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Atmospheric marine particle concentrations impact cloud properties, which strongly impact the amount of solar radiation reflected back into space or absorbed by the ocean surface. While satellites can provide a snapshot of current conditions at the overpass time, models are necessary to simulate temporal variations in both particle and cloud properties. However, poor model accuracy limits the reliability with which these tools can be used to predict future climate. Here, we leverage the comprehensive ocean ecosystem and atmospheric aerosol–cloud dataset obtained during the third deployment of the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES3). Airborne and ship-based measurements were collected in and around a cold-air outbreak during a 3 d (where d stands for day) intensive operations period from 17–19 September 2017. Cold-air outbreaks are of keen interest for model validation because they are challenging to accurately simulate, which is due, in part, to the numerous feedbacks and sub-grid-scale processes that influence aerosol and cloud evolution. The NAAMES observations are particularly valuable because the flight plans were tailored to lie along Lagrangian trajectories, making it possible to spatiotemporally connect upwind and downwind measurements with the state-of-the-art FLEXible PARTicle (FLEXPART) Lagrangian particle dispersion model and then calculate a rate of change in particle properties. Initial aerosol conditions spanning an east–west, closed-cell-to-clear-air transition region of the cold-air outbreak indicate similar particle concentrations and properties. However, despite the similarities in the aerosol fields, the cloud properties downwind of each region evolved quite differently. One trajectory carried particles through a cold-air outbreak, resulting in a decrease in accumulation mode particle concentration (−42 %) and cloud droplet concentrations, while the other remained outside of the cold-air outbreak and experienced an increase in accumulation mode particle concentrations (+62 %). The variable meteorological conditions between these two adjacent trajectories result from differences in the local sea surface temperature in the Labrador Current and surrounding waters, altering the stability of the marine atmospheric boundary layer. Further comparisons of historical satellite observations indicate that the observed pattern occurs annually in the region, making it an ideal location for future airborne Lagrangian studies tracking the evolution of aerosols and clouds over time under cold-air outbreak conditions.

Marine biogenic particle contributions to atmospheric aerosol concentrations are not well understood though they are important for determining cloud optical and cloud-nucleating properties. Here we examine the relationship between marine aerosol measurements (with satellites and model fields of ocean biology) and meteorological variables during the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES). NAAMES consisted of four field campaigns between November 2015 and April 2018 that aligned with the four major phases of the annual phytoplankton bloom cycle. The FLEXible PARTicle (FLEXPART) Lagrangian particle dispersion model is used to spatiotemporally connect these variables to ship-based aerosol and dimethyl sulfide (DMS) observations. We find that correlations between some aerosol measurements with satellitemeasured and modeled variables increase with increasing trajectory length, indicating that biological and meteorological processes over the air mass history are influential for measured particle properties and that using only spatially coincident data would miss correlative connections that are lagged in time. In particular, the marine non-refractory organic aerosol mass correlates with modeled marine net primary production when weighted by 5 d air mass trajectory residence time (r = 0:62). This result indicates that nonrefractory organic aerosol mass is influenced by biogenic volatile organic compound (VOC) emissions that are typically produced through bacterial degradation of dissolved organic matter, zooplankton grazing on marine phytoplankton, and as a by-product of photosynthesis by phytoplankton stocks during advection into the region. This is further supported by the correlation of non-refractory organic mass with 2 d residence-time-weighted chlorophyll a (r = 0:39), a proxy for phytoplankton abundance, and 5 d residencetime-weighted downward shortwave forcing (r = 0:58), a requirement for photosynthesis. In contrast, DMS (formed through biological processes in the seawater) and primary marine aerosol (PMA) concentrations showed better correlations with explanatory biological and meteorological variables weighted with shorter air mass residence times, which reflects their localized origin as primary emissions. Aerosol submicron number and mass negatively correlate with sea surface wind speed. The negative correlation is attributed to enhanced PMA concentrations under higher wind speed conditions. We hypothesized that the elevated total particle surface area associated with high PMA concentrations leads to enhanced rates of condensation of VOC oxidation products onto PMA. Given the high deposition velocity of PMA relative to submicron aerosol, PMA can limit the accumulation of secondary aerosol mass. This study provides observational evidence for connections between marine aerosols and underlying ocean biology through complex secondary formation processes, emphasizing the need to consider air mass history in future analyses.

