Hygroscopic Particles Research Articles

Microencapsulated propolis in chewing gum production

Propolis is a resin with high antibacterial and antioxidant properties that honey bees (Apis mellifera L.) gather and then harmonize through their metabolic secretions. Propolis is beneficial to dental health because it contains antibacterial activity against the Streptococcus mutans, which causes tooth decay. The aim of this study is to use microencapculated propolis extract (MPE) in chewing gum formulation for the first time, in order to protect propolis from environmental effects during production and to ensure controlled release in mouth. Spray drying method was applied where maltodextrin acted as coating material. The particle structure of MPE was investigated by scanning electron microscope. The encapsulation efficiency was determined as 62.19%. The hygroscopicity and moisture content of the MPE particles was found as 31.50% and 7.98% respectively. The DPPH scavenging ability, total flavonoid and total phenolic content of the MPE appeared lower than propolis extract. The total flavonoid content of MPE was calculated as 29.06 mg/kg and total phenolic content as 89.17 mg/kg. In vitro antimicrobial activity against Streptococcus mutans of 9% MPE containing gum samples was measured highest with 4.20 mm zone diameter. Hunter b* value increased with increasing MPE concentration. The final MPE incorporated chewing gum appear to be highly functional.
 Keywords: Propolis; Spray drying; Chewing gum; Functional food

A comprehensive SERS, SEM and EDX study of individual atmospheric PM2.5 particles in Chengdu, China

Characterization of atmospheric fine particulate matter (PM2.5) in large cities has important implications for the study of their sources and formation mechanisms, as well as in developing effective measures to control air pollution. Herein, we report a holistic physical and chemical characterization of PM2.5 by combining surface-enhanced Raman scattering (SERS) with scanning electron microscopy (SEM) and electron-induced X-ray spectroscopy (EDX). PM2.5 particles were collected in a suburban area of Chengdu, a large city in China with a population over 21 million. A special SERS chip composed of inverted hollow Au cone (IHAC) arrays was designed and fabricated to allow direct loading of PM2.5 particles. SERS and EDX were used to reveal the chemical composition, and particle morphologies were analyzed from SEM images. SERS data of atmospheric PM2.5 indicated qualitatively the presence of carbonaceous particulate matter, sulfate, nitrate, metal oxides and bioparticles. The EDX showed the presence of the elements C, N, O, Fe, Na, Mg, Al, Si, S, K, and Ca in the collected PM2.5. Morphology analysis showed that the particulates were mainly in the form of flocculent clusters, spherical, regular crystal shaped or irregularly shaped particles. Our chemical and physical analyses also revealed that the main sources of PM2.5 are automobile exhaust, secondary pollution caused by photochemical reactions in the air, dust, emission from nearby industrial exhaust, biological particles, other aggregated particles, and hygroscopic particles. SERS and SEM data collected during three different seasons showed that carbon-containing particles are the principal sources of PM2.5. Our study demonstrates that the SERS based technique, when combined with standard physicochemical characterization methods, is a powerful analytical tool to determine the sources of ambient PM2.5 pollution. Results obtained in this work may be valuable to the prevention and control of PM2.5 pollution in air.

Photooxidation potential of fine particles from desulfurization flue gas aerosol

In China, desulfurization flue gas from coal-fired power plants (CFPPs) has been the focus of research for many years. In this study, for the first time, we simulated the emission process of flue gas and prepared photocatalysts by deposition. An innovative photocatalytic perspective was applied to study the composition, photocatalytic properties, hygroscopicity, and redox properties of the fine particles from flue gas aerosols. The characterization results of XRD, HRTEM, and XPS of freeze-dried desulfurized flue gas samples indicated that aerosol condensation occurred during the smoke plume transportation process, resulting in the formation of well-crystallized fine particles primarily consisting of soluble salts and metal oxides. The UV–vis and ESR results confirmed that desulfurized flue gas has UV light utilization and photocatalytic potential for producing oxidation groups such as hydroxyl radicals and superoxide radicals in light. The subsequent photocatalyst experiments revealed an 85.4 % reduction of 7 ppm isoprene in 30 min; ozone production (373 μg/m3) was observed under more complicated conditions (NO2 = 80 ppb, VOCs = 7 ppm). As a result of the study, a new research approach was proposed for exploring the photoactive potential of flue gas aerosol produced by CFPPs, as well as its impact on the concentration of VOCs and ozone. The finding provided a new possible pathway for desulfurization flue gas emissions and a reference for futureresearch on the observation and control of atmospheric pollution.

