Aerosol Growth Research Articles

Modeling Aerosol Growth for a Solvent Blend

Management of aerosol in amine scrubbing requires the modeling of aerosol growth at the conditions of the absorber and water wash. If the amine aerosol grows well above 3 µm, it is collected through impaction on the packing and mist eliminator. Although aerosol models for single amine solvents have been developed, none have been reported for blended amine solvents. The presence of a second amine in the bulk liquid might affect the aerosol growth mechanism, composition, and size. Without a model for an amine blend, it is difficult to identify operating conditions that can grow the aerosol to larger sizes for mitigation. This work simulates aerosol growth for a proprietary solvent blend of amines AM1 and AM2. The model calculates the size and composition of a single aerosol drop traveling through the absorber and water wash. It tracks the water, CO2, AM1, and AM2 in the bulk gas phase and aerosol phase with mass transfer occurring at the bulk gas-drop interface and the bulk gas-solvent interface. The approach to modeling aerosol growth in the absorber and water wash involved two steps: (1) process modeling with ProTreat® and Aspen Plus® and (2) aerosol transport modeling with gPROMS®. The simulation was performed for two run conditions with 90% CO2 capture but different lean solvent and intercooling temperatures. The results show that the aerosol grows by picking up water, CO2, and AM1 in the absorber, and only water in the water wash. The aerosol grows to sizes well above 3 µm at the absorber mid-point, absorber outlet, and water wash outlet and will be easily collected. It grows fastest at cold zones in the absorber where the gas and aerosol are not in equilibrium. More packing and colder circulation water in the water wash were found to increase aerosol growth.

Aerosol emissions from water-lean solvents for post-combustion CO2 capture

Advanced water-lean solvents (WLS) for post-combustion CO2 capture have been gaining interest due to their ability to reduce the parasitic penalty from energy needed for solvent regeneration. Commercial implementation of these novel CO2 capture technologies hinges on successful control of amine emissions.RTI conducted a parametric study of fundamental and operational variables influence on overall amine aerosol and vapor emissions from our water-lean solvent eCO2Sol™ using our 6-kW equivalent bench-scale gas absorption system. The parametric testing used a simulated flue gas with 15 % CO2, 2.3–4.2 % H2O, and 0–6 ppm sulfite (SO3) to examine the impact of the presence of aerosols to the capture performance and amine emissions from the system. The SO3 reacts with water in the flue gas to create H2SO4, which forms liquid aerosol droplets and provide nucleation sites for growth of aerosols. Scanning Mobility Particle Sizer and Aerodynamic Particle Sizer instruments monitored the aerosol particle size distribution.Parametric testing results suggested that the presence of the aerosols in the flue gas could increase the overall amine emissions by 10X compared to the baseline emissions from WLS’s vapor pressure. Principal component analysis (PCA) and projection to latent squares (PLS) developed models to predict the aerosol-based amine emissions from process data. The predictive PLS model had a correlation coefficient (Q2) of 0.92 and could predict the aerosol-based emissions from the NAS process with ±15 % accuracy (average absolute deviation, AAD). The PLS regression model also identified key variables affecting aerosol-based emissions from WLS.

Influence of vegetation on occurrence and time distributions of regional new aerosol particle formation and growth

