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Rapid development of agriculture and fossil fuel combustion greatly increased US reactive nitrogen emissions to the atmosphere in the second half of the 20th century, resulting in excess nitrogen deposition to natural ecosystems. Recent efforts to lower nitrogen oxides emissions have substantially decreased nitrate wet deposition. Levels of wet ammonium deposition, by contrast, have increased in many regions. Together these changes have altered the balance between oxidized and reduced nitrogen deposition. Across most of the United States, wet deposition has transitioned from being nitrate-dominated in the 1980s to ammonium-dominated in recent years. Ammonia has historically not been routinely measured because there are no specific regulatory requirements for its measurement. Recent expansion in ammonia observations, however, along with ongoing measurements of nitric acid and fine particle ammonium and nitrate, permit new insight into the balance of oxidized and reduced nitrogen in the total (wet + dry) US nitrogen deposition budget. Observations from 37 sites reveal that reduced nitrogen contributes, on average, ∼65% of the total inorganic nitrogen deposition budget. Dry deposition of ammonia plays an especially key role in nitrogen deposition, contributing from 19% to 65% in different regions. Future progress toward reducing US nitrogen deposition will be increasingly difficult without a reduction in ammonia emissions.

Atmospheric oxidation capacity is the basis for converting freshly emitted substances into secondary products and is dominated by reactions involving hydroxyl radicals (OH) during daytime. In this study, we present in situ measurements of ROx radical (hydroxy OH, hydroperoxy HO2, and organic peroxy RO2) precursors and products; the measurements are carried out in four Chinese megacities (Beijing, Shanghai, Guangzhou, and Chongqing) during photochemically polluted seasons. The atmospheric oxidation capacity is evaluated using an observation-based model and radical chemistry precursor measurements as input. The radical budget analysis illustrates the importance of HONO and HCHO photolysis, which account for ∼50 % of the total primary radical sources. The radical propagation is efficient due to abundant NO in urban environments. Hence, the production rate of secondary pollutants, that is, ozone (and fine-particle precursors (H2SO4, HNO3, and extremely low volatility organic compounds, ELVOCs) is rapid, resulting in secondary air pollution. The ozone budget demonstrates its high production in urban areas; also, its rapid transport to downwind areas results in rapid increase in local ozone concentrations. The O3–NOx–VOC (volatile organic compound) sensitivity tests show that ozone production is VOC-limited and that alkenes and aromatics should be mitigated first for ozone pollution control in the four studied megacities. In contrast, NOx emission control (that is, a decrease in NOx) leads to more severe ozone pollution. With respect to fine-particle pollution, the role of the HNO3–NO3 partitioning system is investigated using a thermal dynamic model (ISORROPIA 2). Under high relative humidity (RH) and ammonia-rich conditions, nitric acid converts into nitrates. This study highlights the efficient radical chemistry that maintains the atmospheric oxidation capacity in Chinese megacities and results in secondary pollution characterized by ozone and fine particles.

Recent research [Wang et al., Nature 581, 184–189 (2020)] indicates nitric acid (NA) can participate in sulfuric acid (SA)–ammonia (NH₂) nucleation in the clean and cold upper free troposphere, whereas NA exhibits no obvious effects at the boundary layer with relatively high temperatures. Herein, considering that an SA–dimethylamine (DMA) nucleation mechanism was detected inmegacities [Yao et al., Science 361, 278–281 (2018)], the roles of NA in SA-DMA nucleation are investigated. Different from SA-NH₂ nucleation, we found that NA can enhance SA-DMA–based particle formation rates in the polluted atmospheric boundary layer, such as Beijing in winter, with the enhancement up to 80-fold. Moreover, we found that NA can promote the number concentrations of nucleation clusters (up to 27-fold) and contribute 76% of cluster formation pathways at 280 K. The enhancements on particle formation by NA are critical for particulate pollution in the polluted boundary layer with relatively high NA and DMA concentrations.

