Extended Aerosol Inorganics Model Research Articles

Impacts of the East Asia monsoon on the PM2.5 acidity in Hanoi

This study investigates the acidity of particulate matter with an aerodynamic diameter less than 2.5 μm (PM2.5) in Hanoi, the capital city of Vietnam, and its influencing factors by analyzing measured concentrations of water-soluble inorganic ions (WSIIs) in 107 24-h samples collected between June 2021 and February 2022. The average sulfate, nitrate, and ammonium concentrations were 5.79, 3.55, and 5.46 μg m−3, respectively. Among the samples, 83 exhibited alkaline aerosols characterized by elevated ammonium-to-sulfate molar concentration ratios (ASMCR) ranging from 2.3 to 9.6, while 24 showed acidic characteristics with ASMCR ranging from 0.5 to 2.5. By employing the Extended Aerosol Inorganic Model E-AIM III, with major WSIIs as the input parameters, the estimated aerosol acidity pH ranged from 8.6 to 10.9 for alkaline aerosols and below 4.5 for acidic aerosols. These variations in ASMCR and aerosol pH highlight the distinct acidic and alkaline aerosol source regions affecting Hanoi's PM2.5 by the monsoon air masses. The Potential Source Contribution Function (PSCF) maps of NH4+ and SO42− revealed the Northeast and Southeast monsoon upwind sources of acidic aerosols over China's Guangdong coastal region and Vietnam's offshore, respectively. Alkaline aerosols were observed during the Northeast and Southwest monsoons, with upwind sources originating in South China, Thailand, and Cambodia. The emergence of the ammonium-rich source areas in South China may be attributed to China's sustained emission control measures, targeting acidic SO2 and NOx emissions while leaving alkaline NH3 emissions largely unaffected. This research provided insights into the intricate relationships between regional emissions, long-range transport, and local meteorological-driven emissions, offering valuable guidance for effective air quality management in urban environments.

Inorganic Nitrogen Gas‐Aerosol Partitioning in and Around Animal Feeding Operations in Northeastern Colorado in Late Summer 2021

AbstractAmmonia (NH3) from animal feeding operations (AFOs) is an important source of reactive nitrogen in the US, but despite its ramifications for air quality and ecosystem health, its near‐source evolution remains understudied. To this end, Phase I of the Transport and Transformation of Ammonia (TRANS2Am) field campaign was conducted in the northeastern Colorado Front Range in summer 2021 and characterized atmospheric composition downwind of AFOs during 10 research flights. Airborne measurements of NH3, nitric acid (HNO3), and a suite of water‐soluble aerosol species collected onboard the University of Wyoming King Air research aircraft present an opportunity to investigate the sensitivity of particulate matter (PM) formation to AFO emissions. We couple the observations with thermodynamic modeling to predict the seasonality of ammonium nitrate (NH4NO3) formation. We find that during TRANS2Am northeastern Colorado is consistently in the NH3‐rich and HNO3‐limited NH4NO3 formation regime. Further investigation using the Extended Aerosol Inorganics Model reveals that summertime temperatures (mean: 23°C) of northeastern Colorado, especially near the surface, inhibit NH4NO3 formation despite high NH3 concentrations (max: ≤114 ppbv). Finally, we model spring/autumn and winter conditions to explore the seasonality of NH4NO3 formation and find that cooler temperatures could support substantially more NH4NO3 formation. Whereas NH4NO3 only exceeds 1 μg m−3 ∼10% of the time in summer, modeled NH4NO3 would exceed 1 μg m−3 61% (88%) of the time in spring/autumn (winter), with a 10°C (20°C) temperature decrease relative to the campaign.

