Volatile Organic Compounds (VOC), particularly of biogenic origin emitted by vegetation and soils, play an important role in the global Organic Aerosol (OA) budget. The introduction of field-deployable Aerosol Mass Spectrometers in the early 2000s, combined with statistical analysis of their mass spectra, has significantly improved our understanding of the impact of secondary processes on fine-mode aerosol concentrations. While delivering innovative and significant insights, those analyses usually fail to explicitly identify precursors/mechanisms. In this context, this work focuses on laboratory-generated secondary OA (SOA) of biogenic VOC and its spectral analysis through a new generation of aerosol mass spectrometers, notably a Proton Transfer Reaction Mass Spectrometer coupled to a CHemical Analysis of aeRosol ONline (PTRMS-CHARON) inlet. Aerosol particles were formed in the new DouAir atmospheric chamber via isoprene (ISOP) OH oxidation, monoterpene O-3 (limonene, MT), and sesquiterpene O-3 ( beta -caryophyllene, SQT) oxidation. ISOP experiments targeted "low-NO" environments, typically remote forested tropical areas, via epoxidiols formation (ISOP-IEPOX-SOA), or through an alternative branching favored in the absence of acidic seed particles (ISOP-non-IEPOX-SOA) and "high NO" environments, representative in urban and polluted regions (ISOP-NO-SOA). Experiments showed that those five SOA formation pathways (ISOP-IEPOX-SOA, ISOP-non-IEPOX-SOA, ISOP-NO-SOA, and the ozonolysis reactions of MT and SQT) exhibited distinguishable spectra, with identifiable tracer ions, such as m/z 119.07 (C5H10O3), m / z 137.081 (C5H12O4) for ISOP-IEPOX-SOA, C5H10O4 ( m/z 135.070), C5H10O6 ( m / z 167.055) for ISOP-non-IEPOX-SOA, and m / z 85.028 (C4H4O2) for NO-SOA pathways, as well as molecules with C-7-C-10 and C-7-C-15 structures, including characteristic fragments, identified during MT and SQT oxidation experiments, respectively. Notably, m/z 83.049 (C5H6O) was detected in both low-NO isoprene pathways, suggesting a broader diagnostic role. These laboratory findings depict promising results for ambient near-real-time biogenic SOA source apportionment, notably in forested and/or urbanized areas.

Black carbon (BC) from maritime emissions plays a critical role by influencing radiation, cloud processes, and atmospheric dynamics in the marine atmosphere. These impacts depend on BC concentration and mixing state with other aerosol components. However, in situ observations of BC over oceans remain scarce, and the influence of the marine environment on the evolution of BC mixing state is not well understood. Here, we present shipborne measurements aboard the research sailing yacht S/Y Eugen Seibold during 10 Atlantic Ocean cruises. The data set spans 1,120 of measurement hours from near-coastal regions to remote ocean areas. In oceanic regions extending from tens to thousands of kilometers offshore, 1-min averaged BC concentrations were typically around 100 ng m(-3), suggesting a well-mixed marine background. Despite the relatively small variability in BC mass concentrations, the mixing state of BC exhibits substantial differences between nearshore and remote oceanic regions. High number fractions (>50%) of BC particles without core-shell morphologies, characterized by BC externally attached to non-BC materials, were observed in near-coastal regions and decreased to similar to 20% in remote oceanic regions. In ship-impacted regions, small freshly emitted BC particles tend to coagulate with other aerosol particles and forming non-core-shell attached structures, while high relative humidity (RH > 85%) tends to promote the formation of thick coatings. Our results provide new insights into climate-relevant properties and underscore the importance of coagulation and hygroscopic processing for the mixing state of BC in the marine atmosphere.

