Hygroscopic Particles Research Articles

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.

New particle formation and its CCN enhancement in the Yangtze River Delta under the control of continental and marine air masses

The aim of this investigation was to figure out the impacts of air mass origins on new particle formation (NPF) events and their cloud condensation nuclei (CCN) enhancement in a polluted atmosphere of the Yangtze River Delta (YRD). We measured the NPF gaseous precursors, 1 nm to 10 μm particle number size distribution, and particle hygroscopicity during a 1-month intensive field campaign in summer at a suburban site. We observed nine NPF events during the campaign, and found that sulfuric acid (H2SO4) played a key role in particle nucleation and subsequent growth. Air masses during the NPF events can be classified into continental (CT), continental-marine (CT-MR) and marine (MR) types, based on their backward trajectories. In CT air masses, the average concentrations of H2SO4 ((5.4 ± 1.0) × 107 cm−3) was 1 times higher than that in CT-MR ((3.2 ± 0.8) × 107 cm−3) and MR air masses ((2.8 ± 0.9) × 107 cm−3). Therefore, new particles in CT air masses were formed (nucleation rate J1.5, 157.4 ± 19.1 cm−3 s−1) and grew rapidly (growth rate GR3–25, 8.1 ± 6.9 nm h−1), which enhanced the number concentrations of 50–100 nm particles by a factor of 2–3. Additionally, the average hygroscopicity parameter of NPs in CT air masses (0.26 ± 0.02) was higher than that in CT-MR (0.13 ± 0.01) and MR (0.15) air masses, leading to a lower critical dry diameter for CCN activation. As a result, the CCN population enhancement due to NPF in continental air masses were 2–5 times higher than that in marine air masses at the suburban site of the YRD.

Measurement report: Firework impacts on air quality in Metro Manila, Philippines, during the 2019 New Year revelry

Abstract. Fireworks degrade air quality, reduce visibility, alter atmospheric chemistry, and cause short-term adverse health effects. However, there have not been any comprehensive physicochemical and optical measurements of fireworks and their associated impacts in a Southeast Asia megacity, where fireworks are a regular part of the culture. Size-resolved particulate matter (PM) measurements were made before, during, and after New Year 2019 at the Manila Observatory in Quezon City, Philippines, as part of the Cloud, Aerosol, and Monsoon Processes Philippines Experiment (CAMP2Ex). A high-spectral-resolution lidar (HSRL) recorded a substantial increase in backscattered signal associated with high aerosol loading ∼440 m above the surface during the peak of firework activities around 00:00 (local time). This was accompanied by PM2.5 concentrations peaking at 383.9 µg m−3. During the firework event, water-soluble ions and elements, which affect particle formation, growth, and fate, were mostly in the submicrometer diameter range. Total (>0.056 µm) water-soluble bulk particle mass concentrations were enriched by 5.7 times during the fireworks relative to the background (i.e., average of before and after the firework). The water-soluble mass fraction of PM2.5 increased by 18.5 % above that of background values. This corresponded to increased volume fractions of inorganics which increased bulk particle hygroscopicity, kappa (κ), from 0.11 (background) to 0.18 (fireworks). Potassium and non-sea-salt (nss) SO42- contributed the most (70.9 %) to the water-soluble mass, with their mass size distributions shifting from a smaller to a larger submicrometer mode during the firework event. On the other hand, mass size distributions for NO3-, Cl−, and Mg2+ (21.1 % mass contribution) shifted from a supermicrometer mode to a submicrometer mode. Being both uninfluenced by secondary aerosol formation and constituents of firework materials, a subset of species were identified as the best firework tracer species (Cu, Ba, Sr, K+, Al, and Pb). Although these species (excluding K+) only contributed 2.1 % of the total mass concentration of water-soluble ions and elements, they exhibited the highest enrichments (6.1 to 65.2) during the fireworks. Surface microscopy analysis confirmed the presence of potassium/chloride-rich cubic particles along with capsule-shaped particles in firework samples. The results of this study highlight how firework emissions change the physicochemical and optical properties of water-soluble particles (e.g., mass size distribution, composition, hygroscopicity, and aerosol backscatter), which subsequently alters the background aerosol's respirability, influence on surroundings, ability to uptake gases, and viability as cloud condensation nuclei (CCN).

