Stable nitrogen isotope analysis serves as a powerful tool for tracing the sources and transformation pathways of nitrogen-containing aerosols, which significantly influence atmospheric chemistry, climate change, and environmental processes. This review comprehensively synthesizes the application of stable nitrogen isotopes for the analysis of various forms of nitrogen in atmospheric aerosols, including ammonium, nitrate, and organic nitrogen, as well as atmospheric nitrous acid (HONO). We summarize key measurement techniques and analytical frameworks, with a particular emphasis on stable isotope mixing models-especially Bayesian methods-for source apportionment and the evaluation of isotope fractionation. The review delineates distinct stable nitrogen isotope signatures of major sources (e.g., fossil fuel combustion, agriculture, biomass burning) and elucidates the spatiotemporal variability of isotopic compositions driven by anthropogenic activities and natural processes. Despite advances in the stable isotope analysis of nitrogen-containing aerosols, several challenges remain, particularly concerning nitrogen isotope fractionation processes and the complexity of organic nitrogen species. Finally, we propose that future studies should refine the database of isotope characteristics from various sources, enhance the analytical precision of measurement techniques, and integrate multi-method approaches to better understand nitrogen cycles and mitigate the environmental impacts of nitrogen-containing aerosols.

While ultraviolet-driven photochemistry influences organic aerosols in typical indoor environments, the impact of high-energy medical X-raysdespite their greater energy and direct health relevanceremains unexplored in radiotherapy rooms. Here, we employed Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to characterize the organic molecular compositions of fine particles (PM2.5) in three radiotherapy rooms and adjacent waiting areas. Results revealed a 1.3-2.3-fold increase in molecular formulas under X-ray exposure, driven by fragmentation-oligomerization cycles. Diurnal oxygen-to-carbon (O/C) ratios varied from 0.31 to 0.35 in the daytime and from 0.53 to 0.61 at night in three radiotherapy rooms, which indicate a light-modulated oxidation. Critically, PAH precursors, dominated by CHO/CHON species with double bond equivalence (DBE) ≥ 10, were enriched by 1.31-2.83-fold in radiotherapy environments. These compounds correlated strongly with oxidative stress biomarkers, implying potential health risks. Mechanistically, fragmentation and oligomerization prevail, likely enhancing gas-phase oxidation and particle-phase dimerization. Our findings necessitate air purification targeting reactive organics in medical radiation facilities to mitigate exposure risks.

Marker components are critical for accurately identifying fine particle (PM2.5) sources. Current marker identification is subjective, and the widely adopted markers to distinguish sub-types of biomass burning (BB) emissions are impossible. Here, PM2.5 samples from the combustion of ten biomass types were collected, and 102 chemical components were analyzed. Results indicated that the mass fractions of K⁺, K, and levoglucosan (LEV) varied in 0.1

Abstract Primary emissions and secondary formation are potential sources of free amino acids (FAAs) in atmospheric aerosols, but their relative importance and governing mechanisms remain unclear. Leveraging COVID-19 lockdown as a natural experiment with reduced primary emissions and variable atmospheric oxidation, we investigated FAA sources and evolution in PM2.5 in Beijing. Neutral glycine consistently dominated, forming primarily via secondary aqueous-phase processes driven by relative humidity and aerosol liquid water content, especially in winter. In spring, the level of hydrophilic FAAs increased but acid–base chemistry enriched basic amino acids with sharp aerosol pH shifts, unlike pure plant-derived enhancement. Hydrophobic alanine, second to glycine, became dominant under enhanced primary dust/combustion events, matching glycine-to-alanine (Gly/Ala) spatial ratio decreases: city-to-rural with primary anthropogenic emissions and background with primary productivity. Our findings demonstrate that FAA profiles provide a useful indicator of both source shifts and aerosol physicochemical processing, offering critical insights into the atmospheric nitrogen cycle.

