Nitrate Aerosol Research Articles

Aerosol formation pathways from aviation emissions

Aviation emissions are responsible for an estimated 24,000 premature mortalities annually and 3.5% of anthropogenic radiative forcing (RF). Emissions of nitrogen and sulfur oxides (NOx and SOx) contribute to these impacts. However, the relative contributions and mechanisms linking these emissions to formation and impacts of secondary aerosols (as opposed to direct aerosol emissions) have not been quantified, including how short-lived aerosol precursors at altitude can increase surface-level aerosol concentrations. We apply global chemistry transport modeling to identify and quantify the different chemical pathways to aerosol formation from aviation emissions, including the resulting impact on radiative forcing. We estimate a net aerosol radiative forcing of –8.3 mWm−2, of which –0.67 and –7.8 mWm−2 result from nitrate and sulfate aerosols respectively. We find that aviation NOx causes –1.7 mWm−2 through nitrate aerosol forcing but also –1.6 mWm−2 of sulfate aerosol forcing by promoting oxidation of SO2 to sulfate aerosol. This accounts for 21% of the total sulfate forcing, and oxidation of SO2 due to aviation NOx is responsible for 47% of the net aviation NOx attributable RF. Aviation NOx emissions in turn account for 41% of net aviation-aerosol-attributable RF (non-contrail). This is due to ozone-mediated oxidation of background sulfur and the ‘nitrate bounce-back’ effect, which reduces the net impact of sulfur emissions. The ozone-mediated mechanism also explains the ability of cruise aviation emissions to significantly affect surface aerosol concentrations. We find that aviation NOx emissions cause 72% of aviation-attributable, near-surface aerosol loading by mass, compared to 27% from aviation SOx emissions and less than 0.1% from direct emission of black carbon. We conclude that aviation NOx and SOx emissions are the dominant cause of aviation-attributable secondary inorganic aerosol radiative forcing, and that conversion of background aerosol precursors at all altitudes is amplified by enhanced production of aviation attributable oxidants at cruise altitudes.

Impact of shipping emissions on air pollution and pollutant deposition over the Barents Sea

Arctic warming leading to reduced summertime sea-ice is likely to lead to increased local shipping especially along the Northeast Passage near the northern coasts of Norway and Russia, which are shorter than the traditional southerly routes. Here, the regional chemistry-transport model WRF-Chem is used to examine the effects of shipping emissions on levels of air pollutants and deposition fluxes over the Barents Sea both for present-day and future conditions, based on a high growth scenario. Present-day shipping emissions are found to have already substantial effects on ozone concentrations, but limited effects on sulphate and nitrate aerosols. Predicted future changes in ozone are also important, particularly in regions with low nitrogen oxide concentrations, and results are sensitive to the way in which diversion shipping is distributed due to non-linear effects on photochemical ozone production. Whilst modest future increases in sulphate and nitrate aerosols are predicted, large enhancements in dry deposition of sulphur dioxide and wet deposition of nitrogen compounds to the Barents Sea are predicted. Such levels of future nitrogen deposition would represent a significant atmospheric source of oceanic nitrogen affecting sensitive marine ecosystems.

The Effect of Transportation and Wildfires on the Spatiotemporal Heterogeneity of PM2.5 Mass in the New York-New Jersey Metropolitan Statistical Area.

Declining ambient PM2.5 concentrations have been attributed to fuel consumption standards and emission controls of secondary sulfate and nitrate aerosol precursors from transportation and industrial sectors. As a result, the relative contribution of PM2.5 sources is modified, shifting PM2.5 trends, physicochemical characteristics, and health effects. Carbonaceous fine aerosol account for most of PM2.5 mass in the US. This study aims to examine the spatiotemporal trends of ambient PM2.5 levels and their association with primary PM2.5 emissions from anthropogenic activities and fires in the New York/New Jersey metropolitan statistical area (NYNJ MSA) airshed. PM2.5 mass concentrations were obtained from the U.S. Environmental Protection Agency (USEPA) Air Data. Ambient PM2.5 mass levels declined on average by 47%, at a rate of −0.61 ± 0.01 μg/m3/year in urban locations and −0.25 ± 0.01 μg/m3/year in upwind and peri-urban locations over the 2007 to 2017 period. The strong spatial gradient in 2007, with high PM2.5 levels in urban locations and low PM2.5 levels in peri-urban locations gradually weakened by 2013 but re-appeared in 2017. Over the same period, primary PM2.5 emissions declined by 52% from transportation, 15% from industrial, and 8% from other anthropogenic sources corresponding to a decrease of 0.8, 0.9, and 0.6 μg/m3 on ambient PM2.5 mass, respectively. Wildland and prescribed fires emissions increased more than 3 times adding 0.8 μg/m3 to ambient PM2.5 mass. These results indicate that (i) fire emissions may impede the effectiveness of existing policies to improve air quality and (ii) the chemical content of PM2.5 may be changing to an evolving mixture of aromatic and oxygenated organic species with differential toxicological responses as compared to inert ammonium sulfate and nitrate salts.