A Large Impact of Cooking Organic Aerosol (COA) on Particle Hygroscopicity and CCN Activity in Urban Atmosphere

AbstractCooking organic aerosol (COA) constitutes a considerable proportion of fine particles in populated area, while there is a lack of understanding on its impact on climate. Using a data set from field observations in Beijing, we characterize the variations of COA in fine particulate matter (PM) and its effect on aerosols hygroscopicity and cloud condensation nuclei (CCN) activity. The observed mass concentrations of COA from two campaigns of winter 2016 and summer 2017 were 5.5 ± 5.6 µg m−3 and 1.9 ± 2.1 µg m−3, respectively, corresponding to mass fraction of COA to PM1 and total OA of 8% ± 9% and 17% ± 13% in winter, 10% ± 9% and 19% ± 13% in summer. The strong COA sources are evidenced by apparent peak values in mass concentrations at lunch and dinner times. With increase of volume fraction of COA from ∼10% during noncooking time to ∼40% during dinner time, hygroscopic parameter of OA (κorg) is decreased from ∼0.21 to ∼0.06 (corresponding to a decrease of overall hygroscopic parameter of aerosols, κ, from ∼0.25 to ∼0.20), showing a critical role of COA in altering aerosols hygroscopicity. Further evaluation shows that the decrease in aerosols hygroscopicity caused by COA will reduce its CCN activity significantly (characterized by critical supersaturation, Sc, increasing from ∼0.25%–0.5% to ∼0.4%–0.7%). The current circumstance, which presents no apparent downward trends of COA in Beijing during last decades but much greater levels of COA in China than those in other regions around the world, suggests the great significance to account for the effect of COA on regional climate in models.

Estimation of hygroscopic growth properties of source-related sub-micrometre particle types in a mixed urban aerosol

Knowledge of hygroscopic properties is essential to prediction of the role of aerosol in cloud formation and lung deposition. Our objective was to introduce a new approach to classify and predict the hygroscopic growth factors (Gfs) of specific atmospheric sub-micrometre particle types in a mixed aerosol based on measurements of the ensemble hygroscopic growth factors and particle number size distribution (PNSD). Based on a non-linear regression model between aerosol source contributions from PMF applied to the PNSD data set and the measured Gf values (at 90% relative humidity) of ambient aerosols, the estimated mean Gf values for secondary inorganic, mixed secondary, nucleation, urban background, fresh, and aged traffic-generated particle classes at a diameter of 110 nm were found to be 1.51, 1.34, 1.12, 1.33, 1.09 and 1.10, respectively. It is found possible to impute (fill) missing HTDMA data sets using a Random Forest regression on PNSD and meteorological conditions.

Hygroscopic growth of aerosol particles consisted of oxalic acid and its internal mixture with ammonium sulfate for the relative humidity ranging from 80% to 99.5%

The hygroscopicity of atmospheric aerosol particles plays an important role in their effects on the environment and the public health. Oxalic acid (OA) has been recognized as the dominant dicarboxylic acid in the urban and the remote aerosol particles, so a number of studies have investigated the hygroscopicity of aerosols consisted of pure OA and its mixture with inorganic salt. However, few experimental studies have focused on the hygroscopicity of nanoscale particles at high relative humidity (RH) below saturation of water vapor (i.e., RH = 90–100%) due to the limitation of traditional measuring instruments. In this work, the hygroscopic growth factor (GF) of the aerosol particles composed of pure OA and internally mixed OA-ammonium sulfate (AS) at RH = 80–99.5% were studied using a high humidity tandem differential mobility analyzer (HHTDMA). The experimental results were used to verify the applicability of models including the ideal solution model, the extended aerosol inorganics model (E-AIM) combining with the revised universal quasi-chemical functional group activity coefficients (UNIFAC-Peng) model, the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model and the Zdanovskii–Stokes–Robinson (ZSR) model. According to the experimental results, the effect of RH, the composition and the initial dry particle diameter (D0) on the hygroscopic growth factor (GF) and the effective hygroscopicity parameter (κ) of particles at RH = 80–99.5% were analyzed in details. It has been found that for pure OA particles with D0 = 80/100 nm, the prediction of the UNIFAC-Peng model and the AIOMFAC model becomes more accurate than the ideal solution model with the increasing of RH, while all models cannot describe the GF of smaller particles at RH = 80–99.5%. For mixed OA-AS aerosol particles, the chemical reaction is an important reason for the discrepancies between the HHTDMA results and the model prediction. For the pure OA aerosol particles at RH ≤ 96% and particles containing AS at RH ≤ 99.5%, the sensitivity of GF to D0 is positively and negatively correlated with RH and D0, respectively. However, for all aerosol types in this study, the sensitivity of GF to RH is positively correlated with both D0 and RH. In addition, the κ based on the measurement is clearly dependent on D0 (at RH = 95–99.5%), RH and the particle composition (at RH = 80–99.5%). The measured hygroscopicity of aerosol particles at high RH might be useful in addressing challenges in solving the discrepancies between the κ measured under subsaturated and supersaturated conditions.