Understanding the composition, sources, and transformation of rainwater dissolved organic matter (DOM) is critical for elucidating surface-atmosphere material transport and associated ecosystem impacts. This three-year (2020-2022) Hangzhou study characterized anthropogenically influenced rainwater DOM via integrated spectroscopic and modeling approaches. Results revealed strong anthropogenic imprints, with mean dissolved organic carbon (DOC) of 2.9 mg C L-1 and secondary ions dominating rainwater chemistry. Post-pandemic economic recovery in 2022 amplified fossil-fuel-combustion signatures and sulfate dominance, alongside a doubled DOC deposition flux (1.0 g C m- (Iavorivska et al., 2016)). Summer DOC flux peaked at 0.32 g C m-2, driven by increased rainfall and secondary processes. Rainwater DOM featured optical indices FI, BIX, and HIX of 1.75 +/- 0.17, 1.09 +/- 0.31, and 1.32 +/- 0.80, indicating low humification and aging degrees with anthropogenic and biological contributions. Parallel Factor Analysis (PARAFAC) identified two humic-like substances (HULIS) and two protein-like substances (PRLIS) with distinct oxidation states. Intense oxidation drove summer peaks of highly oxidized HULIS/PRLIS, with the contribution of oxidized PRLIS increasing from 10% (2020-2021) to 26% (2022). Principal Component Analysis (PCA) emphasized strengthened fossil-fuel emissions and ozone oxidation impacts in 2022, while Positive Matrix Factorization (PMF) modeling confirmed secondary processes as dominant DOM sources, with ozone oxidation accounting for 60.7% in 2022, inducing molecular condensation and fluorescence spectrum redshift. These findings advance understanding of wet-deposition DOM in megacities, clarify the dual influences of anthropogenic activities and environmental processes, and provide quantitative constraints to inform air quality management strategies in anthropogenically perturbed regions.

Abstract Aqueous-phase reactions between carbonyls and reactive nitrogen species (e.g., ammonia/amine) are critical for forming secondary nitrogen-containing organic compounds (NOCs) and brown carbon (BrC). However, current atmospheric chemical paradigms center almost exclusively on low-molecular-weight carbonyls such as glyoxal, leaving the role of high-molecular-weight carbonyls (HMWCs, with 4 ≤ C ≤ 30) unexplored. Here, we utilize advanced nontargeted molecular fingerprinting (PFBHA derivatization and FT-ICR MS) to unveil an unexpected and highly diverse pool of 1,242 unique carbonyl formulas in cloud water. These HMWCs, predominantly distributed in the C8–C20 range, exhibit a disproportionately high signal intensity (up to ∼35%) despite representing only a small fraction (5.4–13.9%) of the total signal diversity. Using a mechanistic “precursor-product pair” framework, we demonstrate that these HMWCs contribute to nearly one-third of the total NOC diversity (including various light-absorbing N-heterocycles) via Maillard-like reactions. This contribution is strikingly comparable to that of well-studied low-molecular-weight dicarbonyls, i.e., glyoxal and methylglyoxal. We further show that this HMWC-driven pathway covaries with precursor availability and cloud water pH. These findings suggest that incorporating HMWC-mediated NOC formation into future atmospheric models may improve the representation of secondary BrC formation and its associated climate effects.

Organosulfur compounds are important constituents of atmospheric aerosols and have been extensively investigated through field observations, laboratory experiments, and atmospheric modeling. However, the mechanisms underlying their formation and atmospheric abundance remain incompletely understood. Ubiquitous in the atmosphere, micrometer-sized droplets provide unique interfacial reaction environments that may facilitate organosulfur formation. Here, we show that sulfite-derived inorganic sulfur species react with oxygenated volatile organic compounds in microdroplets to generate organosulfur compounds under catalyst-free conditions without external energy input. Theoretical calculations suggest that interfacial electric fields facilitate organosulfur formation by lowering reaction activation barriers. Furthermore, several organosulfur species identified in laboratory experiments were also detected in ambient aerosols collected from an urban site and a high-altitude mountain station, supporting the potential atmospheric relevance of this pathway. Our findings provide insights into the role of microdroplet interface in organosulfur formation and their potential contribution to atmospheric aerosol chemistry. Atmospheric microdroplets facilitate organosulfur formation from inorganic sulfur and oxygenated volatile organic compounds, revealing their potential role in atmospheric aerosol chemistry.

