Properties Of Atmospheric Aerosols Research Articles

Microphysical Analysis of Maritime and Continental Aerosols: Determination of Hygroscopic Growth Factors Using OPAC Data and Modeling Approaches

Study’s Excerpt/Novelty This study presents an application of mass-based hygroscopicity models to analyze microphysical properties of atmospheric aerosols from continental and maritime sources using data from the Optical Properties of Aerosols and Clouds (OPAC). By examining hygroscopic growth factors and effective radii at eight different relative humidities, the research highlights the significant variation in growth factors between maritime and continental aerosols, with maritime clean aerosols exhibiting a substantially higher growth factor at 99% RH. The study's robust statistical analysis, confirmed by R² values greater than 90% and significance levels below 0.05, demonstrates the model's efficacy for atmospheric modeling and remote sensing applications. Full Abstract The interplay of marine and continental sources governed the atmospheric aerosols over coastal areas. The transport of aerosols from continental sources into sea surfaces through deposition or diffusion is what causes the fast reduction of continental aerosols. A mass based based hygroscopicity models were applied to the data extracted from the Optical Properties of Aerosols and Clouds (OPAC). The microphysical properties obtained were radii, density, refractive index, mass, volume, and sphericity of the atmospheric aerosols of continental and maritime aerosols at eight different relative humidity of 0%, 50%, 70%, 80%, 90%, 95%, 98%, and 99%. Using the microphysical properties, hygroscopic growth factors, and effective radii of the mixtures, mass growth factor Gm and diameter growth factor DG were determined, and also the parameter Km for mass based of the aerosols were determined using multiple regression analysis with SPSS 16.0 at each relative humidity. The results show that Gm for maritime clean is higher than other aerosols, with a value of 34.46 at 99% RH, while the lowest value is for continental average, with a value of 5.03 at 99% RH. Also, R2 for the model is greater than 90%. The significance and P-values are less than 0.05; therefore, the model is good for atmospheric modeling and remote sensing.

Design and development of a portable tuneable radiation source from UV to IR for in situ calibration of radiometers measuring atmospheric aerosol properties.

Abstract We present the results of development of a portable tuneable radiation source (TuPS II) covering the broad spectral range from 300 nm up to 1700 nm. It was developed to provide SI-traceable in-situ calibration of bandpass functions of radiometers measuring both sky radiance and sky irradiance in the surface-based remote sensing networks for column integrated atmospheric aerosol properties. The TuPS II provides quasi-monochromatic radiation with spectral bandwidth smaller than 0.2 nm and central wavelength uncertainty lower than 0.05 nm for a specified number of aerosol wavelength bands ranging from to 300 nm to 1700 nm. In its design it represents an evolution with extended spectral capabilities of the TuPS (Smid, M. et al., 2021) that was successfully used to characterize Dobson spectrophotometers in the UV spectral range.

Parameterization of Dust Emissions from Heaps and Excavations Based on Measurement Results and Mathematical Modelling

Assessment of the concentrations of dust pollution resulting from both measurements at reference stations and those determined using mathematical modelling requires accurate identification of the sources of emission. Although the concentration of dust results from several complex transport processes, as well as chemical and microphysical transformations of aerosols, sources of emissions may have a significant impact on the local level of pollution. This pilot study aimed to use measurements of the concentrations of dust (with the specification of the PM10 and PM2.5 fractions) made over a heap/excavation and its surroundings using an airship equipped with equipment for testing the optical and microphysical properties of atmospheric aerosols, and a ground station located at the facility. On the basis of the measurements, the function of the source of emissions of dust was estimated. According to our study, the yearly emission of dust varies between 42 470 and 886 289 kg for PM10, and between 42 470 and 803 893 for PM2.5 (minimum and maximum values). A model of local air quality was also used, which allowed us to verify the parameterization of emissions of dust pollutants for the PM10 and PM2.5 fractions from heaps and excavations based on the modelling results.