Vertical profile of particle hygroscopicity and CCN effectiveness during winter in Beijing: insight into the hygroscopicity transition threshold of black carbon.

The hygroscopicity and ability of aerosol particles to act as cloud condensation nuclei (CCN) is important in determining their lifetime and role in aerosol-cloud interactions, thereby influencing cloud formation and climate. Previous studies have used the aerosol hygroscopic properties measured at the ground to evaluate the influence on cloud formation in the atmosphere, which may introduce uncertainty associated with aerosol hygroscopicity variability with altitude. In this study, the CCN behaviour and hygroscopic properties of daily filter collections of PM2.5 from three different heights (8, 120, 260 m) on a tower in Beijing were determined in the laboratory using water, water/methanol and methanol as the atomization solvents. Whilst there was substantial temporal variability in particle concentration and composition, there was little obvious difference in aerosol CCN and hygroscopic behaviour at different heights, although the planetary boundary layer height (PBLH) reduced to below the tower height during the nighttime, suggesting that use of surface hygroscopicity measurements is sufficient for the estimation of aerosol particle activation in clouds. Additionally, the critical coating thickness (in terms of mass ratio of coating/refractory BC, MRc) defining the BC transition between being hydrophobic to hydrophilic, was determined by combining hygroscopic tandem differential mobility analyser (H-TDMA), centrifugal particle mass analyzer (CPMA) and single particle soot photometer (SP2) measurements. The MRc of 250 nm BC-containing particles increased from a background value of between 0.8 and 1.6 to around 4.6 at the onset of the growth event of nanoparticles, decreasing monotonically back to the background level as the event progressed. This indicates that large particles do not act as an effective pre-existing condensation sink of the hygroscopic vapours during the nanoparticle growth events, leading to the 250 nm BC particles requiring more coating materials to transition between being hydrophobic and hydrophilic. These findings show that large particles may be less important in suppressing the new particle formation and subsequent growth in the atmosphere.

Hygroscopic property of inorganic salts in atmospheric aerosols measured with physisorption analyzer

Hygroscopicity is capable of significantly affecting atmospheric visibility, radiative forcing, and cloud condensation nuclei process. Herein, a physisorption analyzer was adopted to determine the hygroscopicity of inorganic salt particles in atmospheric aerosols (e.g., NaNO3, NaClO4, MgSO4, K3PO4, K2HPO4, and KH2PO4) at 298 K. By regulating the ambient relative humidity (RH), qualitative and quantitative information of the mentioned particles in humidifying and dehumidifying cycles was acquired. The deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) were measured, and supersaturation was observed and recorded during the dehumidification process. The water-to-salt molar ratios (WSR) of the mentioned inorganic salts depending on RH were determined, and the hygroscopic results of NaNO3 and NaClO4 particles were well consistent with the results achieved with other methods or by theoretical prediction. This study fitted WSR-to-RH equations suitable for NaNO3, NaClO4, MgSO4, K3PO4, and K2HPO4 particles. The adsorption heats for monolayer water molecules in the mentioned inorganic salt particles are calculated. The present study lays a solid foundation for gaining insights into the effects of RH on atmospheric aerosols.

