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.

India experiences severe air pollution driven by human activities. The role of anthropogenic chlorine is significant yet underexplored, with its mechanisms poorly understood and impacts largely unquantified, despite its importance in atmospheric oxidation and secondary pollutant formation. Using the GEOS-Chem chemical transport model, we quantify the impact of human-derived chlorine emissions on particulate chloride (pCl−), particulate matter (PM2.5), ClNO2, and O3 in the boundary layer over India. Comprehensive model simulations reveal major chlorine hotspots affecting nearly ~ 700 million people across the Indo-Gangetic Plain (IGP). The PM2.5 concentration increases due to pCl− formation (principally NH4Cl). Annual mean pCl− and ClNO2 increase by 4-fold and 3-fold, respectively. Regionally and seasonally, enhancements range from 0.04 − 3.6 μg m−3 for pCl−, 7-273 ppt for ClNO2, and -0.47-0.44 ppb for O3 with strongest effects in autumn and winter. Compared to other polluted hotspots in the world, for example China, O3 showed a lower sensitivity to chlorine emissions over India. Anthropogenic chlorine significantly influences India’s air quality, underscoring the need to include chlorine emission inventories and chemistry in models.

Black carbon is a short-lived climate forcer that occurs as a continuum of particulate matter with varying physical, chemical and optical properties. However, current global climate assessments treat black carbon as a single compound, overlooking the distinct properties of its subtypes and introducing substantial uncertainty in determining its climate impact. Here we evaluate the climate relevance of this overlooked heterogeneity using approximately 2,500 thermal–optical measurements of carbon fractions across emission sources to develop a global inventory distinguishing between less light-absorbing char and more light-absorbing soot from 1750 to 2019. We observed an increase in the soot fraction of total anthropogenic black carbon emissions, from 16% in 1750 to 35% in 2019, primarily driven by increased fossil fuel consumption. This shift, aligning with sediment records, suggests that contemporary black carbon emissions possess greater global warming potential per unit mass than those at the onset of the industrial era. Our findings underscore the need to resolve black carbon subtypes in climate models and prioritize soot-rich sources for mitigation. The soot fraction of global black carbon emissions increased from 16% in 1750 to 35% in 2019, mainly driven by fossil fuel consumption, suggesting a higher warming potential per unit mass, according to an analysis of atmospheric measurements.

Abstract. Sea spray aerosol (SSA) influences climate through direct and indirect interactions with radiation. However, the magnitudes of these interactions remain poorly constrained, in part due to a lack of understanding of the influences of sea surface temperature (SST) on SSA production. There is no agreed-upon dependence of SSA production on SST despite numerous field, laboratory, and modeling investigations. In this study, these disagreements are addressed through a simple theoretical framework that describes the interfacial processes and contextualizes previous work. Next, we characterize the connection between SST, seawater bubble concentrations, SSA number concentrations, and SSA emission fluxes using measurements in the Scripps Ocean-Atmosphere Research Simulator (SOARS). This isolated ocean-atmosphere interaction system incorporates wind, waves, and SST controls to produce wave breaking under realistic and controlled conditions. In SOARS, increasing SST from 2 to 23 °C suppressed total subsurface bubble concentrations (between 6.17 and 830 µm) by a factor of 1.5, SSA number concentrations (between 0.008 and 20 µm) by a factor of 3, and SSA accumulation mode emission flux by a factor of 4. Using these trends, we derive SST-dependent number and mass emission flux correction factors for SSA source functions in climate models. While prior studies report both increases and decreases with SST, these controlled wind-wave-SST experiments demonstrate that increasing SST suppresses SSA production. Resolving this SST dependence is critical, as it directly alters marine aerosol burdens, cloud condensation nuclei, and radiative forcing, and provides a needed constraint missing from current parameterizations.