The high electric field at the microdroplet interface plays a crucial role in regulating aqueous aerosol chemistry. However, the impact of charge at the air-water interface on sulfur-containing aerosols remains unclear. Here we show, using various microdroplet systems, how interfacial reactions of sulfur-containing aerosols proceed under different charge conditions. Field observations were also conducted across multiple regions to characterize the molecular composition of aqueous sulfur-containing aerosols. In small negatively charged droplets, sulfite is spontaneously oxidized to sulfate, with an estimated rate comparable to reported rates of sulfite oxidation by common atmospheric oxidants. Meanwhile, sulfur radicals like sulfate radicals are generated, leading to the rapid formation of organosulfates. In large positively charged droplets, sulfate is spontaneously reduced to sulfite radicals and sulfite, promoting the formation of organosulfonates. These results provide key insights into how charge at the air–water interface affects sulfur aerosols and contribute to our understanding of air pollution. Interfacial charge regulates sulfur redox chemistry in aqueous aerosols, with negative charge promoting sulfite oxidation and positive charge driving sulfate reduction, according to microdroplet experiments. Primary Handling Editors: Theodora Nah and Alice Drinkwater.

Abstract Understanding the sources of emerging PM 2.5 pollution is crucial for developing effective air quality management strategies. This study combines positive matrix factorization (PMF) with random forest (RF) classification to reveal a detailed PM 2.5 source identification for both day and night samples collected in Tianjin in 2022, during winter (including the Beijing Olympics period) and summer. When resolving the overlap of organic compounds between combustion and collinear sources, this model achieved accuracy, precision, recall, and F1 scores in a range from 85% to 91% on the independent test data set. Additionally, scenario simulations are applied to investigate the impacts from air pollution control strategies and large‐scale events on different emission sources. This methodology demonstrates the potential of combining receptor models, machine learning, and chemical analysis to identify overlapping air pollution sources, when samples are limited and conventional tracers are not available for PMF. In general, our results enhance the discrimination of the primary contributors to emerging air pollution from both traditional energy and sources, which can further support more flexible and season‐specific pollution control policies.

Bioaerosols constitute a critical component of atmospheric aerosols, exerting profound impacts on climate, ecosystems, and public health. Laser-induced fluorescence (LIF) techniques allow real-time detection of fluorescent aerosol particles (FAPs), which is one of the most commonly used online bioaerosol monitoring approaches, although certain non-biological fluorescent particles interfere. Current studies on the concentration, size distribution, and sources of bioaerosols in specific environments, such as agro-ecosystems, remain limited, hindering accurate assessment of their environmental effects. In this study, a LIF-based Wideband Integrated Bioaerosol Sensor (WIBS-4A) was deployed in a typical agricultural region of the North China Plain (NCP) during autumn and winter. Results showed the number concentration (650 +/- 566 L-1) and the fraction in total particles (16 +/- 12%) of FAPs in autumn were significantly higher than those in winter (470 +/- 850 L-1, 9.6 +/- 8.4%), with substantial temporal variability. Autumn FAPs were dominated by the coarser mode (peaking at 1-3 mu m), likely associated with pollen and fungal spores, whereas winter FAPs prevailed in the finer mode (peaking at similar to 1 mu m), suggesting contributions from mixed sources, including combustion-related FAPs and possibly some small biological particles (e.g., bacteria). Diurnally, FAPs peaked in the early morning and late afternoon, probably driven by biological rhythms and planetary boundary layer dynamics. Moderate relative humidity (60%-90%) may favor biological aerosol release and was associated with enhanced FAPs, whereas high RH (>90%) promoted wet scavenging of FAPs. Source apportionment based on positive matrix factorization analysis revealed clear seasonal contrasts in sources. Autumn FAPs were primarily associated with biological emissions, with additional contributions from biomass burning and soil/dust, whereas winter FAPs were dominated by combustion-related sources, accompanied by biological sources and soil/dust. This study elucidates the dynamics and sources of bioaerosols in a typical agro-ecosystem in the NCP, providing foundational data for modeling regional atmospheric biogeochemical cycles and assessing health and climate effects.