Mathematical derivation and physical interpretation of particle size-resolved activation ratio based on particle hygroscopicity distribution: Application on global characterization of CCN activity

Size-resolved particle activation ratio (SPAR), the size-dependent ability of aerosol particles to form cloud condensation nuclei (CCN), is generally parameterized by a formula with sigmoidal shape with three key parameters. However, our understanding about the intrinsic relationship between the sigmoidal shape function and aerosol hygroscopicity distribution remains incomplete. In this study, the relationship between SPAR and particle hygroscopicity distribution is mathematically derived and relationships between key parameters for SPAR and particle hygroscopicity distribution are manifested, making it clear for the physical understanding of key CCN spectral characteristics. On the basis of the mathematically derived SPAR parameterization, a simplified method to calculate SPAR parameters (including maximum activation fraction (MAF), critical diameter and its heterogeneity of CCN-active particles) based on hygroscopicity distribution data is proposed. This simplified method is applied to Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) field measurement datasets around the world for the meta-analysis of regional and seasonal characteristics of CCN activity. It was found that, for CCN-active particles in polluted regions, the MAF can reach lower values, and the variations of both hygroscopicity and its heterogeneity can be larger, compared with those in clean regions. In summer, the MAF and the hygroscopicity of CCN can reach higher values than those in winter. In addition, the variations of SPAR parameters among different campaigns are large and thus measurements of CCN Counter or HTDMA are needed to specify the SPAR parameters at a specific site during a specific period. The bulk CCN activity at different SSs are also calculated based on the calculated SPAR by considering the measured particle number size distribution. The mathematical derivation and physical interpretation of SPAR parameterization in this study can further the understanding of CCN activity based on particle hygroscopicity distribution. The proposed simplified method to calculate SPAR parameters based on HTDMA measurements allows meta-analysis of SPAR parameters and helps expand the CCN activity datasets around the world.

Impact of in-cloud aqueous processes on the chemical compositions and morphology of individual atmospheric aerosols

Abstract. The composition, morphology, and mixing structure of individual cloud residues (RES) and interstitial particles (INT) at a mountaintop site were investigated. Eight types of particles were identified, including sulfate-rich (S-rich), S-organic matter (OM), aged soot, aged mineral dust, aged fly ash, aged metal, refractory, and aged refractory mixture. A shift of dominant particle types from S-rich (29 %) and aged soot (27 %) in the INT to aged refractory mixture (23 %) and S-OM (22 %) in the RES is observed. In particular, particles with organic shells are enriched in the RES (27 %) relative to the INT (12 %). Our results highlight that the formation of more oxidized organic matter in the cloud contributes to the existence of organic shells after cloud processing. The fractal dimension (Df), a morphologic parameter to represent the branching degree of particles, for soot particles in the RES (1.82 ± 0.12) is lower than that in the INT (2.11 ± 0.09), which indicates that in-cloud processes may result in less compact soot. This research emphasizes the role of in-cloud processes in the chemistry and microphysical properties of individual particles. Given that organic coatings may determine the particle hygroscopicity, activation ability, and heterogeneous chemical reactivity, the increase of OM-shelled particles upon in-cloud processes should have considerable implications.

Specific Gravity of Inner and Outer Larch Bark

Larch bark is an interesting resource for the production of insulation panels. As it consists of a sugar-rich inner bark and an outer bark containing more durable components, there is the requirement to separate these compartments. Additionally, bark is often mixed with wooden pieces after industrial debarking processes. In this study, the wet density, dry density, and specific gravity of wood, whole bark, and inner and outer bark are investigated using the pycnometer method, which has been proven to be adequate for the volume measurement of irregularly shaped, light objects such as bark flakes. Soaked with water, the density of the inner bark is highest, followed by wood, and the lightest is the outer bark. Because of different moisture contents, the wet density is not directly comparable. The outer bark sucked up less water than the inner bark. Focusing on the specific gravity, the wood is the heaviest, followed by the outer bark and the inner bark. The differences are significant for both methods, displaying a promising physical basis for separation methods based on density differences. These might be a means to pick out more durable and less hygroscopic outer bark particles from a bark mixture in order to produce optimized bark composites.