The Amazon rainforest plays a crucial role in the global climate system, hydrological cycle, and earth's energy balance. As one of the planet's least industrialized regions, it allows investigation of organic aerosol formation and constituents under almost pristine conditions. Nevertheless, human activities are known to affect this ecosystem - especially during the dry seasons. In this study, ambient aerosol samples collected at the Amazon Tall Tower Observatory (ATTO) during two dry and two wet seasons were characterized by high-resolution mass spectrometry (HR-MS). Comprehensive non-targeted data evaluation was applied to identify thousands of molecular formulae. Most were found to be associated with oxidation products of isoprene and monoterpenes, highlighting the predominance of biogenic secondary organic aerosols (SOA) at ATTO. The chemical composition exhibited distinct seasonal patterns with more processed organic compounds during the dry season, which can be explained by an increase of later-generation oxidation products due to reduced wet deposition and enhanced long-range transport. Mono- and polycyclic heteroaromatic components from biomass burning (BB) sources were enhanced during the dry seasons and the second wet season. The wet seasons were generally characterized by less oxidized compounds, associated with freshly formed SOA particles. Height-resolved measurements showed biogenic emissions with higher concentrations of early terpene oxidation products at lower altitudes. Overall, our results provide new insights into the molecular characteristics and seasonality of organic particulate matter at ATTO, helping to constrain the sources and interactions of aerosols, clouds, and precipitation in the Amazon rainforest.

At kilometer-scale resolution, convective systems start to be explicitly resolved in atmospheric models, albeit coarsely. This allows a more process-based analysis of certain aspects of aerosol–cloud interactions in tropical regions. Convective clouds are a ubiquitous feature above the Amazon rainforest and develop under strongly contrasting aerosol conditions, with particle number concentrations during the dry season often exceeding those in the wet season by an order of magnitude.In this context, we explore aerosol and convective cloud processes over the Amazon rainforest by analyzing case studies that combine observations and km-scale cloud-resolving simulations with interactive aerosols in a limited-area configuration. Regional simulations are performed at approximately 1.6 km horizontal resolution using the Icosahedral Nonhydrostatic (ICON) model coupled to the one-moment aerosol scheme HAM-lite. The realism of the simulations is evaluated through comparison with a combination of ground-based, satellite, and aircraft observations.For the wet season, we analyze a case study based on flight RF15, conducted with the German research aircraft HALO during the CAFE-Brazil (Chemistry of the Atmosphere: Field Experiment in Brazil; CAFE-BR) campaign in 2022–2023. Three simulations are presented for this case: a best-estimate factual simulation and two counterfactual sensitivity experiments representing background “green ocean” conditions and heavy aerosol loading associated with biomass burning during dry season periods. For the dry season, we also revisit two research flights from the ACRIDICON-CHUVA 2014 campaign, representing one clean and one polluted case, to further assess the representation of aerosol–cloud processes under different convective regimes. Combining these cases, we discuss the impact of changing aerosol environments on convective systems and draw conclusions relevant to a transition toward a post-fossil aerosol regime.

Abstract The transatlantic transport of dust and smoke aerosols from Africa to South America is a large‐scale, year‐round process that affects atmospheric and nutrient cycling in the Amazon rainforest. We analyze daily variations in black carbon at the Amazon Tall Tower Observatory (ATTO) to investigate how Atlantic synoptic‐scale meteorology influences its long‐range transport. Black carbon fluctuations during the Amazon wet season were not fully explained by air mass trajectory length or direction. Instead, regional‐scale rainfall emerged as the key driver of shifts between clean and polluted days at ATTO. Rainfall maxima along trajectories preceded clean days, indicating effective wet scavenging. Composite analysis linked these rain events to synoptic systems like U.S. cold air outbreaks and South Atlantic high‐pressure anomalies, which enhance moisture convergence and rainfall, promoting aerosol removal. Climate‐driven shifts in tropical Atlantic circulation could alter aerosol and nutrient transport to the Amazon, with unknown impacts on rainforest productivity and resilience.