Characterization of particulate organic nitrates in the Yangtze River Delta, East China, using the time-of-flight aerosol chemical speciation monitor

Particulate organic nitrates (pON) are important species in the atmosphere that influence reactive nitrogen cycling and ozone (O3) formation. Previous studies on pON focused on forest regions involving anthropogenic NOx emissions. Therefore, the investigation and quantitation of pON in polluted urban regions remain limited. Here, we conducted sampling campaigns in four seasons using an Aerodyne time-of-flight aerosol chemical speciation monitor in the SORPES station in Nanjing, East China. pON were calculated using a constrained-PMF method, which was validated using high-resolution aerosol mass spectrometer-based estimates. The results reveal that pON contribute significantly to the aerosol nitrate in the polluted Yangtze River Delta, and the fractions of pON to total nitrate are >21% in the summer, spring, and autumn, and >13% in the winter. The pON contribution to the total nitrate decreases as the fine particulate matter (PM2.5) loading increases but remaines significant at the peak of pollution. This indicates that the pON formation is controlled by the gas-phase chemistry, which is pronounced during low PM2.5 pollution. pON are also associated with organic matter (OM), and its contribution to the OM is independent of the PM2.5 loading. The contributions to OM are 36.2%, 25.6%, 24.1%, and 13.9% during the winter, autumn, spring, and summer, respectively. Our study demonstrates that pON are potentially important components of atmospheric PM2.5 in urban areas.

A case study of heavy PM2.5 secondary formation by N2O5 nocturnal chemistry in Seoul, Korea in January 2018: Model performance and error analysis

Heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) plays an important role in nighttime nitrate (NO3−) formation in urban areas, and sometimes influences the occurrence of heavy PM2.5 pollution the next day in the Seoul Metropolitan Area (SMA), Korea. Here, we discuss the heavy PM2.5 wintertime episode of January 13–15, 2018, which was mainly induced by nighttime N2O5 heterogeneous reaction in the SMA. In our case, we confirmed that nighttime N2O5 hydrolysis is the most critical factor in the rapid formation of aerosol nitrate at high levels during the night, which prevailed in the morning of the next day. Our Integrated Process Rate (IPR) analysis showed that nighttime nitrate production in the episode was almost solely attributable to N2O5 chemistry, with hourly mean production rates of 0.8 ± 0.4 μg/m3 per hour in SMA, which is comparable to the daytime nitrate photochemical production rates of 0.9 ± 0.5 μg/m3 per hour. We also carried out a series of assessment of N2O5–driven nitrate formation sensitivity, and relevant errors were quantified by applying different N2O5 uptake coefficients in the WRF-CMAQ model. The potential errors of nighttime-average nitrate concentrations induced by N2O5 uptake process were assessed from a linear perspective for the planetary boundary layer (PBL) variances caused by four different PBL parameterization schemes: YSU, ACM2, MYJ, and QNSE. The potential error ranges by N2O5 uptake process were analyzed to be 2.3 to 3.5 μg/m3 (~10% relative to the nighttime-average), while biases of PBL simulations from 4 parameterization schemes were 2.3 ± 1.0 μg/m3, showing similar ranges in our episode. Although N2O5–driven heavy PM2.5 episodes do not occur often in SMA, our findings suggest the importance of N2O5 chemistry in vigorous wintertime nitrate formation and operational prediction errors of such PM2.5 episodes, under the premise of enhanced PBL simulation capabilities.