Water activity and surface tension of aqueous ammonium sulfate and D-glucose aerosol nanoparticles

Abstract. Water activity (aw) and interfacial energy or surface tension (σ) are key thermodynamic parameters to describe the hygroscopic growth of atmospheric aerosol particles and their ability to serve as cloud condensation nuclei (CCN), thus influencing the hydrological cycle and climate. Due to size effects and complex mixing states, however, these parameters are not well constrained for nanoparticles composed of organic and inorganic compounds in aqueous solution. In this study, we determined aw and σ by differential Köhler analysis (DKA) of hygroscopic growth measurement data for aerosol particles smaller than 100 nm composed of aqueous ammonium sulfate (AS), D-glucose (Gl), and their mixtures. High-precision measurements of hygroscopic growth were performed at relative humidities (denoted RH) ranging from 2.0 % to 99.6 % with a high-humidity tandem differential mobility analyzer (HHTDMA) in three complementary modes of operation: hydration, dehydration, and restructuring. The restructuring mode (hydration followed by dehydration) enabled the transformation of initially irregular particles into compact globules and the determination of mass equivalent diameters. The HHTDMA-derived growth factors complemented by DKA allows for determination of water activity and surface tension from dilute to highly supersaturated aqueous solutions that are not accessible with other methods. Thus, for mixed AS / Gl nanoparticles with mass ratios of 4:1 and 1:1, the upper limit of solute mass fraction (Xs) was 0.92 and 0.98, respectively. For pure AS and Gl, the DKA-derived aw is in good agreement with electrodynamic balance and bulk measurement data. For AS particles, our aw data also agree well with the Extended Aerosol Inorganics Model (E-AIM III) over the entire concentration range. In contrast, the UNIFAC model as a part of AIOMFAC (Zuend et al., 2011) was found to overestimate aw in aqueous Gl particles, which can be attributed to unaccounted intermolecular interactions. For mixed AS and Gl nanoparticles, we observed a non-monotonic concentration dependence of the surface tension that does not follow the predictions by modeling approaches constructed for mixed inorganic/organic systems. Thus, AS / Gl particles with a 1:1 mass ratio exhibited a strong decrease of σ with increasing solute mass fraction, a minimum value of 56.5 mN m−1 at Xs≈0.5, and a reverse trend of increasing σ at higher concentrations. We suggest that D-glucose molecules surrounded by ammonium sulfate ions tend to associate, forming non-polar aggregates, which lowers the surface tension at the air–droplet interface. We analyzed the uncertainty in the DKA-derived water activity and surface tension, related to the instrumental errors as well as to the morphology of the nanoparticles and their phase state. Our studies have shown that under optimal modes of operation of HHTDMA for moderate aqueous concentrations, the uncertainty in aw and σ does not exceed 0.2 %–0.4 % and 3 %–4 %, respectively, but it increases by an order of magnitude in the case of highly concentrated nanodroplet solutions.

Application of physisorption analyzer in measuring the hygroscopicity of atmospheric aerosols

The hygroscopicity represents a vital physicochemical attribute of aerosols, governing their phase state, acidity, viscosity, chemical reactivity, and other properties, thereby influencing various atmospheric processes, including visibility, radiative forcing, and the formation of cloud condensation nuclei. In this study, the hygroscopicity of common inorganic aerosol particles (NH4)2SO4 and NaCl in the atmosphere was determined using a physisorption analyzer under 298 K conditions, exploring its application prospects in measuring atmospheric aerosol hygroscopicity. By measuring the adsorption of particles to water vapor at different relative humidity (RH) levels, the variation curve of the aerosol particle hygroscopic mass growth factor (MGF) and deliquescence relative humidity (DRH) were obtained. Comparisons with vacuum FTIR methods, the Extended Aerosol Inorganic Model (EAIM), and relevant literature data showed good agreement. This research paves the way for future applications of physisorption analyzer in the study of atmospheric aerosols.

Chip-Based Dark-filed Microscopy for Sensing the Dynamical Changes of Single Aerosol Nanoparticles

The hygroscopicity of aerosol nanoparticles with the high number concentration in atmosphere significantly affects atmospheric chemistry and visibility, global climate, and human health. Therefore, it is critical to analyze the hygroscopicity of aerosol particles in an atmospheric ambient, where the aerosol's chemical compositions and physical states determine its capability of water uptake. In this work, with the advantages of non-destructive detection, highly sensitivity, and wide-field imaging, a planar photonic chip-based dark-field microscope is developed for real-time imaging and sensing the hygroscopic growth dynamics of single aerosol nanoparticles (diameter around 100 nm) in a label-free manner. The experimental results of single-component and multi-components are well consistent with the calculation results based on extended aerosol inorganic model. This photonic-chip based dark-field microscope can be extended to investigate the physicochemical reactions of various aerosols in real atmospheric environment.

