CCN Activation Research Articles

CCN Activation and Droplet Growth in Pi Chamber Simulations with Lagrangian Particle–Based Microphysics

Abstract Numerical simulations of turbulent moist Rayleigh–Bénard convection driving CCN activation and droplet growth in the laboratory Pi chamber are discussed. Supersaturation fluctuations come from isobaric mixing of warm and humid air rising from the lower boundary with colder air featuring lower water vapor concentrations descending from the upper boundary. Lagrangian particle–based microphysics is used to represent the growth of haze CCN and cloud droplets with kinetic, solute, and surface tension effects included. Dry CCN spectra in the range between 2- and 200-nm radii from field observations are considered. Increasing the total CCN concentration from pristine to polluted conditions results in an increase in the droplet concentration and reduction in the mean droplet radius and spectral width. These are in agreement with Pi chamber observations and numerical simulations, as well as with numerous past studies of CCN cloud-base activation in natural clouds. The key result is that a relatively small fraction of the available CCN is activated in the Pi chamber fluctuating supersaturations, from about a half in the pristine case to only a 10th in the polluted case. The activation fraction as a function of the dry CCN radius is similar in all simulations, close to zero at the CCN small end, increasing to a maximum at CCN radius around 50 nm, and decreasing to close to zero at the large CCN end. This is explained as too small supersaturations to activate small CCN as in natural clouds and insufficient time to allow large CCN reaching the critical radius. Significance Statement Impact of turbulence on the formation and growth of cloud droplets is an important cloud physics problem. Laboratory experiments in the Michigan Technological University cloud chamber provide key insights into this problem. Numerical simulations of cloud chamber processes discussed in this paper complement laboratory experiments by providing insights difficult or impossible to obtain in the laboratory. The study contrasts the formation and growth of cloud droplets in the laboratory cloud chamber with processes taking place in natural clouds. The differences documented in this paper pose questions concerning the impact of turbulence on the formation and growth of cloud droplets as a result of interactions of clouds with their environment.

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.

Spatiotemporal Variations of the Effects of Aerosols on Clouds and Precipitation in an Extreme‐Rain‐Producing MCS in South China

AbstractPrevious studies focus mostly on the storm‐scale‐averaged precipitation responses to aerosols. Yet, the spatiotemporal variations of the aerosol effects can lead to localized and short‐duration precipitation changes that are more relevant for improving rainfall forecasts. Here, we investigate the cloud and precipitation responses to aerosols during different life stages and in subregions with various cloud top heights of an extreme‐rain‐producing mesoscale convective system (MCS) in South China using the coupled WRF‐Chem model. Results show mostly similar MCS‐averaged precipitation responses between the polluted and clean conditions due to compensations among the subregions. However, the spatiotemporally discretized changes are divergent. Specifically, during the developing stage, aerosols increase precipitation in all subregions through generating larger precipitating hydrometers produced from the accretion of more cloud droplets. The most prominent precipitation enhancement occurs in the subregion with the strongest clean‐condition rainfall. In the mature stage, the CCN activation abates, and so does the aerosol‐induced precipitation increase. In the mixed‐phase (−40°C < Ttop < 0°C) and cold (Ttop ≤ −40°C) cloud top subregions, aerosols also reduce the melting of the ice‐phase precipitating hydrometers, which process becomes more important to precipitation formation during the mature stage. Therefore, the drop of rainfall rate is more significant in these ice‐phase‐involved subregions, bringing the polluted precipitation to be less than that in the clean condition during the mature stage. The substantial spatiotemporal variations of the aerosol effects and the early intensification of heavy precipitation suggest the importance of incorporating aerosols in the modeling and prediction of regional heavy rainfall events.