Mineral dust absorbs and scatters solar and infrared radiation, thereby affecting the radiance spectrum at the surface and top-of-atmosphere and the atmospheric heating rate. While half of the outgoing thermal radiation is emitted in the far infrared (FIR, 15-100 mu m), knowledge of the optical properties and thermal radiative effects of dust is currently limited to the mid-infrared region (MIR, 3-15 mu m). In this study we performed pellet spectroscopy measurements to evaluate the MIR and FIR contribution to dust absorbance and explore the variability and spectral diversity of the dust signature within the 2.5-25 mu m range. Thirteen dust samples re-suspended from parent soils with contrasting mineralogy were investigated, including low and mid latitude dust (LMLD) sources in Africa, America, Asia, and Middle East, and high latitude dust (HLD) from Iceland. Results show that the absorbance of dust in the FIR up to 25 mu m is comparable in intensity to that in the MIR. Also, spectrally different absorption (position and shape of the peaks) is observed for Icelandic dust compared to LMLD, due to differences in mineralogical composition. Corroborated with the few available literature data on absorption properties of natural dust and single minerals up to 100 mu m wavelength, these data suggest the relevance of MIR and FIR interactions to the dust radiative effect for low to high latitude sources. Furthermore, the dust spectral signatures in the MIR and FIR could potentially be used to characterise the mineralogy and differentiate the origin of airborne particles based on infrared remote sensing observations.

The ocean-atmosphere interface covers 71% of the globe and plays a key role in critical processes that regulate climate. In addition to serving as the primary sink for excess carbon dioxide and heat, oceans emit sea spray aerosols through wind-wave interactions. Sea spray aerosols (SSA), the most abundant natural aerosols, must be quantitatively characterized in terms of their emissions and physicochemical properties to improve their representation in climate models and refine predictions of their climate impacts. Replicating ambient ocean-atmosphere conditions in the laboratory offers a direct method for quantifying SSA transfer processes. The Scripps Ocean-Atmosphere Research Simulator (SOARS) produces SSA under highly controlled and realistic conditions by replicating wind-wave interactions. This study uses SOARS to generate and measure SSA number density distributions under relevant open ocean conditions. Total aerosol concentrations in SOARS approach 105 cm-3 under a 10-m extrapolated wind speed of 18 m s-1. Probability density functions of the number density distributions in SOARS align well with previous field and laboratory measurements, indicating that SOARS is an accurate simulator of SSA emissions. By reproducibly generating atmospherically realistic sea spray aerosols under controlled wind-wave conditions, SOARS reveals how wind speed and the whitecap fraction influence aerosol emissions and size distributions. These findings provide a laboratory-based framework for bridging field and model representations of primary marine aerosol production across the global ocean.

In central Amazonia, aerosol sources, weather, and chemical processes create a highly variable aerosol population. The aerosols' optical properties, shaped by composition and size, determine sunlight interaction and the regional radiation budget. Previous studies observed differences in the particles' physical properties during smoke events and described their vertical gradients during clean periods. However, a complete characterization of these properties at two height levels considering both seasons is still missing. This study connects aerosol optical measurements from the Amazon Tall Tower Observatory (ATTO), at 60 and 325 m heights, to particle composition and sources, characterizing different aerosol populations, assessing their vertical gradients, and associating them with the influence of various emission sources and atmospheric processes. A seasonally segregated clustering method was applied to five years of optical data (2018–2023), allowing for the identification of periods with low biomass-burning impact, long-range transport (LRT) events, and regional pollution episodes. Aerosols from Saharan dust events showed the highest real and imaginary refractive index, along with a large inorganic mass fraction (around 26 %), which differs from typical Amazonian conditions. Furthermore, regional biomass-burning emissions during the dry season promoted elevated fine-mode particle concentrations (median 2250 cm−3), dominated by absorbing carbonaceous material. These particles also showed the maximum mass scattering efficiency, which was consistently higher at the 60 m height, underscoring the importance of vertical transport and aerosol aging processes. These results indicate that the clustering method can discriminate between aerosol populations and elucidate differences between particles of different sources and processes influencing the Amazonian atmosphere.

