Abstract. Accurate simulation of volatile organic compounds (VOCs) remains a persistent challenge in chemical transport models (CTMs), limiting their ability to diagnose species-specific emissions and their role in ozone (O3) formation. Most models rely on lumped surrogate species, which limits their ability to represent speciated VOCs and directly compare with observations. In this study, we develop an enhanced version of the Community Multiscale Air Quality model, termed CMAQ-PAMS, which explicitly incorporates 54 VOC species targeted by the Photochemical Assessment Monitoring Stations (PAMS) network in Taiwan. We evaluate model performance during a representative high-ozone event in fall 2021 and apply a top-down calibration approach using hourly VOC data from 12 PAMS sites. The original simulation (OrigSIM) significantly misrepresents key species, largely due to reliance on U.S.-based speciation profiles. After adjustment, the modified simulation (ModSIM) shows substantial improvements in both individual species concentrations and group-level composition (e.g., alkanes, aromatics). Notably, acetylene, a key tracer of incomplete combustion, was underestimated in OrigSIM but successfully recovered in ModSIM. Despite accounting for only ∼ 32 % of total VOC emissions, PAMS species contribute up to 52 % of modeled domestic O3 formation, highlighting their disproportionate impact in VOC-limited regimes. Additionally, the CMAQ-PAMS framework enables the use of diagnostic ratios (e.g., propylene / acetylene) to identify emission sources and assess air mass aging. These findings underscore the importance of localized VOC profiling and demonstrate that the PAMS-constrained CMAQ-PAMS model provides a more chemically detailed and observationally anchored platform for ozone modeling and regulatory applications.

Abstract. Particulate nitrate (NO3-) is a key target for air pollution control; however, its response to NOx emission reductions remains uncertain, particularly in cold climates. This study assesses long-term trends in fine- and coarse-mode NO3- (f-NO3- and c-NO3-) from 1990 to 2019 across seven Canadian cities, using data from by the National Air Pollution Surveillance (NAPS) network. The analysis reveals disproportionate trends between NO3- and NOx emissions nationwide. In Edmonton, annual mean f-NO3- decreased by approximately 60 % between 2007 and 2019, while provincial NOx emissions declined by only 10 %–20 %. Similar patterns were observed in five of the six other cities during the most recent decade. These disproportionate trends are attributed to reductions in primary f-NO3- emissions, localized dispersion processes, and wind anomalies modulated by Arctic Oscillation. In contrast, all cities exhibited a temporary increase in f-NO3- during 1998–2007, coinciding with early NOx control measures and consistent with an unintended enhancement of primary f-NO3- emissions formed within stationary combustion plumes. c-NO3- was largely insensitive to NOx reduction in most cities (except for Edmonton), with variability governed primarily by neutralization reactions with alkaline aerosols rather than by the availability of gaseous HNO3. These findings provide insight into the weak or absent response of f-NO3- to NOx emission reductions observed globally, particularly in cold-climate regions.

Abstract. Sulfuric acid (H2SO4) is a key precursor in atmospheric new particle formation and cluster early growth. However, long-term measurement of it is only available at a few sites. Although several proxies for estimating H2SO4 concentration have been proposed, they are always site-specific. Therefore, both reliable H2SO4 measurement and proxies with wider application are highly needed. Here, we conducted a long-term H2SO4 measurement in urban Beijing during 2019–2021, and derived three H2SO4 proxies based entirely on its formation and loss pathways. Results show that daytime H2SO4 concentration is 2.0–7.4×106 molec. cm−3 and shows an overall decline with an average annual decrease of 14 %. This decline is mainly due to the ongoing SO2 emission controls. Daytime H2SO4 shows a clear seasonal variation that tracks UVB. Nighttime H2SO4 concentration is 1.6–6.3×105 molec. cm−3, with higher levels in warmer seasons due to stronger sources and lower condensation sink (CS). The diurnal variations of H2SO4 across seasons follow those of photo-oxidation-related parameters, such as UVB, OH radical, and photolysis rate of NO2 (J(NO2)). All of the three proxies can reproduce H2SO4 concentration during 10:00–14:00 LT. Importantly, they can estimate H2SO4 concentration at a boreal forest site in Hyytiälä, Finland, suggesting their applicability to sites with diverse environments. Furthermore, the parameters used in UVB-PM2.5 based proxy are available at most observational sites. Further application of this proxy could provide H2SO4 concentrations covering many regions worldwide, which may further facilitate research on atmospheric nucleation and secondary aerosol growth of these sites.

