of nitrate (δ18O‐NO3−, δ17O‐NO3−, and δ15N‐NO3−) in PM2.5 were conducted in Hangzhou from 9 October 2015 to 24 August 2016. Our results showed that oxygen anomaly of nitrate (Δ17O‐NO3−: 20.0‰–37.9‰) and nitrogen isotope of nitrate (δ15N‐NO3−: −2.9‰ to 18.1‰) values were higher in winter and lower in summer. Based on Δ17O‐NO3− observation and a Bayesian model, NO3 radical chemistry was found to dominate the nitrate formation in winter, while photochemical reaction (NO2 + OH) was the main pathway in summer. After considering the nitrogen isotopic fractionation in the NOx(g)‐NO3−(p) conversion, the average contributions of coal combustion, vehicle exhausts, biomass burning, and soil emission were 50% ± 9%, 19% ± 12%, 26% ± 15%, and 5% ± 4%, respectively, to nitrate aerosols during the whole sampling period. Coal combustion was the most important nitrate source in Hangzhou, especially in winter (∼56%). The contribution of soil emission increased significantly in summer due to active soil microbial processes under high temperature environment. Plain Language Summary: Inorganic nitrate is a crucial component of fine particulate matter in haze events in China. Understanding the formation mechanisms and sources of nitrate was critical to control the air pollution. In this study, measurements of multiple isotope compositions of nitrate showed that oxygen anomaly and nitrogen isotope of nitrate are distinct in winter and summer. Then, nitrate radical chemistry was found to dominate the nitrate formation in winter, while photochemical reaction (nitrogen dioxide with hydroxyl radical) was the main pathway in summer. After considering the nitrogen isotopic fractionation in the conversion from nitrogen oxides to nitrate particle, coal combustion was the most important nitrate source in Hangzhou, especially in winter (56%) and the contribution of soil emission increased significantly in summer under high temperature environment. Key Points: NO3 radical chemistry was dominant in nitrate production in the polluted atmosphere in HangzhouThe isotopic fractionation effect in the NOx(g)‐NO3−(p) conversion showed a greater impact on the source apportionment of nitrate aerosolsCoal combustion was the main source of nitrate aerosols in Hangzhou [ABSTRACT FROM AUTHOR]

Using copper nitrate as raw material, basic copper nitrate with various morphologies were prepared by ammonia precipitation method. The effects of temperature, ammonia-passing time and ammonia-passing rate on the microscopic morphology of the product and product yield during the reaction were studied. Under the optimal reaction conditions, that is, the reaction time of 40 min, the reaction temperature of 90 °C, and the ammonia passing rate of 500 mL/min, the product yield reaches 50%, and the product morphology is hexagonal flake-like, with good dispersibility and the particle size distribution is close to the normal distribution. It is found in the product that a two-dimensional nano-network structure formed by the close arrangement of nano-scale basic copper nitrate particles, with nano-scale micropores distributed in it. The Morphology and CASTEP program were used to theoretically analyze the growth habit of basic copper nitrate, the calculated results are consistent with the experimental results, the change of (001) crystal surface exposure caused by temperature is an important reason for the irregular macroscopic morphology. [ABSTRACT FROM AUTHOR]

Particulate nitrate (NO 3-) is one of the dominant components of fine particles in China, especially during pollution episodes, and has a significant impact on human health, air quality, and climate. Here a comprehensive field campaign that focuses on the atmospheric oxidation capacity and aerosol formation and their effects in the Yangtze River Delta (YRD) was conducted from May to June 2019 at a regional site in Changzhou, Jiangsu Province in China. The concentrations of NO 3- , OH radicals, N 2 O 5 , NO 2 , O 3 , and relevant parameters were measured simultaneously. We showed a high NO 3- mass concentration with 10.6 ± 8.9 µ g m -3 on average, which accounted for 38.3 % of total water-soluble particulate components and 32.0 % of total PM 2.5 , followed by the proportion of sulfate, ammonium, and chloride by 26.0 %, 18.0 %, and 2.0 %, respectively. This result confirmed that the heavy nitrate pollution in eastern China happened not only in winter, but also in summer. This study's high nitrate oxidation ratio (NOR) emphasized the solid atmospheric oxidation and fast nitrate formation capacity in the YRD. It was found that OH + NO 2 during daytime dominated nitrate formation on clean days, while N 2 O 5 hydrolysis vastly enhanced and became comparable with that of OH + NO 2 during polluted days (67.2 % and 30.2 %, respectively). The updated observed-constraint Empirical Kinetic Modeling Approach (EKMA) was used to assess the kinetic controlling factors of both local O 3 and NO 3- productions, which indicated that the O 3 -targeted scheme (VOCs : NO x = 2:1) is adequate for mitigating the O 3 and nitrate pollution coordinately during summertime in this region. Our results promote the understanding of nitrate pollution mechanisms and mitigation based on field observation and model simulation and call for more attention to nitrate pollution in the summertime. [ABSTRACT FROM AUTHOR]

