In Europe, heatwave conditions associated with southern synoptic flows can sometimes be combined with episodes of dust plumes coming from the Saharan desert. Change in the atmospheric composition of aerosols alters the radiation observed near the ground, resulting in impacts on other variables such as air temperature. This study focuses on the heatwave experienced by the Paris region from 15 to 19 June 2022, combined with the passage of a Saharan dust plume. To investigate the aerosols impact on local weather conditions and thermal comfort, three Meso-NH atmospheric numerical simulations are used: one without dust aerosol (C0), one with dust aerosols defined from CAMS reanalysis (C1), and one with twice the concentration of CAMS dust aerosols (C2). Simulation C1 is validated against observations from the PANAME-Urban experimental campaign. The time evolution of Aerosol Optical Depth and incoming solar radiation at the surface is well reproduced, with improvement in the resulting air temperature and boundary layer height when taking aerosols into consideration. The radiative effect of dust aerosols results in a decrease in incoming solar radiation and air temperature of up to 75 W m−2 and 1 °C, respectively. At 16:00 UTC, it results in a thermal comfort improvement of up to 1 °C in sunny urban and suburban areas. However, in shaded suburban areas, the increase in diffuse solar radiation and humidity and the decrease in wind speed induced by the dust aerosols counterbalance the air temperature decrease, resulting in no thermal comfort improvement. This result highlights the need to integrate these insights into operational heat-health warning systems – by coupling aerosol forecasts with bioclimatic indices.
Biogenic volatile organic compounds (BVOCs) substantially influence regional photochemical air pollution, climate, and the carbon cycle. However, observational constraints on BVOC emissions from urban or suburban forests in Asian megacity regions under humid subtropical climates remain limited. In this study, we conducted multi-year intermittent, multi-height BVOC observations at a 30 m flux tower in a suburban Tokyo forest dominated by Quercus serrata. Spatial variability was examined by combining tower measurements with supplemental drone-based sampling. The measurements were compared with estimates from the Model of Emissions of Gases and Aerosols from Nature (MEGAN). Isoprene volume mixing ratios increased during the warm-season observations from May to October, accounting for over 90 % of the measured BVOC composition during peak summer, while monoterpenes remained low with weak vertical gradients. Isoprene exhibited distinct vertical volume mixing ratio gradients peaking within the canopy, with the daily average emission fluxes ranging from −0.05 to 15.30 mgm-2h-1. Drone-based measurements indicated horizontal variability in isoprene volume mixing ratios of approximately 10 %–30 % within 30 m of the tower. Flux estimates derived from tower and drone measurements differed by approximately 30 %, suggesting that small-scale spatial heterogeneity and height differences can affect gradient-based flux estimates. MEGAN generally overestimated the observed fluxes, particularly during the warm-season observations. These results demonstrate the potential of combining tower-based vertical profiling with supplemental drone-based horizontal sampling to evaluate BVOC fluxes and their spatial representativeness. Although the intermittent sampling design limits comprehensive seasonal and interannual interpretations, this study provides methodological insights for future tower- and drone-based BVOC observations and emission model evaluation based on canopy-scale measurements.
Ice-nucleating particles (INPs) play a crucial role in Earth's weather and climate by influencing cloud properties and precipitation. However, their abundance in the free troposphere, vertical distribution, and transport mechanisms are not well-characterized. This study presents immersion INP measurements from filter samples collected aboard the High Altitude and LOng range research aircraft (HALO) in the troposphere and the lower stratosphere (up to 14.5 km) over Europe during the CIRRUS-HL (Cirrus in High Latitudes) campaign in summer 2021. By sampling cloud particle residuals and aerosol particles in the inflow and outflow of deep convective clouds (DCCs), and performing offline INP analysis, we shed light on the vertical transport and segregation of INPs in DCCs. INP-temperature spectra of convective inflow included both INPs active at high (T > −15 °C) and low temperatures (T < −20 °C). In contrast the outflow spectra only featured INPs active at low temperatures. We explain the observed INP segregation in the updraft with precipitation scavenging of INPs active at high temperatures. INPs active at lower temperatures (T < −20 °C), however, are efficiently transported upwards into the free troposphere. There, ambient temperatures are below −40 °C, i.e., temperatures far below the temperatures at which these INPs initiate immersion freezing. In the DCC outflow, INP concentrations exceed the upper tropospheric background concentration by at least two orders of magnitude. These INPs are then available for ice formation in mid and upper tropospheric clouds.
