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Airborne mineral dust particles can act as natural seeds for cirrus clouds in the upper troposphere. However, the atmospheric abundance of dust is unconstrained in cirrus-forming regions, hampering our ability to predict these radiatively important clouds. Here we present global-scale measurements of dust aerosol abundance in the upper troposphere and incorporate these into a detailed cirrus-formation model. We show that dust aerosol initiates cirrus clouds throughout the extra-tropics in all seasons and dominates cirrus formation in the Northern Hemisphere (75–93% of clouds seasonally). Using a global transport model with improved dust treatment, we also explore which of Earth’s deserts are the largest contributors of dust aerosol to cirrus-forming regions. We find that the meteorological environment downstream of each emission region modulates dust atmospheric lifetime and transport efficiency to the upper troposphere so that source contributions are disproportionate to emissions. Our findings establish the critical role of dust in Earth’s climate system through the formation of cirrus clouds.

We present the first data on the concentration of sea-salt aerosol throughout most of the depth of the troposphere and over a wide range of latitudes, which were obtained during the Atmospheric Tomography (ATom) mission. Sea-salt concentrations in the upper troposphere are very small, usually less than 10 ng per standard m3 (about 10 parts per trillion by mass) and often less than 1 ng m−3. This puts stringent limits on the contribution of sea-salt aerosol to halogen and nitric acid chemistry in the upper troposphere. Within broad regions the concentration of sea-salt aerosol is roughly proportional to water vapor, supporting a dominant role for wet scavenging in removing sea-salt aerosol from the atmosphere. Concentrations of sea-salt aerosol in the winter upper troposphere are not as low as in the summer and the tropics. This is mostly a consequence of less wet scavenging in the drier, colder winter atmosphere. There is also a source of sea-salt aerosol over pack ice that is distinct from that over open water. With a well-studied and widely distributed source, sea-salt aerosol provides an excellent test of wet scavenging and vertical transport of aerosols in chemical transport models.

The size of aerosol particles has fundamental effects on their chemistry and radiative effects. We explore those effects using aerosol size and composition data in the lowermost stratosphere along with calculations of light scattering. In the size range between about 0.1 and 1.0 µm diameter (accumulation mode), there are at least two modes of particles in the lowermost stratosphere. The larger mode consists mostly of particles produced in the stratosphere, and the smaller mode consists mostly of particles transported from the troposphere. The stratospheric mode is similar in the Northern and Southern Hemisphere, whereas the tropospheric mode is much more abundant in the Northern Hemisphere. The purity of sulfuric acid particles in the stratospheric mode shows that there is limited production of secondary organic aerosol in the stratosphere, especially in the Southern Hemisphere. Out of eight sets of flights sampling the lowermost stratosphere (four seasons and two hemispheres) there were three with large injections of specific materials: volcanic, biomass burning, or dust. The stratospheric and tropospheric modes have very different roles for radiative effects on climate and for heterogeneous chemistry. Because the larger particles are more efficient at scattering light, most of the radiative effect in the lowermost stratosphere is due to stratospheric particles. In contrast, the tropospheric particles can have more surface area, at least in the Northern Hemisphere. The surface area of tropospheric particles could have significant implications for heterogeneous chemistry because these particles, which are partially neutralized and contain organics, do not correspond to the substances used for laboratory studies of stratospheric heterogeneous chemistry. We then extend the analysis of size-dependent properties to particles injected into the stratosphere, either intentionally or from volcanoes. There is no single size that will simultaneously maximize the climate impact relative to the injected mass, infrared heating, potential for heterogeneous chemistry, and undesired changes in direct sunlight. In addition, light absorption in the far ultraviolet is identified as an issue requiring more study for both the existing and potentially modified stratosphere.