Relative Humidity Dependence of Growth Factor and Real Refractive Index for Sea Salt/Malonic Acid Internally Mixed Aerosols

AbstractSea salt aerosols (SSAs) often mix with organic matter via sea‐to‐air emission, photochemical oxidation, and atmospheric transport, which change particle hygroscopicity and real refractive index (RRI), thereby the radiative effects. However, the influence of water uptake and organic matters on radiative effects is not well understood. In this study, relative humidity (RH)‐dependent growth factor (GF), light scattering enhancement factor, RRI (nmea), and radiative forcing efficiency (RFE) in the visible spectrum of SSAs/malonic acid (MA) internally mixed aerosols were investigated. Our results showed that MA‐mediated SSAs readily absorbed water even at low RH but suppressed hygroscopic growth at high RH. The discrepancy between Zdanovskii‐Stokes‐Robinson predictions and measured GF indicated that the suppressing effect on size caused by produced malonate salts in chloride depletion became more evident as MA content increased. RRI in the visible spectrum decreased as MA got enriched and approached toward water's RRI at high RH, resulting in a less cooling effect. The increasing RH reduced RFE both by decreasing RRI and increasing particle size substantially. Ignoring the effect of MA on particle size and RRI under varied RH, errors in RFE at 525 nm up to 25% could be induced. Volume mixing rule performs well in predicting the RRI of mixed aerosols in dry conditions, while the retrieved difference of 0.061 between volume mixing RRI nvol and nmea would make the error in RFE up to 1.15 W·m−2·Aerosol optical depth−1 at 87% RH. Our study helps understand the impact of water content and dicarboxylic acids on the direct radiative effects of SSAs.

Seasonal variation of particle hygroscopicity and its impact on cloud-condensation nucleus activation in the Beijing urban area

Hygroscopicity is a key parameter for characterizing the ability of particles to be activated as cloud-condensation nuclei (CCN). In this study, we present a 10-month size-resolved measurement of the hygroscopicity of particles with diameters of 50, 100 and 200 nm in the Beijing urban area using a Hygroscopicity-Tandem Differential Mobility Analyzer (H-TDMA). Compared with that in other studies conducted in China, the hygroscopicity parameter (κ) in this study was relatively higher than that in the Pearl River Delta region but lower than that in the Yangtze River Delta region. As the mass fraction of inorganic matter (especially nitrate) increased, the particles became more hygroscopic during spring and summer, as influenced by the chemical composition. Polluted southerly air masses also enhanced particle hygroscopicity. The CCN concentration was calculated based on κ-Köhler theory, with a moderate supersaturation of 0.4%. The critical diameter of particles as potential CCNs was smaller in spring than that in the other seasons. The critical diameters of aerosols in autumn, winter and spring were 142, 123 and 114 nm, respectively, with calculated CCN concentrations of 1683, 1909 and 1765 cm−3, respectively. Activation ratios, calculated as CCN divided by condensation nuclei (CN), were 0.13, 0.15 and 0.18 in autumn, spring and summer, respectively. During episodes of heavy pollution, the CCN concentration highly depended on the particle mass loading, as the accumulation mode dominated the particle number size distribution and the chemical composition was quasi-homogeneous. However, under other conditions, both particle size and chemical composition were important. This study revealed the relationship between particle hygroscopicity, CCN and air pollution level, and it will be useful for evaluating the environmental and climatic effects of aerosols.

Particle Number Size Distribution of Wintertime Alpine Aerosols and Their Activation as Cloud Condensation Nuclei in the Guanzhong Plain, Northwest China

AbstractThe particle number size distribution (PNSD) and hygroscopicity on alpine sites can directly reflect the interaction between aerosol and cloud in the free troposphere, and narrow the errors caused by extrapolation from the ground observation. In this study, PNSD and cloud condensation nuclei (CCN) concentration (NCCN) were observed at the summit of Mt. Hua from December 16, 2020, to January 23, 2021. The result showed that the modes and hygroscopicity parameters (κ) of aerosols were significantly affected by the height and direction of the originated air masses. The particle in nucleation mode from new particle formation erupted frequently in 13:00–18:00 local time due to the intense photochemistry and had a potential contribution to CCN through the process of the growth with the ratio of 0.83 nm·h−1, which was affected by air masses, meteorological conditions, and gaseous pollutants. Subsequently, the hygroscopicity κ varied from 0.22 to 0.13 with the supersaturation from 0.2% to 1.0% as more components that were difficult to dissolve or had low hygroscopicity. The comprehensive analysis of PNSD and hygroscopicity showed that smaller k from two‐parameter power model corresponded to particles with higher hygroscopicity or larger diameter, while larger k matched to the hydrophobic or ultrafine particles. In addition, the interactions between aerosol and cloud were estimated more accurately and the uncertainty of indirect effect was shrunk. Overall, we provided insights into PNSD and hygroscopicity of alpine aerosol particles by direct observation on Mt. Hua, which may be important to other elevated regions as well.