Effects of nighttime heterogeneous reactions on the formation of secondary aerosols and ozone in the Pearl River Delta

<p indent="0mm">Fine particulate matter (PM_2.5) and ozone (O₃) are currently the most concerning air pollutants. Long-term exposure to PM_2.5 and O₃ can induce cardiovascular diseases, presbyopia, and preterm birth. Acute exposure to PM_2.5 and O₃ can also result in hypertension and decreased heart rate variability. In recent years, PM_2.5 pollution abatement in China has achieved remarkable results, but O₃ pollution issues have emerged. In the troposphere, O₃ and PM_2.5 have a close association, i.e., O₃ can oxidize gaseous precursors to form sulfate, nitrate, ammonium and secondary organic aerosols in PM_2.5, and PM_2.5 affects the generation and removal of O₃ by providing surfaces for heterogeneous reactions. Recent studies show that PM_2.5 and O₃ have a significant positive correlation in the photochemically active season in the Pearl River Delta (PRD) region. However, it is still difficult to achieve cooperative control for PM_2.5 and O₃ due to our limited knowledge of their coupling relationship. To date, most studies have focused on the photochemical mechanism in the daytime, and our understanding of the chemical processes at night is insufficient. As an important reactive intermediate, N₂O₅ mainly accumulates through the nocturnal reaction of NO₂ and O₃ and is removed by heterogeneous reactions to produce HNO₃. These processes directly affect ozone production and particulate matter formation. In addition to losing NO_{<italic>x</italic>}, the heterogeneous reaction of N₂O₅ would lead to particulate nitrate formation and photolabile species (i.e., ClNO₂) release. ClNO₂ is a reservoir of NO₂ and chlorine, providing a connection between nitrogen oxide pollution and halogen activation. The formation of ClNO₂ represents an activation process of chlorine from the aerosol phase to the gas phase. ClNO₂ photolysis releases chlorine radicals and oxidizes VOCs, which could increase radical levels and promote ozone formation. Moreover, NO₂ released via ClNO₂ photolysis also promotes O₃ formation in the morning. In coastal areas, sufficient particle chlorine in aerosols might be driven by sea salt chloride interactions, which lead to the formation of ClNO₂. Therefore, with high concentrations of nitrogen oxides, nighttime heterogeneous N₂O₅ reactions would make a substantial contribution to PM_2.5 and O₃ in coastal regions. Although several studies have shown that heterogeneous N₂O₅ reactions play an important role in the formation of PM_2.5 and O₃, related observations in China are limited. In this study, N₂O₅ and ClNO₂ were measured in Shenzhen during the season with severe ozone pollution in October 2018. Iodine chemical ionization time-of-flight mass spectrometry (TOF-CIMS, Aerodyne Inc.) was used to measure N₂O₅ and ClNO₂. Based on collocated measurements of gas- and particle-phase pollutants, we quantitatively calculated the nighttime heterogeneous reactions and assessed their reactivities. The results show that the highest concentrations of N₂O₅ and ClNO₂ could be up to 1524 and 477 pptv <sc>(5 min</sc> average, 1 pptv = <sc>1 pptv=10^–3 mm³/m³)),</sc> respectively, and they were affected by precursors. The concentrations of N₂O₅ and ClNO₂ had pronounced peaks from sunset to sunrise. The nitrate formed via N₂O₅ reactions at night contributed 24%−60% of the total nighttime nitrate particles. In particular, the concentration of ClNO₂ had a noticeable peak in the morning, and the photolysis of ClNO₂ generated Cl radicals, which quickly removed alkanes, and the maximum removal efficiency was 2−3 times higher than that of OH radicals. In addition, the diurnal variation in VOCs with different reactivities also implies that chlorine radicals participate in the VOC oxidation cycle in the morning. This reflects that Cl radical-induced oxidation reactions could promote the generation of ozone and secondary organic aerosols through the HO_{<italic>x</italic>} radical cycle. Our study shows that active nighttime heterogeneous reactions can significantly promote the formation of PM_2.5 and O₃ in the coastal areas of the PRD region. Thus, it is urgent to call for more studies in the future to coordinate the control of PM_2.5 and O₃ in the PRD region.