Aircraft measurements, laboratory photolysis experiments and modelling calculations reveal a mechanism for the recycling of nitric acid into nitrogen oxides; this enables observations to be reconciled with model studies, and suggests that particulate nitrate photolysis could be a substantial tropospheric nitrogen oxide source. Rapid NO x recycling in marine air Nitrogen oxides play a central role in tropospheric chemistry, but current understanding of the processes responsible for their formation and removal from the atmosphere is insufficient to reconcile model studies with observations. This paper presents aircraft measurements, laboratory photolysis experiments and modelling calculations that reveal a mechanism for the rapid recycling of nitric acid to nitrous acid and nitrogen oxides in the clean marine boundary layer via particulate nitrate photolysis. The authors suggest that this process could be an important tropospheric nitrogen oxide source in remote oceanic regions with implications for tropospheric oxidant and secondary atmospheric aerosol formation. Nitrogen oxides are essential for the formation of secondary atmospheric aerosols and of atmospheric oxidants such as ozone and the hydroxyl radical, which controls the self-cleansing capacity of the atmosphere 1 . Nitric acid, a major oxidation product of nitrogen oxides, has traditionally been considered to be a permanent sink of nitrogen oxides 1 . However, model studies predict higher ratios of nitric acid to nitrogen oxides in the troposphere than are observed 2 , 3 . A ‘renoxification’ process that recycles nitric acid into nitrogen oxides has been proposed to reconcile observations with model studies 2 , 3 , 4 , but the mechanisms responsible for this process remain uncertain 5 , 6 , 7 , 8 , 9 . Here we present data from an aircraft measurement campaign over the North Atlantic Ocean and find evidence for rapid recycling of nitric acid to nitrous acid and nitrogen oxides in the clean marine boundary layer via particulate nitrate photolysis. Laboratory experiments further demonstrate the photolysis of particulate nitrate collected on filters at a rate more than two orders of magnitude greater than that of gaseous nitric acid, with nitrous acid as the main product. Box model calculations based on the Master Chemical Mechanism 10 , 11 suggest that particulate nitrate photolysis mainly sustains the observed levels of nitrous acid and nitrogen oxides at midday under typical marine boundary layer conditions. Given that oceans account for more than 70 per cent of Earth’s surface, we propose that particulate nitrate photolysis could be a substantial tropospheric nitrogen oxide source. Recycling of nitrogen oxides in remote oceanic regions with minimal direct nitrogen oxide emissions could increase the formation of tropospheric oxidants and secondary atmospheric aerosols on a global scale.

The concentration of fine particulate matter (PM 2.5 ) across China has decreased by 30–50% over the period 2013–2018 due to stringent emission controls. However, the nitrate component of PM 2.5 has not responded effectively to decreasing emissions of nitrogen oxides and has actually increased during winter haze pollution events in the North China Plain. Here, we show that the GEOS-Chem atmospheric chemistry model successfully simulates the nitrate concentrations and trends. We find that winter mean nitrate would have increased over 2013–2018 were it not for favourable meteorology. The principal cause of this nitrate increase is weaker deposition. The fraction of total inorganic nitrate as particulate nitrate instead of gaseous nitric acid over the North China Plain in winter increased from 90% in 2013 to 98% in 2017, as emissions of nitrogen oxides and sulfur dioxide decreased while ammonia emissions remained high. This small increase in the particulate fraction greatly slows down deposition of total inorganic nitrate and hence drives the particulate nitrate increase. Our results suggest that decreasing ammonia emissions would decrease particulate nitrate by driving faster deposition of total inorganic nitrate. Decreasing nitrogen oxide emissions is less effective because it drives faster oxidation of nitrogen oxides and slower deposition of total inorganic nitrate. Reduction of ammonia emissions may be effective in reducing the nitrate component of fine particulate matter air pollution across the North China Plain, according to the simulation of nitrate trends using the GEOS-Chem atmospheric chemistry model.