Abstract. Aerosols over Earth's remote and spatially extensive ocean surfaces have important influences on planetary climate. However, these aerosols and their effects remain poorly understood, in part due to the remoteness and limited observations over these regions. In this study, we seek to understand factors that shape marine aerosol size distributions and composition in the Northwest Atlantic Ocean region. We use the GEOS-Chem-TOMAS model to interpret measurements collected from ship and aircraft during the four seasonal campaigns of the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES) conducted between 2015 and 2018. Observations from the NAAMES campaigns show enhancements in aerosol total number concentration at atmospheric altitudes of about 1 km, most pronounced during the phytoplankton bloom maxima (May/June). Our simulations, combined with NAAMES ship and aircraft measurements, suggest several key factors contribute to aerosol number and size in the Northwest Atlantic lower troposphere, with significant regional-mean (40–60° N, 20–50° W) aerosol-cloud albedo indirect effects (AIE) and direct radiative effects (DRE) during the phytoplankton bloom. These key factors and their associated radiative effects in the region are: (1) particle formation above/near the marine boundary layer (MBL) top (AIE: −3.37 W m−2, DRE: −0.62 W m−2), (2) particle growth due to marine secondary organic aerosol (MSOA) as the nascent particles subside into the MBL, enabling them to become cloud-condensation-nuclei-size particles (AIE: −2.27 W m−2, DRE: −0.10 W m−2), (3) particle formation/growth due to the products of dimethyl sulfide, above/within the MBL (−1.29 W m−2, DRE: −0.06 W m−2), and (4) ship emissions (AIE: −0.62 W m−2, DRE: −0.05 W m−2). Our results suggest a synergy of particle formation near the MBL top and growth by MSOA that contributes strongly to cloud-condensation-nuclei-sized particles with significant regional radiative effects in the Northwest Atlantic. Future work is needed to understand the sources and temperature-dependence of condensable marine vapors forming MSOA and to understand the species that can form new particles at the boundary layer top and grow these particles as they descend into the marine boundary layer.

The western North Atlantic Ocean is strongly influenced by continental outflow, making it an ideal region to study the atmospheric transition from a polluted coastline to the marine environment. Utilizing eight transit flights between the NASA Langley Research Center (LaRC) in Hampton, Virginia, and the remote island of Bermuda from NASA's Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE), we examine the evolution of trace gas and aerosol properties off the US East Coast. The first pair of flights flew along the wind trajectory of continental outflow, while the other flights captured a mix of marine and continental air mass sources. For measurements within the boundary layer (BL), there was an offshore decline in particle N<100nm , N>100nm , CH 4 , CO, and CO 2 concentrations, all leveling off around ∼900 km offshore from the LaRC. These trends are strongest for the first pair of flights. In the BL, offshore declines in organic mass fraction and increases in sulfate mass fraction coincide with increasing hygroscopicity based on f (RH) measurements. Free troposphere measurements show a decline in N<100nm , but other measured parameters are more variable when compared to the prominent offshore gradients seen in the BL. Pollution layers exist in the free troposphere, such as smoke plumes, that can potentially entrain into the BL. This work provides detailed case studies with a broad set of high-resolution measurements to further our understanding of the transition between continental and marine environments. [ABSTRACT FROM AUTHOR]

Marine boundary layer clouds play a critical role in Earth's energy balance. Their microphysical and radiative properties are highly impacted by ambient aerosols and dynamic forcings. In this study, we evaluate the representation of these clouds and related aerosol–cloud interaction processes in the single-column version of the E3SM climate model (SCM) against field measurements collected during the NASA ACTIVATE campaign over the western North Atlantic, as well as intercompare results with high-resolution process level models. We show that E3SM SCM reproduces the macrophysical properties of post-frontal boundary layer clouds in a cold-air outbreak (CAO) case well. However, it generates fewer but larger cloud droplets compared to aircraft measurements. Further sensitivity tests show that the underestimation of both aerosol number concentration and vertical velocity variance contributes to this bias. Aerosol–cloud interactions are examined by perturbing prescribed aerosol properties in E3SM SCM with fixed dynamics. Higher aerosol number concentration or hygroscopicity leads to more numerous but smaller cloud droplets, resulting in a stronger cooling via shortwave cloud forcing. This apparent Twomey effect is consistent with prior climate model studies. The cloud liquid water path shows a weakly positive relation with cloud droplet number concentration due to precipitation suppression. This weak aerosol effect on cloud macrophysics may be attributed to the dominant impact of strong dynamical forcing associated with the CAO. Our findings indicate that the SCM framework is a key tool to bridge the gap between climate models, process level models, and field observations to facilitate process level understanding. [ABSTRACT FROM AUTHOR]