Wasser an einer Grenzfläche – eingeschlossen in Nanoporen oder zwischen Schichten – weist einzigartige strukturelle und dynamische Eigenschaften auf, die sich deutlich von denen von Bulkwasser unterscheiden. In geschichteten 2D‐Materialien wie MXenen wird angenommen, dass eingeschlossenes Wasser deren Oberflächenchemie durch lokale Oxidation beeinflusst und zu Redoxprozessen beim elektrochemischen Zyklieren beiträgt. Die chemische Beschaffenheit von eingeschlossenem Wasser und dessen Wechselwirkung mit der Oberflächenchemie von MXenen ist jedoch nach wie vor unklar. Hier verwenden wir Rastertransmissionsröntgenmikroskopie (STXM), um die chemische Wechselwirkung von Wasser in einzelnen Ti 3 C 2 T x ‐MXene‐Flocken in feuchten und wässrigen Umgebungen mit einer Ortsauflösung von 50 nm in situ zu untersuchen. An der Sauerstoff‐K‐Kante entdecken wir, dass Wasser, das in Taschen und Falten in weniglagigen MXene‐Flocken eingeschlossen ist, andere Wasserstoffbrückenbindungen aufweist als im MXene‐Zwischenlagenraum eingeschlossenes Wasser. Weiterhin enthüllen wir wasserinduzierte lokale Redoxreaktionen von Ti‐Atomen, die bei Wechselwirkung mit flüssigem Wasser und alkalischen Ionenneutralelektrolyten ungleichmäßig auf den MXene‐Flocken verteilt, aber unter Einwirkung eines sauren Elektrolyten teilweise reversibel sind.

Biological particles are a substantial component of Amazonian aerosols, yet their community composition, spatiotemporal variability, and atmospheric impacts remain poorly understood. Here, we present a comprehensive study of archaeal, bacterial, and fungal bioaerosol communities in the Amazon rainforest, collected at 42 m and 323 m height above ground at the Amazon Tall Tower Observatory (ATTO). Archaea exhibited overall low prevalence, dominated by potential marine clades. Under rainforest background conditions, the bacterial community varied strongly with time and height, indicating an inhomogeneous source distribution and strong long-range influence. In contrast, the fungal bioaerosol community exhibited low variability over time and height, suggesting persistent, widespread, and uniform regional sources. During an African dust intrusion, the bacterial community composition changed drastically. A strong increase in relative sequence abundance of bacteria belonging to the phylum Bacillota, known to be abundant in Saharan dust, suggests a substantial influx of bacterial bioparticles and genetic material from Africa. In contrast, the fungal community remained unaffected, likely due to a masking effect of local emissions. During the dust event, the total mass concentrations of coarse mode aerosol particles (>1 µm) and of atmospheric endotoxins, that can trigger respiratory diseases, increased by factors up to 4. This study serves as a key foundation to unravel the mechanisms of bioparticle and dust cycling and their effects on biodiversity, climate, and public health during Earth history up to current and future environmental conditions in the Anthropocene.

Fog formation over tropical forests remains poorly characterized, despite its potential role in bioaerosol dispersion and ecosystem processes. Here, we analyzed fog samples collected at the Amazon Tall Tower Observatory using flow cytometry and culture-based techniques to characterize viable microbial communities. Microbial cell concentrations varied over an order of magnitude across 13 fog events, reaching up to 8 & times; 104 cells per ml of fog water. Flow cytometry consistently detected metabolically active cells, while culturing and mass spectrometry-based identification yielded eight viable bacterial species and seven fungal taxa. The bacteria Serratia marcescens, Ralstonia pickettii and Sphingomonas paucimobilis exhibited seasonal variations in prevalence. The fungal species identified were primarily mesophilic saprophytes and endophytes, commonly associated with soil and plant surfaces. Our findings indicate that fog harbors viable microbes, including Serratia marcescens and Ralstonia pickettii, which may imply a relevance of fog for microbial dispersal, colonization and nutrient cycling in the Amazon rainforest.

ABSTRACT Microbial communities at the ocean-atmosphere interface play a vital role in nutrient, aerosol, and water cycling, yet their large-scale biogeography remains scarcely studied. Here we assessed microbial cell abundance and community structure in air and surface ocean samples along a 14,400 km transect from the polar circle to the equator. Air and surface ocean microbiomes were taxonomically distinct across the North East Atlantic. The microbiome in the air had a lower local community richness compared to the surface ocean, yet was more diverse across larger geographical scales. Our results suggest that terrestrial-derived air masses affected air and surface ocean communities. Air microbial communities showed a latitudinal diversity gradient with richness and cell abundance increasing from the polar circle to the equator. We observed an exchange of microbial lineages between ocean and air, however, most lineages were confined to one realm, suggesting lineage-selective aerosolization and deposition of microbial cells.