Nitrate and secondary organic aerosol dominated particle light extinction in Beijing due to clean air action

Air quality in China has been continuously improved since clean air action in 2013, yet the visibility was not improved simultaneously. Here we employed a new method by integrating highly time-resolved aerosol compositions with particle light extinction (bext) into positive matrix factorization to quantify the different contributors to visibility degradation during four seasons in Beijing. Our results show that ammonium nitrate-related factor contributed dominantly to bext during all seasons (31–48%) and played more significant roles during low-visibility periods. Secondary organic aerosol (SOA) was an important contributor of bext (27–35%) in autumn and spring while primary OA related sources were more important in winter (37%). An increase in aerosol mass extinction efficiency and similarly important roles of ammonium nitrate and SOA in visibility degradation were also observed during the COVID-19 lockdown period. Our results point towards a future challenge in improving visibility in China due to the increased contributions of nitrate and SOA in PM2.5. Future emission controls with a priority to decrease nitrate would benefit both air quality and visibility.

Impact of the COVID-19 lockdown on the chemical composition and sources of urban PM2.5

The lockdown measures caused by the COVID-19 pandemic substantially affected air quality in many cities through reduced emissions from a variety of sources, including traffic. The change in PM2.5 and its chemical composition in downtown Toronto, Canada, including organic/inorganic composition and trace metals, were examined by comparing with a pre-lockdown period and respective periods in the three previous years. During the COVID-19 lockdown, the average traffic volume reduced by 58%, whereas PM2.5 only decreased by 4% relative to the baselines. Major chemical components of PM2.5, such as organic aerosol and ammonium nitrate, showed significant seasonal changes between pre- and lockdown periods. The changes in local and regional PM2.5 sources were assessed using hourly chemical composition measurements of PM2.5. Major regional and secondary PM2.5 sources exhibited no clear reductions during the lockdown period compared to pre-lockdown and the previous years. However, cooking emissions substantially dropped by approximately 61% due to the restrictions imposed on local businesses (i.e., restaurants) during the lockdown, and then gradually increased throughout the recovery periods. The reduction in non-tailpipe emissions, characterized by road dust and brake/tire dust, ranged from 37% to 61%, consistent with the changes in traffic volume and meteorology across seasons in 2020. Tailpipe emissions dropped by approximately 54% and exhibited even larger reductions during morning rush hours. The reduction of tailpipe emissions was statistically associated with the reduced number of trucks, highlighting that a small fraction of trucks contributes disproportionally to tailpipe emissions. This study provides insight into the potential for local benefits to arise from traffic intervention in traffic-dominated urban areas and supports the development of targeted strategies and regulations to effectively reduce local air pollution.

Influence of atmospheric deposition on biogeochemical cycles in an oligotrophic ocean system

Abstract. The surface mixed layer (ML) in the Mediterranean Sea is a well-stratified domain characterized by low macronutrients and low chlorophyll content for almost 6 months of the year. In this study we characterize the biogeochemical cycling of nitrogen (N) in the ML by analyzing simultaneous in situ measurements of atmospheric deposition, nutrients in seawater, hydrological conditions, primary production, heterotrophic prokaryotic production, N2 fixation and leucine aminopeptidase activity. Dry deposition was continuously measured across the central and western open Mediterranean Sea, and two wet deposition events were sampled, one in the Ionian Sea and one in the Algerian Basin. Along the transect, N budgets were computed to compare the sources and sinks of N in the mixed layer. In situ leucine aminopeptidase activity made up 14 % to 66 % of the heterotrophic prokaryotic N demand, and the N2 fixation rate represented 1 % to 4.5 % of the phytoplankton N demand. Dry atmospheric deposition of inorganic nitrogen, estimated from dry deposition of nitrate and ammonium in aerosols, was higher than the N2 fixation rates in the ML (on average 4.8-fold). The dry atmospheric input of inorganic N represented a highly variable proportion of biological N demand in the ML among the stations, 10 %–82 % for heterotrophic prokaryotes and 1 %–30 % for phytoplankton. As some sites were visited on several days, the evolution of biogeochemical properties in the ML and within the nutrient-depleted layers could be followed. At the Algerian Basin site, the biogeochemical consequences of a wet dust deposition event were monitored through high-frequency sampling. Notably, just after the rain, nitrate was higher in the ML than in the nutrient-depleted layer below. Estimates of nutrient transfer from the ML into the nutrient-depleted layer could explain up to a third of the nitrate loss from the ML. Phytoplankton did not benefit directly from the atmospheric inputs into the ML, probably due to high competition with heterotrophic prokaryotes, also limited by N and phosphorus (P) availability at the time of this study. Primary producers decreased their production after the rain but recovered their initial state of activity after a 2 d lag in the vicinity of the deep chlorophyll maximum layer.