Remote marine regions comprise a high fraction of Earth's surface, but in situ vertically resolved measurements over these locations remain scarce. Here we use airborne data during 15 vertical spiral soundings (0.15–8.5 km) over Bermuda during the NASA Aerosol Cloud meTeorology Interactions over the western ATlantic Experiment (ACTIVATE) to investigate the impact of different source regions on the vertical structure of trace gases, aerosol particles, and meteorological variables over 1000 km offshore of the US East Coast. Results reveal significant differences in vertical profiles of variables between three different air mass source categories (North America, Ocean, Caribbean/North Africa) identified using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model: (i) the strongest pollution signature is from air masses from the North America category, while the weakest one is from the Ocean category; (ii) North America air has the highest levels of CO, CH 4 , submicron particle number concentration, aerosol mass spectrometer (AMS) mass, and organic mass fraction along with smoke layers in the free troposphere (FT); (iii) Ocean air has the highest relative amount of nitrate, non-sea-salt sulfate, and oxalate, which are key acidic species participating in chloride depletion; (iv) air masses from the Caribbean/North Africa showed a pronounced coarse aerosol signature in the FT and reduced aerosol hygroscopicity, which is associated with dust transport; and (v) there is considerable vertical heterogeneity for almost all variables examined, including higher O 3 and submicron particle concentrations with altitude, suggesting that the FT is a potential contributor of both constituents in the marine boundary layer. This study highlights the importance of considering air mass source origin and vertical resolution to capture aerosol and trace gas properties over remote marine areas. [ABSTRACT FROM AUTHOR]

Biogenic sources contribute to cloud condensation nuclei (CCN) in the clean marine atmosphere, but few measurements exist to constrain climate model simulations of their importance. The chemical composition of individual atmospheric aerosol particles showed two types of sulfate-containing particles in clean marine air masses in addition to mass-based Estimated Salt particles. Both types of sulfate particles lack combustion tracers and correlate, for some conditions, to atmospheric or seawater dimethyl sulfide (DMS) concentrations, which means their source was largely biogenic. The first type is identified as New Sulfate because their large sulfate mass fraction (63% sulfate) and association with entrainment conditions means they could have formed by nucleation in the free troposphere. The second type is Added Sulfate particles (38% sulfate), because they are preexisting particles onto which additional sulfate condensed. New Sulfate particles accounted for 31% (7 cm-3) and 33% (36 cm-3) CCN at 0.1% supersaturation in late-autumn and late-spring, respectively, whereas sea spray provided 55% (13 cm-3) in late-autumn but only 4% (4 cm-3) in late-spring. Our results show a clear seasonal difference in the marine CCN budget, which illustrates how important phytoplankton-produced DMS emissions are for CCN in the North Atlantic.