The Southern Ocean (SO) is one of the cloudiest regions on Earth. However, cloud radiative effects are not well represented over the SO in atmospheric models, which is mainly due to an underestimation of aerosols. To address this and other fundamental and pressing open questions on the interaction of atmospheric radiation, aerosol nucleation and growth, cloud formation and impacts over the SOI, the HALO-South aircraft mission was conducted in September and October 2025 based in Christchurch, Aotearoa New Zealand. HALO stands for High Altitude and Long Range Research Aircraft. HALO-South covered the full cycle of processes from aerosol formation, cloud evolution, and radiative interaction with a special focus on the characteristics and effects of mixed-phase clouds. The instrumental payload of HALO included a unique and comprehensive in-situ and remote sensing suite of instruments. It was designed to collect data to improve our understanding of fundamental atmospheric processes and to extrapolate and upscale the results using satellite data and global climate models in order to resolve long-standing measurement-modelling discrepancies. In addition, the ground-based stations in Tāwhaki and Invercargill with remote sensing and in-situ long term measurements will extend the data to a larger scale in time. The first analysis of the campaign shows promising insights into cloud and aerosol processes over the SO, which will be presented and discussed.Acknowledgments: This work was supported by the DFG (Deutsche Forschungsgemeinschaft, German Research Foundation) Priority Program SPP 1294, the Max Planck Society, Priority Program SPP 1294, the German Aerospace Center (DLR)

Abstract. Atmospheric aerosols play a crucial role in Earth’s climate system, yet their spatio-temporal distribution, particularly in the free troposphere (FT) and upper troposphere–lower stratosphere (UTLS), remains poorly constrained, a major source of uncertainty in estimates of aerosol radiative forcing. To address this, we perform ECHAM/MESSy Atmospheric Chemistry (EMAC) model simulations with a newly developed setup, bridging the tropospheric and stratospheric regimes. Model output is evaluated against a comprehensive suite of observations of aerosol mass, number concentrations, and optical properties, showing good agreement across vertical layers and most geographical regions. The evaluated simulations provide a unified description of global distributions of key aerosol species, their composition, and number concentrations from the Earth's surface to the stratosphere. Simulated aerosol mass exhibits a global minimum between 400 and 200 hPa, marking the transition between FT and UTLS, with particle numbers peaking at similar altitudes or slightly higher in the tropics. Primary particles contribute less than 3.5 % to aerosol mass in the stratospheric overworld up to 10 hPa, substantially less than suggested by previous modelling studies and in closer agreement with recent observations. Stratospheric aerosol mass is dominated by sulfate, with a notable contribution (~15 %) from secondary organic aerosol throughout the global lower stratosphere. This work provides new constraints on aerosol distributions in the FT and UTLS, which remain underrepresented in global modelling studies, and enables future research on aerosol-climate interactions in this critical atmospheric regime.

The large extension and diversity of the Brazilian Amazon biome hampers the assessment of the regional-scale carbon budget based solely on local observations. Considering the shortage of observations, this study aims to examine the carbon fluxes throughout the Brazilian Amazon biome using a process-based model (JULES, Joint UK land environment simulator). A sensitivity analysis detected five critical model parameters for the Amazon tropical broadleaf evergreen forest, optimized using carbon flux and meteorological data from four forest sites. The simulations with the new parametrization were compared with JULES default parameter values and with simulations of the Vegetation Photosynthesis and Respiration Model (VPRM). Net ecosystem exchange (NEE) and gross primary production (GPP) estimates were improved at all sites, reaching a Root Mean Squared Error (RMSE) about 30 % lower in comparison to the default version. The optimized parameter values varied among the four sites, indicating that a single parameterization for the whole Amazonia may not be adequate. JULES model parameters were spatialized for the Brazilian Amazonia, based on canopy height and leaf area index gridded data. Applying JULES with spatially dependent parameterization for the year 2021 resulted in a carbon sink of -1.34 Pg C yr-1. Regional differences were observed in the carbon fluxes, with a carbon source of 0.75 kg C m-2 yr-1 in the southwest and north, likely explained by increased ecosystem respiration in older and taller forests.