Abstract. Biogenic volatile organic compounds (BVOCs) are important precursors of secondary organic aerosols and tropospheric ozone, yet their emissions are commonly modelled at the plant functional type (PFT) level, which obscures substantial interspecies variability. This limitation is particularly acute for China, where high forest diversity and ambitious afforestation initiatives are expected to reshape national BVOC budgets. Here, we present a species-explicit BVOC emissions inventory for China covering 234 tree species (https://doi.org/10.5281/zenodo.20396128, Liu et al., 2026). The inventory was developed within the Model of Emissions of Gases and Aerosols from Nature (MEGAN) v3.2 modeling framework, with refined inputs comprising species-specific emission factors and high-resolution forest composition data for China. Total forest BVOC emissions were estimated at 10.26 Tg in 2019, with summer emissions accounting for about 50 % of the annual total and a clear decreasing gradient from southern to northern China. Emissions were highly concentrated among a limited number of species: the five largest contributors, Pinus massoniana, Quercus liaotungensis, Phyllostachys edulis, Cunninghamia lanceolata, and Quercus variabilis accounted for 41.7 % of total emissions while occupying only 25.4 % of forest area. At the compound-class level, Quercus liaotungensis dominated isoprene emissions, whereas Pinus massoniana was the leading contributor to monoterpene emissions. Applying this species-explicit framework to two future afforestation scenarios with identical planted areas but contrasting species composition, we found that BVOC emissions may increase by 4.65 Tg yr⁻¹ with the biomass-maximization tree species and by 5.10 Tg yr⁻¹ under the most environmental-suitability species. The dominant compound class and species contributors differed markedly between scenarios, indicating that afforestation-driven BVOC responses depend strongly on species selection. These results demonstrate the importance of incorporating species-specific emission traits BVOC models and suggest that future afforestation strategies could substantially reshape both the magnitude and chemical composition of biogenic emissions, with implications for atmospheric chemistry and air quality.

Brown carbon (BrC) impacts global climate through solar radiation absorption, yet its optical effects (especially those of water-insoluble fractions) and molecular links between chromophores and sources remain poorly constrained, hindering climate assessments. To address these gaps, this study systematically investigated BrC's optical properties, molecular compositions (focusing on nitroaromatic compounds, NACs) and sources in Nanjing, East China, across four seasons (November 2014-July 2015), by comparing water-soluble (WS-BrC) and methanol-soluble BrC (MS-BrC, total BrC proxy). MS-BrC exhibited 2.1-fold higher average light absorption (11.9 ± 7.7 Mm-1) than WS-BrC (5.8 ± 3.8 Mm-1), with its direct radiative effect relative to black carbon (10.5 ± 3.9 %) being 2.5 times that of WS-BrC (4.8 ± 2.6 %), highlighting the long-overlooked climate significance of water-insoluble BrC. Nine NAC species (9.1 ± 7.9 ng m-3) contributed 0.33 ± 0.14 % to BrC absorption-2.4-17 times their mass contribution, with distinct seasonal shifts: nitrocatechols (NCs) dominated winter/autumn, whereas nitrosalicylic acids (NSAs) prevailed spring/summer. A NAC-constrained Positive Matrix Factorization (PMF) model identified distinct seasonal BrC sources. For MS-BrC, dominant sources were multiphase chemical processes (27.0 %) and biomass burning (25.0 %); for WS-BrC, biomass burning (28.4 %), and photochemical processes (25.2 %) prevailed. Combined secondary formation contributed ∼47 % to BrC light absorption, higher than in northwest China. During aerosol pollution, BrC light absorption and radiative effects intensified, with biomass burning contributions rising from 21.6 % to 36.6 %. These findings advance understanding of water-insoluble BrC's climate role, refine BrC source apportionment via molecular tracers, and offer a robust basis for climate models and targeted air pollution control.