Abstract. Light-absorbing particles on surface ice in ablation areas can accelerate glacier melting and shrinkage. A Single Soot Particle Photometer was used to measure black carbon (BC) mass concentrations (MBC) in the ablation area of Potanin Glacier, Mongolia during summer. Surface-ice MBC values (42–555 ng g−1) greatly exceeded those of surface snow (5–22 ng g−1), snow and rain (2–6 ng g−1), and surface melt water (2–11 ng g−1). Vertical profiles of MBC revealed high surface-layer concentrations, suggesting impurities trapped in the granular ice: the particularly low-density layer of the weathering crust surface. In the ablation area, MBC values of granular ice decreased with lower elevation: 134–601 ng g−1 at the 3317 m site and 8–96 ng g−1 at the 3078 m site. The fraction of residual surface BC to BC contained in lost water over a year, R, was calculated using the yearly BC deposition flux and water ablation weight Aw. Average R values were 0.17 and 0.011, respectively, at 3317 and 3078 m. Aw were 246 and 325 gw.e.cm-2, suggesting that the granular ice retains BC particles best in the upstream ablation area, showing concomitantly less capability with increasing ablation. Enriched BC on the ablation area surface comprises recent BC deposits and BC from the glacier's lower layer after rising during decades or more. Those BC emissions and deposits can therefore affect both future and present ablation area melting processes.

Abstract. Per- and polyfluoroalkyl substances (PFAS) are recognised as atmospheric contaminants, yet processes governing their aerosol formation, size distribution, and interactions with atmospheric particle surfaces remain unknown. We investigated aerosolisation and size-resolved behaviour of 25 PFAS covering short-, medium-, and long-chain perfluoroalkyl carboxylic acids (PFCA), perfluoroalkane sulfonates, fluorotelomer sulfonates and emerging alternatives. Experiments were conducted under controlled chamber conditions using a water–organic solvent system, in the absence/presence of the model bacterium Pseudomonas fluorescens seed to investigate the potential influence of microbial presence on PFAS behaviour. Most PFAS exhibited unimodal mass–size distributions peaking at 0.3 µm, indicating dominant association with the fine mode. Sulfonated PFAS showed broadly similar aerosol-phase concentrations regardless of carbon-chain length, whereas PFCA displayed increasing aerosolisation with chain length. Perfluorooctane sulfonic acid (PFOS) showed additional ultrafine enrichment, 6:2 fluorotelomer sulfonate (6:2 FTS) and sodium 4,8-dioxa-3H-perfluorononanoate (NaDONA) exhibited broader size profiles, suggesting compound-specific effects linked to volatility and interfacial behaviour. Pseudomonas fluorescens seed did not enhance PFAS aerosol concentrations through condensation or heterogeneous uptake onto bacterial particles or shift in modal diameters, and no enrichment was observed at bacterial size mode, indicating limited PFAS–bioaerosol association under the tested conditions. Multiple-Path Particle Dosimetry (MPPD) modelling based on the measured size distributions predicted substantial deposition of the aerosol-bound PFAS in the pulmonary region, particularly for compounds enriched in ultrafine particles. Our findings indicate that PFAS aerosol behaviour in mixed-solvent systems is controlled primarily by physical droplet generation and evaporation, with implications for airborne transport and inhalation exposure from contaminated aqueous sources.

Abstract. Turbulence influences snow microphysics and precipitation formation, while simultaneously degrading polarimetric radar measurements through broadening of the canting angle distribution. We investigate these interactions in the Colorado Rocky Mountains, where an orographic turbulent layer consistently forms in the lee of Gothic Mountain during precipitation events. To isolate microphysical signals from turbulence-induced artifacts, we apply a novel framework contrasting radar observations above and below the turbulent layer. The dataset combines polarimetric W-band and collocated Ka-band radar measurements with surface in situ observations from the Video In Situ Snowfall Sensor (VISSS). All observations were collected during the CORSIPP project, part of the ARM SAIL campaign (winter 2022/2023). Aggregation is identified as a dominant process within the turbulent layer, occurring primarily between −12 and −15 °C. It is responsible for reflectivity (Ze) increase of up to 20 dBZ km−1 and reduction of the mean particle fall velocity. Enhanced KDP and sZDRmax further suggest secondary ice production through ice-collisional fragmentation, generating anisotropic splinters. Riming also occurs frequently, with Ze increases up to 15 dBZ km−1 and systematically increasing mean particle fall velocity. Riming inside the turbulent layer was often observed at temperatures below −10 °C, indicating the presence of supercooled liquid at cold conditions. Statistical analysis revealed that the turbulent layer is frequently collocated with supercooled liquid water layers near the Gothic Mountain summit. Our findings demonstrate how radar polarimetry may be safely used to investigate microphysical processes inside a turbulent layer and highlight the impact of orographic turbulence on snow microphysics and precipitation enhancement.