This study discusses an analysis of combined airborne and ground observations of particulate nitrate (NO3 - (p)) concentrations made during the wintertime DISCOVER-AQ study at one of the most polluted cities in the United States, Fresno, CA in the San Joaquin Valley (SJV) and focuses on development of understanding of the various processes that impact surface nitrate concentrations during pollution events. The results provide an explicit case-study illustration of how nighttime chemistry can influence daytime surface-level NO3 - (p) concentrations, complementing previous studies in the SJV. The observations exemplify the critical role that nocturnal chemical production of NO3 - (p) aloft in the residual layer (RL) can play in determining daytime surface-level NO3 - (p) concentrations. Further, they indicate that nocturnal production of NO3 - (p) in the RL, along with daytime photochemical production, can contribute substantially to the build-up and sustaining of severe pollution episodes. The exceptionally shallow nocturnal boundary layer heights characteristic of wintertime pollution events in the SJV intensifies the importance of nocturnal production aloft in the residual layer to daytime surface concentrations. The observations also demonstrate that dynamics within the RL can influence the early-morning vertical distribution of NO3 - (p), despite low wintertime wind speeds. This overnight reshaping of the vertical distribution above the city plays an important role in determining the net impact of nocturnal chemical production on local and regional surface-level NO3 - (p) concentrations. Entrainment of clean free tropospheric air into the boundary layer in the afternoon is identified as an important process that reduces surface-level NO3 - (p) and limits build-up during pollution episodes. The influence of dry deposition of HNO3 gas to the surface on daytime particulate nitrate concentrations is important but limited by an excess of ammonia in the region, which leads to only a small fraction of nitrate existing in the gas-phase even during the warmer daytime. However, in late afternoon, when diminishing solar heating leads to a rapid fall in the mixed boundary layer height, the impact of surface deposition is temporarily enhanced and can lead to a substantial decline in surface-level particulate nitrate concentrations; this enhanced deposition is quickly arrested by a decrease in surface temperature, which drops the gas-phase fraction to near zero. The overall importance of enhanced late afternoon gas-phase loss to the multiday build-up of pollution events is limited by the very shallow nocturnal boundary layer. The case study here demonstrates that mixing down of NO3 - (p) from the RL can contribute a majority of the surface-level NO3 - (p) in the morning (here, ~80%), and a strong influence can persist into the afternoon even when photochemical production is maximum. The particular day-to-day contribution of aloft nocturnal NO3 - (p) production to surface concentrations will depend on prevailing chemical and meteorological conditions. Although specific to the SJV, the observations and conceptual framework further developed here provide general insights into the evolution of pollution episodes in wintertime environments.

Mesoscale organization of marine convective clouds into linear or clustered states is prevalent across the tropical and subtropical oceans, and its investigation served as a guiding focus for a series of process study flights conducted as part of the Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) during summer 2020, 2021, and 2022. These select ACTIVATE flights involved a novel strategy for coordinating two aircraft, with respective remote sensing and in situ sampling payloads, to probe regions of organized shallow convection for several hours. The main purpose of this measurement report is to summarize the aircraft sampling approach, describe the characteristics and evolution of the cases, and provide an overview of the datasets that can serve as a starting point for more detailed modeling and analysis studies. Six flights are described, involving a total of 80 dropsonde profiles that capture the environment surrounding clustered shallow convection. The flights include detailed observations of the vertical structure of cloud systems, comprising up to 20 in situ sampling levels. Four cases involved deepening convection rooted in the marine boundary layer that developed vertically to 2–5 km with varying precipitation amounts, while two cases captured more complex and developed cumulus congestus systems extending above 5 km. In addition to the thermodynamic and dynamic characterization afforded by dropsonde and in situ measurements, the datasets include cloud and aerosol microphysics, trace gas concentrations, aerosol and droplet composition, and cloud and aerosol remote sensing from high-spectral-resolution lidar and polarimetry. [ABSTRACT FROM AUTHOR]

Organosulfates are important secondary organic aerosol (SOA) components and good tracers for aerosol heterogeneous reactions. However, the knowledge of their spatial distribution, formation conditions, and environmental impact is limited. In this study, we report two organosulfates, an isoprene-derived isoprene epoxydiols (IEPOX) (2,3-epoxy-2-methyl-1,4-butanediol) sulfate and a glycolic acid (GA) sulfate, measured using the NOAA Particle Analysis Laser Mass Spectrometer (PALMS) on board the NASA DC8 aircraft over the continental U.S. during the Deep Convective Clouds and Chemistry Experiment (DC3) and the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS). During these campaigns, IEPOX sulfate was estimated to account for 1.4% of submicron aerosol mass (or 2.2% of organic aerosol mass) on average near the ground in the southeast U.S., with lower concentrations in the western U.S. (0.2-0.4%) and at high altitudes (

[This corrects the article DOI: 10.1371/journal.pone.0121207.].

The Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) is a six-year (2019-2024) NASA Earth-Venture Suborbital-3 (EVS-3) mission to robustly characterize aerosol-cloud-meteorology interactions over the western North Atlantic Ocean (WNAO) during winter and summer seasons, with a focus on marine boundary layer clouds. This characterization requires understanding the aerosol life cycle (sources and sinks), composition, transport pathways, and distribution in the WNAO region. We use the GEOS-Chem chemical transport model driven by the MERRA-2 reanalysis to simulate tropospheric aerosols that are evaluated against in situ and remote sensing measurements from Falcon and King Air aircraft, respectively, as well as ground-based and satellite observations over the WNAO during the winter (Feb. 14 - Mar. 12) and summer (Aug. 13 - Sep. 30) field deployments of ACTIVATE 2020. Transport of pollution in the boundary layer behind cold fronts is a major mechanism for the North American continental outflow to the WNAO during Feb.-Mar. 2020. While large-scale frontal lifting is a dominant mechanism in winter, convective lifting significantly increases the vertical extent of major continental outflow aerosols in summer. Turbulent mixing is found to be the dominant process responsible for the vertical transport of sea salt within and ventilation out of the boundary layer in winter. The simulated boundary layer aerosol composition and optical depth (AOD) in the ACTIVATE flight domain are dominated by sea salt, followed by organic aerosol and sulfate. Compared to winter, boundary layer sea salt concentrations increased in summer over the WNAO, especially from the ACTIVATE flight areas to Bermuda, because of enhanced surface winds and emissions. Dust concentrations also significantly increased in summer because of long-range transport from North Africa. Comparisons of model and aircraft submicron non-refractory aerosol species (measured by an HR-ToF-AMS) vertical profiles show that intensive measurements of sulfate, nitrate, ammonium, and organic aerosols in the lower troposphere over the WNAO in winter provide useful constraints on model aerosol wet removal by precipitation scavenging. Comparisons of model aerosol extinction (at 550 nm) with the King Air High Spectral Resolution Lidar-2 (HSRL-2) measurements (at 532 nm) and CALIOP/CALIPSO satellite retrievals (at 532 nm) indicate that the model generally captures the continental outflow of aerosols, the land-ocean aerosol extinction gradient, and the mixing of anthropogenic aerosols with sea salt. Large enhancements of aerosol extinction at ~1.5-6.0 km altitudes from long-range transport of the western U.S. fire smoke were observed by HSRL-2 and CALIOP during Aug.-Sep. 2020. Model simulations with biomass burning (BB) emissions injected up to the mid-troposphere (vs. within the BL) better reproduce these remote-sensing observations, Falcon aircraft organic aerosol vertical profiles, as well as AERONET AOD measurements over eastern U.S. coast and Tudor Hill, Bermuda. High aerosol (mostly coarse-mode sea salt) extinction near the top (~1.5-2.0 km) of the marine BL along with high relative humidity and cloud extinction were typically seen over the WNAO (< 35°N) in the CALIOP aerosol extinction profiles and GEOS-Chem simulations, suggesting strong hygroscopic growth of sea salt particles and sea salt seeding of marine boundary layer clouds. Contributions of different emission types (anthropogenic, BB, biogenic, marine, and dust) to the total AOD over the WNAO in the model are also quantified. Future modeling efforts should focus on improving parameterizations for aerosol wet scavenging and sea salt emissions, implementing realistic BB emission injection height, and applying high-resolution models that better resolve vertical transport. [ABSTRACT FROM AUTHOR]

The western North Atlantic Ocean is strongly influenced by continental outflow, making it an ideal region to study the atmospheric transition from a polluted coastline to the marine environment. Utilizing eight transit flights between NASA Langley Research Center (LaRC) in Hampton, Virginia and the remote island of Bermuda from NASA's Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE), we examine the evolution of trace gas and aerosol properties off the U.S. East Coast. The first pair of flights flew along the wind trajectory of continental outflow, while the other flights captured a mix of marine and continental air mass sources. For measurements within the boundary layer (BL), there was an offshore decline in particle N<100 nm, N>100 nm, CH4, CO, and CO2 concentrations, all leveling off around ~900 km offshore from LaRC. These trends are strongest for the first pair of flights. In the BL, offshore declines in organic mass fraction and increases in sulfate mass fraction coincide with increasing hygroscopicity based on f(RH) measurements. Free troposphere measurements show a decline in N<100 nm but other measured parameters are more variable when compared to the prominent offshore gradients seen in the BL. Pollution layers exist in the free troposphere, such as smoke plumes, that can potentially entrain into the BL. This work provides detailed case studies with a broad set of high-resolution measurements to further our understanding of the transition between continental and marine environments. [ABSTRACT FROM AUTHOR]