Aerosol pH and Ion Activities of HSO4– and SO42– in Supersaturated Single Droplets

Accurate determination of acidity (pH) and ion activities in aqueous droplets is a major experimental and theoretical challenge for understanding and simulating atmospheric multiphase chemistry. Here, we develop a ratiometric Raman spectroscopy method to measure the equilibrium concentration of sulfate (SO42–) and bisulfate (HSO4–) in single microdroplets levitated by aerosol optical tweezers. This approach enables determination of ion activities and pH in aqueous sodium bisulfate droplets under highly supersaturated conditions. The experimental results were compared against aerosol thermodynamic model calculations in terms of simulating aerosol ion concentrations, ion activity coefficients, and pH. We found that the Extended Aerosol Inorganics Model (E-AIM) can well reproduce the experimental results. The alternative model ISORROPIA, however, exhibits substantial deviations in SO42– and HSO4– concentrations and up to a full unit of aerosol pH under acidic conditions, mainly due to discrepancies in simulating ion activity coefficients of SO42––HSO4– equilibrium. Globally, this may cause an average deviation of ISORROPIA from E-AIM by 25 and 65% in predicting SO42– and HSO4– concentrations, respectively. Our results show that it is important to determine aerosol pH and ion activities in the investigation of sulfate formation and related aqueous phase chemistry.

Offline analysis of the chemical composition and hygroscopicity of submicrometer aerosol at an Asian outflow receptor site and comparison with online measurements

Abstract. Filter-based offline analysis of atmospheric aerosol hygroscopicity coupled to composition analysis provides information complementary to that obtained from online analysis. However, its application itself and comparison to online analysis have remained limited to date. In this study, daily submicrometer aerosol particles (PM0.95, 50 % cutoff diameter 0.95 µm) were collected onto quartz fiber filters on Okinawa Island, a receptor of East Asian outflow, in the autumn of 2015. The chemical composition of water-soluble matter (WSM) in PM0.95, PM0.95 itself, and their respective hygroscopicities were characterized through the offline use of an aerosol mass spectrometer and a hygroscopicity tandem differential mobility analyzer. Thereafter, results were compared with those obtained from online analyses. Sulfate dominated the WSM mass (59 %), followed by water-soluble organic matter (WSOM, 20 %) and ammonium (13 %). WSOM accounted for most (91 %) of the mass of extracted organic matter (EOM) and the atomic O-to-C ratios (O:C) of WSOM and EOM were high (mean ± standard deviation were 0.84 ± 0.08 and 0.78 ± 0.08, respectively), both of which indicate highly aged characteristics of the observed aerosol. The hygroscopic growth curves showed clear hysteresis for most samples. At 85 % relative humidity (RH), the calculated hygroscopicity parameter κ values of the WSM (κWSM), WSOM, EOM, and PM0.95 (κPM0.95) were 0.50 ± 0.03, 0.22 ± 0.12, 0.20 ± 0.11, and 0.47 ± 0.03, respectively. An analysis using the thermodynamic Extended Aerosol Inorganics Model (E-AIM) shows, on average, that inorganic salts and WSOM contributed 88 % and 12 %, respectively, of the κWSM (or κPM0.95). High similarities were found between offline and online analysis for chemical compositions that are related to particle hygroscopicity (the mass fractions and O:C of organics and the degree of neutralization) and also for aerosol hygroscopicity. As possible factors governing the variation in κWSM, the influences of WSOM abundance and the neutralization of inorganic salts were assessed. At high RH (70 %–90 %), the hygroscopicity of WSM and PM0.95 was affected considerably by the presence of organic components; at low RH (20 %–50 %), the degree of neutralization could be important. This study not only characterized aerosol hygroscopicity at the receptor site of East Asian outflow but also shows that offline hygroscopicity analysis is an appropriate method, at least for aerosols of the studied type. The results encourage further applications to other environments and to more in-depth hygroscopicity analysis, in particular for organic fractions.