Aerosols influence Earth's energy balance and hydrological cycle as cloud condensation nuclei (CCN), yet uncertainties persist in how anthropogenic emissions alter their abundance and climate-relevant properties. Abrupt, large-scale reductions in human activities provided a natural experiment to quantify anthropogenic impact on aerosol-cloud-climate interactions in coastal India. Combining chemical and microphysical measurements under drastically reduced and subsequently reintroduced emission scenarios, we reveal that CCN concentrations increased by 80-250% postlockdown. This surge coincided with increased new particle formation (NPF) event frequency and enhanced particle growth rates. Postlockdown air masses shifted from marine to continental sources, revealing that anthropogenic organic matter (OM), despite lower hygroscopicity, dominated particle growth to CCN-active sizes, offsetting hygroscopicity limitations. These findings demonstrate how shifts in anthropogenic activity can strongly impact aerosol-cloud interaction potential, even under varying air mass influences, and provide a reference for understanding the atmospheric effects of future air quality interventions.

Long-term and high-precision measurements of the mole fraction of greenhouse gases (GHG), together with their isotopic composition, are of fundamental importance to understand land–atmosphere interactions. Current flask monitoring programs have important information gaps in large regions of the Earth, particularly in the southern hemisphere and in continental tropical regions. Here, we report on the initiation of a monitoring program and the resulting dataset of high-precision GHG measurements at the Amazon Tall Tower Observatory (ATTO), located in the central Amazon region of Brazil. In September 2021, we installed an automated flask sampler designed and built by the Integrated Carbon Observation System (ICOS) to collect air samples in 3 L flasks at a height of 324 m above ground level (ma.g.l.). Samples are collected weekly, during a one-hour integration time between 13:00 and 14:00 h LT (17:00–18:00 UTC). The flasks are shipped to Jena, Germany, for analyses of CO2, CO, CH4, N2O, H2, SF6, δ13C−CO2, Δ14C−CO2, δ18O−CO2, δ13C−CH4, δ2H−CH4, O2/N2, and Ar/N2 at the laboratories of the Max Planck Institute for Biogeochemistry (MPI-BGC). Measurements from this monitoring program provide reference information for this site and act as an additional independent quality control for other measurements in the region. The record of SF6 and simulations based on a regional atmospheric transport model suggest that the footprint of the measurements is predominantly from the southeasterly and northeasterly directions. The time series of the different gas species measured in this monitoring program are being made publicly available through the ATTO data portal (https://doi.org/10.17871/ATTO.574.13.2365, Sierra et al., 2025).

Aerosols, with their direct and indirect effects impacting the climate, have been established to significantly perturb Earth's radiative budget and hydrological cycle. The climate impact of aerosols is complex and multifaceted, with various factors influencing the combined net effect. The intricacies of aerosol effects, mainly through aerosol-cloud interactions, necessitate precise measurements to reduce the uncertainty in forecasting future climate fluctuations1. Studying their characteristics in pristine settings can provide an enhanced scientific understanding of aerosol impact in background conditions, as opposed to polluted ones2. With this motivation, we conducted a comprehensive field measurement campaign during the second phase of the COVID-induced lockdown in Munnar, a relatively clean high-altitude site in the Western Ghats of India. Munnar is surrounded by lush tea plantations and extensive forest reserves, and tea production and tourism are the major human activities in the area. However, suspended tourist activities due to the pandemic and frequent precipitation during monsoon enabled us to study the ambient aerosol characteristics in near-natural conditions3. This study presents results from the size-resolved Cloud Condensation Nuclei (SR-CCN) measurements conducted along with aerosol size distribution and chemical composition at the Natural Aerosol and Bioaerosol High Altitude Laboratory (NABHA; 10.09 N, 77.06 E; 1605m asl) during the Southwest Monsoon season between June-October 2021. The median number concentration for 10–450nm particles was observed to be 533cm-3, with 357cm-3and 908cm-3 as first and third quartiles, respectively, similar to other pristine locations, such as Amazonia during the wet season4. The average non-refractory particulate matter (NR-PM1) concentration was 2.28±1.81 µg/m3 (mean ± one standard deviation). The SR-CCN measurements were carried out for set supersaturations between 0.1% and 0.85% for particles ranging between 20-350 nm in diameter. The critical dry diameter varied from 60 to 150nm for highest to lowest supersaturation, similar to previously reported studies elsewhere4,5. During the campaign, the efficiency spectra of CCN often reached unity despite organic aerosols dominating the submicron aerosol composition.Further, hygroscopicity, a particle size and composition function, was investigated using the kappa-Köhler theory. The hygroscopicity parameter, kappa, derived from SR-CCN measurements(kCCN) varied between 0.26 and 0.57. kCCN did not exhibit much variation in the Aitken mode regime (60-80nm) but increased in the accumulation mode (100-160nm), suggesting higher hygroscopic fraction in larger (aged) particles. Assuming a linear mixing of organic and inorganic aerosols, chemically derived hygroscopicity (kchem) was comparable to kCCN, following similar diurnal variation. Further details will be presented.References:1.Lohmann, U. & Ferrachat, S. Impact of parametric uncertainties on the present-day climate and on the anthropogenic aerosol effect. AtmosChemPhys (2010).2.Andreae, M. O. Aerosols Before Pollution. Science (2007).3.Navasakthi, S., Pandey, A., Bhari, J. S. & Sharma, A. Significant variation in air quality in South Indian cities during COVID-19 lockdown and unlock phases. EnvironMonitAssess (2023).4.Gunthe, S. S. et al. Cloud condensation nuclei in pristine tropical rainforest air of Amazonia: size-resolved measurements and modeling of atmospheric aerosol composition and CCN activity. AtmosChemPhys (2009).5.Singh, A. et al. Rapid growth and high cloud-forming potential of anthropogenic sulfate aerosol in a thermal power plant plume during COVID lockdown in India. NPJClimAtmosSci (2023).