Addressing observational gaps in aerosol parameters using machine learning: Implications to aerosol radiative forcing

The role of atmospheric aerosols has remained hugely uncertain due to lack of long-term systematic observations. This is particularly the case in the global emission hotspot the Indo-Gangetic Plain. In this paper, we apply a machine learning (ML) approach to fill gaps in aerosol observations by combining meteorological reanalysis, and satellite-observations. Aerosol Optical Depth (AOD), Single Scattering Albedo (SSA), and Asymmetry Parameter (AP) are critical parameters for understanding the atmospheric aerosol properties, radiative forcing and climate patterns. We considered AOD, SSA, and AP over an urban site, Kanpur, in the Indo-Gangetic Plain, which is a global hot-spot of high aerosol loading. We used ML technique to fill the observational gaps in AOD, SSA, and AP at 440 nm. Accurate ground-based daily observations of AOD, SSA, and AP from AERONET (Aerosol Robotic Network) over a period of ∼2 decades (2001–2022) are used in the analysis, which have ∼ 37%, ∼62%, and ∼58% data gap, respectively. To reduce observational gaps, an ML model was trained with reanalysis (meteorological parameters) and satellite (angstrom exponent and absorbing aerosol index) datasets by optimally tuning the hyperparameters. The model tested on unseen data showed that it could simulate the AOD (coefficient of determination, R2 = 0.51 and refined index of agreement, RIA = 0.66), SSA (R2 = 0.60 and RIA = 0.70), and AP (R2 = 0.64 and RIA = 0.71), successfully with moderate accuracies: root mean square error of 0.25, 0.018, and 0.018, respectively. This magnitude of error in AOD is comparable to the errors, 0.26 and 0.25, in satellite (MODIS-Moderate Resolution Imaging Spectroradiometer) derived and reanalysis (MERRA-2- Modern-Era Retrospective analysis for Research and Applications, Version 2) AODs, respectively for the study region. When MODIS AOD was included as a predictor in the ML model, it could reproduce AOD with much smaller errors of 0.14, reproducing 84% of total variability. Thus, based on available input data, we reduced observational gaps of daily AOD, SSA, and AP by ∼10% (∼2 years), ∼23% (∼5 years), and ∼21% (∼4 years), respectively, at the study region. Aerosol Radiative Forcing (ARF) at the top-of-atmosphere was estimated using AOD, SSA, and AP both with and without gap-filling. We found that data gaps can significantly impact the estimations of aerosol climate forcing. A considerable change in ARF, ∼ ± 3 Wm−2 indicating that the observational gaps can mislead our understanding of aerosols' impact on climate. This points toward the need for more comprehensive measurements with minimal observational gaps. Our study highlights the possibility of compensating observational gaps by an optimised ML model and that such innovative approach can potentially lead to better understanding of impact of aerosols on climate. Further, the present study could be of immense interest for the community as our validated approach can be applied for other pollutants and for other data-scarce regions in the South and Southeast Asia.

Impact of Anthropogenic Aerosol Transport on Cloud Condensation Nuclei Activity During Summertime in Qilian Mountain, in the Northern Tibetan Plateau

AbstractIn this study, we conducted field campaigns at two mountain‐top observatories on the Qilian Mountains (QLM) in the northeastern Tibetan Plateau of China, which plays a vital role in sustaining water resources for the downstream arid and semi‐arid regions. The two observatories, Waliguan Baseline Observatory (WLG) and Laohugou station (LHG), are situated on the eastern and western edge of the QLM, respectively. The objectives of this study were to examine the properties of atmospheric aerosols, CCN concentrations (NCCN) at varying supersaturation levels (SS = 0.2%–1.0%), and the hygroscopic nature of aerosols in the QLM, especially during the formation of orographic cloud. Notably, the average aerosol concentration and NCCN was approximately 2–3 factors higher at WLG compared to LHG. The chemical compositions of aerosols were primarily dominated by sulfate and organic aerosol (OA) at these two sites. A very high hygroscopicity parameter of aerosol calculated using chemical composition was observed at these two sites (0.37 ± 0.03 at LHG vs. 0.30 ± 0.04 at WLG). Enhanced aerosol loading episodes impacted by anthropogenic emission were observed at these two sites. Exploration on high‐loading cases at each site, we found that the CCN activity was dominated by aerosol size, but the chemical processes of aerosol during the formation of orographic cloud could also be important in CCN activity, especially for low SS conditions. These findings collectively underscore the significant impact of anthropogenic air plumes on CCN concentrations in the QLM and their potential influence on precipitation patterns.