As part of the third NASA Aircraft Particle Emissions Experiment (APEX-3, November 2005), whole air samples were collected to determine the emission rates of volatile organic compounds (VOCs) from aircraft equipped with three different gas-turbine engines (an Allison Engine 3007-A1E, a Pratt-Whitney 4158, and a Rolls-Royce RB211-535E4B). Samples were collected 1 m behind the engine exhaust plane of the engines while they were operated at powers ranging from idle up to 30% of maximum rated thrust.Exhaust emission indices (mass emitted per kilogram of fuel used) for CO and non-methane hydrocarbons (NMHCs) were calculated based on enhancements over background relative to CO 2. Emissions of all NMHCs were greatest at low power with values decreasing by an order of magnitude with increasing power. Previous studies have shown that scaling idle hydrocarbon emissions to formaldehyde or ethene (which are typically emitted at a ratio of 1-to-1 at idle) reduces variability amongst engine types. NMHC emissions were found to scale at low power, with alkenes contributing over 50% of measured NMHCs. However, as the power increases hydrocarbon emissions no longer scale to ethene, as the aromatics become the dominant species emitted. This may be due in part to a shift in combustion processes from thermal cracking (producing predominantly alkenes) to production of new molecules (producing proportionally more aromatics) as power increases. The formation of these aromatics is an intermediate step in the production of soot, which also increases with increasing power. The increase in aromatics relative to alkenes additionally results in a decrease in the hydroxyl radical reactivity and ozone formation potential of aircraft exhaust.Samples collected 30 m downwind of the engine were also analyzed for NMHCs and carbonyl compounds (acetone, 2-butanone and C 1-C 9 aldehydes). Formaldehyde was the predominant carbonyl emitted; however, the ratio of ethene-to-formaldehyde varied between the aircraft, possibly due to the sampling of transient emissions such as engine start-up and power changes. A large portion of the measured emissions (27-42% by mass) in the plume samples was made up of hazardous air pollutants (HAPs) with oxygenated compounds being most significant. © 2012.

Marine boundary-layer clouds play a critical role in the Earth's energy balance. Their microphysical and radiative properties are highly impacted by ambient aerosols and dynamical forcings. In this study, we evaluate the representation of these clouds and related aerosol-cloud interactions processes in the single-column version of E3SM climate model (SCM), against field measurements collected during the NASA ACTIVATE campaign over the western North Atlantic, as well as intercompare with high-resolution process-level models. Results show that E3SM-SCM, driven by the ERA5 reanalysis, reproduces the cloud properties as good as the high-resolution WRF simulations. For stronger surface forcings combined with a weaker subsidence taken from a WRF cloud-resolving simulation, both E3SM-SCM and WRF large-eddy simulation produce thicker clouds. This indicates that a proper combination of large-scale dynamics, sub-grid scale parameterizations, and model configurations is needed to obtain optimal performance of cloud simulations. In the E3SM-SCM sensitivity tests with fixed dynamics but perturbed aerosol properties, higher aerosol number concentration leads to more numerous but smaller cloud droplets, resulting in a stronger shortwave cloud forcing (i.e., stronger radiative cooling). This apparent Twomey effect is consistent with prior climate model studies. Cloud liquid water path shows a weakly positive relation with cloud droplet number concentration associated with precipitation suppression, which is different from the nonlinear relation approximated from prior observations and E3SM studies, warranting future investigation. Our findings indicate that the SCM framework is a key tool to bridge the gap between climate models, high-resolution models, and field observations to facilitate process-level understanding. [ABSTRACT FROM AUTHOR]