Improving BC Mixing State and CCN Activity Representation With Machine Learning in the Community Atmosphere Model Version 6 (CAM6)

AbstractRepresenting mixing state of black carbon (BC) is challenging for global climate models (GCMs). The Community Atmosphere Model version 6 (CAM6) with the four‐mode version of the Modal Aerosol Module (MAM4) represents aerosols as fully internal mixtures with uniform composition within each aerosol mode, resulting in high degree of internal mixing of BC with non‐BC species and large mass ratio of coating to BC (RBC, the mass ratio of non‐BC species to BC in BC‐containing particles). To improve BC mixing state representation, we coupled a machine learning (ML) model of BC mixing state index trained on particle‐resolved simulations to the CAM6 with MAM4 (MAM4‐ML). In MAM4‐ML, we use RBC to partition accumulation mode particles into two new modes, BC‐free particles and BC‐containing particles. We adjust RBC to make the modeled BC mixing state index (χmode) match the one predicted by the ML model (χML). On a global average, the mass fraction of BC‐containing particles in accumulation mode decreases from 100% (MAM4‐default) to 48% (MAM4‐ML). The globally averaged χmode decreases from 78% (MAM4‐default) to 63% (MAM4‐ML, 19% reduction) and agrees well with χML (66%). The RBC decreases by 52% for accumulation mode and better agrees with observations. The hygroscopicity drops by 9% for BC‐containing particles in accumulation mode, leading to a 20% reduction in the BC activation fraction. The surface BC concentration increases most (6.9%) in the Arctic, and the BC burden increases by 4%, globally. Our study highlights the application of the ML model for improving key aerosol processes in GCMs.

Effect of Single and Double Moment Microphysics Schemes and Change in Cloud Condensation Nuclei, Latent Heating Rate Structure Associated with Severe Convective System over Korean Peninsula

To investigate the impact of advanced microphysics schemes using single and double moment (WSM6/WDM6) schemes, numerical simulations are conducted using Weather Research and Forecasting (WRF) model for a severe mesoscale convective system (MCS) formed over the Korean Peninsula. Spatial rainfall distribution and pattern correlation linked with the convective system are improved in the WDM6 simulation. During the developing stage of the system, the distribution of total hydrometeors is larger in WDM6 compared to WSM6. Along with the mixing ratio of hydrometeors (cloud, rain, graupel, snow, and ice), the number concentration of cloud and rainwater are also predictable in WDM6. To understand the differences in the vertical representation of cloud hydrometeors between the schemes, rain number concentration (Nr) from WSM6 is also computed using particle density to compare with the Nr readily available in WDM6. Varied vertical distribution and large differences in rain number concentration and rain particle mass is evident between the schemes. Inclusion of the number concentration of rain and cloud, CCN, along with the mixing ratio of different hydrometers has improved the storm morphology in WDM6. Furthermore, the latent heating (LH) profiles of six major phase transformation processes (condensation, evaporation, freezing, melting, deposition, and sublimation) are also computed from microphysical production terms to deeply study the storm vertical structure. The main differences in condensation and evaporation terms are evident between the simulations due to the varied treatment of warm rain processes and the inclusion of CCN activation in WDM6. To investigate cloud–aerosol interactions, numerical simulation is conducted by increasing the CCN (aerosol) concentration in WDM6, which simulated comparatively improved pattern correlation for rainfall simulation along with intense hydrometer distribution. It can be inferred that the change in aerosol increased the LH of evaporation and freezing and affected the warming and cooling processes, cloud vertical distribution, and subsequent rainfall. Relatively, the WDM6 simulated latent heating profile distribution is more consistent with the ERA5 computed moisture source and sink terms due to the improved formulation of warm rain processes.

Aerosol-CCN characteristics and dynamics associated with a pre-monsoon dust storm over a high-altitude site in Western Ghats, India.

Aerosol-CCN characteristics and dynamics during a pre-monsoon dust storm (April 6-11, 2015) over a high-altitude site ((17.92°N, 73.66°E, and 1348m above mean sea level (MSL)) in Western Ghats, India, has been studied using ground-based observations, satellite, and reanalysis datasets. Spatial distribution of dust surface mass concentration along with the back trajectory analysis showed the Arabian Desert area (Rub-Al-khali desert) as the source region and strong westerly winds transported the dust particles toward the Indian subcontinent. High values noticed in the surface PM10 (PM2.5), i.e., ~ 450 (~ 130) µg m-3, MODIS AOD550nm (0.6), and MERRA 2 dust surface mass concentration (5 × 10-7kgm-3) along MODIS true color images confirmed the dust storm event on April 6, 2015 over the observational site. Size-segregated aerosol number concentration measured from ground-based observations showed the dominance of Aitken, accumulation, and coarse mode particles during dust period. CCN concentrations at 0.1, 0.3, 0.5, 0.7, and 0.9% SS were analyzed. A low value of CCN concentration and activation fraction (~ 0.3) near surface was noticed during dust storm day, suggesting insoluble mineral dust particle being transported. Analyzed vertical velocity during pre-dust period showed downdraft between 900 and 750hPa, suggesting dust transport from upper altitudes toward the observational site. WRF-Chem model simulation also captured the dust storm event, and the results are in good agreement with the observation with a significance of 95% confidence level.