Black carbon (BC) aerosol plays an important role in the Earth’s climate system because it absorbs solar radiation and therefore potentially warms the climate; however, BC can also act as a seed for cloud particles, which may offset much of its warming potential. If BC acts as an ice nucleating particle (INP), BC could affect the lifetime, albedo, and radiative properties of clouds containing both supercooled liquid water droplets and ice particles (mixed-phase clouds). Over 40% of global BC emissions are from biomass burning; however, the ability of biomass burning BC to act as an INP in mixed-phase cloud conditions is almost entirely unconstrained. To provide these observational constraints, we measured the contribution of BC to INP concentrations ([INP]) in real-world prescribed burns and wildfires. We found that BC contributes, at most, 10% to [INP] during these burns. From this, we developed a parameterization for biomass burning BC and combined it with a BC parameterization previously used for fossil fuel emissions. Applying these parameterizations to global model output, we find that the contribution of BC to potential [INP] relevant to mixed-phase clouds is ∼5% on a global average.

Single-particle mass spectrometry (SPMS) instruments characterize the composition of individual aerosol particles in real time. Their fundamental ability to differentiate the externally mixed particle types that constitute the atmospheric aerosol population enables a unique perspective into sources and transformation. However, quantitative measurements by SPMS systems are inherently problematic. We introduce a new technique that combines collocated measurements of aerosol composition by SPMS and size-resolved absolute particle concentrations on aircraft platforms. Quantitative number, surface area, volume, and mass concentrations are derived for climate-relevant particle types such as mineral dust, sea salt, and biomass burning smoke. Additionally, relative ion signals are calibrated to derive mass concentrations of internally mixed sulfate and organic material that are distributed across multiple particle types. The NOAA Particle Analysis by Laser Mass Spectrometry (PALMS) instrument measures size-resolved aerosol chemical composition from aircraft. We describe the identification and quantification of nine major atmospheric particle classes, including sulfate–organic–nitrate mixtures, biomass burning, elemental carbon, sea salt, mineral dust, meteoric material, alkali salts, heavy fuel oil combustion, and a remainder class. Classes can be sub-divided as necessary based on chemical heterogeneity, accumulated secondary material during aging, or other atmospheric processing. Concentrations are derived for sizes that encompass the accumulation and coarse size modes. A statistical error analysis indicates that particle class concentrations can be determined within a few minutes for abundances above ∼10 ng m−3. Rare particle types require longer sampling times. We explore the instrumentation requirements and the limitations of the method for airborne measurements. Reducing the size resolution of the particle data increases time resolution with only a modest increase in uncertainty. The principal limiting factor to fast time response concentration measurements is statistically relevant sampling across the size range of interest, in particular, sizes D < 0.2 µm for accumulation-mode studies and D > 2 µm for coarse-mode analysis. Performance is compared to other airborne and ground-based composition measurements, and examples of atmospheric mineral dust concentrations are given. The wealth of information afforded by composition-resolved size distributions for all major aerosol types represents a new and powerful tool to characterize atmospheric aerosol properties in a quantitative fashion.

Global observations and model studies indicate that new particle formation (NPF) in the upper troposphere (UT) and subsequent particles supply 40 %–60 % of cloud condensation nuclei (CCN) in the lower troposphere, thus affecting the Earth's radiative budget. There are several plausible nucleation mechanisms and precursor species in this atmospheric region, which, in the absence of observational constraints, lead to uncertainties in modeled aerosols. In particular, the type of nucleation mechanism and concentrations of nucleation precursors, in part, determine the spatial distribution of new particles and resulting spatial distribution of CCN from this source. Although substantial advances in understanding NPF have been made in recent years, NPF processes in the UT in pristine marine regions are still poorly understood and are inadequately represented in global models. Here, we evaluate commonly used and state-of-the-art NPF schemes in a Lagrangian box model to assess which schemes and precursor concentrations best reproduce detailed in situ observations. Using measurements of aerosol size distributions (0.003 < Dp < 4.8 µm) in the remote marine troposphere between ∼0.18 and 13 km altitude obtained during the NASA Atmospheric Tomography (ATom) mission, we show that high concentrations of newly formed particles in the tropical UT over both the Atlantic and Pacific oceans are associated with outflow regions of deep convective clouds. We focus analysis on observations over the remote Pacific Ocean, which is a region less perturbed by continental emissions than the Atlantic. Comparing aerosol size distribution measurements over the remote Pacific with box model simulations for 32 cases shows that none of the NPF schemes most commonly used in global models, including binary nucleation of sulfuric acid and water (neutral and ion-assisted) and ternary involving sulfuric acid, water, and ammonia, are consistent with observations, regardless of precursor concentrations. Through sensitivity studies, we find that the nucleation scheme among those tested that is able to explain most consistently (21 of 32 cases) the observed size distributions is that of Riccobono et al. (2014), which involves both organic species and sulfuric acid. The method of Dunne et al. (2016), involving charged sulfuric acid–water–ammonia nucleation, when coupled with organic growth of the nucleated particles, was most consistent with the observations for 5 of 32 cases. Similarly, the neutral sulfuric acid–water–ammonia method of Napari (2002), when scaled with a tuning factor and with organic growth added, was most consistent for 6 of 32 cases. We find that to best reproduce both nucleation and growth rates, the mixing ratios of gas-phase organic precursors generally need to be at least twice that of SO2, a proxy for dimethyl sulfide (DMS). Unfortunately, we have no information on the nature of oxidized organic species that participated in NPF in this region. Global models rarely include organic-driven nucleation and growth pathways in UT conditions where globally significant NPF takes place, which may result in poor estimates of NPF and CCN abundance and contribute to uncertainties in aerosol–cloud–radiation effects. Furthermore, our results indicate that the organic aerosol precursor vapors may be important in the tropical UT above marine regions, a finding that should guide future observational efforts.