Microbes in the sea surface microlayer (SML) are key to connecting the ocean and the atmosphere, affecting the exchange of matter, momentum, and heat at the interface. However, their sources have never been quantified systematically. Seawater has long been deemed their major source, whereas atmospheric deposition is regarded as trivial or merely providing additional nutrients. Here, combining atmospheric observations and quantitative budget analyses, we show that during the Indonesian peatland wildfire events the smoke can directly deposit abundant microbes into the SML, which can be comparable to the estimated supply from seawater and potentially diversify the microbial community of the SML. This land–air–ocean interaction is relevant for global climate, as it may induce previously unknown effects on the air–sea interactions, especially in an increasingly warming future with more intensifying wildfires.

We present results from ATTO-Campina, a new permanent observational site in central Amazon, about 4 km from the ATTO towers. Operational since 2020, ATTO-Campina characterizes atmospheric, cloud and rainfall properties through remote sensing. The goal is to provide continuous, complementary measurements to the ATTO towers, addressing the rainforest’s complex gas-aerosol-cloud-precipitation dynamics. Using a 3.5-year dataset, we classified convective clouds into three types: shallow cumulus ( ShCu ), congestus ( Con ) or ( Deep ) clouds. The shallow-to-deep transition takes about three hours, starting with ShCu formation at 11:00 local time. The accumulated rainfall peak follows at about 16:00. Only weak downdrafts are present in the upper troposphere where previous studies indicate new particle formation (NPF) occurrence. Strong downdrafts are mostly limited to heights below 5 km. Con and Deep convective days have higher concentrations of ultrafine aerosol and lower concentrations of accumulation-mode particles compared to ShCu . Convective clouds also significantly modify gas mixing ratios. Deep convective clouds are associated with high near-surface O 3 , consistent with downward transport from the midtroposphere. Our results showcase the added detail achieved by integrating data from the ATTO towers and ATTO-Campina sites. Together, these sites support better understanding of interconnected gas-aerosol-cloud-precipitation processes in the Amazon and their evolution under climate change.