Improving the representation of HONO chemistry in CMAQ and examining its impact on haze over China.

We compare Community Multiscale Air Quality (CMAQ) model predictions with measured nitrous acid (HONO) concentrations in Beijing, China for December 2015. The model with the existing HONO chemistry in CMAQ severely under-estimates the observed HONO concentrations with a normalized mean bias of -97%. We revise the HONO chemistry in the model by implementing six additional heterogeneous reactions in the model: reaction of nitrogen dioxide (NO2) on ground surfaces, reaction of NO2 on aerosol surfaces, reaction of NO2 on soot surfaces, photolysis of aerosol nitrate, nitric acid displacement reaction, and hydrochloric acid displacement reaction. The model with the revised chemistry substantially increases HONO predictions and improves the comparison with observed data with a normalized mean bias of -5%. The photolysis of HONO enhances day-time hydroxyl radical by almost a factor of two. The enhanced hydroxyl radical concentrations compare favourably with observed data and produce additional sulfate via the reaction with sulfur dioxide, aerosol nitrate via the reaction with nitrogen dioxide, and secondary organic aerosols via the reactions with volatile organic compounds. The additional sulfate stemming from revised HONO chemistry improves the comparison with observed concentration; however, it does not close the gap between model prediction and the observation during polluted days.

Nonlinear responses of particulate nitrate to NO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt; emission controls in the megalopolises of China

Abstract. Nitrate is an increasingly important component of fine particulate matter (PM2.5) in Chinese cities. The production of nitrate is not only related to the abundance of its precursor, but it is also supported by the atmospheric photochemical oxidants, raising a new challenge for the current emission control actions in China. This paper uses comprehensive measurements and a regional meteorology–chemistry model with optimized mechanisms to establish the nonlinear responses between particulate nitrate and the emission controls of nitrogen oxides (NOx) in the megalopolises of China. Nitrate is an essential component of PM2.5 in eastern China, accounting for 9.4 %–15.5 % and 11.5 %–32.1 % of the PM2.5 mass for the warm and cold seasons. The hypothetical NOx emission reduction scenarios (−10 % to −80 %) during summer–autumn result in almost linearly lower PM2.5 by −2.2 % in Beijing–Tianjin–Hebei (BTH) and −2.9 % in Yangtze River Delta (YRD) per 10 % reduction of NOx emissions, whereas they lead to a rather complicated response of PM components in winter. Wintertime nitrate is found to increase by +4.1 % in BTH and +5.1 % in YRD per 10 % reduction of NOx emissions, with nearly unchanged nitric acid (HNO3) and higher dinitrogen pentoxide (N2O5) intermediate products produced from the increased atmospheric oxidant levels. An inflexion point appears at 30 %–50 % NOx emission reduction, and a further reduction in NOx emissions is predicted to cause −10.5 % reduction of nitrate for BTH and −7.7 % for YRD per 10 % reduction of NOx emissions. In addition, the 2012–2016 NOx control strategy actually leads to no changes or even increases of nitrate in some areas (8.8 % in BTH and 14.4 % in YRD) during winter. Our results also emphasize that ammonia (NH3) and volatile organic compounds (VOCs) are effective in controlling nitrate pollution, whereas decreasing the sulfur dioxide (SO2) and NOx emissions may have counterintuitive effects on nitrate aerosols. This paper helps understand the nonlinear aerosol and photochemistry feedbacks and defines the effectiveness of proposed mitigations for the increasingly serious nitrate pollution in China.

Halogens Enhance Haze Pollution in China.

Severe and persistent haze events in northern China, characterized by high loading of fine aerosol especially of secondary origin, negatively impact human health and the welfare of ecosystems. However, current knowledge cannot fully explain the formation of this haze pollution. Despite field observations of elevated levels of reactive halogen species (e.g., BrCl, ClNO2, Cl2, HBr) at several sites in China, the influence of halogens (particularly bromine) on haze pollution is largely unknown. Here, for the first time, we compile an emission inventory of anthropogenic bromine and quantify the collective impact of halogens on haze pollution in northern China. We utilize a regional model (WRF-Chem), revised to incorporate updated halogen chemistry and anthropogenic chlorine and bromine emissions and validated by measurements of atmospheric pollutants and halogens, to show that halogens enhance the loading of fine aerosol in northern China (on average by 21%) and especially its secondary components (∼130% for secondary organic aerosol and ∼20% for sulfate, nitrate, and ammonium aerosols). Such a significant increase is attributed to the enhancement of atmospheric oxidants (OH, HO2, O3, NO3, Cl, and Br) by halogen chemistry, with a significant contribution from previously unconsidered bromine. These results show that higher recognition of the impact of anthropogenic halogens shall be given in haze pollution research and air quality regulation.