Hygroscopicity of aerosol particles composed of surfactant SDS and its internal mixture with ammonium sulfate at relative humidities up to 99.9%

For atmospheric aerosol particles, the ubiquitous surfactants can significantly influence their hygroscopicity. However, the hygroscopicity of nanoscale particles containing surfactants are not well understood under subsaturated conditions, especially at high relative humidity (RH = 90–100%), due to the limitation of traditional instruments and the diversity of surfactants. Here, the hygroscopicity of nanoscale particles composed of a strong surfactant sodium dodecyl sulfate (SDS) and its mixture with ammonium sulfate (AS) was measured using the high humidity tandem differential mobility analyzer (HHTDMA) at RH up to about 99.9%. Three different models based on Köhler theory but different representations of surfactant characteristics were employed to reproduce the measurement outcomes. Results show that SDS particles start gradual hygroscopic growth after RH reaching 97.5%. For SDS-AS mixed particles with 25% w/w SDS, prompt deliquescence and gradual deliquescence are observed for smaller (20 and 40 nm) and larger particles (100 and 200 nm), respectively. For particles with ≥50% w/w SDS, the onset deliquescence is difficult to found. In general, the model that comprehensively accounts for the surfactant properties including bulk-to-surface partitioning, surface tension reduction, micellization and the effect of RH on hygroscopicity parameter, ĸ (for SDS-AS particles) provides the best predictions for the experimental hygroscopicity of SDS and SDS-AS particles at subsaturation conditions. Besides, factors affecting the accuracy of theoretical model are discussed in detail. The ĸ derived from subsaturated measurement is strongly dependent on RH and dry diameter.

Identifying the hygroscopic properties of fine aerosol particles from diverse sources in urban atmosphere and the applicability in prediction of cloud nuclei

Aerosols hygroscopicity is generally differentiating for particles among various sources, leading to complex effects on visibility impairment and cloud formation. However, the hygroscopicity of aerosol particles from diverse sources is not yet fully understood. Here, combining field observations at an urban site in Beijing and a positive matrix factorization model, we identified aerosols hygroscopicity of six different sources, nucleation, traffic, cooking, aging primary, residential heating, and regional secondary, which corresponds to hygroscopic growth factors (Gfs) of 1.23, 1.15, 1.17, 1.44, 1.48, and 1.47, respectively at relative humidity of 90%. On polluted days, Gf values of particles for the accumulation-mode particles were higher than that under clean conditions. This is largely due to the hygroscopic sources represented by aged and secondary processes showed the dominant role in aerosol population. While on clean days, the Gfs of 40-nm particles showed larger values, that is closely associated with the photochemical-nucleation-sourced particles. By further applying the Gfs together with aerosol spectrum to predict cloud nuclei concentration (CCN), we find the predicted and observed CCN number concentration are well correlated at typical supersaturation in cloud. The study further reveals the dominant roles of secondary processes, contributing up to 90% of the total cloud nuclei. Primary particles from traffic/cooking activities contributed only ∼5%, although they account for 30–40% of the total particle number concentrations in urban Beijing during the studied period. The results indicate the source apportionment of Gfs can well interpret the variation in hygroscopicity of aerosols under different atmospheric conditions and evaluate their effects on cloud formation.

Quantification of gas, ash, and sulphate aerosols in volcanic plumes from open path Fourier transform infrared (OP-FTIR) emission measurements at Stromboli volcano, Italy