Characteristics and optical properties of atmospheric aerosols based on long-term AERONET investigations in an urban environment of Pakistan

Radiative balance, local climate, and human health are all significantly influenced by aerosol. Recent severe air pollution over Lahore, a city in Pakistan calls for more thorough research to determine the negative impacts brought on by too many aerosols. To study regional aerosol characteristics and their differences from various aspects, in-depth and long-term (2007–2020) investigations of the columnar aerosol properties over the urban environment of Lahore were carried out by using AERONET data. The Aerosol Optical Depth (AOD400) and Angstrom Exponent (AE400–870) vary from low values of 0.10 to a maximum value of 4.51 and from 0.03 to 1.81, respectively. The huge differences in the amount of AOD440 as well as AE440–870 show the large fluctuation of aerosol classes because of various sources of their emission. During the autumn and winter seasons, the decreasing trend of the optical parameters of aerosols like Single Scattering Albedo (SSA) and Asymmetry Parameter (ASY) with increasing wavelength from 675 to 1020 nm indicates the dominance of light-absorbing aerosols (biomass burning (BB) and industrial/urban (UI). Due to the long-distance dust movement during spring, summer, and autumn, coarse mode particles predominated in Lahore during the study period. Dust type (DD) aerosols are found to be the dominant one during spring (46.92%), summer (54.31%), and autumn (57.46%) while urban industry (BB/UI) was dominant during the winter season (53.21%). During each season, the clean continental (CC) aerosols are found to be in negligible amounts, indicating terrible air quality in Lahore City. The present research work fills up the study gap in the optical properties of aerosols in Lahore and will help us understand more fully how local aerosol fluctuation affects regional climate change over the urban environment of Lahore.

Mass and Light Absorption Properties of Atmospheric Carbonaceous Aerosols over the Outflow Regions of Indo-Gangetic Plain

The uncertainty associated with carbonaceous aerosols in climate models becomes an obstacle to evaluating their harmful environmental effects. Brown Carbon (BrC) has recently become an eye-catching carbonaceous aerosol in ambient air because of its strong light absorption properties. The optical properties of soluble BrC are well studied in South Asia, one of the largest emitters. However, the absorption property of strong light-absorbing insoluble BrC is highly uncertain because of their complex mixing. Besides, the simultaneous apportionment of light-absorbing carbonaceous aerosol constituents as a function of wavelengths is seldom explored. The present study aims to study mass as well as apportionment of absorption properties of Black Carbon (BC) and different BrC chromophores using a multi-wavelength thermal-optical carbon analyzer over the least explored Eastern part of India to fill the knowledge gap. The output suggested that the primary emission sources dominate the carbonaceous aerosol mass; thus, the primary emission reductions will be helpful in improving the local air quality. The BrC is the dominant light-absorbing carbonaceous aerosol in the ultraviolet wavelengths. Besides, significant light absorption by BrC chromophores in near-infrared wavelengths (up to ∼50% of total aerosol absorption) suggested the absorption by BrC in longer wavelengths cannot be neglected. The insoluble fraction of BrC (Tar-BrC), which has a property identical to BC, contributes significantly to light absorption and is more than 50% of total absorption by aerosols in shorter ≤635 nm wavelengths. The present study also revealed that the contribution of Tar-BrC is more pronounced in the Indo-Gangetic Plain (IGP) outflow region than in the IGP emission source regions.

Far infrared radiative properties of Mars atmospheric aerosols and their application to Mars Climate Sounder retrievals of aerosol profiles, aerosol columns and surface temperatures