Reforestation and afforestation alter climate not only through biogeophysical processes such as changes in surface albedo, evapotranspiration and near-surface turbulence, but also by modifying emissions of biogenic volatile organic compounds (BVOCs) that drive biogenic secondary organic aerosol (BSOA) formation. Using an Earth system model coupled with an advanced aerosol module, we quantify how biogeophysical feedback from vegetation change influences BVOC emissions, BSOA burden and aerosol radiative effects under future land‑use scenarios. Our results reveal that biogeophysical feedback either amplifies or offsets BSOA cooling, depending on regional climate-vegetation interactions. In regions where reduced surface albedo dominates, increasing temperature and BVOC emissions enhance BSOA burden and its radiative cooling. Conversely, in regions where updrafts and cloud formation are enhanced, reduced surface radiation suppresses BVOC emissions and offsets BSOA increases from vegetation changes alone. Globally, these types of feedback amplify BVOC emission changes in 52% of reforested areas but suppress them elsewhere, intensifying spatial heterogeneity in aerosol climate effects. These divergent feedback pathways introduce strong spatial heterogeneity and non-linearity into the BSOA-climate response. Incorporating such biogeophysical modulation of BSOAs is essential for designing reforestation strategies that maximize climate mitigation benefits.

Cropland expansion has been the most notable change in global land use since industrialization. However, assessments of radiative forcing from land-use change have generally neglected the effects of cropland expansion on secondary organic aerosol. Here we perform a series of cropland expansion sensitivity experiments with an Earth system model that incorporates advanced secondary organic aerosol processes, including organic new particle formation. Our model results show an 10

The interactions of metabolically active atmospheric microorganisms with cloud organic matter can alter the atmospheric carbon cycle. Upon deposition, atmospheric microorganisms can influence microbial communities in surface Earth systems. However, the metabolic activities of cultivable atmospheric microorganisms in settled habitats remain less understood. Here, we cultured typical bacterial and fungal species isolated from the urban atmosphere using tryptic soy broth (TSB) and Sabouraud dextrose broth (SDB), respectively, and investigated their exometabolites to elucidate their potential roles in biogeochemical cycles. Molecular compositions of exometabolites were analyzed using ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry. Annotation through the Kyoto Encyclopedia of Genes and Genomes database helped identify metabolic processes. Results showed that bacterial and fungal strains produced exometabolites with lower H / C and higher O / C ratios compared with both consumed and resistant compounds. As CHON compounds are abundant in both TSB (85 %) and SDB (78 %), CHON compounds also constituted over 50 % of the identified exometabolite formulas. Bacterial strains produced more abundant CHONS compounds (25.2 %), while fungal exometabolites were rich in CHO compounds (31.7 %). These microbial exometabolites predominantly comprised aliphatic/peptide-like and carboxyl-rich alicyclic molecule (CRAM)-like compounds. Significant variations in metabolites were observed among different microbial strains. Bacteria exhibited proficiency in amino acid synthesis, while fungi were actively involved in amino acid metabolism, transcription, and expression processes. Lipid metabolism, amino acid metabolism, and carbohydrate metabolism varied widely among bacterial strains, while fungi exhibited notable differences in carbohydrate metabolism and secondary metabolism. This study provides new insights into the transformation and potential oxidative capacity of atmospheric microorganisms concerning organic matter at air–land/water interfaces. These findings are pivotal for assessing the biogeochemical impacts of atmospheric microorganisms in clouds or following their deposition.

Brown carbon (BrC) is a type of light‐absorbing organic carbon and its structural characteristics have significant effects on atmospheric radiative forcing and global climate change. In this work, the main absorbing components of BrC were separated into three fractions and each fraction was analyzed individually and systematically. The compositional and structural characteristics of BrC across different wavelengths were studied, and the potential BrC compounds were revealed. Overall, nearly 100 light‐absorbing compounds were identified. The result showed that aromatic and heterocyclic compounds were main contributors to BrC absorption in the wavelength from 220 to 450 nm, which play an important role in radiative forcing. Interestingly, carbonyl groups dominated the BrC absorption in the wavelength from 200 to 220 nm. Although BrC compounds have minimal direct radiative impact due to limited solar radiation at such a wavelength, it can still play important roles in atmospheric photochemistry by participating in light‐induced oxidation process and lead to severe photochemical pollution. Additionally, carboxyl‐rich alicyclic molecules and lignin‐like substances all promoted the absorptivity of BrC, and nitrogenous substituents further enhanced such a process. These results were further extrapolated to various environmental conditions, which indicated that urban BrC probably contained more carbonyl groups. Conversely, more heterocyclic substances were observed in the forest and ocean areas, which illustrated high burden of radiative forcing in these areas. Overall, our work reveals that the BrC absorption is largely dependent on their molecule properties in different wavelengths, and such a correlation can guide and facilitate the BrC absorption analysis.