SO2 and NO2 are the critical precursors in forming sulfate and nitrate in ambient particles. We studied the mechanism of sulfate and nitrate formation during the co-uptake of NO2 and SO2 into NaCl droplets at different RHs under irradiation and dark conditions. A significant formation of nitrate attributable to NO2 hydrolysis was observed during the NO2 uptake under all conditions, and its formation rate increases with decreasing RH. The averaged NO2 uptake coefficient, γNO2, from the unary uptake of NO2 into NaCl droplets under dark are 1.6 × 10-5, 1.9 × 10-5, and 3.0 × 10-5 at 80 %, 70 %, and 60 % RH, respectively. Chloride photolysis and nitrate photolysis play a crucial role in sulfate formation during the co-uptake. Nitrate photolysis generates reactive species (e.g., OH radicals, NO2, and N(III)) that directly react with S(IV) to produce sulfate. The generated OH radicals from nitrate photolysis can also react with chloride ions to form reactive chlorine species and then sulfate. To parameterize the role of nitrate photolysis and chloride photolysis in forming sulfate, the SO2 uptake coefficient, γSO2, as a function of nitrate photolysis rate, P NO3- (= j NO3- × [NO3-]), and chloride photolysis rate, P Cl- (= j Cl- × [Cl-]), as γSO2 = 0.41 × P NO3- + 0.34 × P Cl- was derived. Our findings open up new perspectives on the formation of secondary aerosol from the combined effect of nitrate photolysis and chlorine chemistry. [ABSTRACT FROM AUTHOR]

The DTA signal was studied in comparison with X-ray phase analysis of potassium nitrate particles in carbon nanotubes. It is shown that, for potassium nitrate particles in nanotubes, the temperature range of the existence of the KNO3 ferroelectric phase expands up to 300 K and the ferroelectric and low-temperature paraelectric phases coexist. [ABSTRACT FROM AUTHOR]

Ammonia (NH3) is an important atmospheric constituent. It plays a role in air quality and climate through the formation of ammonium sulfate and ammonium nitrate particles. It has also an impact on ecosystems through deposition processes. About 85 % of NH3 global anthropogenic emissions are related to food and feed production and, in particular, to the use of mineral fertilizers and manure management. Most global chemistry transport models (CTMs) rely on bottom-up emission inventories, which are subject to significant uncertainties. In this study, we estimate emissions from livestock by developing a new module to calculate ammonia emissions from the whole agricultural sector (from housing and storage to grazing and fertilizer application) within the ORCHIDEE (Organising Carbon and Hydrology In Dynamic Ecosystems) global land surface model. We detail the approach used for quantifying livestock feed management, manure application, and indoor and soil emissions and subsequently evaluate the model performance. Our results reflect China, India, Africa, Latin America, the USA, and Europe as the main contributors to global NH3 emissions, accounting for 80 % of the total budget. The global calculated emissions reach 44 TgNyr-1 over the 2005–2015 period, which is within the range estimated by previous work. Key parameters (e.g., the pH of the manure, timing of N application, and atmospheric NH3 surface concentration) that drive the soil emissions have also been tested in order to assess the sensitivity of our model. Manure pH is the parameter to which modeled emissions are the most sensitive, with a 10 % change in emissions per percent change in pH. Even though we found an underestimation in our emissions over Europe (-26 %) and an overestimation in the USA (+56 %) compared with previous work, other hot spot regions are consistent. The calculated emission seasonality is in very good agreement with satellite-based emissions. These encouraging results prove the potential of coupling ORCHIDEE land-based emissions to CTMs, which are currently forced by bottom-up anthropogenic-centered inventories such as the CEDS (Community Emissions Data System). [ABSTRACT FROM AUTHOR]

The article notes that photolysis of particulate nitrate is a source of Nitrous acid (HONO) and Nitrous oxides (NOx). It is reported that HONO is a substantial source of NOx which further regulates the abundance of atmospheric oxidants and is essential for the formation of secondary atmospheric aerosols. It is further reported that recycling of nitric acid to nitrogen oxides on nitrate aerosol could have unexplored implications for the trends and distributions of atmospheric oxidants.