Marine boundary-layer clouds play a critical role in the Earth's energy balance. Their microphysical and radiative properties are highly impacted by ambient aerosols and dynamical forcings. In this study, we evaluate the representation of these clouds and related aerosol-cloud interactions processes in the single-column version of E3SM climate model (SCM), against field measurements collected during the NASA ACTIVATE campaign over the western North Atlantic, as well as intercompare with high-resolution process-level models. Results show that E3SM-SCM, driven by the ERA5 reanalysis, reproduces the cloud properties as good as the high-resolution WRF simulations. For stronger surface forcings combined with a weaker subsidence taken from a WRF cloud-resolving simulation, both E3SM-SCM and WRF large-eddy simulation produce thicker clouds. This indicates that a proper combination of large-scale dynamics, sub-grid scale parameterizations, and model configurations is needed to obtain optimal performance of cloud simulations. In the E3SM-SCM sensitivity tests with fixed dynamics but perturbed aerosol properties, higher aerosol number concentration leads to more numerous but smaller cloud droplets, resulting in a stronger shortwave cloud forcing (i.e., stronger radiative cooling). This apparent Twomey effect is consistent with prior climate model studies. Cloud liquid water path shows a weakly positive relation with cloud droplet number concentration associated with precipitation suppression, which is different from the nonlinear relation approximated from prior observations and E3SM studies, warranting future investigation. Our findings indicate that the SCM framework is a key tool to bridge the gap between climate models, high-resolution models, and field observations to facilitate process-level understanding. [ABSTRACT FROM AUTHOR]

As one of the dominant sinks of aerosol particles, wet scavenging greatly influences aerosol lifetime and interactions with clouds, precipitation, and radiation. However, wet scavenging remains highly uncertain in models, hindering accurate predictions of aerosol spatiotemporal distributions and downstream interactions. In this study, we present a flexible, computationally inexpensive method to identify meteorological variables relevant for estimating wet scavenging using a combination of aircraft, satellite, and reanalysis data augmented by trajectory modeling to account for air mass history. We assess the capabilities of an array of meteorological variables to predict the transport efficiency of black carbon (TE BC) using a combination of nonlinear regression, curve fitting, and k -fold cross-validation. We find that accumulated precipitation along trajectories (APT) – treated as a wet scavenging indicator across multiple studies – does poorly when predicting TE BC. Among different precipitation characteristics (amount, frequency, intensity), precipitation intensity was the most effective at estimating TE BC but required longer trajectories (>48 h) and including only intensely precipitating grid cells. This points to the contribution of intense precipitation to aerosol scavenging and the importance of accounting for air mass history. Predictors that were most able to predict TE BC were related to the distribution of relative humidity (RH) or the frequency of humid conditions along trajectories, suggesting that RH is a more robust way to estimate TE BC than APT. We recommend the following alternatives to APT when estimating aerosol scavenging: (1) the 90th percentile of RH along trajectories, (2) the fraction of hours along trajectories with either water vapor mixing ratios >15 g kg -1 or RH >95 %, and (3) precipitation intensity along trajectories at least 48 h along and filtered for grid cells with precipitation >0.2 mm h -1. Future scavenging parameterizations should consider these meteorological variables along air mass histories. This method can be repeated for different regions to identify region-specific factors influencing wet scavenging. [ABSTRACT FROM AUTHOR]

Chloride (Cl-) displacement from sea salt particles is an extensively studied phenomenon with implications on human health, visibility, and the global radiation budget. Past works have investigated Cl- depletion over the northwest Atlantic (NWA), yet an updated, multiseasonal, and geographically expanded account of sea salt reactivity over the region is needed. This study uses chemically resolved mass concentrations and meteorological data from the airborne Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) to quantify seasonal, spatial, and meteorological trends in Cl- depletion and to explore the importance of quantifying (1) non-sea salt sources of Na+ and (2) mass concentrations of lost Cl- instead of relative amounts displaced. Lost Cl- mass concentrations are lowest in December–February and March, moderate around Bermuda in June, and highest in May (median losses of 0.04, 0.04, 0.66, and 1.76 µg m-3, respectively), with losses in May high enough to potentially accelerate tropospheric oxidation rates. Inorganic acidic species can account for all Cl- depletion in December–February, March, and June near Bermuda, yet none of the lost Cl- in May, suggesting organic acids may be of importance for Cl- displacement in certain months. Contributions of dust to Na+ are not important seasonally but may cause relevant overestimates of lost Cl- in smoke and dust plumes. Higher percentages of Cl- depletion often do not correspond to larger mass concentrations of lost Cl-, so it is highly recommended to quantify the latter to place depletion reactions in context with their role in atmospheric oxidation and radiative forcing. [ABSTRACT FROM AUTHOR]