Nitrate aerosol formation and source assessment in winter at different regions in Northeast China

To fully identify the formation and source characteristics of aerosol nitrate during winter which accompanied by frequent haze in Northeast China, the nitrate concentrations, δ15N–NO3- and δ18O–NO3- characteristics of PM2.5 samples from four sites of different types including the (a) urban area, (b) suburban area, (c) suburban-rural transition area and (d) rural area in Northeast China were analyzed in this work. δ15N isotope fractionation (εN) during the conversion of atmospheric NOX (δ15N-NOX) to nitrate (δ15N–NO3-) was discussed on the basis of the estimated contributions of the gas phase oxidation pathway of OH radical (·OH) (γOH) and the heterogeneous hydrolysis pathway of N2O5 (γN2O5) to nitrate formation. Besides, the contribution of biological soil, coal combustion, biomass burning and motor vehicles to NOX emissions had also been analyzed using Bayesian isotope mixing model. Results showed that different types of sites had inconsistent source contributions. Obviously, coal combustion plays an absolutely indispensable role in the production of NOX in all the sites, especially in rural areas (d), where coal combustion contributed up to 89.8%. In addition, biomass burning still has a great effect on NOX emissions in the Northeast, showing a contribution of 5.2%–35.8%. It is worth noting that in urban site (a) where the tropospheric O3 concentration (34±9 μg m−3) was significantly lower than the other regions ((b): 54 ± 16 μg m−3, (c): 101 ± 21 μg m−3, (d): 98 ± 17 μg m−3), the contribution of the ·OH pathway to nitrate was as high as 69.4%, which was very different from the other three sites. The different formation mechanisms of nitrate in urban and non-urban may be due to different O3. Haze not only affects the sources of nitrate, but also changes the contribution of nitrate formation. Except for rural site (d), the contribution of the heterogeneous reaction (N2O5 pathway) of the other three sites during the haze period was greater (a: 35.1%, b: 60.0%, c: 59.0%, d: 60.8%) than in the non-haze period (a: 22.5%, b: 44.7%, c: 41.9%, d: 64.2%).

An investigation of aqueous ammonium nitrate aerosols with soft X-ray spectroscopy

Aqueous aerosols are important in atmospheric chemistry, drug delivery, and human health. X-ray photoelectron spectroscopy (XPS), a surface-sensitive technique and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy which informs on the bulk, is deployed to study the solvation of ammonium nitrate (NH4NO3). Aerosolised solutions of NH4NO3 were introduced into a photoelectron spectrometer via an aerodynamic lens, and interrogated with soft X-ray photons, the resulting electrons were imaged via velocity map imaging. Density functional theory calculations were performed to compare with the measured binding energies of NH4 + and NO3 − solvated in water. The results reveal that the nitrate anion has a slight propensity for the surface, while both ammonium and nitrate ions are present in equal measure in the bulk.

Assessment of the emission mitigation effect on the wintertime air quality in the Guanzhong Basin, China from 2013 to 2017

With the completion of the Air Pollution Prevention and Control Action Plan (Action Plan) in the Guanzhong Basin (GZB), the wintertime fine particulate matters (PM2.5) concentration has decreased by 50.0 μg m−3 (33.4%) from 2013 to 2017. Considering the significant impacts of meteorological conditions on particulate pollution, the effectiveness of anthropogenic emissions abatement on the PM2.5 reduction in the GZB remain unclear. In this study, the WRF-Chem model was used to quantitatively evaluate the effectiveness of emissions mitigation on the particulate pollution in the GZB in December from 2013 to 2017. The model typically showed an affordable consistence with observations and simulation in meteorological parameters, atmospheric pollutants, and aerosol components. Sensitivity results revealed that emissions reduction decreases PM2.5 concentrations by around 17% in the GZB, about half of the observed PM2.5 decreased in December from 2013 to 2017, showing significant role of meteorological conditions in modulating particulate pollution. Emissions mitigation also decreased SO2, CO and O3 concentrations by around 55%, 17%, and 7%, respectively, but increased NO2 concentrations by 27%. Additionally, the PM2.5 decrease in the GZB is mainly contributed by the reduction of sulfate and primary organic aerosols, caused by implementation of stringent SO2 mitigation measures and reduced coal use. However, the mitigation of elemental carbon, dust and nitrate aerosols are not significant, particularly with regarding to nitrate aerosols, which is influenced by increase of NO2 and decrease of sulfate. Specified emissions mitigation strategies need to be designed to further lower the PM2.5 level in the GZB.