Limb sounding of thermal emission in the infrared wavelength range is a powerful technique for measuring temperature and aerosols in the martian atmosphere. However, the long optical path may provide challenges to limb retrievals in high aerosol conditions. These can be mitigated by considering limb measurements in the far infrared, where opacities of most aerosols are lower than in the mid-infrared. We present analyses of radiative properties of Mars dust and water ice aerosols at far infrared wavelengths based on measurements by the Mars Climate Sounder (MCS) in limb geometry at mid- and far infrared wavelengths. For dust aerosols, derived far infrared radiative properties show a homogeneous behavior that is consistent with particle sizes in the order of 1 μm effective radius. Far infrared radiative properties for water ice aerosols exhibit a larger variability in local time and region, leading to significant differences between the aphelion cloud belt and the north polar hood, with the resulting parameters suggesting particle sizes around 3 μm or larger. Using the derived parameters, we develop a method for retrieving aerosol profiles from MCS limb measurements that combines information from mid- and far infrared spectroscopic channels. The use of far infrared channels enables aerosol profile retrievals from limb measurements that typically reach about a scale height deeper into the atmosphere than would be possible using mid-infrared channels only. The extended vertical range of the aerosol profiles allows the derivation of aerosol column optical depths through vertical integration, with dust column derivations in global or large-scale regional dust storms also being available by extrapolating dust profiles below the lowest retrievable altitude of a limb measurement. The quantification of aerosol columns allows us to retrieve surface brightness temperatures from MCS on-planet viewing measurements that are corrected for atmospheric contributions. We show that differences between surface brightness and top-of-the-atmosphere temperatures are typically within 20 K, with surface brightness temperatures generally being warmer (colder) than top-of-the-atmosphere temperatures at daytime (nighttime), except at high latitudes.

Characteristics of Absorbing Aerosols in Mexico City: A Study of Morphology and Columnar Microphysical Properties

This paper presents an analysis of the morphology and columnar microphysical properties of atmospheric aerosols in Mexico City (MC) for the period 2022–2023. The morphological study focused on the structure description of soot particles and tar balls (TB). By transmission electron microscope (TEM) and scanning electrode microscope (SEM), voluminous soot aggregates mixed with TBs were observed. The chemistry shows that both soot and TBs are mostly carbonaceous species with well-defined morphologies. On the other hand, the columnar aerosol microphysical properties recovered from AERONET show that the particles have a bimodal aerosol size distribution (ASD) with two modes: fine and coarse. The ASD remains constant without showing significant seasonal changes, only with some variability for coarse particles. The aerosol optical depth (AOD) value is significantly high, typical of urban areas. The real (n) and imaginary (k) parts of the complex refractive index (CRI) were obtained from the photometric measurements. The CRI values show seasonal variations, with spring being the season with the highest values for n, while the highest values for k were measured in winter.

Probing Reflection from Aerosols with the Near-infrared Dayside Spectrum of WASP-80b

The presence of aerosols is intimately linked to the global energy budget and the composition of a planet’s atmosphere. Their ability to reflect incoming light prevents energy from being deposited into the atmosphere, and they shape the spectra of exoplanets. We observed five near-infrared secondary eclipses of WASP-80b with the Wide Field Camera 3 (WFC3) aboard the Hubble Space Telescope to provide constraints on the presence and properties of atmospheric aerosols. We detect a broadband eclipse depth of 34 ± 10 ppm for WASP-80b. We detect a higher planetary flux than expected from thermal emission alone at 1.6σ, which hints toward the presence of reflecting aerosols on this planet’s dayside, indicating a geometric albedo of A g < 0.33 at 3σ. We paired the WFC3 data with Spitzer data and explored multiple atmospheric models with and without aerosols to interpret this spectrum. Albeit consistent with a clear dayside atmosphere, we found a slight preference for near-solar metallicities and for dayside clouds over hazes. We exclude soot haze formation rates higher than 10−10.7 g cm−2s−1 and tholin formation rates higher than 10−12.0 g cm−2s−1 at 3σ. We applied the same atmospheric models to a previously published WFC3/Spitzer transmission spectrum for this planet and found weak haze formation. A single soot haze formation rate best fits both the dayside and the transmission spectra simultaneously. However, we emphasize that no models provide satisfactory fits in terms of the chi-square of both spectra simultaneously, indicating longitudinal dissimilarity in the atmosphere’s aerosol composition.

Long-term spatiotemporal variation in atmospheric aerosol properties over Türkiye based on MERRA-2 reanalysis data: aerosol classification based on city type.

