Cloud Condensation Nuclei Activity Research Articles

A New Method for Size‐Resolved Aerosol CCN Activity Measurement at Low Supersaturation in Pristine Atmosphere

AbstractUnderstanding aerosol‐cloud interactions under pristine atmospheric conditions is crucial for meaningful global climate change predictions. However, ability of aerosol particles to act as cloud condensation nuclei (CCN) activity of aerosols in pristine atmosphere remains limited, mainly because of the difficulty of measuring it in pristine environments. Given the extremely low CCN number concentration under low supersaturations (SSs) and the complex impact of the kinetic limitation in CCN measurements, size‐resolved CCN activity measurements at low SSs cannot be achieved with traditional methods using differential mobility analyzers and a CCN counter (CCNC) because of the low sample efficiency and the impact of particles with multiple charges. To overcome this difficulty, we propose a new method for measuring size‐resolved CCN activity at low aerosol number concentrations. The new method is based on a combined system comprising an aerodynamic aerosol classifier (AAC) and a CCNC (AAC‐CCNC system). A control program was developed to achieve high‐resolution scans of the SS and particle size within a reasonable time frame. A data inversion scheme, including corrections for the transfer function and kinetic limitation, was developed to obtain accurate size‐resolved CCN activity. In July‐August 2022, the new method was used to measure the size‐resolved CCN activity on the Tibetan Plateau. This new method can advance the size‐resolved CCN measurements, thereby enhancing our understanding of aerosol‐cloud interactions in pristine atmosphere.

Impact of Anthropogenic Aerosol Transport on Cloud Condensation Nuclei Activity During Summertime in Qilian Mountain, in the Northern Tibetan Plateau

AbstractIn this study, we conducted field campaigns at two mountain‐top observatories on the Qilian Mountains (QLM) in the northeastern Tibetan Plateau of China, which plays a vital role in sustaining water resources for the downstream arid and semi‐arid regions. The two observatories, Waliguan Baseline Observatory (WLG) and Laohugou station (LHG), are situated on the eastern and western edge of the QLM, respectively. The objectives of this study were to examine the properties of atmospheric aerosols, CCN concentrations (NCCN) at varying supersaturation levels (SS = 0.2%–1.0%), and the hygroscopic nature of aerosols in the QLM, especially during the formation of orographic cloud. Notably, the average aerosol concentration and NCCN was approximately 2–3 factors higher at WLG compared to LHG. The chemical compositions of aerosols were primarily dominated by sulfate and organic aerosol (OA) at these two sites. A very high hygroscopicity parameter of aerosol calculated using chemical composition was observed at these two sites (0.37 ± 0.03 at LHG vs. 0.30 ± 0.04 at WLG). Enhanced aerosol loading episodes impacted by anthropogenic emission were observed at these two sites. Exploration on high‐loading cases at each site, we found that the CCN activity was dominated by aerosol size, but the chemical processes of aerosol during the formation of orographic cloud could also be important in CCN activity, especially for low SS conditions. These findings collectively underscore the significant impact of anthropogenic air plumes on CCN concentrations in the QLM and their potential influence on precipitation patterns.

Efficient droplet activation of ambient black carbon particles in a suburban environment

Abstract. The cloud condensation nuclei (CCN) activity of black carbon (BC) particles importantly determines their impacts on cloud microphysics and atmospheric lifetime. This process is crucially influenced by the number of hygroscopic coating materials that BC acquire during the ageing process. It remains a challenge for ambient measurements to capture this process and link this with CCN activity of BC. Here, we directly measured the droplet activation diameter (D50) and activation fraction of BC-containing (BCc) particles (Fact,BC) in suburban Beijing using coupled measurements of size-resolved number concentrations of CCN at configured water supersaturation (SS) and BCc particles. The number concentration of BCc particles was found to peak at diameter 180–210 nm after acquiring coatings, larger than that for all particles (50–150 nm). Consequently, the initially smaller BC particles become enlarged and more hygroscopic, thereby exhibiting CCN activities than other particles. The Fact,BC increased from 42 % to 69 % in number and from 67 % to 85 % in mass as SS increased from 0.1 % to 0.2 % but tended to reach a plateau when SS > 0.2 %. Notably, Fact,BC and D50 linearly correlated with equivalent photochemical age, at a rate of +2 % h−1 and −3 nm h−1, respectively. The results suggest BCc particles from anthropogenic sources can readily serve as CCN at a relatively low SS, and more than half of the BC population can be activated within a few hours, indicating that the surface-sourced BC can efficiently incorporate into clouds and potentially exert important indirect radiative impacts.