Ice cloud microphysical properties, such as phase, shape, size and density, introduce significant uncertainties and biases that affect our understanding of how clouds influence weather and climate. The upcoming spaceborne sub-millimeter radiometers are expected to help in reducing the ice microphysics induced uncertainties in observations. Yet our knowledge about ice microphysics in this spectrum is still limited, and comparison/validation work is still sparse. This paper delivers a comprehensive cross-instrument closure study using active, passive remote sensing and in-situ cloud probe measurements collected during the NASA's IMPACTS field campaign. Through combined use of two radars, one lidar and in-situ cloud probe measurements, a comprehensive best reference “truth” is generated, which is then used to validate collocated sub-mm CoSSIR radiance measurements. We found out that only through a realistic vertical hydrometeor type classification that all types of measurements can reach a closure with minimal discrepancies between simulations and observations, which is critical for generating high-quality retrievals for frozen hydrometeors. We further present a comprehensive exploration of the scientific merit of polarimetric measurements in improving frozen hydrometeor microphysics retrievals. For the first time, we can validate previous theoretical predictions that sub-mm polarimetric signals can be used to retrieve ice particle habit and size. Moreover, we find it is possible to differentiate detailed vertical structure of hydrometeor types using both polarimetric and radiance measurements. This paper paves concrete steps in assuring timely delivery of high-quality science products for the sub-mm radiometer missions as well as possibilities of new science products beyond the mission requirements.
Recent studies suggest that significant aerosol formation occurs in the tropical upper troposphere (UT). However, the impact of these particles at lower levels remains poorly understood. We present results from multi-year global EMAC simulations investigating the downward transport of UT tracers and their resulting spatial distribution. Nineteen idealized tracers were released in the tropical UT and subjected to resolved-scale advection, parameterized convection, turbulent mixing, and wet/dry deposition. Transport diagnostics are highly sensitive to source extent: the age of air at 500 hPa is ≈ 45 d for tropical-wide tracers, compared to over 250 d for regional continental sources, reflecting the importance of mixing, dilution, and source-receptor geometry. A complementary time-to-threshold diagnostic reveals faster transport pathways, with all source regions exhibiting descent times shorter than 7 d to reach 10 % of the source average. Advection dominates vertical transport, with convective and vertical diffusion parameterizations contributing marginally. Injection height exerts a stronger influence on descent time than parameterized transport or particle size in the 20–100 nm range. Tracer maxima are typically advected east of their source centers, resulting in significant concentrations (≈ 10 %–15 % of source values) in the mid-troposphere. Offline calculations show that, for initial particle numbers below ≈ 6×107 kg−1, the mid-troposphere values predicted by the model are reduced by less than 10 % when coagulation is considered, but substantial deviations occur at higher concentrations. These results provide quantitative constraints on particle transport efficiency and inform expectations for aerosol distributions following UT nucleation events.