Refractory black carbon (rBC) aerosol particles strongly influence Arctic atmospheric radiative transfer, making it essential to understand their microphysical properties and mixing state. However, in-situ investigations on microphysical properties and mixing state of rBC particles over the central Arctic marine boundary layer are scarce. To address this gap, we carried out a comprehensive investigation of rBC particles in the central Arctic onboard the RV Polarstern during the ATWAICE cruise. Our results revealed pronounced spatial and temporal variability in microphysical properties of rBC in the Arctic marine boundary layer, governed by transport pathways and removal mechanisms. Under pristine background conditions, rBC mass concentrations were at their lowest (median similar to 0.4-0.6 ng m(-3)). The mass median diameter of rBC cores was found to increase with latitude, from the lowest value (similar to 156 nm) in lower-latitude regions influenced by higher anthropogenic emissions to similar to 220 nm in the high Arctic, consistent with the persistence of aged aerosols under background conditions. Warm airmass intrusions into the Arctic atmosphere were found to bring polluted anthropogenic aerosols into this pristine environment with an eightfold increase in rBC mass concentrations (median similar to 3.4 ng m(-3), rBC(max) similar to 74 ng m(-3)). A dominant influence of biomass-burning emissions from Eurasia during the warm airmass intrusion, which coincided with a shift toward larger rBC cores (similar to 264 nm) and moderate coating thickness. The light absorption enhancement of rBC estimated using core-shell Mie theory remained low during warm airmass intrusions (similar to 1-1.2) than under background conditions (similar to 1.1-1.6), underscoring a strong dependence of rBC radiative effects in the central Arctic on source regions and aging/processing during long-range transport. This study highlights the complexity of rBC aging and mixing state in the central Arctic, driven by variable source characteristics and summertime processing conditions and will help to increase the accuracy in representing rBC in climate models.

Primary biological aerosols such as pollen, fungal spores, bacteria and plants debris have traditionally been associated with the coarse particle mode. In contrast, small organic particles in the submicron range have largely been attributed to secondary formation processes, as few primary biogenic sources were known [1-3]. Due to their hygroscopic properties, bioaerosols may act as cloud condensation nuclei (CCN) and ice nuclei (IN), potentially influencing cloud formation and precipitation [4]. In the Amazon rainforest, coarse particles are typically present at lower number concentrations, whereas fine organic particles are more abundant and thus are known to contribute significantly to cloud microphysics under certain conditions [1,5].In this study, we investigate a previously overlooked primary biogenic source of organic aerosol droplets linked to spore release by many fungi and lichens, with measurements conducted at the Amazon Tall Tower Observatory (ATTO) site in Brazil [6]. Many lichenized and non-lichenized Ascomycota release spores actively, building pressure in their reproductive cells through osmolyte-driven water influx until the spores are suddenly expelled.We combined controlled laboratory experiments with ambient field measurements to characterize particles emitted during this process. Particle size distributions were measured in isolated chamber experiments using two complementary particle sizers covering a broad size range, providing information on both particle size and emission strength. Field experiments gave insights into emission patterns and triggers under natural tropical forest conditions. Droplets were additionally collected by impaction for further microscopic and chemical analyses. The chemical composition was determined using scanning transmission X-ray microscopy with near-edge X-ray absorption and fine structure (STXM-NEXAFS) spectroscopy, as well as high performance liquid chromatography (HPLC) with electrospray ionisation ultra-high resolution orbitrap mass spectrometry (ESI-UHR-Orbitrap-MS).This integrated approach allows us to assess the size, chemical composition, and emission strength of fungal aerosol emissions. The findings provide new insights into the contribution of sub- and supermicron fungal emissions to organic aerosol populations and their potential implications for atmospheric processes.[1] Pöschl, U., et al. (2010). Rainforest aerosols as biogenic nuclei of clouds and precipitation in the Amazon. Science, 329, 1513–1516. https://doi.org/10.1126/science.1191056[2] Barbosa, C. G. G., et al. (2022). Amazon rainforest aerosols: Characterization and implications for climate. npj Climate and Atmospheric Science, 5, 73. https://doi.org/10.1038/s41612-022-00294-y[3] Graham, B., et al. (2003). Source attribution and seasonality of Amazon aerosol: Implications for cloud formation. Journal of Geophysical Research: Atmospheres, 108. https://doi.org/10.1029/2003JD004049[4] Pöhlker, M. L., et al. (2023). Global organic and inorganic aerosol hygroscopicity and its effect on radiative forcing. Nature Communications, 14(1), 6139. https://doi.org/10.1038/s41467-023-41695-8[5] Moran-Zuloaga, D., et al. (2018). Long-term study on coarse mode aerosols in the Amazon rainforest with frequent intrusion of Saharan dust plumes. Atmospheric Chemistry and Physics, 18(13), 10055–10088. https://doi.org/10.5194/acp-18-10055-2018[6] Andreae, M. O., et al. (2015). The Amazon Tall Tower Observatory (ATTO): Overview of pilot measurements on ecosystem ecology, meteorology, trace gases, and aerosols. Atmospheric Chemistry and Physics, 15(18), 10723–10776. https://doi.org/10.5194/acp-15-10723-2015