Acidity of Size-Resolved Sea-Salt Aerosol in a Coastal Urban Area: Comparison of Existing and New Approaches

Aging of sea-salt aerosol and the corresponding phase partitioning behavior of HCl/Cl– were monitored and studied in the Halifax Fog and Air Quality Study (HaliFAQS) field campaign in Canada in the early summer of 2019. Ionic chemical composition of ambient aerosol from total suspended particles (TSPs) to 10 nm was sampled and measured into 14 size ranges, which showed that aerosol at the sampling location was mainly composed of sea-salt constituents with little influence from anthropogenic emissions of secondary inorganic precursors. The size-resolved pH of aged sea-salt aerosol was calculated by Extended Aerosol Inorganic Model (E-AIM) IV and a newly derived method based on the measurement of NH3/NH4+ and HNO3/NO3– or HCl/Cl– coupled phase partitioning behavior. The latter pH calculation method does not require information about the aerosol liquid water content or complete water-soluble ion measurement, compensating for the role of oxidized organics to influence the proton activity. In comparison, E-AIM calculation requires the input of all major hygroscopic species. The size-resolved pH calculated by E-AIM IV generally agrees with the one calculated by NH3–HCl coupled phase partitioning. The sensitivities of HCl and HNO3 phase partitioning to aerosol pH are studied with both observational data and conceptual modeling, which gives evidence that the phase partitioning of HCl is more sensitive and therefore more reflective of aerosol pH changes than HNO3 in sea-salt dominated atmospheres where particles typically have pH > 3. The higher concentration and more reliable measurement also make HCl a more suitable choice to track pH changes in this study.

Competitive Uptake of Dimethylamine and Trimethylamine against Ammonia on Acidic Particles in Marine Atmospheres.

Alkaline gases such as NH3 and amines play important roles in neutralizing acidic particles in the atmosphere. Here, two common gaseous amines (dimethylamine (DMA) and trimethylamine (TMA)), NH3, and their corresponding ions in PM2.5 were measured semicontinuously using an ambient ion monitor-ion chromatography (AIM-IC) system in marine air during a round-trip cruise of approximately 4000 km along the coastline of eastern China. The concentrations of particulate DMA, detected as DMAH+, varied from <4 to 100 ng m-3 and generally decreased with increasing atmospheric NH3 concentrations. Combining observations with thermodynamic equilibrium calculations using the extended aerosol inorganics model (E-AIM) indicated that the competitive uptake of DMA against NH3 on acidic aerosols generally followed thermodynamic equilibria and appeared to be sensitive to DMA/NH3 molar ratios, resulting in molar ratios of DMAH+ to DMA + DMAH+ of 0.31 ± 0.16 (average ± standard deviation) at atmospheric NH3 concentrations over 1.8 μg m-3 (with a corresponding DMA/NH3 ratio of (1.8 ± 1.0) × 10-3), 0.80 ± 0.15 at atmospheric NH3 concentrations below 0.3 μg m-3 (with a corresponding DMA/NH3 ratio of (1.3 ± 0.6) × 10-2), and 0.56 ± 0.19 in the remaining cases. Particulate TMA concentrations, detected as TMAH+, ranged from <2 to 21 ng m-3 and decreased with increasing concentrations of atmospheric NH3. However, TMAH+ was depleted concurrently with the formation of NH4NO3 under low concentrations of atmospheric NH3, contradictory to the calculated increase in the equilibrated concentration of TMAH+ by the E-AIM.

Technical note: Real-time diagnosis of the hygroscopic growth micro-dynamics of nanoparticles with Fourier transform infrared spectroscopy