Activation properties of aerosol particles as cloud condensation nuclei at urban and high-altitude remote sites in southern Europe

Understanding the activation properties of aerosol particles as cloud condensation nuclei (CCN) is important for the climate and hydrological cycle, but their properties are not fully understood. In this study, the CCN activation properties of aerosols are investigated at two different sites in southern Spain: an urban background station in Granada and a high altitude mountain station in the Sierra Nevada National Park, with a horizontal separation of 21 km and vertical separation of 1820 m.CCN activity at the urban environment is driven by primary sources, mainly road traffic. Maximum CCN concentrations occurred during traffic rush hours, although this is also when the activation fraction is lowest. This is due to the characteristics of the rush hour aerosol consisting of ultrafine and less hygroscopic particles. In contrast, the mountain site exhibited larger and more hygroscopic particles, with CCN activity driven by the joint effect of new particle formation (NPF) and vertical transport of anthropogenic particles from Granada urban area by orographic buoyant upward flow. This led to the maximum concentrations of CCN and aerosol particles occurring at midday at the mountain site. Clear differences in the diurnal evolution of CCN between NPF events and non-event days were observed at the Sierra Nevada station, demonstrating the large contribution of NPF to CCN concentrations, especially at high supersaturations. The isolated contribution of NPF to CCN concentration has been estimated to be 175% higher at SS = 0.5% relative to what it would be without NPF. We conclude that NPF could be the major source of CCN at this mountain site.Finally, two empirical models were used to parameterize CCN concentration in terms of aerosol optical or physical parameters. The models can explain measurements satisfactorily at the urban station. At the mountain site both models cannot reproduce satisfactorily the observations at low SS.

Differences in fine particle chemical composition on clear and cloudy days.

Clouds are prevalent and alter PM2.5 mass and chemical composition. Cloud-affected satellite retrievals are often removed from data products, hindering estimates of tropospheric chemical composition during cloudy times. We examine surface fine particulate matter (PM2.5) chemical constituent concentrations in the Interagency Monitoring of PROtected Visual Environments network during Cloudy and Clear Sky times defined using Moderate Resolution Imaging Spectroradiometer (MODIS) cloud flags from 2010-2014 with a focus on differences in particle hygroscopicity and aerosol liquid water (ALW). Cloudy and Clear Sky periods exhibit significant differences in PM2.5 and chemical composition that vary regionally and seasonally. In the eastern US, relative humidity alone cannot explain differences in ALW, suggesting emissions and in situ chemistry exert determining impacts. An implicit clear sky bias may hinder efforts to quantitatively to understand and improve model representation of aerosol-cloud interactions.

Nitrogen pulse jet cleaning of the pleated filter cartridge clogged with adhesive hygroscopic dusts

The pleated filter cartridges are widely used in industrial production to control particulate emissions and recover valuable particulate matter, where pulse jet cleaning is a major method for filter regeneration. However, for strong hygroscopic and adhesive dust such as Polyacrylamides (PAM), during pulse-jet cleaning the compressed air forms a low temperature zone at the nozzle and will release water to the environment, causing the particles to adhere on the filter after absorbing moisture, resulting in ineffective filter regeneration. Therefore, a higher cleaning pressure or other methods are required to clean the filter surface. In this article, we have proposed a method to collect adhesive dust using dry gas injection and verified its feasibility through pilot experiments. The compressed air injection source was replaced with nitrogen, the curve of system pressure drop shows that nitrogen has good performance in the peeling of adhesive dust from the surface of the pleated filter cartridge. This highlights the effect of the moisture content in the cleaning compressed air on the regeneration of filters clogged with adhesive hygroscopic particles. Therefore, the feasibility of the nitrogen injection was validated, which provides a basis for the future systematic research of industry to collect adhesive dust.

Comparison of aerosol properties derived from sampling and near-horizontal lidar measurements using Mie scattering theory.

Aerosol optical properties are measured near the surface level using sampling instruments and a near-horizontal lidar. The values of the aerosol extinction coefficient inside the instruments are derived from nephelometer and aethalometer data, while the ambient values are measured from the lidar. The information on aerosol size distribution from optical particle counters is used to simulate extinction coefficients using the Mie scattering theory, with corrections on the humidity growth of hygroscopic particles. By applying this method to the continuous data obtained from November to December 2018 at Chiba, Japan, we elucidate the temporal variations of near-surface aerosol properties, including the complex refractive index, single scattering albedo, and Angstrom exponent. The results indicate how aerosol particles change their properties between the dry, instrumental conditions and relatively humid setting of the ambient atmosphere.