Due to their complex aerosol characteristics, it is crucial to analyze the trends and properties of atmospheric aerosols over the eastern Mediterranean countries. This study comprehensively evaluates Aerosol Optical Depth (AOD) and Angström Exponent (AE) trends and aerosol classification over Türkiye, using the MERRA-2 reanalysis data from 1980 to 2019. The spatial distributions of AOD and AE were determined across various temporal scales, including multiannual, 5-year intervals, seasonal, and monthly periods. The analysis of the spatial distribution of AOD values revealed that the mean values in the northwestern areas, ranging from 0.20 to 0.25, were comparatively higher than those observed in the eastern regions, which ranged from 0.10 to 0.15. Between 1980 and 1994, the AOD values gradually increased, followed by a subsequent decline from 1995 to 2019. Based on 5-year intervals between 1980 and 2019, the coastal regions exhibited higher AOD values than the inland areas. Specifically, higher AOD values were noted between May and August, whereas lower values were observed during autumn and winter. Additionally, higher AE values were detected over the northwestern region, while the southeastern region had the lowest AE values, particularly during spring, attributed to the frequent occurrence of dust transport events in this area. The AOD and AE values were also examined in different city types, using the population thresholds of the European Commission. The global city category consisting only of Istanbul showed the highest AOD values across all seasons, while the category of very small cities, which includes 12 cities, had the lowest AOD values. Furthermore, this study investigated the contributions of dominant aerosol categories across various city types based on multiannual and seasonal variations of AOD and AE. The results showed that mixed and continental aerosols had higher portions across all city types. However, biomass burning/industrial and mixed aerosol categories were more prominent in global and large cities. Overall, this study provides a comprehensive overview of the atmospheric aerosol properties in Türkiye and can serve as a useful guide for researchers intending to conduct future studies utilizing AOD and AE data obtained through MERRA-2 aerosol diagnosis.

Infrared Reflection-Absorption Spectroscopy of α-Keto Acids at the Air-Water Interface: Effects of Chain Length and Headgroup on Environmentally Relevant Surfactant Films.

A variety of organic surfactants are found at air-water interfaces in natural environments, including on the surfaces of aqueous aerosols. The structure and morphology of these organic films can have profound impacts on material transfer between the gas and condensed phases, the optical properties of atmospheric aerosol, and chemical processing at air-water interfaces. Combined, these effects can have significant impacts on climate via radiative forcing, but our understanding of organic films at air-water interfaces is incomplete. Here, we examine the impact of the polar headgroup and alkyl tail length on the structure and morphology of organic monolayers at the air-water interfaces. First, we focus on the substituted carboxylic acids, α-keto acids, using Langmuir isotherms and infrared reflection absorption spectroscopy (IR-RAS) to elucidate key structures and phase behaviors of α-keto acids with a range of surface activities. We show that the structure of α-keto acids, both soluble and insoluble, at water surfaces is a compromise between van der Waals interactions of the hydrocarbon tail and hydrogen bonding interactions involving the polar headgroup. Then, we use this new data set regarding α-keto acid films at water surfaces to examine the role of the polar headgroup on organic films using a similar substituted carboxylic acid (α-hydroxystearic acid), an unsubstituted carboxylic acid (stearic acid), and an alcohol (stearyl alcohol). We show that the polar headgroup and its hydrogen bonding interactions can significantly affect the orientation of amphiphiles at air-water interfaces. Here, we provide side-by-side comparisons of Langmuir isotherms and IR-RA spectra for a set of environmentally relevant organic amphiphiles with a range of alkyl tail lengths and polar headgroup structures.

Atmospheric nanoparticles hygroscopic growth measurement by a combined surface plasmon resonance microscope and hygroscopic tandem differential mobility analyzer

Abstract. The hygroscopic growth of atmospheric aerosols plays an important role in regional radiation, cloud formation, and hence climate. Aerosol hygroscopic growth is often characterized by hygroscopic tandem differential mobility analyzers (HTDMAs), and Xie et al. (2020) recently demonstrated that hygroscopic growth measurements of a single particle are possible using a surface plasmon resonance microscope-azimuthal rotation illumination (SPRM-ARI). The hygroscopic properties of ambient aerosols are not uniform and often exhibit large relative humidity (RH) and size variabilities due to different chemical compositions and mixing states. To better understand the contribution of different aerosol components and establish a link between the apparent hygroscopic properties of single particles and bulk aerosols, we conduct combined hygroscopic growth measurements using a SPRM-ARI and an HTDMA as a case study to prove the concept (experimental information: 100–200 nm, during noontime on 28 September 2021 and 22 March 2022 in Hefei, China). According to the distinct hygroscopic growth behavior from single-particle probing using a SPRM-ARI, the individual particles can be classified into three categories defined as non-hygroscopic (NH), less hygroscopic (LH), and more hygroscopic (MH). The mean growth factor (GF) of the three categories can be utilized to reproduce the GF distribution obtained from the HTDMA measurement. The chemical compositions of individual particles from the three categories are identified to be organic carbon (OC), soot (mainly elemental carbon), fly ash, and secondary aerosols (mainly OC and sulfate), using scanning electron microscopy (SEM) with an energy-dispersive spectrometer (EDS). The coupled SPRM–HTDMA measurement suggests a size-dependent variation of aerosol chemical components, i.e., an increase of OC fraction with increasing particle sizes, which agrees reasonably well with the chemical compositions from collected aerosol samples. This likely links the hygroscopic properties of individual particles to their bulk hygroscopic growth and chemical composition.