Abstract. Aqueous-phase secondary organic aerosols (aqSOA) constitute a large fraction of SOA, thereby exerting significant influence on air quality, climate, and human health. However, its formation pathways under real ambient conditions in Chinese urban regions remain insufficiently constrained. We conducted field measurements of particulate matters (PM) composition by deploying high-resolution aerosol mass spectrometry in a suburban environment during autumn in Nanjing, China. The characteristics and formation pathways of aqSOA are comprehensively investigated by using Positive Matrix Factorization (PMF) method. Our results show that aqSOA accounted for 27.6 % of oxidized organic aerosols, exhibiting elevated O:C ratios (0.78) and strong correlations with nitrate and aerosol liquid water (ALW). The important role of acid-catalyzed reactions is also revealed by the enhanced production of aqSOA at lower aerosol pH conditions. Under elevated nitrate and ALW levels, a pronounced morning aqSOA peak was frequently observed; whereas a noon-time aqSOA peak was also observed on several days, likely governed by photochemistry and aqueous-phase reactions. These findings highlight the critical roles of nitrate, ALW, acidity, and photochemistry in driving aqSOA production in polluted urban environments. This study advances the mechanistic understanding of aqSOA formation and provides insights into the mitigation of SOA in the Eastern China.

Abstract. The number of ice crystals formed during the contrail's jet phase has a long-lasting impact on the life cycle and radiative forcing of contrail cirrus clouds. For conventional kerosene combustion, suitable parameterizations for early ice crystal number have been developed and employed in general circulation models that are used to estimate the climate impact of contrail cirrus. However, a parameterization for the number of ice crystals formed is lacking for hydrogen combustion. To develop such a parameterization, we present a comprehensive set of contrail formation simulations using the particle-based Lagrangian Cloud Module in a box model approach. Unlike kerosene combustion, no soot particles are emitted. Thus, ice crystals are assumed to form on ambient aerosols entrained into the exhaust plume. The total number of entrained particles primarily governs the nonlinear depletion of water vapor. Consequently, the impact of coarse-mode particles is negligible due to their low abundance. Additionally, ice crystal formation from multiple aerosol populations can be reconstructed from single-population simulations using population-specific properties (size and hygroscopicity) and the total number concentration. We also identify atmospheric conditions where homogeneous droplet nucleation can be safely neglected as potential ice formation pathway. Based on more than 20 000 simulations covering a broad range of atmospheric conditions and aerosol properties, we identify a regime where ice crystal formation becomes nearly independent of ambient relative humidity, aerosol size, and hygroscopicity. Our results provide a basis for a data-driven parameterization of ice crystal number in contrails from hydrogen combustion, to be presented in a companion paper.

Abstract. Non-dust emissions have been increasingly recognized as important contributors to atmospheric iron (Fe), influencing marine productivity through enhanced bioavailable Fe inputs. However, accurately quantifying the contributions and spatiotemporal variability of non-dust sources remains challenging due to relatively low time-resolution of traditional filter-based analytical methods. In this study, the contributions of non-dust emissions to atmospheric total and soluble Fe in the Northwest Pacific were quantified based on online measurements from three ship-based observation campaigns in 2021–2022. A Positive Matrix Factorization (PMF) model was applied for source apportionment. Results showed non-dust emissions were notable contributors to atmospheric total Fe, representing 24 %–41 % of total Fe in PM10 and 30 %–56 % in PM2.5 samples across different cruise legs. Importantly, their contributions to soluble Fe were significantly higher, reaching 88 %–97 % in PM10 and 85 %–98 % in PM2.5 samples. Among non-dust sources, land anthropogenic emissions contributed substantially to both total and soluble Fe, whereas ship emission contributed a small portion to total Fe but was a major source of soluble Fe, particularly in summer, when its contribution reached 79 % of soluble Fe in PM10 samples in coastal regions. Additionally, Fe from non-dust sources exhibited stronger spatial variability than dust source. The concentrations of land anthropogenic Fe differed by 3–5 times between coastal and open-ocean areas during the same cruises, while ship-derived Fe varied by an order of magnitude or more. This study offers critical observational evidence to advance understanding of how diverse emission sources shape atmospheric composition in Asian continental outflow regions.

Abstract. Dust transport around the Tibetan Plateau (TP) plays a key role in regional climate and air quality, yet its seasonal and diurnal variability remains insufficiently understood. Here, we presented the spatiotemporal characteristics of dust transport in the TP region combining satellite observations, reanalysis datasets and ground-based measurements. A new method for dust mass concentration based on CATS (Cloud-Aerosol Transport System) lidar was developed and showed strong consistency with multiple products in both spatial and temporal scales. Our results revealed persistent dust transport belts on both the northern and southern sides of TP, with peaks in spring. The amounts of dust flux transported to TP were estimated for different directional sources, seasons and heights. The vertical variation and amount of dust backflow in Taklamakan Desert were analyzed. The diurnal characteristics of vertical resolved dust flux were presented in three-hour interval and four sections around TP. Overall, this study deepened the understanding of the dust climatology over the TP region from a satellite perspective.