The delivery of water ice to the Martian surface by passive degassing

Passive degassing, or volatile release from persistent, non-eruptive volcanic activity, may have contributed significantly to the Martian ice budget over the planet's history by providing a pathway for volatile species, such as water vapor, to outgas from the interior into the atmosphere and accumulate on the surface. Passive degassing from Martian volcanoes can result in ice accumulations in unusual locations (e.g., outside of the typical cold traps like the Tharsis Rise and polar ice caps). Such locally derived water could cover the volcanic source in an ice-rich veneer that may later serve as a source of ice for subsequent interactions between impactors or lava. Using the Laboratoire de Météorologie Dynamique Generic Planetary Climate Model, we model the spatial distribution of ice that results from passive degassing from five major Martian volcanic centers, Cerberus Fossae, Apollinaris Mons, Elysium Mons, Hadriacus Mons, and Pityusa Patera, and assess the sensitivity of these results to a range of volcanic, atmospheric, orbital, environmental, and numerical parameters. Previous studies have placed estimates on the amount of water released from volcanic outgassing (e.g., Carr, 1987; Craddock and Greeley, 2009; Grott et al., 2011; Carr and Head, 2015), but this is the first study to track the water released from specific volcanoes and to determine the spatial distribution of resulting ice deposits. We find that volcanic variables, such as mass flux and duration of degassing, primarily drive the thickness of volcanogenic ice fields –– higher mass fluxes create the thickest ice deposits (up to ∼210 mm around the volcanic center for a mass flux 106 kg s−1) and durations of >6 months are required for ice to reach near-global distributions. Ice accumulation around the degassing source is maximized if the latitude of passive degassing occurs near the south pole (i.e., Pityusa Patera) due to its exposure to circulation patterns that promote ice deposition in that region. Changes in the season, dust visible optical depth, and eccentricity can also impact the thickness of ice, with ice accumulation around the volcanic source being maximized in the spring, with higher dust visible optical depths, and when the eccentricity is low, due to more favorable temperatures and atmospheric circulation. Variances in the obliquity can also influence ice fields by modulating which latitudes receive more surface ice accumulation. The amount of activated cloud condensation nuclei (CCN) in the atmosphere has a lesser impact on the thickness and spatial distribution of ice fields than other parameters, with lower amounts of CCN leading to narrower, but slightly thicker deposits compared to higher amounts of CCN. The mean surface pressure and longitude of perihelion both affect the timing of ice fields. The surface pressure influences how long water lingers in the atmosphere before depositing to the surface, while the longitude of perihelion controls the time of year in which the thickest ice deposits accumulate around the degassing source. Ice fields are highly sensitive to the model resolution. Higher model resolutions allow for a more detailed representation of physical processes, such as cloud formation and ice deposition, while lower resolution model runs provide a more approximate guide to the representation of these processes and may underestimate surface ice thicknesses around the degassing source. The thickest deposits of ice form around the source of passive degassing, but later migrates and restabilizes in cold traps after degassing has ceased, unless it is protected against sublimation by the deposition of dust or pyroclastic material. Assuming the ice is protected, recurrent episodes of passive degassing could have facilitated the gradual accumulation of ice deposits several meters thick over time.