Abstract. The occurrence frequency of regional atmospheric new aerosol particle formation and consecutive growth events (fNPF) were studied with respect to vegetation activity, aerosol properties, air pollutants and meteorological data in Budapest over the time interval from 2008 to 2018. The data set evaluated contained results of in situ measurements on the land surface that were mostly performed at the Budapest platform for Aerosol Research and Training Laboratory, of satellite-based products recorded by MODIS on Terra and of modelled vegetation emission-related properties from an advanced regional biogeochemical model. The annual mean relative occurrence frequencies were considerable (with an overall mean of 21 %), remained at a constant level (with an overall SD of 5 %) and did not exhibit tendentious change over the years. The shape of the distributions of monthly mean fNPF exhibited large variability from year to year, while the overall average distribution already possessed a characteristic pattern. The structure of the new particle formation (NPF) occurrence distributions was compared to those of environmental variables including concentrations of gas-phase H2SO4, SO2, O3, NO, NO2, CO, PM10 mass and NH3; particle numbers in the size fractions of 6–1000, 6–100 and 100–1000 nm; condensation sink; air temperature (T); relative humidity (RH); wind speed (WS); atmospheric pressure (P); global solar radiation (GRad); gross primary production (GPP) of vegetation; leaf area index (LAI); and stomatal conductance (SCT). There were no evident systematic similarities between fNPF on the one hand and all of the variables studied on the other hand, except for H2SO4 and perhaps NH3. The spring maximum in the NPF occurrence frequency distribution often overlapped with the time intervals of positive T anomaly in vegetated territories. The link between the potential heat stress exerted on plants in sultry summer intervals and the summer fNPF minimum could not be proven. The relevance of environmental variables was assessed by their ratios on NPF event days and on non-event days. The gas-phase H2SO4 concentration showed the largest monthly ratios, followed by O3. The WS, biogenic precursor gases and SO2 can generally favour NPF events, although their influence seemed to be constrained. An association between the fNPF and vegetation growth dynamics was clearly identified.

Explosive Secondary Aerosol Formation during Severe Haze in the North China Plain.

Severe haze events with exceedingly high-levels of fine aerosols occur frequently over the past decades in the North China Plain (NCP), exerting profound impacts on human health, weather, and climate. The development of effective mitigation policies requires a comprehensive understanding of the haze formation mechanisms, including identification and quantification of the sources, formation, and transformation of the aerosol species. Haze evolution in this region exhibits distinct physical and chemical characteristics from clean to polluted periods, as evident from increasing stagnation and relative humidity, but decreasing solar radiation as well as explosive secondary aerosol formation. The latter is attributed to highly elevated concentrations of aerosol precursor gases and is reflected by rapid increases in the particle number and mass concentrations, both corresponding to nonequilibrium chemical processes. Considerable new knowledge has been acquired to understand the processes regulating haze formation, particularly in light of the progress in elucidating the aerosol formation mechanisms. This review synthesizes recent advances in understanding secondary aerosol formation, by highlighting several critical chemical/physical processes, that is, new particle formation and aerosol growth driven by photochemistry and aqueous chemistry as well as the interaction between aerosols and atmospheric stability. Current challenges and future research priorities are also discussed.

Trifluoroacetaldehyde aminolysis catalyzed by a single water molecule: An important sink pathway for trifluoroacetaldehyde and a potential pathway for secondary organic aerosol growth

High-level ab initio calculations and variational transition state theory with small curvature tunneling have been used to study the aminolysis of trifluoroacetaldehyde catalyzed by a single water molecule. The results of energetic studies indicate that the energy barrier of the trifluoroacetaldehyde aminolysis reaction decreases along ammonia, methylamine, and dimethylamine. A single water molecule can significantly reduce the reaction energy barrier of trifluoroacetaldehyde aminolysis. In particular, the reaction involving dimethylamine has the lowest reaction energy barrier and the energy barrier is decreased to be −9.69 kcal/mol in the CF3CHO + (CH3)2NH + H2O reaction relative to CF3CHO, (CH3)2NH, and H2O separated reactants. Kinetic calculation shows that the rate coefficient of CF3CHO + (CH3)2NH⋯H2O ranges from 7.78 × 10−13 to 4.45 × 10−16 cm3⋅molecules−1⋅s−1 at 190–350 K. Here, we find an important CF3CHO elimination pathway, which can compete with the reaction of CF3CHO + OH when the OH concentration is 104 molecules⋅cm−3 and the dimethylamine concentration is higher than 109 molecules⋅cm−3 in the temperature range between 240 and 330 K. In addition, once (CH3)2NCH(OH)CF3 is formed by the reaction of CF3CHO + (CH3)2NH + H2O, it will further promote the growth of secondary organic aerosols.