Field-portable Open Path Fourier Transform Infrared (OP-FTIR) spectrometers can be used to remotely measure the composition of volcanic plumes using absorption spectroscopy, providing invaluable data on total gas emissions. Quantifying the temporal evolution of gas compositions during an eruption helps develop models of volcanic processes and aids in eruption forecasting. Absorption measurements require a viewing geometry which aligns infrared source, plume, and instrument, which can be challenging. Here, we present a fast retrieval algorithm to estimate quantities of gas, ash and sulphate aerosols from thermal emission OP-FTIR measurements, and the results from two pilot campaigns on Stromboli volcano in Italy in 2019 and 2021. We validate the method by comparing time series of SO2 slant column densities retrieved using our method with those obtained from a conventional UV spectrometer, demonstrating that the two methods generally agree to within a factor of 2. The algorithm correctly identifies ash-rich plumes and gas bursts associated with explosions and quantifies the mass column densities and particle sizes of ash and sulphate aerosols (SA) in the plume. We compare the ash sizes retrieved using our method with the particle size distribution (PSD) of an ash sample collected during the period of measurements in 2019 by flying a Remotely Piloted Aircraft System into the path of a drifting ash plume and find that both modes of the bimodal PSD (a fine fraction with diameter around 5–10 μm and a coarse fraction around 65 μm) are identified within our datasets at different times. We measure a decrease in the retrieved ash particle size with distance downwind, consistent with settling of larger particles, which we also observed visually. We measure a decrease in the SO2/SA ratio as the plume travels downwind, coupled with an increase in measured SA particle size (range 2–6 μm), suggesting rapid hygroscopic particle growth and/or SO2 oxidation. We propose that infrared emission spectroscopy can be used to examine physical and chemical changes during plume transport and opens the possibility of remote night-time monitoring of volcanic plume emissions. These ground-based analyses may also aid the refinement of satellite-based aerosol retrievals.

Molecular dynamics study on the surface tension of succinic acid-water nano-aerosol droplets

The surface tension plays a significant role in the hygroscopicity of aerosol particles on a nanoscale. However, it cannot be obtained by using the existing measurement techniques. In this study, we simulate the hygroscopic growth of one single succinic acid (SA) particle by using the molecular dynamic (MD) method. Based on the MD simulation results, the surface tension of the stable SA-water droplet is calculated by using a numerical model. Furthermore, the influencing mechanisms of temperature, diameter and concentration of SA on the surface tension of the nanoscale droplet are investigated. The results show that with the temperature increasing from 260 K to 320 K, the surface tension of the droplet decreases, which is mainly caused by the weakening of the intermolecular forces inside the droplet. Besides, the sensitivity of the surface tension to the temperature increases with the increasing SA concentration, which can be explained by the effect of the temperature and the SA concentration on the radial distribution of SA molecules. With the increase of the particle diameter, the surface tension of droplet first increases and then tends to be constant. The normal components of the Irving-Kirkwood pressure tensors are calculated to explain the effect of diameter and SA on the surface tension. In addition, when the SA concentration is increased, the particle diameter range which has an obvious effect on the surface tension is reduced. Moreover, the surface tension of the nanodroplet is negatively correlated with the SA concentration, and the correlation fits into the logarithmic function form, especially for droplet with a diameter smaller than 6.12 nm. The Szyszkowski equation is employed to fit the relationship between SA concentration and the surface tension of droplet. These findings can provide parameter support for improving the theoretical model of particle hygroscopicity and related kinetic processes. This study emphasizes further research on the surface tension of nano-droplets with more complex components.

Differentiating the Contributions of Particle Concentration, Humidity, and Hygroscopicity to Aerosol Light Scattering at Three Sites in China

AbstractThe scattering of light by aerosol particles dictates atmospheric visibility, which is a straightforward indicator of air quality. It is affected by numerous factors, such as particle number size distribution, particle mass concentration (PM2.5), ambient relative humidity (RH), and chemical composition. The latter two factors jointly influence the aerosol liquid water content (ALWC). Here, the particle backscattering coefficient (βp) under ambient RH conditions is investigated to differentiate and quantify the contributions of aerosol properties and meteorology using comprehensive observational datasets acquired at three megacities in China, that is, Beijing (BJ), Nanjing (NJ), and Guangzhou (GZ). Overall, the temporal variations in βp under ambient RH conditions are consistent with those in ALWC at the three sites. The PM2.5 in BJ is systematically higher than in NJ and GZ, while ambient RH and aerosol hygroscopicity in NJ are much higher than in BJ and GZ. Notable differences in the variations of βp with related factors at the three sites are demonstrated. βp is more sensitive to particle hygroscopicity and mass in NJ and ambient RH in BJ. The relative contributions of these factors to βp at the three sites under different pollution conditions are differentiated and quantified. The factor with the largest impact on the variability in βp shifts from particle mass to ambient RH as air quality deteriorated to heavy pollution in BJ. The opposite is true in NJ. In GZ, the contributions of these factors to changes in βp under different pollution conditions are similar, both dominated by PM2.5.