Relationships between structures of surfactants and their anti-hygroscopicity performance of ammonium nitrate particles

The coating processing condition was used for the research of coating ammonium nitrate particles by means of the surfactants as coating materials which have the same polar heads with different non-polar straight hydrocarbon tails or the same non-polar straight hydrocarbon tails with different polar heads, in order to investigate the relationships between structures of surfactants and their anti-hygroscopicity used in ammonium nitrate particles coated. Furthermore, the relationships and possibly mechanism were analyzed and discussed based on both the experimental results and the selected coating processing, basic property of materials and behaviors of material in coating processes. The experimental results indicated that, the anti-hygroscopicity performance of C16-alchol and C18-acid were best ones within the all groups of selected surfactants with polar heads. The best declines of hygroscopicity of ammonium nitrate particles coated were 29.26% for C16-alcohol and 24.0% for C18-acid. And for the later, the anti-hygroscopicity performance of C18-acid and C20-acid were best ones within the all groups of selected surfactants with longer tails, and that of C16-alcohol and C14-amine were best ones with shorter tails, their best declines were 24.0%, 20.39%, 29.26% and 18.6%, respectively. The mechanism analysis results indicated that the decline of hygroscopicity were influenced by both the coating processing condition and the material properties, such as the polarity, hydroscopicity, solublity in solves and especially the surfactivity of the used surfactants, and the later, i.e. the material properties would play an important role when the processing condition and solvent used in coating ammonium nitrate particles were fixed. Therfore, the property of coating materials which are depending on their structures and the bahaviours of coating material molecules in coating processes dominated commonly and ultimatly anti-hydroscopicity of coated ammonium nitrate particles, under a selected coating processing condition. This work provides new insights on understanding the relationship between structure of surfactants and anti-hygroscopicity performance to further improve the anti-hygroscopicity performance of ammonium nitrate particles.

Effects of temperature and relative humidity on laboratory air filter loading test by hygroscopic salts

Air filters are common control technologies of the atmosphere pollutant, which could be operated under different environments. The operating conditions include different temperatures, relative humidities, and particle states. However, typical filter testing conditions do not strictly regulate the aforementioned parameters; hence there is always a discrepancy between the lab filter testing and the actual operating conditions. Air filter loading results are valuable to predict the filter life, whereas it could be affected by testing conditions. In this study, the effects of temperature and relative humidity on the air filter loading by hygroscopic salts were studied. Potassium chloride, ammonium sulfate, and ammonium nitrate are used to represent the dry solid particles and wet particles in the atmosphere. The testing temperature covers from 5 ℃ to 50 ℃ and the testing RH covers from 30% to 75%. Importantly, the absolute humidity is considered in this study, which depends on both temperature and RH. Besides the experiments, the possible reasons that could affect the filter loading were also discussed. Based on the experiment results and theories discussed, it could be concluded that the testing RH is a major factor while the testing temperature is a minor factor, which affects the hygroscopic particle filter loading, regardless of the particle state (dry/wet). This article not only suggests the need of revising current filter testing standards by considering the air filter operating temperature and relative humidity but also offers an insight into how the atmospheric particles hygroscopicity can be linked with ambient particulate matter mitigation technologies.

Impact of aging process on atmospheric black carbon aerosol properties and climate effects