Impacts of Atmospheric Boundary Layer Vertical Structure on Haze Pollution Observed by Tethered Balloon and Lidar

In this paper, the characteristics of the atmospheric boundary layer (ABL) vertical structure over the North China Plain (NCP) during a comprehensive observation experiment conducted during 15–21 December 2018 were investigated. Observational data were obtained with a large tethered balloon, Doppler wind lidar, and ground-level instruments. The maximum concentration of PM2.5 exceeded 200 µg m−3, and the ratio of PM2.5/PM10 was approximately 0.4 (its maxi-mum was approximately 0.8) during the whole observation period, indicating the explosive growth of dominant fine-mode aerosols in the winter heating season. Elevated concentrations of pollutants decreased the solar irradiance received by the ground, resulting in lower temperature at ground level. Our results illustrate three distinct types of vertical profiles: Type 1 (convective state)—the concentration of PM2.5 decreased nearly linearly with increase of the height below approximately 600 m; Type 2 (stable state)—the PM2.5 concentration sharply decreased from the ground to approximately 200 m; and Type 3 (multilayer structure)—some pollutants were suspended aloft in the upper air layer. Diurnal evolution of the vertical profiles of PM2.5 and their relationship with the changes in meteorological factors were identified. From daytime to nighttime, the vertical profiles evolved from Type 1 to Type 2 or Type 3. All the 33 vertical PM2.5 profiles that we obtained showed a strong relationship with elements of the ABL structure, such as the distributions of winds, the inversion layer, and turbulence activities. A light-wind layer and weak turbulence activity, especially within the inversion layer, contributed greatly to the accumulation of pollutants. Vertical PM2.5 concentration patterns were also greatly affected by local ground-level emission sources and regional transport processes.

Aerosol Emissions of Amine-Based CO2 Absorption System: Effects of Condensation Nuclei and Operating Conditions.

Amine emissions from a post-combustion CO2 capture process can lead to solvent loss and serious environmental issues. The emission characteristics of amine mixtures and influencing factors are seldom reported. This work comprehensively investigated emissions of AMP (2-amino-2-methyl-1-propanol)/MEA (monoethanolamine) from a 3.6 Nm3/h flue gas CO2 capture platform. The condensation nuclei in flue gas dominated the generation of amine aerosols and resulted in a heavy total amine loss of over 1400 mg/Nm3, which is equivalent to 5.88 kg/t CO2 captured under the high nuclei concentration scenario. Inside the absorber, a higher CO2 concentration and lower lean solvent CO2 loading can significantly promote the growth of aerosols due to the intensive reaction of CO2 absorption. The maximum amine emissions were observed at 8-12 vol % CO2. The flue gas temperature and liquid/gas ratio had insignificant effects on aerosol emissions, while amine emissions after the absorber increased 340-500% as the lean solvent temperature increased from 30 to 50 °C. A synergistic control strategy of nuclei pretreatment, operating optimization, and water scrubbing can effectively reduce amine emissions to 4.0 mg/Nm3 MEA and 8.3 mg/Nm3 AMP.

Insights into the chemistry of aerosol growth in Beijing: Implication of fine particle episode formation during wintertime