Evaluation of a low-cost dryer for a low-cost optical particle counter

Abstract. The use of low-cost sensors for air quality measurements has become very popular in the last few decades. Due to the detrimental effects of particulate matter (PM) on human health, PM sensors like photometers and optical particle counters (OPCs) are widespread and have been widely investigated. The negative effects of high relative humidity (RH) and fog events in the mass concentration readings of these types of sensors are well documented. In the literature, different solutions to these problems – like correction models based on the Köhler theory or machine learning algorithms – have been applied. In this work, an air pre-conditioning method based on a low-cost thermal dryer for a low-cost OPC is presented. This study was done in two parts. The first part of the study was conducted in the laboratory to test the low-cost dryer under two different scenarios. In one scenario, the drying efficiency of the low-cost dryer was investigated in the presence of fog. In the second scenario, experiments with hygroscopic aerosols were done to determine to which extent the low-cost dryer reverts the growth of hygroscopic particles. In the second part of the study, the PM10 and PM2.5 mass concentrations of an OPC with dryer were compared with the gravimetric measurements and a continuous federal equivalent method (FEM) instrument in the field. The feasibility of using univariate linear regression (ULR) to correct the PM data of an OPC with dryer during field measurement was also evaluated. Finally, comparison measurements between an OPC with dryer, an OPC without dryer, and a FEM instrument during a real fog event are also presented. The laboratory results show that the sensor with the low-cost dryer at its inlet measured an average of 64 % and 59 % less PM2.5 concentration compared with a sensor without the low-cost dryer during the experiments with fog and with hygroscopic particles, respectively. The outcomes of the PM2.5 concentrations of the low-cost sensor with dryer in laboratory conditions reveal, however, an excess of heating compared with the FEM instrument. This excess of heating is also demonstrated in a more in-depth study on the temperature profile inside the dryer. The correction of the PM10 concentrations of the sensor with dryer during field measurements by using ULR showed a reduction of the maximum absolute error (MAE) from 4.3 µg m−3 (raw data) to 2.4 µg m−3 (after correction). The results for PM2.5 make evident an increase in the MAE after correction: from 1.9 µg m−3 in the raw data to 3.2 µg m−3. In light of these results, a low-cost thermal dryer could be a cost-effective add-on that could revert the effect of the hygroscopic growth and the fog in the PM readings. However, special care is needed when designing a low-cost dryer for a PM sensor to produce FEM similar PM readings, as high temperatures may irreversibly change the sampled air by evaporating the most volatile particulate species and thus deliver underestimated PM readings. New versions of a low-cost dryer aiming at FEM measurements should focus on maintaining the RH at the sensor inlet at 50 % and avoid reaching temperatures higher than 40 ∘C in the drying system. Finally, we believe that low-cost dryers have a very promising future for the application of sensors in citizen science, sensor networks for supplemental monitoring, and epidemiological studies.

Variations of aerosol and cloud vertical characteristics based on aircraft measurements in upstream of Shanghai during the 2020 China international import expo

Vertical information about aerosols and clouds is vital to understanding aerosol transport, aerosol-cloud interactions, and pollution-weather-climate feedback so as to reduce uncertainties in estimating their climatic effects. The combination of sounding, lidar, aircraft, and satellite measurements is widely used to obtain the vertical information of aerosols and clouds. We used an aircraft measurement over southern Anhui, the upstream regions of Shanghai, on 1 November which conducted to ensure good air quality for the Third China International Import Expo to examine the vertical characteristics of aerosol and cloud microphysical properties and their variations before and after cloud seeding. Observations showed aerosols and clouds were vertically stratified. Most aerosols trapped within the boundary layer are small particles with sizes less than 0.12 µm. Aerosol number concentrations (Na) generally decreased with altitude in the cloudless atmosphere, with the largest particles occurring in 2500–3500 m due to dust transported from distant regions and high ambient humidity. Four separate cloud layers with unequal depths dominated by altostratus and nimbostratus appeared at different heights. The maximum cloud droplet concentration (Nc) and the minimum cloud droplet diameter (Dc) that appeared in the mid-level cloud (2246–2482 m) were 107.7 cm−3 and 4.03 μm, respectively, owing to the high proportion of hygroscopic particles. Hygroscopic particles played an important role in the growth of droplets and the activation of cloud condensation nuclei, especially under high ambient humidity. Cloud droplet size spectrum showed a unimodal distribution with a single peak at 5 µm in low- (970–1000 m) and mid-level clouds, but a trimodal distribution with peaks at 7 μm, 12 μm, and 17 μm in the mid-high- and high-level clouds, indicating the broadening of spectra with increasing altitude. An artificial seeding experiment was conducted in the high-level clouds. Big cloud droplets and ice crystals increased significantly after cloud seeding. Meanwhile, cloud particle populations showed less Nc, larger Dc, and a wider size spectrum. Our results suggest that the artificial precipitation experiment promoted rainfall to a certain extent and contributed to the removal of pollutants from upstream regions, which is beneficial to the air quality of Shanghai.