A multiple-charging correction algorithm for a broad-supersaturation scanning cloud condensation nuclei (BS2-CCN) system

Abstract. High time resolution (∼1 s) of aerosol hygroscopicity and CCN activity can be obtained with a broad-supersaturation scanning cloud condensation nuclei (BS2-CCN) system. Based on a commercial CCNC (CCN counter), the newly designed diffusive inlet in the BS2-CCN realizes a broad-supersaturation distribution in a chamber with a stable, low sheath-to-aerosol-flow ratio (SAR). In this way, a monotonic relation between the activation fraction of aerosols (Fact) and critical activation supersaturation (Saerosol) can be obtained. The accuracy of the size-resolved aerosol hygroscopicity, κ, measured by the BS2-CCN system can be, however, hampered by multiply charged particles, i.e., resulting in the overestimation of κ values. As the BS2-CCN system uses multiple and continuous supersaturations in the chamber and the size-resolved Fact value is directly used to derive κ values, the multiple-charging correction algorithm of the traditional CCNC where single supersaturation is applied does not work for the BS2-CCN observation. Here, we propose a new multiple-charging correction algorithm to retrieve the true Fact value. Starting from the largest size bin, a new Fact value at a specific particle diameter (Dp) is updated from a measured activation spectrum after removing both aerosol and the CCN number concentration of multiply charged particles using a kernel function with a given particle number size distribution. We compare the corrected activation spectrum with laboratory aerosols for a calibration experiment and ambient aerosols during the 2021 Yellow-Sea Air Quality Studies (YES-AQ) campaign. It is noted that this algorithm is only applied to the monomodal particle distribution. The difference between corrected and measured κ values can be as large as 0.08 within the measured κ values that range between 0.11 and 0.37 among the selected samples, highlighting that the multiple-charge effect should be considered for the ambient aerosol measurement. Furthermore, we examine how particle number size distribution is linked to the deviation of activation spectra and κ values.

CCN activation of ultrafine biogenic-WSOC under restricted anthropogenic emissions: A study over eastern Himalaya in India

CCN activation of ultrafine biogenic-WSOC under restricted anthropogenic emissions: A study over eastern Himalaya in India

Impact of Cloud-Base Turbulence on CCN Activation: CCN Distribution

Abstract Following our previous investigation of the turbulence impact on cloud-base single-size CCN activation, this study considers a similar problem assuming CCN size distribution obtained from field measurements. The total CNN concentration is taken as either 200 cm−3 to represent clean conditions, or as 2000 cm−3 to represent polluted conditions. CCN is assumed to be sodium chloride. The CCN activation in the rising nonturbulent adiabatic parcel is contrasted with the activation within a rising adiabatic parcel filled with inertial-range homogeneous isotropic turbulence. The turbulent parcel of 643 m3 and the turbulent kinetic energy dissipation rate of 10−3 m−2 s−3 are used in most of the simulations. Results for a range of mean parcel ascent rates, between 0.125 and 8 m s−1, are discussed. Overall, the adiabatic turbulent parcel simulations show results consistent with the adiabatic nonturbulent parcel, with higher activated CCN concentrations for stronger parcel ascent rates. The key difference is a blurriness of the separation between dry CCN size bins featuring activated and nonactivated (haze) CCN, especially for weak mean ascent rates. The blurriness comes from CCN getting activated and subsequently deactivated in the fluctuating supersaturation field, instead of all becoming cloud droplets above the cloud base. This leads to significantly larger spectral widths in turbulent parcel simulations compared to the nonturbulent parcel when activation is completed. Modeling results are discussed in the context of the impact of turbulent fluctuations on CCN activation documented in laboratory experiments using the Pi chamber.