Nucleation particle growth plays a major role in the occurrence of fine particles, yet the mechanism of new particle formation (NPF) remains ambiguous in the complex atmosphere of megacities and hinders the development of measures to mitigate PM2.5 pollution. In this study, the chemistry of ultrafine particles during the growth phase of nucleation events was investigated in urban Beijing from Nov. 15, 2018 to Jan. 15, 2019, using two scanning mobility particle spectrometers (SMPS) systems and an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). During this intense campaign, 11 NPF events were observed and the growth rate (GR) of nanoparticles ranged from 12.5 to 24.5 nm h−1. Four periodic cycles of PM2.5 episodes that included aerosol particle growth to particulate matter pollution were identified. Based on the QGR – QAMS theoretical frame that exploring the balance between the source rate of condensable vapors and the observed growth rate of nanoparticles, we clearly showed the physical and chemical evolution of nano-particle during the growth processes to ambient-atmosphere sizes (>100 nm). Generally, the modal diameter of aerosol particles grew by more than 100 nm (7 out of 11 NPF events) when the nitrate concentration and less-oxidized oxygenated organic aerosol (LO-OOA) were high; however, another class of aerosol particle growth was limited to 50–100 nm (3 out of 11 NPF events) when sulfate was high. Note that the remaining one NPF event could not be identified if it can grow up to 100 nm or not due to the unavailable of observation data during the late growth stage. By linking the aerosol growth with chemical compositions, sulfate and organics were found to be the main contributors during the initial stage of the aerosol growth, while cooking-related OA (COA) enhanced the transition stage, and nitrate and more-oxidized OOA (MO-OOA) dominated the subsequent growth of aerosol to ambient-atmosphere sizes. An important portion of aerosol growth in PM2.5 was controlled by semi-volatile organic vapors, which can partition into the externally condensed phase of the accumulation mode and coarse mode via the physical process of adsorption. Through quantifying the physical and chemical properties of aerosol particle growth, the detail processes of nucleation initiated PM2.5 pollution episodes were evaluated and provided observational evidence on the formation mechanism of winter haze pollution in the megacity of Beijing.

Hygroscopic and chemical properties of aerosol emissions at a major mining facility in Iran: Implications for respiratory deposition

This study characterizes the hygroscopic and chemical nature of aerosols originating from ten locations (4 outdoors and 6 indoors) around the Gol-E-Gohar (GEG) iron ore mine (Iran), including an assessment of how hygroscopic growth alters particulate deposition in the respiratory system. Aerosols collected on filters in three diameter (Dp) ranges (total suspended particulates [TSP], Dp ≤ 10 μm [PM10], and Dp ≤ 2.5 μm [PM2.5]) were analyzed for chemical and hygroscopic characteristics. The water-soluble aerosol composition is dominated by species associated with directly emitted crustal matter such as chloride, sodium, calcium, and sulfate. There was minimal contribution from organic acids and other secondarily formed species such as inorganic salts. Aerosol growth factors at 90% relative humidity varied between 1.39 and 1.72 and exceed values reported for copper mines in the United States where similar data are available. Values of the hygroscopicity parameter kappa (0.19–0.45) were best related to the mass fraction of chloride among all the studied species. Kappa values were generally similar when comparing the three types of samples (TSP, PM2.5, PM10) at each site and also when comparing each of the ten sampling sites. Accounting for hygroscopic growth yields an increase in the deposition fraction for aerosols with a dry Dp between 0.2 and 2 μm based on International Commission on Radiological Protection model calculations, with more variability when examining each of the three individual head airway regions.

Study on Aerosol Growth and Deposition Properties Affected by Steam Condensation

Study on Aerosol Growth and Deposition Properties Affected by Steam Condensation

Transport and chemistry of isoprene and its oxidation products in deep convective clouds

Deep convective clouds can transport trace gases from the planetary boundary layer into the upper troposphere where subsequent chemistry may impact aerosol particle formation and growth. In this modelling study, we investigate processes that affect isoprene and its oxidation products injected into the upper troposphere by an isolated deep convective cloud in the Amazon. We run a photochemical box model with coupled cloud microphysics along hundreds of individual air parcel trajectories sampled from a cloud-resolving model simulation of a convective event. The box model simulates gas-phase chemical reactions, gas scavenging by liquid and ice hydrometeors, and turbulent dilution inside a deep convective cloud. The results illustrate the potential importance of gas uptake to anvil ice in regulating the intensity of the isoprene oxidation and associated low volatility organic vapour concentrations in the outflow. Isoprene transport and fate also depends on the abundance of lightning-generated nitrogen oxide radicals (NOx = NO + NO2). If gas uptake on ice is efficient and lightning activity is low, around 30% of the boundary layer isoprene will survive to the cloud outflow after approximately one hour of transport, while all the low volatile oxidation products will be scavenged by the cloud hydrometeors. If lightning NOx is abundant and gas uptake by ice is inefficient, then all isoprene will be oxidised during transport or in the immediate outflow region, while several low volatility isoprene oxidation products will have elevated concentrations in the cloud outflow. Reducing uncertainties associated with the uptake of vapours on ice hydrometeors, especially HO2 and oxygenated organics, is essential to improve predictions of isoprene and its oxidation products in deep convective outflows and their potential contribution to new particle formation and growth.