Effects of South Asian outflow on aerosol hygroscopicity and cloud droplet activation over the northern Indian Ocean

As a part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB-2018), shipborne measurements of aerosol properties were carried out to study the influence of South Asian outflow on cloud condensation nuclei (CCN) characteristics over the South East Arabian Sea (SEAS) and equatorial Indian Ocean (EIO) during winter, 2018. The poor association between CCN concentration and mass concentrations of the major aerosol species (organic matter, sulfate, and elemental carbon) is observed over SEAS, where airmass is mostly continental. In comparison, the CCN is well-correlated with aerosol mass concentrations over remote EIO. The dependence of CCN activation (at lower supersaturations) on the ultrafine particle fraction differs between SEAS and EIO depending on the mass concentration of organic aerosols. The coexistence of small and hydrophobic aerosols (organics) decreased the CCN activation ratio during ultrafine particle events. The hygroscopicity estimated using the chemical composition of aerosols is comparable to that estimated from CCN measurements over the Indian Ocean, except for the SEAS, where organic aerosols dominate. The effect of outflow aerosols on cloud droplet number concentration (CDNC) is simulated using a parcel model, which showed a good association (R ∼0.96) with CCN measurements over the EIO; however, the estimated CDNC did not follow the CCN variation over the SEAS region. This study highlights the importance of aerosol chemical composition, especially organics, in CCN activation and cloud droplet formation in the South Asian outflow over the Indian Ocean.

Size-resolved cloud condensation nuclei activity of aerosol particles sampled above the mixing layer

Size-resolved cloud condensation nuclei activity of aerosol particles sampled above the mixing layer

Measurement report: Hygroscopicity of size-selected aerosol particles in the heavily polluted urban atmosphere of Delhi: impacts of chloride aerosol

Abstract. Recent research has revealed the crucial role of wintertime, episodic high chloride (H-Cl) emissions in the Delhi region, which significantly impact aerosol hygroscopicity and aerosol-bound liquid water, thus contributing to the initiation of Delhi fog episodes. However, these findings have primarily relied on modeled aerosol hygroscopicity, necessitating validation through direct hygroscopicity measurements. This study presents the measurements of non-refractory bulk aerosol composition of PM1 from an Aerodyne aerosol chemical speciation monitor and for first-time size-resolved hygroscopic growth factors (nucleation, Aitken, and accumulated mode particles) along with their associated hygroscopicity parameters at 90 % relative humidity using a hygroscopic tandem differential mobility analyzer at the Delhi Aerosol Supersite. Our observations demonstrate that the hygroscopicity parameter for aerosol particles varies from 0.00 to 0.11 (with an average of 0.03 ± 0.02) for 20 nm particles, 0.05 to 0.22 (0.11 ± 0.03) for 50 nm particles, 0.05 to 0.30 (0.14 ± 0.04) for 100 nm particles, 0.05 to 0.41 (0.18 ± 0.06) for 150 nm particles, and 0.05 to 0.56 (0.22 ± 0.07) for 200 nm particles. Surprisingly, our findings demonstrate that the period with H-Cl emissions displays notably greater hygroscopicity (0.35 ± 0.06) in comparison to spans marked by high biomass burning (0.18 ± 0.04) and high hydrocarbon-like organic aerosol (0.17 ± 0.05) and relatively cleaner periods (0.27 ± 0.07). This research presents initial observational proof that ammonium chloride is the main factor behind aerosol hygroscopic growth and aerosol-bound liquid water content in Delhi. The finding emphasizes ammonium chloride's role in aerosol–water interaction and related haze/fog development. Moreover, the high chloride levels in aerosols seem to prevent the adverse impact of high organic aerosol concentrations on cloud condensation nuclei activity.

Impacts of the aerosol mixing state and new particle formation on CCN in summer at the summit of Mount Tai (1534m) in Central East China