Sulfates are critical constituents of atmospheric fine particulate matter (PM2.5) that significantly influence air quality and climate dynamics. While stable isotope fractionation analysis is a powerful tool for tracing atmospheric sulfate formation mechanisms, conventional isotopic models rely on idealized complete SO2 oxidation scenarios. This oversimplification introduces systematic errors and neglects vital kinetic isotope effects generated during incomplete SO2 processing. To address this knowledge gap, we established a field-validated analytical framework combining seasonal PM2.5 observations in Nanjing, China, with Bayesian isotope mixing and process-specific Rayleigh fractionation modeling. Our kinetic fractionation-corrected approach accounts for actual atmospheric oxidation processes, revealing that transition-metal ion (TMI)-catalyzed and NO2-mediated pathways dominate secondary sulfate production. Conversely, comparative analysis demonstrates that traditional complete-oxidation models disproportionately diminish TMI pathway contributions. Furthermore, implementing kinetic fractionation corrections successfully resolves systematic biases in source apportionment. We demonstrate that traditional models misrepresent source contributions, overestimating coal combustion by 10 % and underestimating traffic emissions by 8 % during summer photochemical episodes. These findings establish a refined isotopic tracing framework that resolves long-standing calculation discrepancies. Ultimately, this framework delivers essential constraints for atmospheric sulfur cycle modeling and underscores the necessity for multi-pollutant regulation strategies targeting vehicular emissions and co-emitted transition meals.
Nitrosamines are carcinogenic nitrogen-containing atmospheric pollutants that are widely detected in particulate matter. However, their formation mechanisms remain poorly understood. While gas-phase and bulk aqueous pathways have been extensively investigated, the role of heterogeneous interfacial chemistry remains largely unexplored. Herein, we elucidate the molecular mechanisms underlying heterogeneous nitrosamine formation via amine-mediated reactions with dinitrogen tetroxide (N2O4) at the air–water interface using Born–Oppenheimer molecular dynamics and metadynamics simulations. These reactions proceed through two distinct and competing pathways: (i) a kinetically favored, barrierless N-nitrosation pathway in which N2O4 directly reacts with methylamine (MA) or dimethylamine (DMA), yielding nitrosamine cations and nitrate ions (NO3-); and (ii) a MA/DMA-mediated hydrolysis pathway of N2O4 that rapidly generates interfacial HONO (∼ 2–16 ps), providing a potential secondary source of nitrosamines via subsequent HONO-mediated nitrosation with a free-energy barrier of 7.65 kcal mol−1 at 300 K. These findings reveal amine-mediated interfacial chemistry as an important heterogeneous pathway distinct from bulk processes, providing molecular insights into urban reactive nitrogen cycling and improving atmospheric chemical transport models.
Microplastics (MPs) are environmental contaminants of global concern. Although the relevance of the atmosphere in the transport and distribution of MPs worldwide has been acknowledged, country-scale quantitative data on wet and dry MP deposition rates remain limited. We therefore quantified MPs in wet and dry atmospheric deposition samples collected on a four-weekly basis over a one-year period between May 2024 and May 2025 at one urban (Zurich), one suburban (Duebendorf), two rural (Magadino and Payerne) and one mountainous site (Chaumont) in Switzerland. We used focal plane array μ-Fourier transform infrared spectroscopy to identify MPs in the 20–215 µm size range and included a rigorous assessment of the measurement uncertainties. Particle sizes were converted into masses to obtain mass deposition rates. The number- and mass-based MP deposition rates were highest at the urban site, with respective means of 881 MPs m−2 d−1 [95 % confidence interval (CI): 562–1199] and 53 µg m−2 d−1 [CI: 17–107]. The deposition rates were lower and similar among the remaining sites, ranging from 249 to 331 MPs m−2 d−1 [CI: 140–478] and from 13 to 21 µg m−2 d−1 [CI: 4–46]. Based on the determined deposition rates and land-use statistics, an annual deposition of 219 t or 3.8×1014 particles was estimated for MPs of the analyzed 20–215 µm size fraction excluding tire wear particles, in regions <2000 m above sea level across Switzerland. Corresponding annual atmospheric inputs of MPs to Swiss agricultural land and surface waters were estimated at 78 and 10 t, respectively.