Formation pathways and sources of size-segregated nitrate aerosols in a megacity identified by dual isotopes

Nitrate becomes a vital secondary component in atmospheric aerosols, especially in the urban areas with effective control of sulfur emissions. The elucidation of NO3− formation pathways and sources would be very helpful for accurately modeling and controlling the particle pollution. In this study, the dual isotopic compositions (δ15N and δ18O) of size-segregated NO3− aerosols were measured at an urban site of Shanghai in the winter of 2018 and the late summer of 2019. The formation mechanisms and sources of NO3− in five size-modes and different seasons were explored by combining analyses of dual isotopes, ion concentrations, air mass back trajectories and the Bayesian isotopic mixing model. Our results showed that the δ18O-NO3− values in summer (62.1 ± 12.8‰) were generally lower than those in winter (80.1 ± 9.9‰) with the lowest values in Aitken (sub–0.1 μm) and condensation modes (0.1–0.56 μm, 48.6 ± 9.8‰). This suggested that NO2 + ∙OH pathway probably played a significant role in the NO3− formation in summer (56.1 ± 4.8%) especially in Aitken and condensation modes, while N2O5 heterogeneous and NO3 + RH/RSC (hydrocarbons/reduced sulfur compounds) pathways could be critical in winter (55.0 ± 12.6% and 28.3 ± 15.2%, respectively). The isotopic fractionation may play a critical role in the size distribution of δ15N-NO3−. Based on the formation pathways of NO3− and calculated fractionation factors εN, the contribution of major NOx sources to NO3− aerosols in Shanghai was estimated. It was found that diesel engine exhaust contributed to the highest percentages (39.3% and 32.1%) of NO3− aerosols followed by gasoline engine exhaust (30.9%) and coal combustion (29.4%) in summer and winter, respectively. The contribution of coal combustion was almost doubled in winter compared to summer whereas soil emission decreased. Our results suggest that the impact of diesel engine exhaust on particle pollution in eastern Chinese megacities should be emphasized.

Formation Mechanisms and Source Apportionments of Airborne Nitrate Aerosols at a Himalayan-Tibetan Plateau Site: Insights from Nitrogen and Oxygen Isotopic Compositions.

Formation pathways and sources of atmosphere nitrate (NO3-) have attracted much attention as NO3- had detrimental effects on Earth's ecosystem and climate change. Here, we measured nitrogen (δ15N-NO3-) and oxygen (δ18O-NO3- and Δ17O-NO3-) isotope compositions in nitrate aerosols at the Qomolangma station (QOMS) over the Himalayan-Tibetan Plateau (HTP) to quantify the formation mechanisms and emission sources of nitrate at the background site. At QOMS, the enhanced NO3- concentrations were observed in the springtime. The average δ15N-NO3-, δ18O-NO3-, and Δ17O-NO3- values were 0.4 ± 4.9, 64.7 ± 11.5 and 27.6 ± 6.9‰, respectively. Seasonal variations of isotope ratios at QOMS can be explained by the different emissions and formation pathways to nitrate. The average fractions of NO2 + OH and N2O5 + H2O to nitrate production were estimated to be 43 and 52%, respectively, when the NO3 + hydrocarbon (HC)/dimethyl sulfide (DMS) (NO3 + HC/DMS) pathway was assumed to be 5%. Using stable isotope analysis in the R (SIAR) model, the relative contributions of biomass burning (BB), biogenic soil emission, traffic, and coal combustion to nitrate were estimated to be 28, 25, 24, and 23%, respectively, on yearly basis. By FLEXible PARTicle (FLEXPART) dispersion model, we highlighted that NOx from BB emission over South Asia that had undergone N2O5 + H2O processes enhanced the nitrate concentrations in the springtime over the HTP region.