In this study, the aerosol size distributions, cloud condensation nuclei (CCN) number concentration (NCCN), single-particle chemical composition and meteorological data were collected from May 12 to June 8, 2017, at the summit of Mt. Tai. The effects of new particle formation (NPF) events and aerosol chemical components on CCN at Mt. Tai were analyzed in detail. The results showed that, NPF events significantly enhanced the CCN population, and the enhancement effect increased with increasing supersaturation (SS) value at Mt.Tai. NCCN at SS ranging from 0.1 to 0.9 % on NPF days was 10.9 %, 36.5 %, 44.6 %, 53.5 % and 51.5 % higher than that on non-NPF days from 10:00–13:00 as NPF events progressed. The effect of chemical components on CCN activation under the influence of NPF events was greater than that in the absence of NPF events. The correlation coefficients of EC-Nitrate particles (EC-Sulfate particles) and CCN at all SS levels on NPF days were 1.31–1.59 times (1.17–1.35 times) higher than those on non-NPF days. Nitrate particles promoted CCN activation but sulfate particles inhibited activation at Mt. Tai. There are differences or even opposite effects of the same group of particles on CCN activation under the influence of NPF events in different air masses. EC-Sulfate particles inhibited CCN activation at all SS levels for type I but weakly promoted activation at lower SS ranging from 0.1 to 0.3 % and weakly inhibited it at higher 0.9 % SS for type II. OCEC particles significantly inhibited CCN activation for type II, and this effect decreased with increasing SS. OCEC particles only weakly inhibited activation at SS ranging from 0.5 to 0.7 % for type I. OCEC particles only weakly inhibited this process at 0.1 % SS, while they very weakly promoted activation for SS > 0.1 %. This reveals that the CCN activity is not only related to the chemical composition of the particles, but the mixing state also has an important effect on the CCN activity.

CCN activation in homogeneous isotropic turbulence: Response to particle characteristics and environmental conditions

Direct Numerical Simulation (DNS) of turbulent cloud parcels are presented where particles evolve in response to the local values in supersaturation (s) led by turbulent fluctuations. A pseudo-spectral DNS is modified to incorporate aerosol particles and Cloud Condensation Nuclei (CCN) activation applying a droplet growth equation based on κ-Köhler theory that works for the whole range of warm cloud particles, from un-activated deliquesced aerosol particles to activated cloud droplets, that grow by condensation. The Lagrangian microphysics applied ensures that each particle experiences a supersaturation (fluctuations added to the mean) corresponding to its near-particle gas phase surroundings, which differs from the uniform parcel view where an average value of supersaturation, s¯, is applied to every particle in the domain. A non-turbulent Lagrangian parcel model with similar mean thermodynamics and CCN properties is compared with turbulent DNS parcels of varying fluctuation intensities to distinguish the impact of turbulence on CCN activation. It is shown that, in a given distribution, a subset of aerosols respond to fluctuations rather than the mean thermodynamics allowing particles of similar dry size to co-exist as un-activated haze particles as well as cloud droplets. The cloud microphysical properties are analysed in a series of DNS experiments with contrasting thermodynamic and aerosol properties. DNS results agrees with the general understanding of aerosol activation and cloud droplets evolution in response to various background conditions such as pristine and polluted aerosol distributions, composition of CCN with respect to organic - inorganic components and magnitude of vertical velocity. DNS considers turbulence-microphysics interactions in such problems providing additional knowledge on the role of turbulence in clouds. It is argued that incorporating turbulent fluctuations in simulating the CCN activation and droplet growth is well reasoned and a constructive way forward.

Acoustic levitation of pollen and visualisation of hygroscopic behaviour

Abstract. Pollen are hygroscopic; therefore, they have the potential to act as cloud condensation nuclei (CCN) in the atmosphere. This could have uncertain implications for cloud processes and climate as well as plant biodiversity and human health. Previous studies have investigated the hygroscopic swelling of pollen, linked to CCN activity by the κ-Köhler theory, using methods that follow observed mass increase by electrodynamic balance (EDB) or vapour sorption analyser. This study uses an acoustic levitator to levitate pollen grains in the true aerosol phase and uses a macroscope to image the pollen to investigate hygroscopic behaviour when relative humidity (RH) is changed. Two pollen species were studied in this work: Lilium orientalis (oriental lily) and Populus deltoides (eastern cottonwood). Both species were successfully levitated; however, the smaller Populus deltoides showed greater instability throughout experiments. The quality of images taken by the macroscope, and thus calculations of pollen area and aspect ratio, varied significantly and were sensitive to lighting conditions as well as to levitated pollen grain movement and orientation. Experiments with surface-fixed pollen grains were also conducted. They showed evidence that pollen hygroscopic swelling could be observed by the macroscope. The produced results were comparable with previously reported mass increase values. Although less accurate than methods that measure mass changes, the acoustic levitator and macroscope set-up offer an attractive alternative by virtue of being commercial, off-the-shelf, low-cost, and versatile techniques. A key advantage of this method is that it is possible to visually observe particle shape dynamics under varying environmental conditions.