Atmospheric ice-nucleating particles (INPs) are vital for cloud formation, yet the importance of INPs from anthropogenic sources remains poorly understood. We conducted a month-long winter field campaign in Taiyuan (China), a heavily industrialized city, to quantify INP concentrations (NINP) and ice nucleation active site density (ns) of immersion mode INPs, alongside particle size distributions and chemical compositions. Our results indicate that NINP ranged from 0.05 to 13.37 L−1 at and above −15 °C, corresponding to ns values of 105–107 m−2. During the identified desert dust event, both NINP (7.47 L−1; 95 % confidence interval (CI): 6.64–8.41 L−1) and ns (1.57×107 m−2) were substantially higher than during the remaining observation periods without clear desert-dust intrusion (0.61 L−1 and 9.52×105 m−2, respectively), highlighting the strong influence of long-range transported desert dust. In contrast, during pollution periods, NINP showed only weak correlations with urban aerosol components such as sulfate (SO42-), nitrate (NO3-), and organic carbon (OC) (|r|<0.3). Positive matrix factorization (PMF) identified five PM2.5 source factors: industrial emissions, dust-related particles, secondary aerosols, coal combustion and traffic emissions, and fireworks. Although these factors dominated the PM2.5 mass, they showed no significant covariation with NINP. During this winter campaign, NINP variability in Taiyuan was characterized by strong enhancement during the identified desert dust event and weak associations with the major PM2.5 source factors during the remaining observation periods.
Radiatively driven Arctic stratocumulus clouds have important climactic impacts due to their effects on surface radiative balance. The presence of both liquid and ice within Arctic stratocumulus, and their interaction through the Wegener-Bergeron-Findeisen (WBF) process, strongly affects the properties and lifetimes of these clouds. To assess the impacts of mixed-phase microphysical processes in Arctic Stratocumulus, we use a Langrangian cloud microphysical model within a large eddy simulation framework to simulate a single-layer cloud under varying free-tropospheric humidity and above-cloud inversion strength. We also run two simulations in which precipitating ice crystals have their ice nucleating particles (INP) re-injected into the model domain, rather than removed. We find that INP recycling plays a critical role in maintaining the presence of ice in the mixed-phase cloud. The simulations with drier free-tropospheric air experience greater sublimation of ice crystals below cloud, recycling of ice crystals, and a higher ice water path than simulations with more humid free-tropospheric air. We also find that the impact of inversion strength on cloud microphysical characteristics is strongly modulated by free-tropospheric relative humidity, with decreased inversion strength resulting in both increased and decreased liquid water path under high and low free-tropospheric relative humidity, respectively.
Urban emissions impact aerosol–cloud interactions and thereby modify precipitation patterns, yet whether realistic emission perturbations from a mid-sized European city produce detectable effects above internal meteorological variability in a high-background aerosol environment remains an open question. This study investigates the influence of urban aerosol fields on convective precipitation through explicit chemistry-cloud coupling, using a trajectory-based ensemble approach designed to isolate weak aerosol signals from natural variability. Using the coupled COSMO-DCEP-MUSCAT modeling system, we simulate two convective events passing over the city of Leipzig, Germany, with experiments comparing total emissions to zero urban emissions, with five ensemble members for each setting. Cloud droplet activation is calculated from prognostic three-dimensional aerosol fields, providing a physically consistent representation of aerosol–cloud interactions. We use backward trajectory analysis to directly trace air volumes carrying urban emissions from convective clouds back to the region of urban emission sources, enabling objective sampling of individual clouds and isolation of local emission effects. The results reveal case-dependent responses. Under moderate atmospheric instability, urban aerosols locally modify the cloud microphysics and precipitation without altering the overall structure of the convective event. Under stronger initial instability, the urban emissions intensify the precipitation, leading to stronger downdrafts and weaker updrafts, altering the convective event's evolution compared to the zero urban emission scenario. Ensemble analysis demonstrates that emission-induced changes are comparable in amount to internal variability, highlighting the need for multiple realizations and significance testing, and that domain-mean surface precipitation remains within the ensemble spread despite detectable microphysical responses and spatial redistribution.