Nitrate (NO3-) has become a major component of fine particulate matter (PM2.5) during hazy days in China. However, the role of the heterogeneous reactions of dinitrogen pentoxide (N2O5) in nitrate formation is not well constrained. In January 2017, a severe haze event occurred in the Pearl River Delta (PRD) of southern China during which high levels of PM2.5 (∼400 µg m−3) and O3 (∼160 ppbv) were observed at a semi-rural site (Heshan) in the western PRD. Nitrate concentrations reached 108 µg m−3 (1 h time resolution), and the contribution of nitrate to PM2.5 was nearly 40 %. Concurrent increases in NO3- and ClNO2 (with a maximum value of 8.3 ppbv at a 1 min time resolution) were observed in the first several hours after sunset, indicating an intense N2O5 heterogeneous uptake by aerosols. The formation potential of NO3- via N2O5 heterogeneous reactions was estimated to be between 29.0 and 77.3 µg m−3 in the early hours (2 to 6 h) after sunset based on the measurement data, which could completely explain the measured increase in the NO3- concentration during the same time period. Daytime production of nitric acid from the gas-phase reaction of OH+NO2 was calculated with a chemical box model built using the Master Chemical Mechanism (MCM v3.3.1) and constrained by the measurement data. The integrated nocturnal nitrate formed via N2O5 chemistry was comparable to or even higher than the nitric acid formed during the day. This study confirms that N2O5 heterogeneous chemistry was a significant source of aerosol nitrate during hazy days in southern China.

Recent observations of nitrous acid (HONO) in the remote troposphere show much higher concentrations than can be explained through known sources, with important implications for air quality and climate. Laboratory evidence and modelling of field observations suggests that nitrate aerosol photolysis is the likely mechanism providing the additional HONO, offering a rapid route for recycling of NO.sub.x from nitric acid (HNO.sub.3). Previous studies of the global impact of this chemistry have used either very restricted HONO data or a \"top-down\" approach to parameterize the HONO source by reconciling simulated and observed NO.sub.x concentrations. Here, we use multiple, independent tropospheric HONO observations from different locations to parameterize nitrate photolysis, and evaluate its impacts on global atmospheric chemistry using GEOS-Chem. The simulations improve agreement between modelled and observed HONO concentrations relative to previous studies, decreasing the model bias by 5 %-20 %. The remaining (and large) underestimate of HONO in the model is due predominantly to an underestimate of total nitrate aerosol (-95 %) and is reduced to 20 % when accounting for low model nitrate. Despite the low bias in the model HONO, we find that nitrate aerosol photolysis leads to substantial global increases in NO.sub.x, O.sub.3 and OH concentrations, likely beyond the observational constraints. The additional source of NO.sub.x (â¼ 48 Tg N yr.sup.-1 globally) is comparable to total NO.sub.x emissions from all sources (â¼ 55 Tg yr.sup.-1). These HONO observations in the remote troposphere, thus imply a large uncertainty in the NO.sub.x budget and an incomplete understanding of atmospheric chemistry. Improved techniques to measure HONO at the low concentrations typical of remote areas, coupled with more measurements in these areas and improved process level understanding of nitrate photolysis are needed to provide quantitative assessment of its potentially global-scale atmospheric impacts.

Pollution of environmental waters and ecosystems is increasing. Adsorption is an effective technique for water decontamination, but is limited by the cost of commercial adsorbents such as activated carbon. Research has thus focused on the recycling and transformation of biowaste as low-cost, biodegradable adsorbents. In particular, banana peel is promising for commercial use due to its wide availability and efficiency. Here, we review the use of natural banana peel for the biosorption of pollutants from water. We discuss the factors controlling pollutants removal, and the regeneration and reuse of the biosorbent. pH of 5.0 to 7.0 is favorable for the removal of cationic pollutants, while pH of 2.0 to 4.0 is suitable for anionic pollutants. Generally, higher pollutant concentration induces lower removal, whereas higher banana peel dosage induces higher removal. Banana peel exhibits efficient removal of pollutants at various temperatures, with adsorption capacities mostly within 1–100 mg/g. Nitric acid is the most efficient eluent for heavy metal desorption from banana peel. Most studies showed efficient biosorbent reuse up to five cycles and above. We also discuss the thermodynamics, kinetics and isotherms of the adsorption process.