General discussion: Aerosol formation and growth; VOC sources and secondary organic aerosols.

General discussion: Aerosol formation and growth; VOC sources and secondary organic aerosols.

Aerosol Emissions from Water-lean Solvents for Post-combustion CO2 Capture

Advanced water-lean solvents (WLS) for post-combustion CO2 capture have been gaining interest due to their ability to reduce the parasitic penalty from energy needed for solvent regeneration. Commercial implementation of these novel CO2 capture technologies hinges on successful control of amine emissions. RTI conducted a parametric study of fundamental and operational variables influence on overall amine aerosol and vapor emissions from our water-lean solvent eCO2Sol™ using our 6-kW equivalent bench-scale gas absorption system. The parametric testing used a simulated flue gas with 15 % CO2, 2.3–4.2 % H2O, and 0–6 ppm sulfite (SO3) to examine the impact of the presence of aerosols to the capture performance and amine emissions from the system. The SO3 reacts with water in the flue gas to create H2SO4, which forms liquid aerosol droplets and provide nucleation sites for growth of aerosols. Scanning Mobility Particle Sizer and Aerodynamic Particle Sizer instruments monitored the aerosol particle size distribution. Parametric testing results suggested that the presence of the aerosols in the flue gas could increase the overall amine emissions by 10X compared to the baseline emissions from WLS’s vapor pressure. Principal component analysis (PCA) and projection to latent squares (PLS) developed models to predict the aerosol-based amine emissions from process data. The predictive PLS model had a correlation coefficient (Q2) of 0.92 and could predict the aerosol-based emissions from the NAS process with ±15 % accuracy (average absolute deviation, AAD). The PLS regression model also identified key variables affecting aerosol-based emissions from WLS.

Open-source modelling of aerosol dynamics and computational fluid dynamics: Nodal method for nucleation, coagulation, and surface growth

Understanding formation, growth and transport of aerosols is critical to processes ranging from cloud formation to disease transmission. In this work, a numerical algorithm of aerosol dynamics including nucleation, coagulation, and surface growth was coupled with flow and heat transfer equations enabling the solution of three-dimensional multi-physics aerosol processes in an open-source platform. The general dynamic equation was solved by a nodal method where the particle size distribution was represented by a finite number of nodes. The models were verified by comparing four test cases, (1) pure coagulation, (2) nucleation and coagulation, (3) pure surface growth, and (4) a general dynamic equation that includes the three mechanisms provided in literature. A high temperature aerosol flow in a cooled pipe is chosen as a tutorial case of coupled computational aerosol and fluid dynamics. The aerosolGDEFoam code is available at https://openaerosol.sourceforge.io and can be further modified under GNU general public licence. Programme summaryProgramme title: aerosolGDEFoamCPC Library link to programme files: https://doi.org/10.17632/3s368jpdx2.1Developer’s repository link: https://openaerosol.sourceforge.io/Licencing provisions: GNU General Public Licence 3Programming language: C++Nature of problem: aerosolGDEFoam solves the general dynamic equation coupled with flow and heat transfer equations enabling the solution of three-dimensional multi-physics aerosol processes using the open-source computational platform, OpenFOAM [1]. The general dynamic equation describes changes in aerosols due to e.g. nucleation, coagulation and evaporation/condensation, processes which depend on local conditions such as temperature and humidity. A zero-dimensional form of the general dynamic equation from Prakash et al. [2] has been implemented and verified with previously published examples.Solution method: aerosolGDEFoam employs an explicit time-stepping for the time-dependent source terms for aerosol dynamics. The solution methods and schemes provided by OpenFOAM 6 are used for spatial derivatives.