Organic aerosol (OA) is an important constituent of the Earth's atmosphere, yet the extent of its destruction by photolysis remains an active research question. Recent laboratory studies reveal evidence for rapid short‐term photolysis for secondary OA, but the rate declines to negligible levels over time. Here we use the stratosphere to investigate long‐term OA photolysis because of the relatively simple sources and sinks of OA in this region. Airborne campaign observations show that the organic content in organic‐sulfate aerosols remains stable with altitude and time in the stratosphere, indicating no significant photolysis. Satellite observations of the 2020 Australian wildfires reveal OA persists over a year in the stratosphere, consistent with model simulations excluding long‐term photolysis. These findings suggest long‐term OA photolysis is negligible in the real atmosphere. The current Community Earth System Model (CESM) significantly underestimates the abundance of stratospheric OA due to assumed rapid photolysis. We add this well‐validated mechanism into CESM by turning off secondary OA photolysis after it is 50 days old, effectively simulating stratospheric OA consistent with observations. In summary, multiple lines of evidence confirm that the long‐term photolysis of OA is negligible or extremely slow. Incorporating this mechanism into CESM addresses a key model deficiency, improving simulation of stratospheric OA. Plain Language Summary Organic aerosols are important particles in the atmosphere, but it's unclear how much sunlight (photolysis) breaks them down over time. Recent laboratory studies show that older organic aerosols become resistant to photolysis. Using a climate model (CESM) and aircraft observations, we find that older organic aerosols in the stratosphere change very little, suggesting slow or negligible photolysis. Simulations of the 2020 Australian wildfires confirm that if high organic aerosol photolysis happened, the particles would disappear too quickly. We propose a new model strategy that better represents organic aerosol behavior, improving predictions for future climate and stratospheric chemistry research. Key Points Recent laboratory and field studies both indicate that photolysis of secondary organic aerosols diminishes to negligible levels over time The persistence of wildfire and background organic aerosols in the stratosphere shows that they undergo negligible or very slow photolysis A field‐constrained photolysis strategy is derived for use in organic aerosol modeling