How Asian slum emissions impact local microclimates in polluted air masses

AbstractUrban sprawl comprising densely populated slums over South Asian cities yields copious amounts of soot and black carbon from archaic cooking methods involving cow dung cakes and firewood, which remain afloat for over 10–12 h, enabling them to age in a sulphur rich environment. Not only are there toxicological concerns arising out of improper ventilation mechanisms, but there are also other concerns impacting the local microclimate. These emissions mix with other aerosol particles and, when conditions are favourable, are rendered partially soluble, enabling them to activate into cloud condensation nuclei. This study first yields a quantification of the soluble mass fraction and subsequently shows how aerosols from this local area source mix with background aerosol modes to perturb the local cloud microphysics over Chennai, a megacity in Southern India. On‐site sampling was undertaken to find the mass concentrations of the collected deposits separately from cow dung and firewood fuel. Additional micro‐physical attributes, including the morphological indentations that served as a receptacle to contain the accreted sulphate along with the particle size distribution were ascertained through Scanning Electron Microscopy. It is shown that accreted sulphate on carbonaceous particles facilitates CCN activation over the city. We show through large‐eddy simulations (LES) that extensive slum emissions over the study region contribute to the observed local cloud cover and enhanced rain amounts over a densely built‐up area housing the city's most vulnerable citizens.

Co@CoO encapsulated with N-doped carbon nanotubes activated peroxymonosulfate for efficient purification of organic wastewater

Catalytic activation of peroxymonosulfate (PMS) for purifying organic wastewater has been widely studied. However, the activation mechanism either by free radicals or non-radicals alone is not satisfactory efficiency. In this work, a novel advanced oxidation system based on surface oxidized metallic cobalt nanoparticles (Co@CoO) encapsulated with N-doped carbon nanotubes (NCT), i.e., CCN, activated peroxymonosulfate (PMS) was designed. The CCN-PMS system showed excellent performance in the degradation for a wide range of organic pollutants, including antibiotics, dyes, and phenols. The degradation rate of 100 mL 80 mg·L-1 tetracycline hydrochloride (TC) in the system (10 mg CCN + 0.5 mM PMS) reached 92% within 30 min. The mechanism analysis of the CCN activation of PMS reveals that the NCT mainly provided the active sites for non-radical formation, while the Co@CoO mainly provided the active sites for radical formation. In addition, the NCT could not only effectively reduce the leaching of Cox+, but also play a synergistic role with Co@CoO in the degradation of organic pollutants. This work provides new insight into the mechanism of PMS activation by carbon-based catalysts for the efficient purification of organic wastewater.

Contribution of New Particle Formation to Cloud Condensation Nuclei Activity and its Controlling Factors in a Mountain Region of Inland China

AbstractA 1‐month field campaign (May 29–June 29, 2018) was conducted at a mountain site (862 m above sea level) on the Wudang Mountains in China. The particle number size distribution (3–400 nm), size‐resolved cloud condensation nuclei (CCN, at 0.2% and 0.8% SS), and nonrefractory PM1 chemical composition were measured, respectively. The occurrence of the three chosen new particle formation (NPF) events (June 4, 7, and 11) was facilitated by southwest or west winds with a speed of about 3–4 m s−1. The hygroscopicity parameter () value of newly grown particles varied in a wide range (0.25–0.8) during the events, leading to large uncertainties (−98% to 38%) on the NCCN prediction compared to the campaign. During growth of the newly formed particles for the three events, condensation of sulfuric acid vapor accounts for 14%–42%, 2%–8%, and 3%–9%, respectively, indicating that organic vapors may play an important role in particle growth. For plume‐type events with rapid growth, the observed high CCN activity (i.e., June 7 event) may be explained by the contribution of amines and depression of surface tension in the presence of organic surfactants during particle growth. Our study demonstrates that the contribution of NPF to CCN concentration is modulated by many key factors including growth rate, hygroscopicity, concentrations of new particles and preexisting particles, and variation of those factors from one event to another leads to large uncertainties on the CCN prediction.