Aerosol particles formed by new particle formation (NPF) are essential for cloud condensation nuclei and can strongly influence cloud properties and climate. However, the mechanisms behind NPF in the Amazon boundary layer have remained elusive. Classical "banana" NPF events, common in other continental regions, are rarely observed in the Amazon, while most detected sub-50 nm particles have been linked to precipitation- and downdraft-related episodes, often called Amazonian banana events. Here, we analyse a decade of particle number size distributions (10-420 nm) from the Amazon Tall Tower Observatory (ATTO) during the wet season and demonstrate the presence of a distinct phenomenon called Quiet NPF. This process represents a subtle but persistent background particle formation, occurring on days without clear banana-type growth signatures. Using a statistical approach, we show that Quiet NPF links freshly formed 10 nm particles to their subsequent growth into the Aitken mode. This mechanism is characterized by a growth rate of 2.4 +/- 0.1 nm h-1, about half that of Amazonian banana events, but occurs much more frequently. Quiet NPF accounts for similar to 45 % of 10-25 nm particle production during the wet season, revealing an overlooked but important source of nanoparticles that contributes to sustaining Amazonian aerosol populations.

Abstract The Asian summer monsoon establishes a strong connection between near-surface pollution in Southeast Asia and the global atmosphere by linking local emission sources with the large-scale circulation. There is a strong impact on the extratropical lower stratosphere, which is thought to occur mainly via quasi-horizontal export of polluted and moist air from the upper-level Asian monsoon anticyclone (AMA). The recent Probing High Latitude Export of Air from the Asian Summer Monsoon (PHILEAS) campaign focused on investigating this eddy transport and the associated mixing of monsoon-influenced air into the extratropical lower stratosphere through dedicated High Altitude and Long-Range Aircraft (HALO) observations from Oberpfaffenhofen, Germany, and Anchorage, Alaska, in the late summer and early autumn 2023. We summarize the mission’s motivation and objectives, place the Asian monsoon season 2023 into a climatological context, and present some representative observations. The observations during flights from Oberpfaffenhofen demonstrate the significant spatial and temporal AMA variability, which allowed HALO to investigate the displaced lower AMA boundary over the eastern Mediterranean, Israel, and Jordan. The observations during flights from Anchorage highlight the influence of long-range transport of moist and polluted air from the region of the Asian summer monsoon on the composition of the extratropical upper troposphere and lower stratosphere (UTLS), which impacts both ozone chemistry and the climate-relevant radiation budget. Significance Statement This study examines the Asian summer monsoon’s role in linking Southeast Asia’s near-surface atmosphere to the global upper troposphere and lower stratosphere. We find that the Asian summer monsoon plays a key role in transporting moisture, aerosols, and pollutants (including very-short-lived chlorinated compounds) to high-latitude regions. This, in turn, impacts atmospheric chemistry including stratospheric ozone depletion and alters the climate-relevant atmospheric radiation budget. Additionally, long-range transport of aerosols, such as ammonium nitrate, has the potential to influence cloud formation, further affecting climate and weather patterns.