Abstract. Nanoparticles can absorb water to grow, and this process will affect the light-scattering behavior, cloud condensation nuclei properties, lifetime, and chemical reactivity of these particles. Current techniques for calculation of aerosol liquid water content (ALWC) usually restrict the size of particles to be within a certain range, which may result in a large uncertainty when the particle size is beyond the specified range. Furthermore, current techniques are difficult to use to identify the intermolecular interactions of phase transition micro-dynamics during particles' hygroscopic growth process because their limited temporal resolutions are unable to capture complex intermediate states. In this study, the hygroscopic growth properties of nanoparticles with electrical mobility diameters (Dem) of ∼ 100 nm and their phase transition micro-dynamics at the molecular level are characterized in real time by using the Fourier transform infrared (FTIR) spectroscopic technique. We develop a novel real-time method for ALWC calculation by reconstructing the absorption spectra of liquid water and realize real-time measurements of water content and dry nanoparticle mass to characterize hygroscopic growth factors (GFs). The calculated GFs are generally in good agreement with the Extended Aerosol Inorganics Model (E-AIM) predictions. We also explore the phenomenon that the deliquescence points of the ammonium sulfate / sodium nitrate (AS/SN) mixed nanoparticles and the AS / oxalic acid (AS/OA) mixed nanoparticles are lower than that of the pure AS. We further normalize the FTIR spectra of nanoparticles into 2D IR spectra and identify in real time the hydration interactions and the dynamic hygroscopic growth process of the functional groups for AS, AS/SN, and AS/OA nanoparticles. The results show that both SN and OA compounds can lower the deliquescence point of AS, but they affect AS differently. The SN can change but OA cannot change the hydrolysis reaction mechanism of AS during the hygroscopic growth process. Compared with previous studies, we captured more complex processes and the intermediate state of the hygroscopic growth of nanoparticles. This study not only can provide important information with respect to the difference in the phase transition point under different conditions but also can improve current understanding of the chemical interaction mechanism between nanoparticles (particularly for organic particles) and the surrounding medium, which is of great significance for investigation of haze formation in the atmosphere.

Strong Acids or Bases Displaced by Weak Acids or Bases in Aerosols: Reactions Driven by the Continuous Partitioning of Volatile Products into the Gas Phase

Aerosols are ubiquitous in the atmosphere and profoundly affect climate systems and human health. To gain more insights on their broad impacts, we need to comprehensively understand the fundamental properties of atmospheric aerosols. Since aerosols are multiphase, a dispersion of condensed matter (solid particles or liquid droplets, hereafter particles) in gas, partitioning of volatile matter between the condensed and the gas phases is one defining characteristic of aerosols. For example, water content partitioning under different relative humidity conditions, known as aerosol hygroscopicity, has been extensively investigated in the past decades. Meanwhile, partitioning of volatile organic or inorganic components, which is referred to as aerosol volatility, remains understudied. Commonly, a bulk solution system is treated as a single phase, with volatility mainly determined by the nature of its components, and the composition partitioning between solution and gas phase is limited. Aerosols, however, comprise an extensive gas phase, and their volatility can also be induced by component reactions. These reactions occurring within aerosols are driven by the formation of volatile products and their continuous partitioning into the gas phase. As a consequence, the overall aerosol systems exhibit prominent volatility. Noteworthily, such volatility induced by reactions is a phenomenon exclusively observed in the multiphase aerosol systems, and it is trivial in bulk solutions due to the limited extent of liquid-gas partitioning. Take the chloride depletion in sea salt particles as an example. Recent findings have revealed that chloride depletion can be caused by reactions between NaCl and weak organic acids, which release HCl into the gas phase. Such a reaction can be described as a strong acid displaced by a weak acid, which is hardly observed in bulk phase. Generally, this unique partitioning behavior of aerosol systems and its potential to alter aerosol composition, size, reactivity, and other physicochemical properties merits more attention by atmospheric community.This Account focuses on the recent advancements in the research of component reactions that induce aerosol volatility. These reactions can be categorized into four types: chloride depletion, nitrate depletion, ammonium depletion, and salt hydrolysis. The depletion of chloride or nitrate can be regarded as a displacement reaction, in which a strong acid is displaced by a weak acid. Such a reaction releases highly volatile HCl or HNO3 into the gas phase and leads to a loss of chloride or nitrate within the particles. Likewise, ammonium depletion is a displacement reaction in which a strong base is displaced by a weak base, resulting in release of ammonia and substantial changes in aerosol hygroscopicity. In addition, aerosol volatility can also be induced by salt hydrolysis in a specific case, which is sustained by the coexistence of proton acceptor and hydroxide ion acceptor within particles. Furthermore, we quantitatively discuss these displacement reactions from both thermodynamic and kinetic perspectives, by using the extended aerosol inorganic model (E-AIM) and Maxwell steady-state diffusive mass transfer equation, respectively. Given the ubiquity of component partitioning in aerosol systems, our discussion may provide a new perspective on the underlying mechanisms of aerosol aging and relevant climate effects.