The importance of accounting for enhanced emissions of monoterpenes from new Scots pine foliage in models - A Finnish case study

Models to predict the emissions of biogenic volatile organic compounds (BVOCs) from terrestrial vegetation largely use standardised emission potentials derived from shoot enclosure measurements of mature foliage. In these models, the potential of new foliage to emit BVOCs is assumed to be similar, or up to twice as high, as that of mature foliage, and thus new conifers foliage is predicted to have a negligible to minor contribution to canopy BVOC emissions during spring time due to the small foliage mass of emerging and growing needles. Extensive observations have, however, recently demonstrated that the potential of new Scots pine needles to emit several different BVOCs can be up to about 500 times higher than that of the corresponding mature foliage. Thus, we build on these discoveries and investigate the potential impact of considering these enhanced emissions from new Scots pine foliage on estimates of monoterpene emissions and new atmospheric aerosol particle formation and their subsequent growth. We show that the importance of taking the enhanced monoterpene emission potential of new Scots pine foliage into account decreases as a function of season, tree age and latitude, and that new foliage could be responsible for the majority of the whole tree's foliage emissions of monoterpenes during spring time, independently of tree age and location. Our results suggest that annual monoterpene emission estimates from Finland would increase with up to ~25% if the enhanced emissions from new Scots pine foliage were also considered, with the majority being emitted during spring time where also new particle formation has been observed to occur most frequently. We estimate that our findings can lead to increases in predictions of the formation rates of 2 nm particles during spring time by ~75–280% in northern Finland and by ~130–870% in southern Finland. Likewise, simulated growth rates of 2–3 nm particles would increase by ~65–180% in northern Finland and by ~110–520% in southern Finland if the enhanced emissions of monoterpenes from new Scots pine foliage were explicitly considered. Since only one measurement study (Aalto et al., 2014), on which our work builds, has so far found highly pronounced emissions of monoterpenes from new Scots pine foliage compared to those of mature, we conclude that more spring time measurements of new conifers foliage are required for improving emission algorithms in biogenic emission models. • In models, emission potentials of new and mature foliage are assumed to be similar. • VOCs emissions from new vs mature pine foliage can differ by 2–3 orders of magnitude. • Accounting for new foliage emissions significantly impacts annual emission estimates. • Enhanced springtime emissions can lead to enhanced aerosol formation and growth.

Amplification of South Asian haze by water vapour–aerosol interactions

Abstract. Air pollution and wintertime fog over South Asia is a major concern due to its significant implications for air quality, visibility and health. Using a regional climate model coupled with chemistry, we assess the contribution of the hygroscopic growth of aerosols (ambient–dry) to the total aerosol optical depth and demonstrate that the increased surface cooling due to the hygroscopic effects of aerosols further increases the humidity in the boundary layer and thus enhances the confinement of pollutants through aerosol–boundary layer interactions. This positive feedback mechanism plays an important role in the prevalence of wintertime fog and poor air quality conditions over South Asia, where water vapour contributes more than half of the aerosol optical depth. The aerosol–boundary layer interactions lead to moistening of the boundary layer and drying of the free troposphere, which amplifies the long-term trend in relative humidity over the Indo-Gangetic Plain during winter. Hence, the aerosol–water vapour interaction plays a decisive role in the formation and maintenance of the wintertime fog conditions over South Asia, which needs to be considered for planning mitigation strategies.

Measurements of atmospheric aerosol hygroscopic growth based on multi-channel Raman-Mie lidar

In this study, an effective method with a multi-channel Raman-Mie lidar has been proposed to detect the hygroscopic growth of atmospheric aerosol in the boundary layer, and the effect of aerosol hygroscopic growth on the optical and microphysical properties of aerosol particles is studied. The vertical relative humidity profile is obtained by combining calibrated water vapor mixing ratio and temperature profiles from Raman-Mie lidar. Two cases were selected in Hefei to analyze the aerosol hygroscopic growth. Case studies show that the increase of the backscatter coefficient of aerosol is consistent with the increase of relative humidity, and the decrease of Ångstrom exponent indicates the increasing aerosol particle size due to the hygroscopic growth process. The Kasten model is used to fit the aerosol hygroscopic enhancement factor, and the b value from the Kasten parameterization for Case II is much larger than that for Case I, which shows the stronger hygroscopicity in Case II. The results indicate that the multi-channel Raman-Mie lidar has potential in the measurements of the hygroscopic growth of aerosols.