Black carbon (BC) is a key driver of global warming, exerting a strong but highly uncertain effect on the global climate. A key challenge of quantifying BC’s radiative effect is predicting the impact of aging process on BC’s properties. BC is mainly emitted from the incomplete combustion of fossil fuel and biomass burning. After being emitted, it undergoes complex chemical and physical transformations in the atmosphere, through condensation, coagulation and heterogeneous oxidation, which is referred to as BC aging. During aging, BC is gradually coated by other chemical species, which dramatically affects BC properties and alters the corresponding climate effects. The objective of this review is to discuss the current knowledge of BC aging from both laboratory and field measurements. This review first summarizes the methods used in the laboratory experiments and field observations. Then it reviews the effects of aging process on BC mixing state, which comprises both morphology and chemical composition. On this basis, changes to BC light absorption, hygroscopicity, cloud condensation nuclei (CCN) and ice nucleation (IN) activity during the aging process are discussed; these properties are closely related to BC climate effects. Laboratory studies on BC aging are always conducted using single-precursor systems, which do not capture the complex composition of the real atmosphere. In field studies, some state-of-the-art online instruments have been deployed to measure BC properties. However, since the field studies are usually conducted at fixed stations, it is difficult to dynamically track the entire BC aging process. Atmospheric aging process converts BC particles from externally mixed to internally mixed, with other chemical species coated on them. The morphology evolution in the aging process contains three stages: void filling, collapse and subsequent growth, which results in BC morphology changing from the initially chain-like shapes to spherical core-shell structures. The chemical composition of coatings is affected by both emission source and extent of aging, showing an increasing proportion of secondary species during the aging process. BC light absorption is enhanced because of coatings that accumulate during aging. However, the absorption enhancement factors reported in previous studies show large discrepancy because of differences in aging degree, morphology and chemical composition. The particle-to-particle heterogeneity in mixing state can also lead to different enhancement factors for particle populations. Along with the enhancement of light absorption, BC particles hygroscopicity and CCN activity are also enhanced during aging process. However, observational evidence is still lacking. The IN activity of BC is still an open question, and whether aging process enhances or suppresses the IN activity of BC is still debatable. Based on this review of previous studies, we recommend further research on the following topics: (1) Characterize the aging process for BC from different emission sources under realistic combustion conditions. (2) Identify the key factors affecting BC aging in various atmospheric environments. (3) Explore the role of BC particles in regional air pollution. (4) Conduct further studies on the interaction between BC and cloud formation.

Interaction between high humidity and hygroscopic aerosol on the filtration performance of electret media

Interaction between relative humidity (RH) and hygroscopic particles significantly affects the filtration performance of electret media. To investigate the interaction effect between RH and hygroscopic particles on the loading characteristics, electret media were loaded with potassium chloride (KCl) at ∼30% and ∼75% RH, respectively. Then, to study the effect of RH change on the filtration performance, media loaded at low RH were exposed to air with higher RH. Results showed that electret media loaded at 75% RH experienced a continuous efficiency degradation from 74 to 38%, and the pressure drop only increased by 1 Pa. Because the collected particles deposited on fibers as isolated particles, instead of the dendrites occurred at low RH. After exposing loaded samples to air with RH higher than the loading RH, the efficiency and pressure drop decreased significantly: the efficiency and pressure drop decreased from 74 to 44% and from 37 to 16 Pa within 3 min, respectively, when samples preloaded at ∼30% RH were treated with ∼75% RH. Due to the water adsorption at higher RH, the dendritic structures formed by the collected particles on fibers were damaged, affecting the filtration performance. This work will offer important guidance for practical applications of electret filters.

Fouling of composite water vapor transport membranes by aerosol nanoparticles

Composite membranes combining a structural microporous substrate with a dense selective layer are used in various water vapor transport applications. The impact of exposure to aerosols on the water vapor permeance is investigated to develop an understanding of the potential for air pollution to degrade the membrane performance. Samples of commercial membranes were loaded with hygroscopic NaCl and non-hygroscopic graphite nanoparticles. Permeance measurements showed that the deposition of both particle types under dry loading conditions (RH<20%) had minimal influence on the membrane. However, when membranes loaded with hygroscopic particles in dry conditions were subsequently exposed to an elevated humidity (RH>75%) and the associated surface condensation, the membrane permeance reduced by up to 16%. This permeance reduction is shown to be caused by pore narrowing, which results in increased resistance of the microporous substrate. SEM analysis of the fouled membrane surfaces confirmed a reduction in the average pore diameter of degraded samples, proportional to the fouling degree. The reversibility of the fouled membrane permeance, achieved through membrane cleaning, implies that re-crystallization of salt ions, entrained onto the edges of the substrate pores in an aqueous form, is a potential explanation for the permeance changes observed.