The development and evaluation of a sequential aerosol-water measurement system

Aerosol water content (AWC) is associated with aerosol formation, aerosol–cloud interaction, and atmospheric visibility degradation; however, few studies have directly measured the mass of water in aerosols. Studies on AWC have employed modeling techniques [e.g., ISORROPIA II and the Extended Aerosol Inorganics Model (E-AIM) III)], the thermal-ramp Karl-Fisher method, and conducted volume-based measurements by using a hygroscopic tandem differential mobility analyzer (H-TDMA). The current study developed a sequential aerosol-water measurement system (SAWMS) that sequentially collects particles with up to 10 filters and measures AWC immediately after the collection. With this system, the loss of semi-volatile species was lower, and contamination during sample transport was reduced. The AWC measurement results were validated by comparing them with modeling results obtained in this study and the results obtained in previous studies. The measurement results for three aerosolized salts—NaCl, NH4NO3, and (NH4)2SO4—were found to be in favorable agreement with the corresponding modeling results and results from previous studies. The relationship between aerosol mass change (AMC) and relative humidity (RH) was investigated. The deliquescence RH (DRH) was 75%, 63%, and 81% for NaCl, NH4NO3, and (NH4)2SO4, respectively, whereas efflorescence RH was observed only for NaCl at 40%–50% RH. The AWC of mixed inorganic salts exhibited weak mutual DRH at 45%–50% RH. By contrast, the AWC obtained using ISORROPIA II and E-AIM III exhibited a gradual rather than an abrupt change of RH upon deliquescence. The measured AWC values were more similar to the values obtained with ISORROPIA II than to the values obtained with E-AIM III. To compare our mass-based measurements with the volume-based measurements obtained in previous studies using H-TDMAs, we derived the relationships of hygroscopicity parameter and growth factor with AMC. We discovered that the aforementioned relationships were comparable. In summary, the developed SAWMS integrates a sequential aerosol collection device with a gas chromatograph equipped with a thermal conductivity detector; the system performs well in the quantification of AWC and provides an additional chance of the filter for further chemical analysis.

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.

Impact of the particle mixing state on the hygroscopicity of internally mixed sodium chloride-ammonium sulfate single droplets: a theoretical and experimental study.

Sodium chloride (NaCl) is the main constituent of sea-salt aerosols. During atmospheric transport, sea-salt aerosols can interact with gases and other particles including secondary aerosols containing ammonium sulfate ((NH4)2SO4). This paper reports on the deliquescence relative humidity (DRH) of internally mixed sodium chloride-ammonium sulfate (NaCl/(NH4)2SO4) coarse particles by means of an acoustic levitation system fitted with a confocal Raman microscope (CRM). The chemical composition and physical state of individual levitated particles of different initial NaCl mole fractions were monitored during the deliquescence cycle by CRM. Experimental results were compared to the data predicted by the thermodynamic model E-AIM (Extended-Aerosol Inorganics Model). We demonstrated that NH4Cl, Na2SO4 and NH4NaSO4·2H2O are formed in recrystallized particles and coexist with NaCl and (NH4)2SO4. All these products are randomly distributed within the particles. Deliquescence curves described two or three-stage phase transitions depending on the initial composition of the droplet. Significant discrepancies between the model and the laboratory experiments were observed for NaCl mole fractions varying between 0.40 and 0.77 due to a divergence between the predicted and the truly present products in the particles' solid fraction during the humidification cycle.

Characterizations and Potential Formation Pathways of Atmospheric Inorganic Ions at a National Background Site in the Northeastern Qinghai‐Tibet Plateau During Autumn Season