Due to their fast evolution and large natural variability in macro- and microphysical properties, the accurate representation of boundary layer clouds in current climate models remains a challenge. One of the regions with large intermodel spread in the Coupled Model Intercomparison Project Phase 6 ensemble is the western North Atlantic Ocean. Here, statistically representative in situ measurements can help to develop and constrain the parameterization of clouds in global models. To this end, we performed comprehensive measurements of boundary layer clouds, aerosol, trace gases, and radiation in the western North Atlantic Ocean during the NASA Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) mission. In total, 174 research flights with 574 flight hours for cloud and precipitation measurements were performed with the HU-25 Falcon during three winter (February–March 2020, January–April 2021, and November 2021–March 2022) and three summer seasons (August–September 2020, May–June 2021, and May–June 2022). Here we present a statistical evaluation of 16 140 individual cloud events probed by the fast cloud droplet probe and the two-dimensional stereo cloud probe during 155 research flights in a representative and repetitive flight strategy allowing for robust statistical data analyses. We show that the vertical profiles of distributions of the liquid water content and the cloud droplet effective diameter (ED) increase with altitude in the marine boundary layer. Due to higher updraft speeds, higher cloud droplet number concentrations (Nliquid) were measured in winter compared to summer despite lower cloud condensation nucleus abundance. Flight cloud cover derived from statistical analysis of in situ data is reduced in summer and shows large variability. This seasonal contrast in cloud coverage is consistent with a dominance of a synoptic pattern in winter that favors conditions for the formation of stratiform clouds at the western edge of cyclones (post-cyclonic). In contrast, a dominant summer anticyclone is concomitant with the occurrence of shallow cumulus clouds and lower cloud coverage. The evaluation of boundary layer clouds and precipitation in the Nliquid ED phase space sheds light on liquid, mixed-phase, and ice cloud properties and helps to categorize the cloud data. Ice and liquid precipitation, often masked in cloud statistics by a high abundance of liquid clouds, is often observed throughout the cloud. The ACTIVATE in situ cloud measurements provide a wealth of cloud information useful for assessing airborne and satellite remote-sensing products, for global climate and weather model evaluations, and for dedicated process studies that address precipitation and aerosol–cloud interactions. [ABSTRACT FROM AUTHOR]

Nucleation in the free troposphere (FT) and subsequent growth of new particles represent a globally important source of cloud condensation nuclei (CCN). Whereas new particle formation (NPF) has been shown to occur frequently in the upper troposphere over tropical oceans, there have been few studies of NPF at lower altitudes. In addition, the impact of urban emissions and biomass burning on the NPF in tropical marine FT remains poorly understood. In this study, we examine NPF in the lower and mid-troposphere (3–8.5 km) over the tropical ocean and coastal region using airborne measurements during the recent Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP 2 Ex). NPF was mostly observed above 5.5 km and coincided with elevated relative humidity (RH) and reduced condensation sink (CS), suggesting that NPF occurs in convective cloud outflow. The frequency of NPF increases with altitude, reaching ∼ 50 % above 8 km. An abrupt decrease in NPF frequency coincides with early monsoon transition and is attributed to increased CS resulting from reduced convective activity and more frequent transport of aged urban plumes. Surprisingly, a large fraction of NPF events in background air were observed in the early morning, and the NPF is likely made possible by very low CS despite low actinic flux. Convectively detrained biomass-burning plumes and fresh urban emissions enhance NPF as a result of elevated precursor concentrations and scavenging of pre-existing particles. In contrast, NPF is suppressed in aged urban plumes where the reactive precursors are mostly consumed, while CS remains relatively high. This study shows a strong impact of urban and biomass-burning emissions on the NPF in tropical marine FT. The results also illustrate the competing influences of different variables and interactions among anthropogenic emissions, convective clouds, and meteorology, which lead to NPF under a variety of conditions in tropical marine environments. [ABSTRACT FROM AUTHOR]