Secondary aerosol formation alters CCN activity in the North China Plain

Abstract. Secondary aerosols (SAs, including secondary organic and inorganic aerosols, SOAs and SIAs) are predominant components of aerosol particles in the North China Plain (NCP), and their formation has significant impacts on the evolution of particle size distribution (PNSD) and hygroscopicity. Previous studies have shown that distinct SA formation mechanisms can dominate under different relative humidity (RH). This would lead to different influences of SA formation on the aerosol hygroscopicity and PNSD under different RH conditions. Based on the measurements of size-resolved particle activation ratio (SPAR), hygroscopicity distribution (GF-PDF), PM2.5 chemical composition, PNSD, meteorology and gaseous pollutants in a recent field campaign, McFAN (Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain), conducted during the autumn–winter transition period in 2018 at a polluted rural site in the NCP, the influences of SA formation on cloud condensation nuclei (CCN) activity and CCN number concentration (NCCN) calculation under different RH conditions were studied. Results suggest that during daytime, SA formation could lead to a significant increase in NCCN and a strong diurnal variation in SPAR at supersaturations lower than 0.07 %. During periods with daytime minimum RH exceeding 50 % (high RH conditions), SA formation significantly contributed to the particle mass and size changes in a broad size range of 150 to 1000 nm, leading to NCCN (0.05 %) increases within the size range of 200 to 500 nm and mass concentration growth mainly for particles larger than 300 nm. During periods with daytime minimum RH below 30 % (low RH conditions), SA formation mainly contributed to the particle mass and size and NCCN changes for particles smaller than 300 nm. As a result, under the same amount of mass increase induced by SA formation, the increase of NCCN (0.05 %) was stronger under low RH conditions and weaker under high RH conditions. Moreover, the diurnal variations of the SPAR parameter (inferred from CCN measurements) due to SA formation varied with RH conditions, which was one of the largest uncertainties within NCCN predictions. After considering the SPAR parameter (estimated through the number fraction of hygroscopic particles or mass fraction of SA), the relative deviation of NCCN (0.05 %) predictions was reduced to within 30 %. This study highlights the impact of SA formation on CCN activity and NCCN calculation and provides guidance for future improvements of CCN predictions in chemical-transport models and climate models.

Water uptake of subpollen aerosol particles: hygroscopic growth, cloud condensation nuclei activation, and liquid–liquid phase separation

Abstract. Pollen grains emitted from vegetation can release subpollen particles (SPPs) that contribute to the fine fraction of atmospheric aerosols and may act as cloud condensation nuclei (CCN), ice nuclei (IN), or aeroallergens. Here, we investigate and characterize the hygroscopic growth and CCN activation of birch, pine, and rapeseed SPPs. A high-humidity tandem differential mobility analyzer (HHTDMA) was used to measure particle restructuring and water uptake over a wide range of relative humidity (RH) from 2 % to 99.5 %, and a continuous flow CCN counter was used for size-resolved measurements of CCN activation at supersaturations (S) in the range of 0.2 % to 1.2 %. For both subsaturated and supersaturated conditions, effective hygroscopicity parameters, κ, were obtained by Köhler model calculations. Gravimetric and chemical analyses, electron microscopy, and dynamic light scattering measurements were performed to characterize further properties of SPPs from aqueous pollen extracts such as chemical composition (starch, proteins, DNA, and inorganic ions) and the hydrodynamic size distribution of water-insoluble material. All investigated SPP samples exhibited a sharp increase of water uptake and κ above ∼95 % RH, suggesting a liquid–liquid phase separation (LLPS). The HHTDMA measurements at RH >95 % enable closure between the CCN activation at water vapor supersaturation and hygroscopic growth at subsaturated conditions, which is often not achieved when hygroscopicity tandem differential mobility analyzer (HTDMA) measurements are performed at lower RH where the water uptake and effective hygroscopicity may be limited by the effects of LLPS. Such effects may be important not only for closure between hygroscopic growth and CCN activation but also for the chemical reactivity, allergenic potential, and related health effects of SPPs.

Further analysis of CCN data at a mid-Antarctica Peninsula field site

Observations of very low atmospheric cloud condensation nuclei occurred about 10% of the daytime sampling periods at Palmer Station during February 1994. These measurements were not only observed following typical nuclei reducing weather events or phenomena. We do advocate the need and worthiness for a more comprehensive study to assess the frequency of low nuclei concentrations. The comprehensive study would include concurrent CCN activity spectra, individual nuclei or aerosol morphology and corresponding chemical composition, meteorology, atmospheric chemistry, and radiation measurements made simultaneously at least at Palmer. The study will contribute to the understanding of such very low nuclei event occurrence, to a more quantified assessment of their climatic relevance for this region, and to the development of a more viable aerosol measurement system. A suggested functional definition for a viable aerosol measurement system is provided.