Biogenic volatile methylated sulfur (VMS) gases, dimethyl sulfide (DMS) and methanethiol (MeSH), are major precursors of climate-cooling sulfate aerosol, yet their sources, co-emission and fate remain poorly constrained over the Southern Ocean and Antarctica. In this study, we combine atmospheric VMS measurements with biogeochemical and meteorological observations from an austral summer Southern Ocean voyage to examine the drivers of atmospheric VMS concentrations across contrasting ocean-atmosphere regimes. At the Antarctic Ice Edge (62–67° S), DMS dominated the VMS burden (up to 5.7 ppb) with episodic coastal polynya and shelf biological hotspots demonstrating very low MeSH : DMS (0.3 %–4 %). In this regime, DMS variability was strongly related to recent air mass exposure to high surface chlorophyll a (R2 = 0.49), whereas MeSH showed little dependence (R2 < 0.14), consistent with stronger heterotrophic control on MeSH production. Over the open ocean (32–62° S), DMS and MeSH were tightly coupled (R2 = 0.80) with higher MeSH (up to 250 ppt) and MeSH : DMS (∼ 15 %), but chlorophyll a explained little of the variability (R2 = 0.15); instead, physical ocean structure and boundary-layer conditions influenced VMS variability. DMS and DMSO co-varied at the Antarctic Ice-Edge (R2 = 0.69), indicating rapid oxidation via addition reactions with hydroxyl and bromine monoxide radicals at the surface. Overall, our results support developing coupled VMS–biogeochemical parameterisations to better capture aerosol–cloud representation in Southern Ocean climate models, and revising DMS-based parameterisations of MeSH at the Antarctic Ice-Edge, which currently appear to overestimate MeSH contributions to the VMS burden under these conditions.
Cloud droplet number concentration (Nd) in warm liquid clouds plays a crucial role in understanding cloud microphysical processes and the influence of aerosol–cloud interactions (ACI) on Earth's climate. Nd from satellite-retrieved cloud properties such as the cloud optical thickness (τ) and cloud droplet effective radius (re) can be biased due to the three-dimensional (3D) radiative transfer (RT) effects. Using Large-Eddy Simulation (LES) cloud fields and RT simulations, this study investigates how biases in cloud property retrievals caused by 3D-RT effects impact the derived Nd and subsequent regression-based albedo-susceptibility estimates. Our sensitivity studies confirm that the bi-spectral retrievals using the 3.7 µm channel – whose re retrieval is closest to cloud top – shows better agreement with Nd from our LES models, compared to results based on the 1.6 and 2.1 µm retrievals. At native LES resolution, Nd across all absorbing channels is strongly impacted by the 3D-effects, with the magnitude depending on the solar zenith angles (SZAs); on average, for high/low sun conditions Nd under 1D-RT overestimates/underestimate its 3D-RT counterpart, which indicates dominant darkening/brightening effects. At coarser satellite-like resolutions, average statistics between 1D and 3D retrievals agree better, indicating compensation between 3D and plane-parallel averaging assumptions. Furthermore, our regression-based albedo-susceptibility results evaluated at the top of cloud domain show that 3D RT greatly modifies the local α–Nd relationship at native LES resolution, especially under more oblique solar geometry, but the differences between 1D- and 3D-regression-based susceptibility estimates are substantially reduced at coarser resolution and low-to-moderate LWP/τ regimes. These results indicate that although 3D RT can strongly affect domain-top regression-based susceptibility at the LES resolution, its impact is reduced at satellite-like spatial resolution for low to moderate LWP/τ regimes.
Methane is emitted from a range of anthropogenic and natural sources, and identifying these sources is important for emissions monitoring and mitigation. Different sources emit methane with different isotopic signatures. However, these signatures can vary spatially and temporally and are often not well understood. Top-down inverse methods can be used with measurements of methane mole fractions to estimate total emissions of methane from all sources. Here, we present an inverse system for concurrently estimating regional fossil-fuel (FF) and non-fossil-fuel (non-FF) emissions, using isotope ratio observations and considering uncertainty in the isotopic signatures. This method is highly adaptable and could be used to estimate emissions from any number of sources. Synthetic data tests with this method show that this inverse system can accurately estimate FF and non-FF methane emissions across the UK, when isotopic source signatures are fixed at known values. However, emissions estimation becomes less accurate when source signature uncertainties are increased to over approximately 50 % of their likely ranges. In a real-world test of this method, we estimated south-east UK FF and non-FF emissions using high-frequency δ13C-CH4 and δ2H-CH4 observations from one UK site, with source signature uncertainties reflecting our current understanding of these values. Results show only a limited impact on emissions uncertainty and magnitude, when compared with a methane-only inversion. This suggests that both an expansion of the UK network of isotope ratio observations and an improved understanding of isotopic signatures is required for this method to be effective in estimating UK FF and non-FF methane emissions with reduced uncertainty compared to traditional inverse methods.