The Amazon rain forest plays an important role in the biogeochemistry, water cycle, and climate of the South American continent and the Earth system. The Amazon Tall Tower Observatory (ATTO) has been established to study and quantify forest-atmosphere interactions under natural conditions, as well as the transformation of the Amazon ecosystem as a result of increasing perturbations related to deforestation and climate change. Here, we present the design and first results of a custom-made Robotic Lift system, RoLi, installed to automatically measure high-resolution vertical profiles along the 325 m tall ATTO tower at high spatial and temporal resolution at vertical profiling speeds up to 0.5ms-1. The RoLi payload of up to 80kg can be flexibly adjusted and comprises meteorological, trace gas, and aerosol instruments with short inlet lines, minimizing potential wall losses and related artifacts that may occur in longer sampling tubes of tall towers. First measurement results show spatiotemporal patterns in the altitude profiles of temperature, humidity, fog, and aerosol particle concentration and size. This proves RoLi's technical capability to resolve the diel interplay of convectively mixed daytime and stable stratified nighttime conditions. The RoLi data will help to better constrain the gradients and exchange of air masses, gases, and particles across the forest-atmosphere interface and related mixing processes in the lowermost planetary boundary layer.

Biomass burning aerosol (BBA) impacts climate through aerosol-cloud-radiation interactions, but models disagree on the sign and magnitude of BBA radiative effects. We quantify the sensitivity of BBA radiative effects and transport to three BBA-relevant processes and properties: parameterized oxidative aging of organic aerosol (OA), a combined change to black carbon (BC) density and the method for calculating aerosol refractive index, and reduction in OA density. We evaluate Unified Model simulations against two aircraft campaigns from summer 2017 over the Southeast Atlantic. The model generally performs well, such that discrepancies between the observational data sets may sometimes limit the precision of the evaluation. Our newly developed aging parameterization reproduces observed OA:BC mass ratios well and allows modeled OA:BC to decrease with smoke age, but increases bias in aerosol extinction and changes the BBA radiative effect little (+0.12 ). We calculate aerosol refractive index using either a volume-weighted component average or the Maxwell-Garnett (MG) mixing assumption, which represents BC as small inclusions in a host material. Compared to MG mixing, the volume-weighted average refractive index and reduced BC density increase aerosol absorption, substantially increasing the total BBA radiative effect (+2.66 ) and amount of BBA transported across the ocean through BC self-lofting. Reducing OA density to better match literature values changes the total BBA radiative effect by -1.96 . Changes to direct radiative effects exceed changes to cloud radiative effects. Our findings emphasize the sensitivity of aerosol radiative effects and transport to these processes and properties, which we suggest could be improved in climate models.

Abstract Soils are recognized sinks for atmospheric isoprene, but their in situ behavior remains understudied, particularly in the Amazon where emissions are globally significant. Here we show how the 2023 El Niño affected the diel and seasonal variation in soil isoprene fluxes. Under non-stress conditions, the soils acted as a persistent isoprene sink, with uptake driven primarily by ambient isoprene mixing ratios, following a diel cycle and peaking during the dry season. Soil organic matter and litter modulated the fluxes by shaping moisture, temperature, and gas-diffusion responses. During the 2023 El Niño dry season, soil moisture below ~20% constrained soil gas exchange, while associated heat extremes further reduced soil respiration and the soils’ isoprene uptake capacity. As climate change intensifies drought and heat extremes, reduced soil isoprene uptake capacity could impact atmospheric oxidation, aerosol formation, and methane lifetime. Incorporating soil isoprene uptake into atmospheric models is essential for quantifying these feedbacks.