Aerosols over the oceans significantly influence the composition of the Earth’s atmosphere and climate. Over the vast expanse of oceans, aerosols are emitted at high rates, primarily through wave breaking and bubble bursting. Additionally, secondary aerosol particles can be formed by gas-phase reactions. Particles emitted from shipping activities, long-range transport from continents, and potential downward transport from the upper troposphere can also contribute to aerosols in the marine boundary layer (MBL). Hence, marine aerosols represent a highly heterogeneous and complex mixture pivotal in regulating the global radiation budget.This study explores aerosol number size distributions over the North Atlantic Ocean measured on the research vessel S/Y Eugen Seibold. The cruises conducted between June 2020 and September 2021 covered a broad geographic range from polar (~67° N) to tropical (~3° N) waters. Aerosols were sampled at approximately 13 m above the ocean surface using two sets of instrumentation covering particle sizes of 10 nm to 430 nm and 520 nm to 20 µm.Three aerosol size modes, Aitken, accumulation, and coarse, were consistently identified throughout the dataset, aligning with prior research. Additionally, we observed an intermittently occurring nucleation mode and a bimodal coarse mode. While the presence of a nucleation mode suggests aerosol formation over the open ocean, it did not exhibit the typical banana-shaped contour plots usually observed during new particle formation and subsequent growth. Prior research indicates that ships may act as emitters in this size range, in addition to secondary formation from atmospheric gases.Here, we describe and discuss particle number size distributions observed over the open ocean, raising questions about the formation and lifecycle of aerosol modes. Our goal is to track the behavior of nucleation, Aitken, accumulation and coarse modes in the MBL with high temporal resolution, spanning the latitudinal range of the northern Atlantic Ocean. A comprehensive dataset is prepared by incorporating surface ocean data retrieved on board and satellite observations to disentangle the natural and/or anthropogenic origins of aerosols responsible for the observed particle number size distributions in the MBL.

Comprehending the intricate interplay between atmospheric aerosols and water vapour in subsaturated regions is vital for accurate modelling of aerosol–cloud–radiation–climate dynamics. But the microphysical mechanisms governing these interactions with ambient aerosols remain inadequately understood. Here we report results from high-altitude, relatively pristine site in Western-Ghats of India during monsoon, serving as a baseline for climate processes in one of the world’s most polluted regions. Utilizing a novel quartz crystal microbalance (QCM) approach, we conducted size-resolved sampling to analyse humidity-dependent growth factors, hygroscopicity, deliquescence behaviour, and aerosol liquid water content (ALWC). Fine-mode aerosols (≤2.5 μm) exhibited size-dependent interactions with water vapour, contributing significantly to ALWC. Deliquescence was observed in larger aerosols (>180 nm), influenced by organic species, with deliquescence relative humidity (DRH) lower than that of pure inorganic salts. This research highlights the significance of understanding ambient aerosol-water interactions and hygroscopicity for refining climate models in subsaturated conditions.

This study investigates the rain-initiated mixing and variability in the mixing ratio of selected trace gases in the atmosphere over the central Amazon rain forest. It builds on comprehensive data from the Amazon Tall Tower Observatory (ATTO), spanning from 2013 to 2020 and comprising the greenhouse gases (GHGs) carbon dioxide (CO2) and methane (CH4); the reactive trace gases carbon monoxide (CO), ozone (O3), nitric oxide (NO), and nitrogen dioxide (NO2); and selected volatile organic compounds (VOCs). Based on more than 1000 analyzed rainfall events, the study resolves the trace gas mixing ratio patterns before, during, and after the rain events, along with vertical mixing ratio gradients across the forest canopy. The assessment of the rainfall events was conducted independently for daytime and nighttime periods, which allows us to elucidate the influence of solar radiation. The mixing ratios of CO2, CO, and CH4 clearly declined during rainfall, which can be attributed to the downdraft-related entrainment of pristine air from higher altitudes into the boundary layer, a reduction of the photosynthetic activity under increased cloud cover, and changes in the surface fluxes. Notably, CO showed a faster reduction than CO2, and the vertical gradient of CO2 and CO is steeper than for CH4. Conversely, the O3 mixing ratio increased across all measurement heights in the course of the rain-related downdrafts. Following the O3 enhancement by up to a factor of 2, NO, NO2, and isoprene mixing ratios decreased. The temporal and vertical variability of the trace gases is intricately linked to the diverse sink and source processes, surface fluxes, and free-troposphere transport. Within the canopy, several interactions unfold among soil, atmosphere, and plants, shaping the overall dynamics. Also, the mixing ratio of biogenic VOCs (BVOCs) clearly varied with rainfall, driven by factors such as light, temperature, physical transport, and soil processes. Our results disentangle the patterns in the trace gas mixing ratio in the course of sudden and vigorous atmospheric mixing during rainfall events. By selectively uncovering processes that are not clearly detectable under undisturbed conditions, our results contribute to a better understanding of the trace gas life cycle and its interplay with meteorology, cloud dynamics, and rainfall in the Amazon.