Cloud condensation nuclei activity of internally mixed particle populations at a remote marine free troposphere site in the North Atlantic Ocean

This study reports results from research conducted at the Observatory of Mount Pico (OMP), 2225 m above mean sea level on Pico Island in the Azores archipelago in June and July 2017. We investigated the chemical composition, mixing state, and cloud condensation nuclei (CCN) activities of long-range transported free tropospheric (FT) particles. FLEXible PARTicle Lagrangian particle dispersion model (FLEXPART) simulations reveal that most air masses that arrived at the OMP during the sampling period originated in North America and were highly aged (average plume age > 10 days). We probed size-resolved chemical composition, mixing state, and hygroscopicity parameter (κ) of individual particles using computer-controlled scanning electron microscopy with an energy-dispersive X-ray spectrometer (CCSEM-EDX). Based on the estimated individual particle mass from elemental composition, we calculated the mixing state index, χ. During our study, FT particle populations were internally mixed (χ of samples are between 53 % and 87 %), owing to the long atmospheric aging time. We used data from a miniature Cloud Condensation Nucleus Counter (miniCCNC) to derive the hygroscopicity parameter, κCCNC. Combining κCCNC and FLEXPART, we found that air masses recirculated above the North Atlantic Ocean with lower mean altitude had higher κCCNC due to the higher contribution of sea salt particles. We used CCSEM-EDX and phase state measurements to predict single-particle κ (κCCSEM-EDX) values, which overlap with the lower range of κCCNC measured below 0.15 % SS. Therefore, CCSEM-EDX measurements can be useful in predicting the lower bound of κ, which can be used in climate models to predict CCN activities, especially in remote locations where online CCN measurements are unavailable.

Enhancements in Cloud Condensation Nuclei Activity From Turbulent Fluctuations in Supersaturation

AbstractThe effect of aerosols on the properties of clouds is a large source of uncertainty in predictions of weather and climate. These aerosol‐cloud interactions depend critically on the ability of aerosol particles to form cloud droplets. A challenge in modeling aerosol‐cloud interactions is the representation of interactions between turbulence and cloud microphysics. Turbulent mixing leads to small‐scale fluctuations in water vapor and temperature that are unresolved in large‐scale atmospheric models. To quantify the impact of turbulent fluctuations on cloud condensation nuclei (CCN) activation, we used a high‐resolution Large Eddy Simulation of a convective cloud chamber to drive particle‐based cloud microphysics simulations. We show small‐scale fluctuations strongly impact CCN activity. Once activated, the relatively long timescales of evaporation compared to fluctuations causes droplets to persist in subsaturated regions, which further increases droplet concentrations.

Accounting for Cloud Nucleation Activation Mechanism of Secondary Organic Matter from α-Pinene Oxidation Using Experimentally Retrieved Water Solubility Distributions.

Activation of cloud droplets of aerosol particles from biogenic precursors plays a critical role in Earth's climate system. However, the molecular-level understanding of the cloud condensation nuclei (CCN) activation process for secondary organic matter (SOM) is still lacking. Here, we reduced the gap by segregating SOM from α-pinene based on water solubility. The chemical composition and CCN activity of the solubility-segregated fractions of SOM were measured. The results demonstrated for the first time by laboratory experiment that highly oxygenated compounds such as hydroperoxides and highly oxygenated organic molecules are important contributors for the CCN activity of α-pinene SOM. Meanwhile, relatively less water-soluble species were also abundant. Analysis based on the Köhler theory demonstrated that less water-soluble compounds in SOM remain undissolved during the cloud activation process, suggesting that the traditional single-parameter parameterization for CCN activation would not be sufficient for representing the process. In combination with the recent developments in SOM formation chemistry, the present study helps in understanding the interactions between the biosphere and climate.