Annual cycle of hygroscopic properties and mixing state of the suburban aerosol in Athens, Greece

Abstract. The hygroscopic properties of atmospheric aerosol were investigated at a suburban environment in Athens, Greece, from August 2016 to July 2017. The growth factor distribution probability density function (GF-PDF) and mixing state were determined with a hygroscopicity tandem differential mobility analyser (HTDMA). Four dry particle sizes (D0) were selected to be analysed in terms of their hygroscopic properties at 90 % relative humidity. The annual mean GFs for D0 = 30, 50, 80 and 250 nm were found to be equal to 1.28, 1.11, 1.13 and 1.22, respectively. The hygroscopic growth spectra were divided into two distinct hygroscopic ranges: a non- and/or slightly hygroscopic mode (GF &lt; 1.12) and a moderately hygroscopic mode (GF &gt; 1.12), which are representative of a suburban environment influenced by local/urban emissions and background aerosol. The standard deviation σ of the GF-PDF was employed as a measure of the mixing state of ambient aerosol. The 30 nm particles were mostly internally mixed, whereas larger particles were found to be externally mixed, either with a distinct bimodal structure or with partly overlapping modes. Cluster analysis on the hourly dry number size distributions was performed to identify the link between aerosol hygroscopicity and aerosol emission sources and formation processes. The size distributions were classified into five groups, with the “mixed urban and regional background” aerosol (67 %) and the “fresh traffic-related particles” from the neighbourhood urban area (15 %) accounting for more than 80 % of the results. The hygroscopic properties for 50 and 80 nm were found to be similar in all cases, indicating particles of similar nature and origin across these sizes. This was also confirmed through the modal analysis of the average number size distributions for each cluster; the 50 and 80 nm particles were found to belong to the same Aitken mode in most cases. The 250 nm particles (i.e. accumulation mode) were generally more hygroscopic than Aitken particles but less hygroscopic than the 30 nm particles (nuclei mode).

A Comprehensive Review on Study Methods of Aerosol Optical Properties in Different Dimensions

A comprehensive understanding of the optical properties of atmospheric aerosol is essential for a variety of applications, such as optical imaging, optical communication, and remote sensing. In recent years, many theories and numerical simulation methods have been developed to connect aerosol physical-chemical properties to their intrinsic and integral optical properties. Usually, simulations and measurements are intertwined to synergistically attain the retrieval of aerosol optical properties and mitigate or even eliminate the adverse impacts of aerosol during imaging, sensing, or communication. This review covers the fundamental theories of aerosol optical properties, the development of numerical simulations, the instrument-based sampling measurements, and the cutting-edge techniques of remote sensing. Numerical simulations have been progressing from symmetric particles to asymmetric particles over the past two decades, although any simulation method is limited by specific shape and a restricted size parameter range. Thus, this review also examines the most typical advances in aerosol instrumentation that are frequently used to measure the intrinsic optical properties of unknown aerosols. Such obtained properties validate simulations and constitute the basis of integral optical properties. In terms of practical applications, integral optical properties are the most critical knowledge about atmospheric aerosol. Remote sensing measurements, be it ground-based, airborne, or satellite-based, all retrieve integral optical properties of atmospheric aerosol from various perspectives, which are elaborated upon in this review. In conclusion, this review provides an all-encompassing comprehension of aerosol optical properties.