Emissions of shipping have great influences on atmospheric environment and global climate with the growth of maritime trade. Previous studies identified that shipping black carbon (BC) constitutes the main aerosol component that responsible for light absorption, while shipping organic carbon (OC) was generally considered non-absorbing. Recent studies have indicated that organic components derived from shipping emissions exhibit light absorption at short wavelengths (brown carbon, BrC). However, there is a lack of quantification regarding the absorption effects of shipping BrC. This study investigates the radiative absorption effect (RAE) of shipping BrC, updating models with measured shipping BrC light absorption ability. Surface concentrations of shipping BrC contribute ~ 40% to fossil fuel OC. The global annual average RAE of shipping BrC is +0.15 to +0.36 mW m⁻ 2 , peaking in boreal summer. The warming effect of shipping BrC is 8%–19% relative to shipping BC. With the low-sulfur standard, the RAE of shipping BrC increases to +0.52 mW m⁻ 2 , which is close to that of shipping BC (+0.52 mW m⁻ 2 ) and could offset ~ 40% of direct cooling effect of shipping sulfate. For the radiative absorption of shipping aerosols, BrC is an important part, whose contribution is greater with the upgrade of ship fuels. Graphical Abstract

Nitrogen-containing organic compounds (NOCs) threaten air quality and public health by contributing to secondary organic aerosol (SOA) formation and atmospheric toxins. However, despite their importance, NOCs present in vehicle emissions remain poorly understood. To bridge this knowledge gap, this study deployed ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in positive electrospray ionization (ESI+) mode to characterize the molecular composition and dynamic evolution of NOCs in traffic-related aerosols from the entrance and exit of a tunnel in Yichang, China. Two main NOCs groups, CHN+ (13.3-22.6 %) and CHON+ (44.1-55.2 %), were identified. CHN+ compounds, mainly aromatic nitrogen-containing heterocyclics, were light-absorbing and might significantly contribute to SOA. CHON+ species showed high reactivity, with over 70 % undergoing oxidation (42-45 %) or hydrolysis (14-15 %) or both (14-21 %) within the tunnel. After release, they evolved via methylation or hydroxylation, generating high-molecular-weight oxidized derivatives. Moreover, tunnel-specific transformations included dealkylation, oxygenation, and reactions with amines, nitro, and nitroso groups, without decarboxylation due to limited hydroxyl radicals in the dark environments. These findings highlight vehicle emissions as a major NOCs source and provide molecular-level evidence for integrating vehicular NOCs into air quality models and emission control strategies, addressing their impacts on public health and the global nitrogen cycle.

To elucidate the origin and seasonality of atmospheric aerosols in forest areas, simultaneous PM2.5 collection was carried out in two typical forest sites: Changbai Mountain (CB, 42.40° N, 128.11° E), North China and Xishuangbanna (BN, 22.25° N, 100.89° E), South China, at day and night during the summer and winter periods of 2023–2024. Carbonaceous and nitrogenous components, water-soluble inorganic ions (WSII) and stable carbon isotopic composition of total carbon (δ13CTC) were measured in PM2.5. Generally, the contents of carbonaceous and nitrogenous components were higher in winter than in summer, with secondary organic carbon (SOC) and water-soluble organic carbon (WSOC) being higher in daytime than that in nighttime at both CB and BN. The average concentrations of WSII in total samples were 5.36 and 2.23 µg m−3 at CB and BN, respectively. SO42-, NO3- and NH4+ were dominant at CB, while SO42-, NH4+ and Na+ were dominant at BN, which accounted for 86 % and 89 % to the total ions, respectively. δ13CTC ranged from −27.8 ‰ to −22.1 ‰ at CB, while −27.6 ‰ to −24.5 ‰ at BN. Besides biogenic emissions, the emissions from biomass burning and terrestrial and/or marine organisms were major sources of aerosols at both sites. Furthermore, fossil fuel combustion contributed more significantly at CB than at BN in winter. This study sheds better light on the seasonality in chemical composition and origins of PM2.5 in forest areas in North and South China.