Impacts of Aerosol Chemical Composition on Cloud Condensation Nuclei (CCN) Activity during Wintertime in Beijing, China

The cloud condensation nuclei (CCN) activity and aerosol chemical composition were concurrently measured via a scanning mobility CCN analyzer (SMCA) and an Aerodyne Time-of-Flight Aerosol Chemical Speciation Monitor (ACSM), respectively, during wintertime 2022 in Beijing, China. During the observation period, the mean CCN number concentrations ranged from 1345 ± 1270 cm−3 at SS = 0.1% to 3267 ± 2325 cm−3 at SS = 0.3%. The mean critical activation diameters (D50) at SS = 0.1%, 0.2%, and 0.3% were 172 ± 13 nm, 102 ± 8 nm, and 84 ± 7 nm, corresponding to the average hygroscopicity parameters (κCCN) of 0.34, 0.33, and 0.26, respectively. The diurnal variations in D50 suggested that the local primary emissions significantly enhanced D50 at SS = 0.2% and 0.3%, but had less influence on D50 at SS = 0.1% due to the limited size (<150 nm) of particles emitted from primary sources. As PM2.5 concentration increases, the dominant driver of CCN activity transitions from sulfate to nitrate. At a specific SS, D50 decreased with increases in the degree of internal mixing, implying that the elevated internal mixing degree during atmospheric aging was beneficial to CCN activation. In this study, the commonly used f44 (or O:C) was weakly correlated with κorg and failed to describe the variations in κorg. Instead, the variations in κorg can be well parameterized with the Org/BC ratio. The correlation between κ derived from bulk chemical compositions and CCN measurements was substantially improved when this κorg scheme was adopted, emphasizing the importance of considering κorg variations on deriving κchem from aerosol chemical composition.

Visual analysis of model parameter sensitivities along warm conveyor belt trajectories using Met.3D (1.6.0-multivar1)

Abstract. Numerical weather prediction models rely on parameterizations for subgrid-scale processes, e.g., for cloud microphysics, which are a well-known source of uncertainty in weather forecasts. Via algorithmic differentiation, which computes the sensitivities of prognostic variables to changes in model parameters, these uncertainties can be quantified. In this article, we present visual analytics solutions to analyze interactively the sensitivities of a selected prognostic variable to multiple model parameters along strongly ascending trajectories, so-called warm conveyor belt (WCB) trajectories. We propose a visual interface that enables us to (a) compare the values of multiple sensitivities at a single time step on multiple trajectories, (b) assess the spatiotemporal relationships between sensitivities and the trajectories' shapes and locations, and (c) find similarities in the temporal development of sensitivities along multiple trajectories. We demonstrate how our approach enables atmospheric scientists to interactively analyze the uncertainty in the microphysical parameterizations and along the trajectories with respect to the selected prognostic variable. We apply our approach to the analysis of WCB trajectories within extratropical Cyclone Vladiana, which occurred between 22–25 September 2016 over the North Atlantic. Peaks of sensitivities that occur at different times relative to a trajectory's fastest ascent reveal that trajectories with their fastest ascent in the north are more susceptible to rain sedimentation from above than trajectories that ascend further south. In contrast, large sensitivities to cloud condensation nuclei (CCN) activation and cloud droplet collision in the south indicate a local rain droplet formation. These large sensitivities reveal considerable uncertainty in the shape of clouds and subsequent rainfall. Sensitivities to cloud droplets' formation and subsequent conversion to rain droplets are also more pronounced along convective ascending trajectories than for slantwise ascents. The slantwise ascending trajectories are characterized by periods of slower ascent and even descent, during which the sensitivities to the formation of cloud droplets and rain droplets alternate. This alternating pattern leads to large-scale precipitation patterns, whereas convective ascending trajectories do not exhibit this pattern. Thus the primary source for uncertainty in large-scale precipitation patterns stems from slantwise ascents. The strong ascent of convective trajectories leads to large sensitivities of rain mass density to riming and freezing parameters at high altitudes, which are barely present in slantwise ascending trajectories. These sensitivities correspond to uncertainties concerning graupel and hail formation in convective ascents.