Optical trapping and light scattering in atmospheric aerosol science.

Aerosol particles are ubiquitous in the atmosphere, and currently contribute a large uncertainty to climate models. Part of the endeavour to reduce this uncertainty takes the form of improving our understanding of aerosol at the microphysical level, thus enabling chemical and physical processes to be more accurately represented in larger scale models. In addition to modeling efforts, there is a need to develop new instruments and methodologies to interrogate the physicochemical properties of aerosol. This perspective presents the development, theory, and application of optical trapping, a powerful tool for single particle investigations of aerosol. After providing an overview of the role of aerosol in Earth's atmosphere and the microphysics of these particles, we present a brief history of optical trapping and a more detailed look at its application to aerosol particles. We also compare optical trapping to other single particle techniques. Understanding the interaction of light with single particles is essential for interpreting experimental measurements. In the final part of this perspective, we provide the relevant formalism for understanding both elastic and inelastic light scattering for single particles. The developments discussed here go beyond Mie theory and include both how particle and beam shape affect spectra. Throughout the entirety of this work, we highlight numerous references and examples, mostly from the last decade, of the application of optical trapping to systems that are relevant to the atmospheric aerosol.

Remote sensing of aerosol water fraction, dry size distribution and soluble fraction using multi-angle, multi-spectral polarimetry

Abstract. A framework to infer volume water fraction, soluble fraction and dry size distributions of fine-mode aerosol from multi-angle, multi-spectral polarimetry retrievals of column-averaged ambient aerosol properties is presented. The method is applied to observations of the Research Scanning Polarimeter (RSP) obtained during two NASA aircraft campaigns, namely the Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) and the Cloud, Aerosol, and Monsoon Processes Philippines Experiment (CAMP2Ex). All aerosol retrievals are statistically evaluated using in situ data. Volume water fraction is inferred from the retrieved ambient real part of the refractive index, assuming a dry refractive index of 1.54 and by applying a volume mixing rule to obtain the effective ambient refractive index. The uncertainties in inferred volume water fraction resulting from this simplified model are discussed and estimated to be lower than 0.2 and decreasing with increasing volume water fraction. The daily mean retrieved volume water fractions correlate well with the in situ values with a mean absolute difference of 0.09. Polarimeter-retrieved ambient effective radius for daily data is shown to increase as a function of volume water fraction as expected. Furthermore, the effective variance of the size distributions also increases with increasing effective radius, which we show is consistent with an external mixture of soluble and insoluble aerosol. The relative variations of effective radius and variance over an observation period are then used to estimate the soluble fraction of the aerosol. Daily results of soluble fraction correlate well with in situ-observed sulfate mass fraction with a correlation coefficient of 0.79. Subsequently, inferred water and soluble fractions are used to derive dry fine-mode size distributions from their ambient counterparts. While dry effective radii obtained in situ and from RSP show similar ranges, in situ values are generally substantially smaller during the ACTIVATE deployments, which may be due to biases in RSP retrievals or in the in situ observations, or both. Both RSP and in situ observations indicate the dominance of aerosol with low hygroscopicity during the ACTIVATE and CAMP2Ex campaigns. Furthermore, RSP indicates a high degree of external mixing of particles with low and high hygroscopicity. These retrievals of fine-mode water volume fraction and soluble fraction may be used for the evaluation of water uptake in atmospheric models. Furthermore, the framework allows us to estimate the variation in the concentration of fine-mode aerosol larger than a specific dry radius limit, which can be used as a proxy for the variation in cloud condensation nucleus concentrations. This framework may be applied to multi-angle, multi-spectral satellite data expected to be available in the near future.

The light absorbing and molecule characteristic of PM2.5 brown carbon observed in urban Shanghai