Atmospheric aerosol growth rates at different background station types

Highly time-resolved particle number size distributions (PNSDs) were evaluated during 5 years (2013–2017) at four background stations in the Czech Republic located in different types of environments—urban background (Ústí nad Labem), industrial background (Lom), agricultural background (National Atmospheric Observatory Košetice), and suburban background (Prague-Suchdol). The PNSD data was used for new particle formation event determination as well as growth rate (GR) and condensation sink (CS) calculations. The differences or similarities of these parameters were evaluated from perspectives of the different pollution load, meteorological condition, and regional or long-range transport. The median growth rate (4 nm h−1) is very similar at all stations, and the most frequent length of growth lasted between 2 and 4 h. Condensation sink reflects the pollution load at the individual station and their connection to the environment type. The highest median, CS = 1.34 × 10−2 s−1, was recorded at the urban station (Ústí nad Labem), and the lowest (CS = 0.85 × 10−2 s−1) was recorded at the agricultural station (National Atmospheric Observatory Košetice). Conditional probability function polar plots illustrate the influence of source location to GR. These primary potential emission sources involve traffic, operation of a power plant, and domestic heating.

A Review on Laboratory Studies and Field Measurements of Atmospheric Organic Aerosol Hygroscopicity and Its Parameterization Based on Oxidation Levels

The study of organic aerosol hygroscopic growth and cloud droplet activation is crucial for accurately quantifying their climate and environmental impacts. However, the physical mechanisms behind organic aerosol hygroscopicity variations are not well understood. In this review, we summarized laboratory and field measurements of the organic aerosol hygroscopicity parameter κOA, discussed the physical understanding of why κOA was generally positively correlated with organic aerosol oxidation level, summarized proposed κOA parameterization schemes, and examined possible explanations for the marked differences among these parameterization schemes. Recent findings challenged the general cognition that cloud condensation nuclei (CCN) activity of secondary organic aerosol depended largely on solubility, showing it to be mainly controlled by molecular weight, yet the universality of this finding needs to be further examined. It was found that carbon chain length and functional groups had significant impacts on κOA and additional parameters other than O/C ratio need to be included when parameterizing κOA of multifunctional compounds, which is typically the case for ambient atmospheric aerosols. Additionally, laboratory results of secondary organic aerosol suggest that κOA might be highly RH-dependent under sub-saturated conditions, especially for biogenic secondary organic aerosols. This review summarized laboratory and field measurements of atmospheric organic aerosol hygroscopicity parameter κOA and its parameterization schemes. The results demonstrate that representing κOA with a single oxidation level parameter still bears large uncertainty, and physical mechanisms associated with hygroscopic growth and cloud activation processes of organic aerosol still remain unresolved and need further investigation.

Approximated expression of the hygroscopic growth factor for polydispersed aerosols

Hygroscopic growth of aerosols plays an important role in the characterization of atmospheric aerosols. Physico-chemical and optical properties of aerosols are dependent on relative humidity (RH) as well as on their size and composition. In this study, scattering enhancement factors, f(RH), for polydispersed aerosols were approximated. f(RH) of ammonium sulfate and ammonium nitrate (AS) and NaCl aerosols (ss) were considered under the assumption of externally mixed aerosols. f(RH) was calculated using the Mie theory for polydispersed aerosols at a given RH and corresponding water uptake (denoted as fMie(RH)). f(RH) was approximated as a quadratic function of RH (denoted as fapp(RH)). The obtained approximated f(RH) or fapp(RH)) values were compared with the values based on the Mie theory and showed a reasonable agreement between them. To the best of our knowledge, this is the first study that parametrizes the size dependency of f(RH) for log normally distributed aerosols.