Radiative Forcing of Nitrate Aerosols From 1975 to 2010 as Simulated by MOSAIC Module in CESM2‐MAM4

AbstractWe incorporate the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) module in the Community Earth System Model version 2 (CESM2) Community Atmosphere Model Version 6 with interactive chemistry (CAM6‐chem), and couple it with the four mode version of the Modal Aerosol Module (MAM4). The MOSAIC module is used to simulate the thermodynamics of the gas‐aerosol mass exchange, with a special focus on simulating nitrate aerosol. By comparing against ground and satellite observations, we found that the MOSAIC/MAM4 scheme performs reasonably well in simulating spatiotemporal distributions of aerosols, including nitrate aerosol. We conducted a series of model experiments with and without nitrate aerosols, and examined the radiative effect (RE) associated with nitrate aerosols in 1975, 2000, and 2010, and accessed the radiative forcing (RF) of nitrate aerosols between the present day and pre‐industrial periods. Comparing with the nitrate aerosol RE, we predicted relatively small RF of anthropogenic nitrate aerosol from aerosol‐radiation interactions (RFari: −0.014 W m−2) and large RF from aerosol‐cloud interactions (RFaci: −0.219 W m−2). Regional signatures of nitrate RE/RF are noticeable and important: for instance, very small changes in REari in Europe and USA, but 2.8–3 times increases in REari in India and China from 1975 to 2010, while REaci/RFaci in China is a warming effect due to the competing effect between sulfate and nitrate aerosols as cloud condensation nuclei.

Giant clam growth in the Gulf of Aqaba is accelerated compared to fossil populations.

The health of reef-building corals has declined due to climate change and pollution. However, less is known about whether giant clams, reef-dwelling bivalves with a photosymbiotic partnership similar to that found in reef-building corals, are also threatened by environmental degradation. To compare giant clam health against a prehistoric baseline, we collected fossil and modern Tridacna shells from the Gulf of Aqaba, Northern Red Sea. After calibrating daily/twice-daily growth lines from the outer shell layer, we determined that modern individuals of all three species (Tridacna maxima, T. squamosa and T. squamosina) grew faster than Holocene and Pleistocene specimens. Modern specimens also show median shell organic δ15N values 4.2‰ lower than fossil specimens, which we propose is most likely due to increased deposition of isotopically light nitrate aerosols in the modern era. Nitrate fertilization accelerates growth in cultured Tridacna, so nitrate aerosol deposition may contribute to faster growth in modern wild populations. Furthermore, colder winter temperatures and past summer monsoons may have depressed fossil giant clam growth. Giant clams can serve as sentinels of reef environmental change, both to determine their individual health and the health of the reefs they inhabit.

Important Role of NO3 Radical to Nitrate Formation Aloft in Urban Beijing: Insights from Triple Oxygen Isotopes Measured at the Tower.

Until now, there has been a lack of knowledge regarding the vertical profiles of nitrate formation in the urban boundary layer (BL) based on triple oxygen isotopes. Here, we conducted vertical measurements of the oxygen anomaly of nitrate (Δ17O-NO3-) on a 325 m meteorological tower in urban Beijing during the winter and summer. The simultaneous vertical measurements suggested different formation mechanisms of nitrate aerosols at ground level and 120 and 260 m in the winter due to the less efficient vertical mixing under stable atmospheric conditions. Particularly, different chemical processes of nitrate aerosols at the three heights were found between clean days and polluted days in the winter. On clean days, nocturnal chemistry (NO3 + HC and N2O5 uptake) contributed to nitrate production equally with OH/H2O + NO2 at ground level, while it dominated aloft (contributing 80% of nitrate production at 260 m), due to the higher aerosol liquid water content and O3 concentration there. On polluted days, nocturnal reactions dominated the formation of nitrate at the three heights. Particularly, the contribution of the OH/H2O + NO2 pathway to nitrate production increased from the ground level to 120 m might be attributed to the hydrolysis of NO2 to HONO and then further photolysis to OH radicals in the day. In contrast, the proportion of N2O5 + H2O decreased at 260 m, likely due to the low relative humidity aloft that inhibited the N2O5 hydrolysis reactions in the residual layer. Our results highlighted that the differences between meteorology and gaseous precursors could largely affect particulate nitrate formation at different heights within the polluted urban BL.