Anthropogenic sulphate aerosol is a primary forcing agent in the historical period, and its exact impact on North Atlantic SST variability remains an open question. A set of novel, idealised, single-forcing experiments was performed to isolate the physical processes. The medium-resolution (60 km atmosphere, 0.25° ocean) and low-resolution (135 km atmosphere, 1° ocean) of the HadGEM3-GC3.1 model were used to investigate the impact of resolution on the forced response. The SST response at both resolutions is timescale dependent: a fast, large-scale surface cooling is followed by a slow, small-scale subpolar warming lagging by 15–20 years. Warming of the subpolar North Atlantic is due to a strengthening of the Atlantic Meridional Overturning Circulation (AMOC) and is stronger at the medium resolution. This resolution difference is because sea ice growth in the Labrador Sea (LS) is stronger at low resolution, which inhibits air-sea interaction and reduces surface forced dense water formation, leading to a weaker AMOC response. There is also evidence of a stronger AMOC positive feedback involving salt-advection at medium-resolution. These results show that the large-scale North Atlantic response to external forcing can be sensitive to regional differences, such as model climatology of LS sea ice and its response to aerosol cooling.
Accurately modeling the cloud condensation nuclei (CCN) budget is a key factor in reducing uncertainty in aerosol–cloud interactions in Earth system models. Wet deposition – the removal of particles by precipitation – is a major CCN sink, but rainfall can also trigger a replenishment phase via the formation and growth of new particles, partially offsetting losses. However, the ability of general circulation models (GCMs) to capture this precipitation-driven replenishment and size-dependent losses remains under-explored. Here, we evaluate three GCMs representation the size- and time-resolved effects of precipitation on the particle number size distribution (PNSD) and CCN budget, based on correlations between PNSD and precipitation rate along back trajectories from three long-term measurement stations. To better isolate the role of precipitation from confounding factors, we also apply a Machine Learning approach (XGBoost), training one regression model per site and source using a minimal set of physically relevant predictors. Our results show that at the two high-latitude stations, the models underestimate CCN replenishment following precipitation, with too weak new particle formation and growth. At ATTO, two of the models instead overestimate this effect, simulating an immediate CCN source after rainfall. Observations also suggest that CCN removal is weaker during colder conditions, a pattern that models struggle to capture – either overestimating or underestimating the precipitation effect, depending on the model. The XGBoost analysis confirms the key findings of the correlation analysis while helping to correct for likely confounding influences, showing promise for disentangling spurious correlations in model evaluation in process evaluation.
Realistic cloud droplet number concentrations (Nd) are critical for simulating the Earth's radiative budget, yet remain challenging to represent in global models. One reason is that aerosol optical depth (AOD), commonly used for aerosol validation and assimilation, only weakly constrains aerosol number concentrations relevant for cloud formation. We introduce a representation of aerosol-cloud interactions (ACI) into the ECMWF Integrated Forecasting System (IFS), restricted to aerosol activation and the first indirect effect, and use it as a diagnostic tool to relate aerosol properties to cloud observations in the Copernicus Atmosphere Monitoring System (CAMS). Using 18 years of MODIS Nd retrievals, we infer effective aerosol size distribution parameters and evaluate results using independent observations of AOD, Angstrom exponent, size spectra, and top-of-atmosphere shortwave fluxes. After optimisation, CAMS aerosols produce more realistic large-scale Nd patterns, but exhibit regional biases: overestimation over sub-Saharan Africa and underestimation at high latitudes. The African bias is consistent with carbonaceous aerosol emissions from wildfires and can be partially mitigated through optimisation, while the high-latitude bias is likely linked to excessive aerosol scavenging in mixed-phase clouds and cannot be resolved by size adjustments alone. We show that introducing phase-dependent aerosol scavenging substantially improves all-sky shortwave fluxes over the Southern Ocean compared to the ACI implementation in the standard CAMS system. This highlights a direct link between radiative biases and aerosol wet removal processes. Representing ACI in a weather model enables Nd to provide additional constraints on aerosol numbers and related processes globally, revealing limitations in how ageing, vertical transport and wet removal are represented, as these processes are only weakly constrained by standard AOD evaluation.