Both brown carbon (BrC) and the non-absorbing components coated on black carbon (BC) aerosols can enhance the light absorption of BC aerosols. BrC is a complicated mixture of organic compounds and not well characterized, which hinders exploring the links between BrC and optical properties. We conducted an in-depth field study on optical properties of ambient aerosols at a monitoring site in Shanghai, China via real-time monitoring and offline analysis. Results showed that BrC caused light absorption coefficients were 3.3 ± 3.3 Mm−1, 2.2 ± 5.0 Mm−1, 1.2 ± 1.2 Mm−1 at λ = 370, 470 and 520 nm, respectively, accounting for 11%, 10%, 6% of the total aerosol absorption for the corresponding wavelengths. A larger proportion of long-chain aliphatic organosulfates (OSs, CnH2n+2O4S, (CH2)nO5S, (CH2)nO6S) with double bond equivalent (DBE) values of 0 or 1 accounted for 5–20% of the light absorption (λ = 365 nm) for soluble brown carbon (BrC), which were dominating for the days with less N-containing aromatic compounds appearing. Furthermore, the structure of CnH2n+2O4S, (CH2)nO5S, (CH2)nO6S were explored using target MS/MS of HPLC-Q-ToF-MS: (CH2)nO5S series, the most abundant family of OSs, were constructed by functionalizing a saturated hydrocarbon with one sulfate and one carbonyl group. CnH2n+2O4S series were oxidized with only one sulfate group in the aliphatic chain R. (CH2)nO6S series were proposed as aliphatic OSs with one ester group. We speculated aliphatic OSs were formed via acid catalyzed perhydrolysis of hydroperoxides derived from long-chain alkanes releasing from diesel fueled vehicles, followed by the reaction with sulfate anion radicals. Therefore, relevant technologies should be further explored to reduce the impacts from vehicle emissions.

Size-fractionated ultrafine particles and their optical properties produced from heating edible oils in a kitchen laboratory

Cooking oil fume (COF) is an important source of indoor and outdoor air pollutants. COF generates a large number of organic compounds through volatilization and thermal oxidation, mainly including acids, alcohols, aldehydes and polycyclic aromatic hydrocarbons (PAHs), which can contribute 10 %–35 % to airborne organic particles in urban areas. COF not only affects human health owning to their small sizes, but also may absorb incident light due to the presence of brown carbon (BrC) chromophores in organic components. Therefore, we investigated size distributions and light absorption properties of particles produced from heating four types of edible oil. Results showed over 75 % of COF particles belonged to ultrafine particles (UFPs) and capable of absorbing light. The particle number size distributions for heating all edible oils were bimodal lognormal distribution, and the two mode diameters were within 27.9–32.2 nm and 187.7–299.6 nm. Both real-time monitoring and offline analyzing results show the average absorption coefficients of particles generated from heating soybean oil were much greater compare to those of heating other three edible oils. The mean AAE370/520 for heating soybean oil, olive oil, corn oil and peanut oil were 1.877, 1.669, 1.745 and 1.288, respectively, indicating the presence of BrC chromophores. A large proportion of BrC identified by HPLC-DAD-Q-TOF-MS only contain carbon, hydrogen and oxygen, which are CnH2nO2, CnH2n-2O2, CnH2n-4O2 and CnH2n-6O2 (9 <n < 23), may belong to fatty acids. Their total light absorption at λ = 370 nm accounted for 16.75 %–54.56 % of the total absorption of methanol-soluble BrC. The findings provided scientific evidences for the significance of cooking emissions on ambient aerosol properties.

Sulfur radical formation from the tropospheric irradiation of aqueous sulfate aerosols

The sulfate anion radical (SO4•-) is known to be formed in the autoxidation chain of sulfur dioxide and from minor reactions when sulfate or bisulfate ions are activated by OH radicals, NO3 radicals, or iron. Here, we report a source of SO4•-, from the irradiation of the liquid water of sulfate-containing organic aerosol particles under natural sunlight and laboratory UV radiation. Irradiation of aqueous sulfate mixed with a variety of atmospherically relevant organic compounds degrades the organics well within the typical lifetime of aerosols in the atmosphere. Products of the SO4•- + organic reaction include surface-active organosulfates and small organic acids, alongside other products. Scavenging and deoxygenated experiments indicate that SO4•- radicals, instead of OH, drive the reaction. Ion substitution experiments confirm that sulfate ions are necessary for organic reactivity, while the cation identity is of low importance. The reaction proceeds at pH 1-6, implicating both bisulfate and sulfate in the formation of photoinduced SO4•-. Certain aromatic species may further accelerate the reaction through synergy. This reaction may impact our understanding of atmospheric sulfur reactions, aerosol properties, and organic aerosol lifetimes when inserted into aqueous chemistry model mechanisms.