Satellite-based retrievals of tropospheric NO 2 columns are widely used to infer NO x (≡ NO + NO 2) emissions. These retrievals rely on model information for the vertical distribution of NO 2. The free tropospheric background above 2 km is particularly important because the sensitivity of the retrievals increases with altitude. Free tropospheric NO x also has a strong effect on tropospheric OH and ozone concentrations. Here we use observations from three aircraft campaigns (SEAC 4 RS, DC3, and ATom) and four atmospheric chemistry models (GEOS-Chem, GMI, TM5, and CAMS) to evaluate the model capabilities for simulating NO x in the free troposphere and attribute it to sources. NO 2 measurements during the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC 4 RS) and Deep Convective Clouds and Chemistry (DC3) campaigns over the southeastern U.S. in summer show increasing concentrations in the upper troposphere above 10 km, which are not replicated by the GEOS-Chem, although the model is consistent with the NO measurements. Using concurrent NO, NO 2 , and ozone observations from a DC3 flight in a thunderstorm outflow, we show that the NO 2 measurements in the upper troposphere are biased high, plausibly due to interference from thermally labile NO 2 reservoirs such as peroxynitric acid (HNO 4) and methyl peroxy nitrate (MPN). We find that NO 2 concentrations calculated from the NO measurements and NO–NO 2 photochemical steady state (PSS) are more reliable to evaluate the vertical profiles of NO 2 in models. GEOS-Chem reproduces the shape of the PSS-inferred NO 2 profiles throughout the troposphere for SEAC 4 RS and DC3 but overestimates NO 2 concentrations by about a factor of 2. The model underestimates MPN and alkyl nitrate concentrations, suggesting missing organic NO x chemistry. On the other hand, the standard GEOS-Chem model underestimates NO observations from the Atmospheric Tomography Mission (ATom) campaigns over the Pacific and Atlantic oceans, indicating a missing NO x source over the oceans. We find that we can account for this missing source by including in the model the photolysis of particulate nitrate on sea salt aerosols at rates inferred from laboratory studies and field observations of nitrous acid (HONO) over the Atlantic. The median PSS-inferred tropospheric NO 2 column density for the ATom campaign is 1.7 ± 0.44 × 10 14 molec. cm -2 , and the NO 2 column density simulated by the four models is in the range of 1.4–2.4 × 10 14 molec. cm -2 , implying that the uncertainty from using modeled NO 2 tropospheric columns over clean areas in the retrievals for stratosphere–troposphere separation is about 1 × 10 14 molec. cm -2. We find from GEOS-Chem that lightning is the main primary NO x source in the free troposphere over the tropics and southern midlatitudes, but aircraft emissions dominate at northern midlatitudes in winter and in summer over the oceans. Particulate nitrate photolysis increases ozone concentrations by up to 5 ppbv (parts per billion by volume) in the free troposphere in the northern extratropics in the model, which would largely correct the low model bias relative to ozonesonde observations. Global tropospheric OH concentrations increase by 19 %. The contribution of the free tropospheric background to the tropospheric NO 2 columns observed by satellites over the contiguous U.S. increases from 25 ± 11 % in winter to 65 ± 9 % in summer, according to the GEOS-Chem vertical profiles. This needs to be accounted for when deriving NO x emissions from satellite NO 2 column measurements. [ABSTRACT FROM AUTHOR]

Over the past decade, fine particulate matter (PM) pollution in China has been abated significantly, benefiting from strict emission control measures, but particulate nitrate continues to rise. Here, we review the progress in particulate nitrate (pNO 3 −) pollution characterization, nitrate formation mechanisms, and the proposed control strategies in China. The spatial and temporal distributions of pNO 3 − are summarized. The current status of knowledge on the chemical mechanism is updated, and the significance of its formation pathways is assessed by various approaches such as field observation and modelling of nitrate production rate, as well as isotopic analysis. The factors impacting pNO 3 − formation and the corresponding pollution regulation strategies are discussed, in which the importance of atmospheric oxidation capacity and ammonia are addressed. Finally, the challenges and open questions in pNO 3 − pollution control in China are outlined. [Display omitted] [ABSTRACT FROM AUTHOR]