Direct observation for relative-humidity-dependent mixing states of submicron particles containing organic surfactants and inorganic salts

Abstract. Aerosol mixing state plays an important role in heterogeneous reactions and cloud condensation nuclei (CCN) activity. Organic surfactants could affect aerosol mixing state through bulk–surface partitioning. However, the mixing state of surfactant-containing particles remains unclear due to the lack of direct measurements. Here, direct characterizations of the mixing state for 20 kinds of submicron particles containing inorganic salts (NaCl and (NH4)2SO4) and atmospheric organic surfactants (organosulfates, organosulfonates, and dicarboxylic acids) were conducted upon relative humidity (RH) cycling by environmental scanning electron microscopy (ESEM). As the RH increased, the surfactant shells inhibited water diffusion being exposed to the inorganic core, leading to notably increased inorganic deliquescence RH (88.3 %–99.5 %) when compared with pure inorganic aerosol. Meanwhile, we directly observed an obvious Ostwald ripening process (that is, the growth of larger crystals at the expense of smaller ones) in 6 out of 10 NaCl–organic surfactant systems. As a result of water inhibition by the organic surfactant shell, Ostwald ripening in all systems occurred at RH above 90 %, which were higher than the reported RH range for pure NaCl measured at 27 ∘C (75 %–77 %). As RH decreased, eight systems underwent liquid–liquid-phase separation (LLPS) before efflorescence, showing a strong dependence on the organic molecular oxygen-to-carbon ratio (O:C). Quantitatively, LLPS was always observed when O:C≤0.43 and was never observed when O:C>∼0.57. Separation RH (SRH) of inorganic salt–organic surfactant mixtures generally followed the trend of (NH4)2SO4 < NaCl, which is consistent with their salting-out efficiencies reported in previous studies. Solid-phase separations were observed after efflorescence for systems without LLPS. Our results provide a unique insight into the consecutive mixing processes of the inorganic salt–organic surfactant particles, which would help improve our fundamental knowledge of model development on radiative effect.

Aerosol Properties and Their Influences on Marine Boundary Layer Cloud Condensation Nuclei over the Southern Ocean

Five overcast marine stratocumulus cases during the Southern Ocean Clouds Radiation Aerosol Transport Experimental Study (SOCRATES) aircraft field campaign were selected to examine aerosol and cloud condensation nuclei (CCN) properties with cloud influence. The Aitken- and accumulation-mode aerosols contributed approximately 70% and 30% of the total aerosols, respectively. The aerosol properties before and after periods of drizzle were investigated using in situ measurements during one case. Sub-cloud drizzle processes impacted accumulation-mode aerosols and CCN distribution. There was a nearly linear increase in CCN number concentration (NCCN) with supersaturation (S) during the ‘before drizzle’ period, but this was not true for the ‘after drizzle’ period, particularly when S > 0.4%. Using the hygroscopicity parameter (κ) to quantitatively investigate the chemical cloud-processing mechanisms, we found that higher κ values (>0.4) represent cloud-processing aerosols, while lower κ values (<0.1) represent newly formed aerosols. When the supersaturation is less than the Hoppel minimum (0.22%), cloud processing is dominant, whereas sea-spray aerosols are dominant contributors to CCN activation when S exceeds 0.22% but is less than 0.32%, the effective supersaturation threshold. Sea salt is considered a non-cloud-processing aerosol and is large and hygroscopic enough to form cloud droplets.