Analytical procedure for the concomitant analysis of 242 polar and non-polar organic compounds of different functional groups in fog water

Fog droplets consist of a mixture of organic and inorganic compounds resulting from scavenging hygroscopic particles and water-soluble trace gases. Organic matters found in fog water are able to accumulate non-volatile, semi-volatile and volatile compounds of different chemical structures and properties (different functional groups and chemical properties). However, their extraction and analysis are still very limited and infrequent. The paper presents a new analytical procedure based on the combination of liquid–liquid extraction (LLE) on a solid support (XTR Chromabond cartridge). It is developed, optimized and validated for the simultaneous analysis of 38 organic acids, 64 phenols, 55 semi-volatile pesticides, 27 non-volatile pesticides, 16 polycyclic aromatic hydrocarbons (PAHs), 20 organochlorine pesticides (OCPs) and 22 polychlorinated biphenyls (PCBs) residues in fog water. The obtained extract is concentrated, derivatized and then analyzed with different chromatographic techniques. Phenols and acids are injected into gas chromatography-mass spectrometry (GC-MS) preceded by a derivatization reaction with N-tert-butyldimethylsilyl-N-methyltrifluoroacetamide (MtBSTFA). Semi-volatile pesticides, PAHs, PCBs and OCPs are injected into gas chromatography-tandem mass spectrometry (GC-MS/MS), while non-volatile pesticides are injected into liquid chromatography-tandem mass spectrometry (LC-MS/MS). The new protocol is validated for its linearity (r2), recovery (R), precisions (inter and intra-day), limit of detection (LOD) and limit of quantification (LOQ). It shows good linearity (r2 > 0.99) in the concentration ranging from 0.05 to 150 ng/L. The repeatability and reproducibility of measurements expressed as relative standard deviation (% RSD) is found to be satisfactory (% RSD < 20 %). Furthermore, the detection limits obtained are in the low order of ng/L. Then the developed procedure is tested successfully for its efficiency on real fog samples collected from Erstein (Bas-Rhin, Alsace) and Fnaideq (Akkar, North-Lebanon).

Hygroscopicity of ultrafine particles containing ammonium/alkylaminium sulfates: A Köhler model investigation with correction of surface tension

Insights into the hygroscopicity of nanoparticles is critically important in determining their growth and subsequent climate implications. A significant knowledge gap in the traditional Köhler (TK) model is the lack of size dependence of physicochemical properties of nanoparticles. Here, we present the modified Köhler curves of nanoparticles containing ammonium sulfate (AS) and alkylaminium sulfates (AASs), including monomethylaminium sulfate (MMAS), dimethylaminium sulfate (DMAS), and trimethylaminium sulfate (TMAS), by considering the curvature-correction of surface tension (σ) obtained from polarizable molecular dynamic (MD) simulations. It is found that sulfates can cause a non-monotonic relation between σ and droplet size with a maximum at several nanometers. The modified Köhler curves demonstrate that, for sulfate particles with diameters over 10 nm, accounting σ-correction can apparently affect the inverse of critical supersaturation (Sc−1), but still give the consistent Sc−1 order with the model without σ-correction (Sc−1 (AS) > Sc−1 (MMAS) > Sc−1 (DMAS) > Sc−1 (TMAS)). While for ultra-small particles (d = ∼3 nm), the curvature correction of σ is much more significant, which even alters the order of Sc−1 (Sc−1 (MMAS) ≈ Sc−1 (DMAS) > Sc−1 (AS) > Sc−1 (TMAS)). The present study shows that the curvature correction of surface tension is necessary to accurately predict the hygroscopicity of nucleation mode particles, and MMAS/DMAS-containing nanoparticles can demonstrate easier water-uptake than that of AS-containing nanoparticles with ∼3 nm diameter, thereby leading to higher growth rate and stronger climate impacts.