AbstractAtmospheric particulate matter (PM) imposes uncertain impacts on both radiative forcing and human health. Ambient PM has been comprehensively studied in China's megacities, while its compositions, sources, and characterizations in the Qinghai‐Tibet Plateau (QTP) are not fully understood. An autumn observation campaign was conducted during 1–14 October 2013 at a national background monitoring station (3,295 m above sea level, a.s.l.) in the QTP. Real time concentrations of inorganic water‐soluble ions (WSIs) associated with PM2.5 (PM with aerodynamic diameter equal to or less than 2.5 μm) were measured in addition to PM2.5 concentrations, gaseous pollutants, and meteorological parameters. SO42− was the most abundant WSI (10.0 μg/m3) followed by NH4+ (2.0 μg/m3), and NO3− (1.7 μg/m3). Observed WSI concentrations were higher as compared to other QTP monitoring sites and some remote sites. The levels of SO42− were comparable to concentrations of some urban sites. The correlation analysis disclosed the relationship among WSIs and other gaseous pollutants and discussed the possibilities of biomass burning as the potential sources. Extended aerosol inorganic model (E‐AIM) model results revealed that particulate sulfate, nitrate, and ammonium (SNA) existed in both liquid and solid phases. High sulfate and nitrate oxidation ratios indicated strong secondary formation of both SO42− and NO3−. By analyzing the relationship among SOR/NOR, RH, and O3, we found that both photochemical and heterogeneous reactions contributed to the formation of SO42−, while the conversion of NO2 to NO3− was likely to occur via photochemical reactions in the presence of high O3 concentrations and strong sunlight.

In Situ Quantitative Observation of Hygroscopic Growth of Single Nanoparticle Aerosol by Surface Plasmon Resonance Microscopy.

Aerosol particle hygroscopicity is an important factor in visibility reduction, cloud formation, radiation forcing, and the global climate. The high number concentration of nanoparticles (defined as particles with diameters below 100 nm) means that their hygroscopic growth abilities and potential contributions to the climate and environment are significant. Therefore, a rapid and accurate in situ analysis method for single nanoparticle hygroscopic growth in an atmospheric environment is important to characterize the effects of the particle's physical and chemical properties in this process. In this work, surface plasmon resonance microscopy with azimuthal rotation illumination (SPRM-ARI) is used to observe the hygroscopic growth and water content of single nanoparticles in situ. The hygroscopic growth results of a single-component nanoparticle are well matched with the extended aerosol inorganic model (E-AIM) results, and the proposed method remains reliable even when the relative humidity (RH) exceeds 90%. For a bicomponent nanoparticle (with NaCl as the primary content), the presence of a component without deliquescence phase transitions under increasing humidity conditions causes the measured data to differ from both the Zdanovskii-Stokes-Robinson (ZSR) model and E-AIM predictions in the low RH range. However, because of their complete liquefaction, the growth factor (GF) variation of the bicomponent nanoparticle is close to the model predictions in the high RH range. Finally, based on the positive correlation between particle volume and the gray intensity of SPRM-ARI, GF values can be obtained from the cube root of the gray intensity and the actual water content of single nanoparticles can then be derived.

Hygroscopicity of amino acids and their effect on the water uptake of ammonium sulfate in the mixed aerosol particles

Amino acids are important water-soluble nitrogen-containing compounds in atmospheric aerosols. They can be involved in cloud formation due to their hygroscopicity and have significant influences on the hygroscopicity of inorganic compounds, which have not yet been well characterized. In this work, the hygroscopic properties of three amino acids, including aspartic acid, glutamine, and serine, as well as their mixtures with ammonium sulfate (AS) were investigated using a hygroscopicity tandem differential mobility analyzer (HTDMA) system. The gradual water uptake of aspartic acid, glutamine and serine particles indicates that they exist as liquid phase at low RH. When mixing either aspartic acid or glutamine with AS by mass ratio of 1:3, we observed a clear phase transition but with a lower deliquescence relative humidity (DRH) with respect to that of pure AS. This suggests the crystallization of AS in the presence of each of these two amino acids. However, as the mass fractions of these two amino acids increased in the mixed particles, the deliquescence transition process was not obvious. In contrast, the crystallization of AS was efficiently hampered even at low content (i.e., 25% by mass) of serine in the mixed particles. The Zdanovskii-Stokes-Robinson (ZSR) method in general underestimated the hygroscopic growth of any mixtures at RH below 79% (prior to AS deliquescence), suggesting both amino acid and the partially dissolved AS contributed the overall hygroscopicity at RH in this range. Relatively good agreements were reached between the measurements and model predictions using the Extended Aerosol Inorganic Model (E-AIM) assuming solid state AS in the mixed particles for 1:3 aspartic acid-AS and glutamine-AS systems. However, the model failed to simulate the water uptake behaviors of any other systems. It demonstrates that the interactions between components within the aerosols have a significant effect on the phase state of the mixed particles.