Rapid growth and high cloud-forming potential of anthropogenic sulfate aerosol in a thermal power plant plume during COVID lockdown in India

The COVID lockdown presented an interesting opportunity to study the anthropogenic emissions from different sectors under relatively cleaner conditions in India. The complex interplays of power production, industry, and transport could be dissected due to the significantly reduced influence of the latter two emission sources. Here, based on measurements of cloud condensation nuclei (CCN) activity and chemical composition of atmospheric aerosols during the lockdown, we report an episodic event resulting from distinct meteorological conditions. This event was marked by rapid growth and high hygroscopicity of new aerosol particles formed in the SO2 plume from a large coal-fired power plant in Southern India. These sulfate-rich particles had high CCN activity and number concentration, indicating high cloud-forming potential. Examining the sensitivity of CCN properties under relatively clean conditions provides important new clues to delineate the contributions of different anthropogenic emission sectors and further to understand their perturbations of past and future climate forcing.

Characterization of dust-related new particle formation events based on long-term measurement in the North China Plain

Abstract. Mineral dust is a major natural atmospheric aerosol that impacts the Earth's radiation balance. The significant scavenging process of fine particles by the strong wind during the dust episodes provided a relatively clean environment that was favourable for new particle formation (NPF) occurrence. In this study, the NPF occurred following the dust episodes (dust-related NPF), and other cases under clean and polluted conditions were classified based on the long-term particle number size distribution (PNSD) in urban Beijing in spring from 2017 to 2021. It was found that the observed formation (Jobs) and growth rate (GR) of dust-related NPF events were approximately 50 % and 30 % lower than the values of other NPF days, respectively. A typical severe dust storm that originated from Mongolia and swept over northern China on 15–16 March 2021 was analysed, to illustrate how the dust storm influences the NPF event. The maximum hourly mean PM10 mass concentration reached 8000 µg m−3 during this dust storm. The occurrence of an NPF event after a dust storm was facilitated due to the low condensation sink (∼ 0.005 s−1) caused by the strong dilution process of pre-existing particles. However, a downward trend of particle hygroscopicity was found during dust storm and NPF event as compared with the polluted episode, resulting in an increasing trend of the critical diameter at different supersaturations (ss), where aerosols are activated as cloud condensation nuclei (CCN), although the NPF event occurred when dust faded. The critical diameter was elevated by approximately 6 %–10 % (ss = 0.2 % and 0.7 %) during the dust storm, resulting in a lower CCN activation ratio, especially at low supersaturation. Modifications of the nucleation and growth process, as well as the particle-size distribution and hygroscopicity of the dust particles, provide valuable information that reveals the underlying climate and air quality effects of Asian mineral dust.

Optical properties and cloud condensation nuclei activity of brown carbon containing α–dicarbonyls and reduced nitrogen compounds

The aqueous–phase reaction of carbonyls with reduced nitrogen compounds (RNCs) is considered an important pathway for the formation of secondary BrC, and these particles could greatly impact the global climate. However, our knowledge of their properties is still limited. In this study, we measured the optical properties, cloud condensation nuclei (CCN) activity (represented as hygroscopicity parameter κCCN), and effective density (ρeff) under varied reaction conditions for BrC mixtures formed by reactions between two α–dicarbonyls (glyoxal GX and methylglyoxal MG) and three RNCs (ammonium sulfate AS, glycine GC, and dimethylamine DA). Our results showed a wide range of particle ρeff from 1.08 to 1.66 g cm−3, depending on the type of RNCs reactant. At 532 nm wavelength, the real part n of complex refractive index ranged from 1.41 to 1.60 and the imaginary part k of BrC ranged from 0.024 to 0.043, and the absorption cross section for MG mixtures were 20.3–51.0% higher than those of GX mixtures. These values did not appear to vary significantly with mixing molar ratios between RNCs and α–dicarbonyls (10:1, 1:1, and 1:3) for mixtures with the same reactants, but the mass absorption cross section for mixtures after a reaction time of 14 days were 1.57–74.2% higher than those of mixtures after reaction times of 0.5 and 7 days. The κCCN values of different BrC were consistent with the order of κCCN for single RNCs, i.e., AS–containing (0.349–0.428) > GC–containing (0.221–0.352) > DA–containing (0.090–0.131), and showed an overall increasing trend (1.47–59.3%) with the RNCs content. Our results highlight the importance of reaction conditions on secondary BrC properties and are helpful for a more accurate assessment of aerosol radiative forcing.