Organic enrichment in droplet residual particles relative to out of cloud over the northwestern Atlantic: analysis of airborne ACTIVATE data

Abstract. Cloud processing is known to generate aerosol species such as sulfate and secondary organic aerosol, yet there is a scarcity of airborne data to examine this issue. The NASA Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) was designed to build an unprecedented dataset relevant to aerosol–cloud interactions with two coordinated aircraft over the northwestern Atlantic, with aerosol mass spectrometer data used from four deployments between 2020–2021 to contrast aerosol composition below, in (using a counterflow virtual impactor) and above boundary layer clouds. Consistent features in all time periods of the deployments (January–March, May–June, August–September) include the mass fraction of organics and relative amount of oxygenated organics (m/z 44) relative to total organics (f44) increasing in droplet residuals relative to below and above cloud. Detailed analysis comparing data below and in cloud suggests a possible role for in-cloud aqueous processing in explaining such results; an intriguing aspect though requiring more attention is that only approximately a quarter of the cloud cases (29 of 110) showed higher organic mass fractions either below or above cloud. Of those 29 cases, the majority (25) showed higher organic mass fraction below cloud base where the cloud processing signature is presumably more evident as compared to above cloud. These results are consistent with the few past studies analyzing droplet residuals pointing to higher organic enrichment than in adjacent cloud-free areas. The data findings are important as other datasets (e.g., reanalysis) suggest that sulfate is both more abundant than organics (in contrast to this work) and more closely related to drop number concentrations in the winter when aerosol–cloud interactions are strongest. Here we show that organics are more abundant than sulfate in the droplet residuals and that aerosol interaction with clouds potentially decreases particle hygroscopicity due to the increase in organic:sulfate ratio for droplet residuals relative to surrounding cloud-free air. These results are important in light of the growing importance of organics over the northwestern Atlantic in recent decades relative to sulfate owing to the success of regulatory activity over the eastern United States to cut sulfur dioxide emissions.

Hygroscopicity and CCN potential of DMS-derived aerosol particles

Abstract. Dimethyl sulfide (DMS) is emitted by phytoplankton species in the oceans and constitutes the largest source of naturally emitted sulfur to the atmosphere. The climate impact of secondary particles, formed through the oxidation of DMS by hydroxyl radicals, is still elusive. This study investigates the hygroscopicity and cloud condensation nuclei activity of such particles and discusses the results in relation to their chemical composition. We show that mean hygroscopicity parameters, κ, during an experiment for particles of 80 nm in diameter range from 0.46 to 0.52 or higher, as measured at both sub- and supersaturated water vapour conditions. Ageing of the particles leads to an increase in κ from, for example, 0.50 to 0.58 over the course of 3 h (Exp. 7). Aerosol mass spectrometer measurements from this study indicate that this change most probably stems from a change in chemical composition leading to slightly higher fractions of ammonium sulfate compared to methanesulfonic acid (MSA) within the particles with ageing time. Lowering the temperature to 258 K increases κ slightly, particularly for small particles. These κ values are well comparable to previously reported model values for MSA or mixtures between MSA and ammonium sulfate. Particle nucleation and growth rates suggest a clear temperature dependence, with slower rates at cold temperatures. Quantum chemical calculations show that gas-phase MSA clusters are predominantly not hydrated, even at high humidity conditions, indicating that their gas-phase chemistry should be independent of relative humidity.

Impact of Relative Humidity on the vertical distribution of aerosols over India

Water uptake by aerosol particles significantly changes its light-scattering characteristics, alters the physical and optical properties of aerosols. In this article, we present the effect of ambient relative humidity (RH) on the optical properties of surface and columnar aerosols under different RH conditions during different seasons over India. In particular, relation between surface RH and surface aerosol properties, surface RH and aerosol optical depth (AOD) and profile of RH on the AOD is presented. An increase in the surface aerosol scattering coefficient (σscat) and total aerosol number concentration (N(d)) is observed under high humidity (50–100%) condition than low humidity (0–30%). This is more prominent in the winter and pre-monsoon seasons. An increase in the AOD is observed as surface RH increase in all the seasons except in monsoon season where it showed an inverse relation. The profile measurements (Lidar and radiosonde) also showed a consistent increase in AOD (0-6 km) from low to high RH (0-6 km) conditions. This relationship is significant during winter followed by the post-monsoon, pre-monsoon, and monsoon seasons. An increase in AOD with RH is observed regardless of altitude in all seasons, except during the monsoon at higher altitudes (2-6 km). Our findings clearly demonstrated that during the winter and monsoon (pre- and post-monsoon) seasons, aerosols within (above) the boundary layer are more sensitive to the RH. This relationship has been tested in different surface conditions across India using CALIPSO and IMD radiosonde measurements. These measurements also confirmed an increase in AOD with increasing RH in all the regions except East Coast (EC) region. Irrespective of the region, this feature is more prominent at lower altitudes (0–1.5 km) during winter and higher altitudes (1.5-6 km) during other seasons. At higher altitudes, only a few regions (Central India, Indo-Gangetic Plain, and West Coast) showed an inverse relationship between AOD and RH during the monsoon and post-monsoon seasons. Further, our results illustrated the contribution of hygroscopic particles to the RH increases the AOD in a given season. This finding implies that hygroscopic growth caused by water-soluble particles in the lower troposphere can significantly alter the magnitude of aerosol radiative forcing both at the surface and at the top of the atmosphere.