High-humidity tandem differential mobility analyzer for accurate determination of aerosol hygroscopic growth, microstructure, and activity coefficients over a wide range of relative humidity

Abstract. Interactions with water are crucial for the properties, transformation, and climate effects of atmospheric aerosols. Here we present the high-humidity tandem differential hygroscopicity analyzer (HHTDMA) and a new method to measure the hygroscopic growth of aerosol particles with in situ restructuring to minimize the influence of particle shape. With this approach, growth factors can be measured with an uncertainty of 0.3 %–0.9 % over a relative humidity (RH) range of 2 %–99.6 % and with an RH measurement accuracy better than 0.4 %. The HHTDMA instrument can be used in hydration, dehydration, and restructuring modes of operation. The restructuring mode allows us to investigate the effects of drying conditions on the initial microstructure of aerosol particles and specifies the optimal parameters that provide their rearrangements into compact structures with a nearly spherical shape. These optimal parameters were used in hygroscopic growth experiments by combining the restructuring mode with a conventional hydration or dehydration mode. The tandem of two modes allowed us to measure the particle growth factors with high precision as well as to determine the thickness of the water adsorption layer on the surface of compact crystalline particles. To verify the HHTDMA instrument we compared the measured ammonium sulfate growth factors with those obtained from the E-AIM-based Köhler model (E-AIM: Extended Aerosol Inorganics Model). Averaged over the range of 38 %–96 % RH, the mean relative deviations between measurements and model results is less than 0.5 %. We demonstrate this precision by presenting data for glucose, for which bulk thermodynamic coefficients are available. The HHTDMA-derived activity coefficients of water and glucose were obtained for both dilute and supersaturated solutions and are in good agreement with those reported in the literature. The average deviation between the measured activity coefficients and those obtained by the bulk method is less than 4 %. For dilute solution in water with an activity range of 0.98–0.99, the hygroscopicity parameter of glucose and the molal osmotic coefficient were obtained with an uncertainty of 0.4 % and 2.5 %, respectively.

Dissociation and microsolvation of bisulfate anion

AbstractBisulfate anion has vast applications in industrial and environmental chemistry. Its dissociation to sulfate anion has been studied computationally and by size‐selected clusters. In this study, Micro‐Raman spectroscopy is applied at different relative humidity (RH)% on small droplets of aqueous solutions of NaHSO4, NH4HSO4, and H2SO4 to explore the dissociation of HSO4‐ experimentally in bulk and supersaturated state. Our cumulative experimental results support the conclusion obtained from gas‐phase cluster study that HSO4‐ starts dissociating at very low number of water molecules (H2O ~ 1). However, dissociation of HSO4‐ is found dependent on the cation attached with this weak acid. The magnitude of dissociation is found as NH4HSO4 ≈ H2SO4 &gt; NaHSO4. In addition, studying the hydration process of HSO4− in the presence of three different cations enabled us to observe various species in three systems. Theoretical values obtained from extended aerosol inorganic model supports the experimental results.

Sources and atmospheric dynamics of organic aerosol in New Delhi, India: insights from receptor modeling

Abstract. Delhi, India, is the second most populated city in the world and routinely experiences some of the highest particulate matter concentrations of any megacity on the planet, posing acute challenges to public health (World Health Organization, 2018). However, the current understanding of the sources and dynamics of PM pollution in Delhi is limited. Measurements at the Delhi Aerosol Supersite (DAS) provide long-term chemical characterization of ambient submicron aerosol in Delhi, with near-continuous online measurements of aerosol composition. Here we report on source apportionment based on positive matrix factorization (PMF), conducted on 15 months of highly time-resolved speciated submicron non-refractory PM1 (NR-PM1) between January 2017 and March 2018. We report on seasonal variability across four seasons of 2017 and interannual variability using data from the two winters and springs of 2017 and 2018. We show that a modified tracer-based organic component analysis provides an opportunity for a real-time source apportionment approach for organics in Delhi. Phase equilibrium modeling of aerosols using the extended aerosol inorganics model (E-AIM) predicts equilibrium gas-phase concentrations and allows evaluation of the importance of the ventilation coefficient (VC) and temperature in controlling primary and secondary organic aerosol. We also find that primary aerosol dominates severe air pollution episodes, and secondary aerosol dominates seasonal averages.