Secondary Marine Aerosol Plays a Dominant Role over Primary Sea Spray Aerosol in Cloud Formation.

Marine aerosols play a critical role in impacting our climate by seeding clouds over the oceans. Despite decades of research, key questions remain regarding how ocean biological activity changes the composition and cloud-forming ability of marine aerosols. This uncertainty largely stems from an inability to independently determine the cloud-forming potential of primary versus secondary marine aerosols in complex marine environments. Here, we present results from a unique 6-day mesocosm experiment where we isolated and studied the cloud-forming potential of primary and secondary marine aerosols over the course of a phytoplankton bloom. The results from this controlled laboratory approach can finally explain the long-observed changes in the hygroscopic properties of marine aerosols observed in previous field studies. We find that secondary marine aerosols, consisting of sulfate, ammonium, and organic species, correlate with phytoplankton biomass (i.e., chlorophyll-a concentrations), whereas primary sea spray aerosol does not. Importantly, the measured CCN activity (κapp = 0.59 ± 0.04) of the resulting secondary marine aerosol matches the values observed in previous field studies, suggesting secondary marine aerosols play the dominant role in affecting marine cloud properties. Given these findings, future studies must address the physical, chemical, and biological factors controlling the emissions of volatile organic compounds that form secondary marine aerosol, with the goal of improving model predictions of ocean biology on atmospheric chemistry, clouds, and climate.

Relative role of black carbon and sea-salt aerosols as cloud condensation nuclei over a high altitude urban atmosphere in eastern Himalaya

The present study is an attempt to investigate the relative role of black carbon (BC) and sea-salt aerosols on the CCN activation over a high altitude station, Darjeeling (27.1° N and 88.15° E, 2200 m asl) at eastern Himalaya. Aerosols (CN, CCN, BC and PM2.5) were measured during premonsoon and monsoon in 2017 and 2018. A unique sampling strategy and a novel methodology were adopted that enabled us to quantitatively and separately determine the contributions of local emissions (LE), valley wind transport (VWT) and long-range transport (LRT) to BC aerosols and their role in CCN activation. On the other hand, the contributions of transported sea-salt (NaCl) aerosols to CCN activation were also determined when they interact with the local anthropogenic soluble species and when they do not. CCN (0.5% super-saturation) concentrations were found to be increased when BC aerosols were more aged (~ 80 cm−3 and 218 cm−3 increase in CCN for 1 μg m−3 increase in BCLE and BCLRT with activation ratios of 0.17 and 0.55 respectively). Local anthropogenic acidic species (SO42−/H2SO4 (g) and NO3−/HNO3 (g)) interact with NaCl resulting to Cl− depletion. Cl− depletion was increased with the increase in NO3− + SO42−(45% for1 μg m−3increase in NO3− + SO42−) that in turn sharply decreased the AR of NaCl (0.04 for 1% increase in Cl- depletion). On the other hand, higher the NO3− + SO42−, higher were the CCN activation of transported BC which could be due to the soluble coating on BC. The important and interesting fact is that when transported and interacted with anthropogenic soluble species, BC aerosols (though hydrophobic) act as much better CCN than NaCl (though hydrophilic).

Hygroscopic properties and cloud condensation nuclei activity of atmospheric aerosols under the influences of Asian continental outflow and new particle formation at a coastal site in eastern Asia