Biogenic organic compounds in the surface ocean may significantly alter the cloud‐forming ability of sea spray aerosol and thereby affect the amount of solar radiation reaching the ocean surface. Estimates of the organic mass fraction of sea spray vary widely, and some results show a significant dependence on biological activity in the source seawater. We present airborne observations of the organic mass fraction of individual sea spray particles measured using the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument during the Atmospheric Tomography (ATom) mission, which sampled a wide range of latitudes and altitudes over the remote Atlantic and Pacific Oceans across four seasons, from the marine boundary layer to the upper troposphere. The measured sea spray particles of about 0.15–0.7 μm dry diameter showed higher average organic mass fractions at smaller sizes, but values were low overall, with regional integrated submicron means almost always <10%. Atmospheric aging adds organics to sea spray particles, leading to higher mean organic mass fractions (sometimes exceeding 50%) in the free troposphere than in the marine boundary layer. The average submicron sea spray organic mass fractions are on the low end of previously reported values and show weak seasonal variability for most regions. These results imply that recent biological activity in the surface ocean has only weak control over how much organic matter is in nascent submicron sea spray particles over the remote oceans, in contrast to findings from some observational studies and global numerical simulations. Plain Language Summary The physical properties of particles found over the oceans are important to understand because marine particles interact with radiation both directly and by affecting the formation and lifetime of clouds. Both of these roles are important for the climate. Some studies show that biologically formed organic matter in the surface ocean can be a major component of sea spray particles formed at the ocean surface by wave breaking. If this is true broadly over the oceans, then biological cycles may control the climate‐relevant properties of sea spray particles. However, measurements that can directly answer this question are limited. Here we report multi‐season, size‐dependent, ambient global‐scale observations of particle composition in which only sea spray particles are considered. The results show that fresh sea spray particles over the remote oceans have relatively little organic matter, even in biologically productive seasons. This implies that surface ocean biology may have only a small role in governing the properties of fresh sea spray particles over most of the oceans. Key Points Organic compounds make up only a small fraction of submicron sea spray particle mass over the remote oceans Regional seasonal cycles in sea spray organic mass fraction are mostly small in amplitude, indicating weak biological control Atmospheric aging can drive sea spray organic mass fractions significantly higher than in primary emitted sea spray

Natural aerosols in pristine regions form the baseline used to evaluate the impact of anthropogenic aerosols on climate. Sea spray aerosol (SSA) is a major component of natural aerosols. Despite its importance, the abundance of SSA is poorly constrained. It is generally accepted that wind-driven wave breaking is the principle governing SSA production. This mechanism alone, however, is insufficient to explain the variability of SSA concentration at given wind speed. The role of other parameters, such as sea surface temperature (SST), remains controversial. Here, we show that higher SST promotes SSA mass generation at a wide range of wind speed levels over the remote Pacific and Atlantic Oceans, in addition to demonstrating the wind-driven SSA production mechanism. The results are from a global scale dataset of airborne SSA measurements at 150 to 200 m above the ocean surface during the NASA Atmospheric Tomography Mission. Statistical analysis suggests that accounting for SST greatly enhances the predictability of the observed SSA concentration compared to using wind speed alone. Our results support implementing SST into SSA source functions in global models to better understand the atmospheric burdens of SSA.

Each year, commercial ships emit over 1.67 Tg of particulate matter (PM) pollution into the atmosphere. These ships rely on the combustion of heavy fuel oil, which contains high levels of sulfur, large aromatic organic compounds, and metals. Vanadium is one of the metals most commonly associated with heavy fuel oil and is often used as a tracer for PM from ship exhaust. Previous studies have suggested that vanadium-containing PM has impacts on human health and climate due to its toxicological and cloud-formation properties, respectively; however, its distribution in the atmosphere is not fully understood, which limits our ability to quantify the environmental implications of PM emitted by ships. Here, we present data obtained from a Particle Analysis by Laser Mass Spectrometry (PALMS) instrument on the NASA DC-8 aircraft during the 2016–2018 Atmospheric Tomography Mission (ATom) and show that ∼ 1 % of the accumulation mode particles measured in the marine boundary layer of the central Pacific and Atlantic oceans contain vanadium. These measurements, which were made without targeting ship plumes, suggest that PM emitted by ships is widespread in the atmosphere. Furthermore, we observed vanadium-containing ship exhaust particles at altitudes up to 13 km, which demonstrates that not all ship exhaust particles are immediately removed via wet deposition processes. In addition, using laboratory calibrations, we determined that most vanadium-containing ship exhaust particles can contain up to a few weight percent of vanadium. This study furthers our understanding of both the chemical composition and distribution of PM emitted by ships, which will allow us to better constrain the climate, health, and air quality implications of these particle types in the future. We note that these data were collected prior to the 2020 International Maritime Organization (IMO) sulfur regulation and stand as a reference for understanding how ship emissions have evolved in light of these regulations.