Background Concentrated nitric acid can form nitrogen oxides through a spontaneous redox reaction. After inhalation, it has a stimulating effect on the respiratory tract and can cause inhalation lung injury. This lung injury is mainly manifested as non-cardiogenic pulmonary edema in imaging, and pulmonary ultrasound can dynamically monitor the severity of pulmonary edema at the patient’s bedside. Case presentation This article describes a 52-year-old female case who experienced chest tightness and dyspnea 8 h after inhaling nitric acid fumes and was sent to the emergency room. Chest CT suggested diffuse exudation in both lungs, and dynamic B-lines could be seen by bedside pulmonary ultrasound. During the treatment of this patient, we adjusted the dosage of hormones and the oxygen therapy plan in a timely manner according to the evolution of B-lines under ultrasound. After treatment, her clinical symptoms gradually improved, and the re-examination of chest CT showed that the exudation in both lungs was gradually absorbed. Conclusion This case reminds clinicians to be alert to the occurrence of delayed pulmonary edema when treating patients who inhale nitric acid fumes. At the same time, dynamic examination of bedside pulmonary ultrasound is of great value in the treatment of such patients.

We use Aura Microwave Limb Sounder (MLS) trace gas measurements to investigate whether water vapor (H2O) injected into the stratosphere by the Hunga Tonga‐Hunga Ha'apai (HTHH) eruption affected the 2022 Antarctic stratospheric vortex. Other MLS‐measured long‐lived species are used to distinguish high HTHH H2O from that descending in the vortex from the upper‐stratospheric H2O peak. HTHH H2O reached high southern latitudes in June–July but was effectively excluded from the vortex by the strong transport barrier at its edge. MLS H2O, nitric acid, chlorine species, and ozone within the 2022 Antarctic polar vortex were near average; the vortex was large, strong, and long‐lived, but not exceptionally so. There is thus no clear evidence of HTHH influence on the 2022 Antarctic vortex or its composition. Substantial impacts on the stratospheric polar vortices are expected in succeeding years since the H2O injected by HTHH has spread globally. Plain Language Summary The 2022 Hunga Tonga‐Hunga Ha'apai eruption injected vast amounts of water vapor into the stratosphere. Concern arose that this excess water vapor could affect the 2022 Antarctic stratospheric polar vortex and ozone hole: Water vapor plays a crucial role in forming polar stratospheric clouds, which provide surfaces upon which chemical reactions that destroy ozone take place. Enhanced water vapor also affects temperatures, which in turn affect the powerful winds defining the polar vortex boundary. Antarctic polar vortex development began in April–May; by June the intense vortex‐edge winds presented a formidable obstacle to transport. Satellite trace‐gas measurements show that when water vapor from the Hunga Tonga eruption reached the vortex edge in June, it faced an impenetrable barrier and “besieged” the vortex, building up exceptionally strong water vapor gradients across the vortex edge. Water vapor, ozone, and chemicals involved in ozone destruction remained near historical average levels within the vortex through spring 2022. Because excess water vapor spread throughout the south polar regions after vortex breakup, much larger effects on the Antarctic vortex and chemical processing within it are expected in 2023 and beyond, when high water vapor will be entrained into the vortex as it develops. Key Points Microwave Limb Sounder (MLS) trace gas data show that the Hunga Tonga‐Hunga Ha'apai H2O plume was effectively excluded from the 2022 Antarctic polar vortex Antarctic lower stratospheric vortex strength, size, and longevity were among the largest on record, but within the range of previous years Antarctic chemical ozone loss in 2022 was unexceptional, with MLS ozone and related trace gases observed to be near average