Abstract. The chemical composition of fine particulate matter (PM2.5), the size distribution and number concentration of aerosol particles (NCN), and the number concentration of cloud condensation nuclei (NCCN) were measured at the northern tip of Taiwan during an intensive observation experiment from April 2017 to March 2018. The parameters of aerosol hygroscopicity (i.e., activation ratio, activation diameter and kappa of CCN) were retrieved from the measurements. Significant variations were found in the hygroscopicity of aerosols (kappa – κ – of 0.18–0.56, for water vapor supersaturation – SS – of 0.12 %–0.80 %), which were subject to various pollution sources, including aged air pollutants originating in eastern and northern China and transported by the Asian continental outflows and fresh particles emitted from local sources and distributed by land–sea breeze circulations as well as produced by processes of new particle formation (NPF). Cluster analysis was applied to the back trajectories of air masses to investigate their respective source regions. The results showed that aerosols associated with Asian continental outflows were characterized by lower NCN and NCCN values and by higher kappa values of CCN, whereas higher NCN and NCCN values with lower kappa values of CCN were observed in the aerosols associated with local air masses. Besides, it was revealed that the kappa value of CCN exhibited a decrease during the early stage of an event of new particle formation, which turned to an increasing trend over the later period. The distinct features in the hygroscopicity of aerosols were found to be consistent with the characteristics in the chemical composition of PM2.5. This study has depicted a clear seasonal characteristic of hygroscopicity and CCN activity under the influence of a complex mixture of pollutants from different regional and/or local pollution sources. Nevertheless, the mixing state and chemical composition of the aerosols critically influence the aerosol hygroscopicity, and further investigations are necessary to elucidate the atmospheric processing involved in the CCN activation in coastal areas.

Atmospheric aerosol properties at a semi-rural location in southern India: particle size distributions and implications for cloud droplet formation

Particle number size distributions in the diameter range of 5 nm–34 µm were measured in tropical southern India during the transition season from May (pre-monsoon) to June–July (monsoon). Averaged over the entire measurement period, the particle number size distributions were bimodal with a total particle number concentration of NTot = (4.5 ± 2.7) × 103 cm−3 (arithmetic mean ± standard deviation). The ratio of Aitken to accumulation mode particle number concentrations (NAit/NAcc) ranged from 1.06 to 2.07, increasing from May to June–July due to a pronounced decrease in the accumulation mode particles with the onset of the monsoon. Cloud condensation nuclei concentrations at 0.4% supersaturation (CCN0.4) were calculated under the assumption of an average continental hygroscopicity parameter of κ = 0.3. The calculated CCN0.4 concentrations are mostly sensitive to variations in NAcc and vary accordingly between the May and June–July. Using a cloud parcel model, we investigated the formation of cloud droplets using the measured average particle number size distributions, updraft velocities up to 20 m s−1, and initial aerosol particle number concentrations up to 25,000 cm−3. We found different regimes of CCN activation and cloud formation for the month of May (shifting from aerosol-limited regime to transitional regime) and for June–July (transitional regime), which was primarily due to the variability of particle number concentrations in the accumulation mode.

On the submicron aerosol distributions and CCN activity in and around the Korean Peninsula measured onboard the NASA DC-8 research aircraft during the KORUS-AQ field campaign

Aerosol and CCN number concentrations, and aerosol size distributions in and around the Korean Peninsula were measured onboard the NASA DC-8 research aircraft during the KORUS-AQ campaign in May and June of 2016. The average and standard deviation values of total aerosol number concentrations (diameter > 10 nm, NCN10) in the boundary layer (below 1 km altitude) over Seoul, the Korean Peninsula, the Yellow Sea, the East Sea, and the South Sea were 12,318±7808, 10,721±8636, 7599±8583, 5759±6051, and 4186±4777 cm−3, respectively. Corresponding values for CCN number concentration at 0.6% supersaturation (NCCN06) were 3358±1916, 3613±2242, 3422±1707, 2313±1039, and 1819±1328 cm−3, respectively. NCN10 was highly variable according to region and period. In contrast, NCCN06 was relatively invariant regardless of region and period. With the PMF analysis on the aircraft measured aerosol size distribution and NCCN06 data, it was found that aerosol size distributions were composed of several distinct modes and they were different depending on regions. The aerosol hygroscopicity parameter κ was also estimated based on the PMF analysis results. The estimated κ values over Seoul, the Korean Peninsula, and the Yellow Sea were 0.11, 0.11, and 0.36, respectively. Lastly, the CCN closure experiments indicated that CCN concentration could be successfully estimated with the aerosol size distribution data and the estimated κ information from the aircraft measurements. Overall, it was demonstrated that PMF analysis technique could be applied to extract valuable information from limited aircraft measurement dataset.