Multiphase oxidation of sulfur dioxide (SO2) is an important source of sulfate in the atmosphere. There are, however, concerns that protons produced during SO2 oxidation may cause rapid acidification of aerosol water and thereby quickly shut down the fast reactions favored at high pH. Here, we show that the sustainability of sulfate production is controlled by the competing effects of multiphase buffering and acidification, which can be well described by a characteristic buffering time, τbuff. Both GEOS‐Chem simulations and observations show that globally, τbuff is long enough (days) to sustain sulfate production over most populated regions, where the acidification of aerosol water is counteracted by the strong buffering effect of NH4+/NH3. Our results highlight the importance of anthropogenic ammonia emissions and pervasive human influences in shaping the chemical environment of the atmosphere.

The wet-season atmosphere in the central Amazon resembles natural conditions with minimal anthropogenic influence, making it one of the rare preindustrial-like continental areas worldwide. Previous long-term studies have analyzed the properties and sources of the natural Amazonian background aerosol. However, the vertical profile of the planetary boundary layer (PBL) has not been assessed systematically. Since 2017, such a profile assessment has been possible with the 325 m high tower at the Amazon Tall Tower Observatory (ATTO), located in a largely untouched primary forest in the central Amazon. This study investigates the variability of submicrometer aerosol concentration, size distribution, and optical properties at 60 and 325 m in the Amazonian PBL. The results show significant differences in aerosol volumes and scattering coefficients in the vertical gradient. The aerosol population was well-mixed throughout the boundary layer during the daytime but became separated upon stratification during the nighttime. We also found a significant difference in the spectral dependence of the scattering coefficients between the two heights. The analysis of downdrafts and the related rainfall revealed changes in the aerosol populations before and after rain events, with absorption and scattering coefficients decreasing as optically active particles are removed by wet deposition. The recovery of absorption and scattering coefficients is faster at 325 m than at 60 m. Convective events were concomitant with rapid increases in the concentrations of sub-50 nm particles, which were likely associated with downdrafts. We found that the aerosol population near the canopy had a significantly higher mass scattering efficiency than at 325 m. There was also a clear spectral dependence, with values for λ=450, 525, and 635 nm of 7.74±0.12, 5.49±0.11, and 4.15±0.11 m2 g−1, respectively, at 60 m, while at 325 m the values were 5.26±0.06, 3.76±0.05, and 2.46±0.04 m2 g−1, respectively. The equivalent aerosol refractive index results, which were obtained for the first time for the wet season in the central Amazon, show slightly higher scattering (real) components at 60 m compared to 325 m of 1.33 and 1.27, respectively. In contrast, the refractive index's absorptive (imaginary) component was identical for both heights, at 0.006. This study shows that the aerosol physical properties at 60 and 325 m are different, likely due to aging processes, and strongly depend on the photochemistry, PBL dynamics, and aerosol sources. These findings provide valuable insights into the impact of aerosols on climate and radiative balance and can be used to improve the representation of aerosols in global climate models.

Australian mega-wildfires in the summer of 2019-2020 injected smoke into the stratosphere, causing strong ozone depletion in the lower stratosphere. Here, we model the smoke plume and reproduce its unexpected trajectory toward the middle stratosphere at ~35-kilometer altitude. We show that a smoke-charged vortex (SCV) induced and maintained by absorbing aerosols played a key role in lofting pollutants from the lower stratosphere and nearly doubled the southern hemispheric aerosol burden in the middle stratosphere. The SCV caused a redistribution of stratospheric aerosols, which boosted heterogeneous chemistry in the middle stratosphere and enhanced ozone production, compensating for up to 70% of the ozone depletion in the lower stratosphere. As global warming continues, we expect a growing frequency and importance of SCVs in promoting the impacts of wildfires on stratospheric aerosols and chemistry.