PM2.5 Source Apportionment and Implications for Particle Hygroscopicity at an Urban Background Site in Athens, Greece

Aerosol hygroscopicity is a key aerosol property, influencing a number of other physical properties, and the impacts of PM pollution on the environment, climate change, and health. The present work aims to provide insight into the contribution of major PM sources to aerosol hygroscopicity, focusing on an urban background site, with a significant impact from both primary and secondary sources. The EPA PMF 5.0 model was applied to PM2.5 chemical composition and hygroscopicity data collected from August 2016 to July 2017 in Athens, Greece. Source apportionment analysis resulted in six major sources, including four anthropogenic sources (vehicular exhaust and non-exhaust, heavy oil combustion, and a mixed source of secondary aerosol formation and biomass burning) and two natural sources (mineral dust and aged sea salt). The mixed source was found to be the main contributor to PM2.5 levels (44%), followed by heavy oil combustion (26%) and vehicular traffic exhaust and non-exhaust emissions (15%). The aerosol hygroscopic growth factor (GF) was found to be mainly associated with the mixed source (by 36%) and heavy oil combustion (by 24%) and, to a lesser extent, with vehicle exhaust (by 19%), aged sea salt (by 14%), and vehicle non-exhaust (by 6%).

Assessment of NAAPS-RA performance in Maritime Southeast Asia during CAMP2Ex

Abstract. Monitoring and modeling aerosol particle life cycle in Southeast Asia (SEA) is challenged by high cloud cover, complex meteorology, and the wide range of aerosol species, sources, and transformations found throughout the region. Satellite observations are limited, and there are few in situ observations of aerosol extinction profiles, aerosol properties, and environmental conditions. Therefore, accurate aerosol model outputs are crucial for the region. This work evaluates the Navy Aerosol Analysis and Prediction System Reanalysis (NAAPS-RA) aerosol optical thickness (AOT) and light extinction products using airborne aerosol and meteorological measurements from the Cloud, Aerosol, and Monsoon Processes Philippines Experiment (CAMP2Ex) conducted in 2019 during the SEA southwest monsoon biomass burning season. Modeled AOTs and extinction coefficients are compared to those retrieved with a high spectral resolution lidar (HSRL-2). Agreement between simulated and retrieved AOT (R2= 0.78, relative bias =-5 %, normalized root mean square error (NRMSE) = 48 %) and aerosol extinction coefficients (R2= 0.80, 0.81, and 0.42; relative bias = 3 %, −6 %, and −7 %; NRMSE = 47 %, 53 %, and 118 % for altitudes between 40–500, 500–1500, and &gt;1500 m, respectively) is quite good considering the challenging environment and few opportunities for assimilations of AOT from satellites during the campaign. Modeled relative humidities (RHs) are negatively biased at all altitudes (absolute bias =-5 %, −8 %, and −3 % for altitudes &lt;500 500–1500 and &gt;1500 m, respectively), motivating interest in the role of RH errors in AOT and extinction simulations. Interestingly, NAAPS-RA AOT and extinction agreement with the HSRL-2 does not change significantly (i.e., NRMSE values do not all decrease) when RHs from dropsondes are substituted into the model, yet biases all move in a positive direction. Further exploration suggests changes in modeled extinction are more sensitive to the actual magnitude of both the extinction coefficients and the dropsonde RHs being substituted into the model as opposed to the absolute differences between simulated and measured RHs. Finally, four case studies examine how model errors in RH and the hygroscopic growth parameter, γ, affect simulations of extinction in the mixed layer (ML). We find NAAPS-RA overestimates the hygroscopicity of (i) smoke particles from biomass burning in the Maritime Continent (MC) and (ii) anthropogenic emissions transported from East Asia. This work mainly provides insight into the relationship between errors in modeled RH and simulations of AOT and extinction in a humid and tropical environment influenced by a myriad of meteorological conditions and particle types. These results can be interpreted and addressed by the modeling community as part of the effort to better understand, quantify, and forecast atmospheric conditions in SEA.