A large discrepancy of at least 10 Gg yr−1 exists between reported emissions of the potent greenhouse gas HFC-23 (CHF3, trifluoromethane) and emissions derived from atmospheric measurements. In-atmosphere production of HFC-23 from the breakdown of fluorinated source gases such as hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs) contributes to this gap, but only a conservative upper limit has been estimated for the magnitude of this source. This uncertainty is due, in part, to limited experimental measurements of the photolysis quantum yield of trifluoroacetaldehyde (CF3CHO), a key degradation product that forms HFC-23 via photolysis. The parameters governing CF3CHO deposition are also poorly understood. Here, we use a 3D chemistry and transport model, STOCHEM-CRI, to further constrain the magnitude of in-atmosphere HFC-23 production, using recent estimates of source gas emissions and explicitly parametrised photolysis and deposition. Furthermore, we perform an ensemble of simulations to account for the uncertainties in these values. We find that in-atmosphere production of HFC-23 is in the range 0.013–0.035 Gg yr−1, substantially lower than previous estimates. This accounts for < 0.5 % of the discrepancy between reported and measurement-derived emissions, suggesting that this source makes a negligible contribution to the overall HFC-23 budget and that unreported direct emissions are likely responsible for the vast majority of the discrepancy. As part of this work, we also calculate indirect global warming potentials for the HFC-23 source gases HFO-1234ze(E), HFO-1336mzz(Z) and HCFO-1233zd(E) and find that their impact on climate is up to ten times higher than previously reported.
Reactive nitrogen (Nr) species such as particulate ammonium (pNH4+) and nitrate (pNO3-) are critical drivers of air pollution and ecosystem health, yet their transformation in mountain forests remains poorly characterized. We performed a one-week field campaign in a subtropical mountain forest at Xitou, Taiwan, using size-segregated aerosol sampling, stable isotopic techniques, and Bayesian modeling. Functional groups were analyzed by Fourier-transform infrared spectroscopy (FTIR-ATR), and isotopes δ15N and δ18O were measured using gas chromatography-isotope ratio mass spectrometry (GC-IRMS) to quantify pNH4+ source contributions and pNO3- formation pathways. During the sampling week, diurnal patterns of higher daytime particle concentrations were disrupted by a 26 h fog event, which suppressed δ15N-enriched urban plume influx and promoted aqueous-phase uptake of isotopically depleted local gas-phase species. Under clear conditions, size-resolved δ15N-NH4+ exhibited a bell-shaped distribution peaking at the accumulation mode, whereas this gradient flattened during the fog event. Size-resolved δ15N and δ18O signatures of pNO3- revealed two nitrate formation regimes: urban plumes retained O3-driven oxidation signatures with higher δ18O, and rural/local regimes were dominated by RO2-involved processes with greater isotopic depletion and/or biogenic contributions. Bayesian source apportionment constrained by δ15N-NH4+ indicated 50 %–83 % of NH3 emissions originated from combustion-related sources. Concurrently, δ18O source apportionment showed RO2-initiated oxidation dominated daytime pNO3- formation (42 %–95 %) and heterogeneous reactions contributed 6 %–84 % at night. Although based on a short-term campaign capturing a single fog episode, this case study highlights the value of size-resolved isotopic approaches for characterizing reactive nitrogen transport and evolution under contrasting meteorological conditions, providing mechanistic insights into complex environments.