We present a comparison of fast-response instruments installed onboard the NASA DC-8 aircraft that measured nitrogen oxides (NO and NO 2), nitrous acid (HONO), total reactive odd nitrogen (measured both as the total (NO y) and from the sum of individually measured species (Σ NO y)), and carbon monoxide (CO) in the troposphere during the 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign. By targeting smoke from summertime wildfires, prescribed fires, and agricultural burns across the continental United States, FIREX-AQ provided a unique opportunity to investigate measurement accuracy in concentrated plumes where hundreds of species coexist. Here, we compare NO measurements by chemiluminescence (CL) and laser-induced fluorescence (LIF); NO 2 measurements by CL, LIF, and cavity-enhanced spectroscopy (CES); HONO measurements by CES and iodide-adduct chemical ionization mass spectrometry (CIMS); and CO measurements by tunable diode laser absorption spectrometry (TDLAS) and integrated cavity output spectroscopy (ICOS). Additionally, total NO y measurements using the CL instrument were compared with Σ NO y (= NO + NO 2 + HONO + nitric acid (HNO 3) + acyl peroxy nitrates (APNs) + submicrometer particulate nitrate (p NO 3)). Other NO y species were not included in Σ NO y as they either contributed minimally to it (e.g., C 1 –C 5 alkyl nitrates, nitryl chloride (ClNO 2), dinitrogen pentoxide (N 2 O 5)) or were not measured during FIREX-AQ (e.g., higher oxidized alkyl nitrates, nitrate (NO 3), non-acyl peroxynitrates, coarse-mode aerosol nitrate). The aircraft instrument intercomparisons demonstrate the following points: (1) NO measurements by CL and LIF agreed well within instrument uncertainties but with potentially reduced time response for the CL instrument; (2) NO 2 measurements by LIF and CES agreed well within instrument uncertainties, but CL NO 2 was on average 10 % higher; (3) CES and CIMS HONO measurements were highly correlated in each fire plume transect, but the correlation slope of CES vs. CIMS for all 1 Hz data during FIREX-AQ was 1.8, which we attribute to a reduction in the CIMS sensitivity to HONO in high-temperature environments; (4) NO y budget closure was demonstrated for all flights within the combined instrument uncertainties of 25 %. However, we used a fluid dynamic flow model to estimate that average p NO 3 sampling fraction through the NO y inlet in smoke was variable from one flight to another and ranged between 0.36 and 0.99, meaning that approximately 0 %–24 % on average of the total measured NO y in smoke may have been unaccounted for and may be due to unmeasured species such as organic nitrates; (5) CO measurements by ICOS and TDLAS agreed well within combined instrument uncertainties, but with a systematic offset that averaged 2.87 ppbv; and (6) integrating smoke plumes followed by fitting the integrated values of each plume improved the correlation between independent measurements. [ABSTRACT FROM AUTHOR]

While a typical military flare includes a metal fuel (aluminum, magnesium, boron, etc.) to increase combustion temperature and therefore spectral performance, the replacement of these fuels to reduce smoke (non‐light emitting species) is still desired. Nitrocellulose (NC) is known to be a "smokeless" propellant and, in this work, used solely as the fuel component in the pyrotechnic flare composition. Two levels of nitration, 12.2% N and 13.4% N content, are studied and while the influence of nitration content is shown, this work focuses on the role of barium nitrate (Ba(NO3)2) particle size. For formulations with 13.4% N content, reducing the Ba(NO3)2 particle size fraction from 250–300 μm to ≤25 μm results in the dominant wavelength being reduced from 569 nm to 558 nm. Combustion products collected from formulations using the 250–300 μm Ba(NO3)2, were characterized and indicated barium carbonate (BaCO3) as the major condensed phase product. This indicates that the formation of barium oxide (BaO) from the slow decomposition of Ba(NO3)2, coupled with the release of CO2 from NC, likely results in the following reaction: BaO+CO2→BaCO3. While unexpected, this implies that controlling early chemistry during combustion may allow significant improvements in spectral performance by inhibiting non‐light emitting specie formation. [ABSTRACT FROM AUTHOR]