Aerosol Species Research Articles

Secondary aerosol formation during a special dust transport event: impacts from unusually enhanced ozone and dust backflows over the ocean

Abstract. In the autumn of 2019, a 5 d long-lasting dust event was observed using a synergy of field measurement techniques in Shanghai. This particular dust event stood out from others due to its unique characteristics, including low wind speed, high relative humidity, elevated levels of gaseous precursors, and contrasting wind patterns at different altitudes. During this event, three distinct dust stages were identified. The first stage was a typical dust invasion characterized by high concentrations of particulate matters but relatively short duration. In contrast, the second stage exhibited an unusual enhancement of ozone, attributed to compound causes of a weak synoptic system, transport from the ocean, and subsidence of high-altitude ozone downdrafted by dust. Consequently, gas-phase oxidation served as the major formation pathway of sulfate and nitrate. In the third stage of dust, a noteworthy phenomenon known as dust backflow occurred. The dust plume originated from the Shandong Peninsula and slowly drifted over the Yellow Sea and the East China Sea before eventually returning to Shanghai. Evidence of this backflow was found through the enrichment of marine vessel emissions (V and Ni) and increased solubility of calcium. Under the influence of humid oceanic breezes, the formation of nitrate was dominated by aqueous processing. Additionally, parts of nitrate and sulfate were directly transported via sea salts, evidenced by their co-variation with Na+ and confirmed through thermodynamic modeling. The uptake of NH3 on particles, influenced by the contributions of alkali metal ions and aerosol pH, regulated the formation potential of secondary aerosol. By developing an upstream–receptor relationship method, the quantities of transported and secondarily formed aerosol species were separated. This study highlights that the transport pathway of dust, coupled with environmental conditions, can significantly modify the aerosol properties, especially at the complex land–sea interface.

Molecular compositions of marine organic aerosols over the Bohai and Yellow Seas: Influence of primary emission and secondary formation

To understand the influences of continental emissions on marine aerosols from East Asia, the total suspended particle (TSP) samples were collected from the Yellow Sea (YS) and Bohai Sea (BS) of China through a cruise campaign during the spring of 2018. >100 individual organic compounds were identified and determined in the samples, and they were classified into 6 major groups according to the functionality and sources. The mass concentrations of total quantified organic compounds in BS (307.6 ± 268.3 ng m−3) was approximately 3 times those of YS (111.0 ± 97.3 ng m−3). Organic compounds from biomass burning (23.2 ± 34.2 ng m−3), marine/microbial source (23.1 ± 6.2 ng m−3) and fossil fuel combustion (21.4 ± 11.3 ng m−3) were the most abundant in the YS samples, whereas primary biological origins (90.0 ± 107.1 ng m−3), biomass burning (52.1 ± 39.1 ng m−3) and fossil fuel combustion (41.9 ± 20.1 ng m−3) were the most important organic species in BS marine aerosols. The concentrations of total biogenic secondary organic aerosol (BSOA) tracers including six isoprene SOA (SOAI), three monoterpene SOA (SOAM), and one β-caryophyllene SOA (SOAC) were 2.4 ± 1.5 ng m−3 and 7.3 ± 4.4 ng m−3 in the YS and BS samples, respectively. A strong correlation between BSOA tracers and levoglucosan was shown in the marine aerosol samples, indicating that biomass burning emissions could also promote BSOA formations. The spatial distribution of chemical and source characteristics suggests a relatively large impacts of continental emissions to the BS aerosols. Based on the tracer-based method, the biomass burning (YS: 4.9 ± 6.8%; BS: 8.8 ± 8.0%) was the largest contributor to OC, followed by primary biogenic OC (YS: 0.59 ± 0.44%; BS: 2.9 ± 3.0%) and biogenic secondary OC (YS: 0.45 ± 0.23%; BS: 1.0 ± 0.59%). This study suggests important contributions of biomass burning emissions to organic aerosols in the YS and BS marine atmosphere, although marine source, fossil fuel combustion and secondary formation was also substantial.

Mixing state and effective density of aerosol particles during the Beijing 2022 Olympic Winter Games

Abstract. Mixing state and density are two key parameters of aerosol particles affecting their impacts on radiative forcing and human health. Here a single-particle aerosol mass spectrometer in tandem with a differential mobility analyzer and an aerodynamic aerosol classifier was deployed during the Beijing 2022 Olympic Winter Games (OWG) to investigate the impacts of emission controls on particle mixing state and density. Our results show the dominance of carbonaceous particles comprising mainly total elemental carbon (Total-EC, 13.4 %), total organic carbon (Total-OC, 10.5 %) and Total-ECOC (47.1 %). Particularly, the particles containing organic carbon and sulfate were enhanced significantly during OWG, although those from primary emissions decreased. The composition of carbonaceous particles also changed significantly which was characterized by the decreases in EC mixed with nitrate and sulfate (EC-NS), EC mixed with potassium nitrate (KEC-N), and amine-containing particles and increase in ECOC mixed with nitrate and sulfate (ECOC-NS). This result indicates that emission controls during OWG reduced the mixing of EC with inorganic aerosol species and amines yet increased the mixing of EC with organic aerosol. The average effective density (ρeff) of aerosol particles (150–300 nm) was 1.15 g cm−3 during the non-Olympic Winter Games (nOWG), with higher values during OWG (1.26 g cm−3) due to the increase in secondary particle contribution. In addition, the ρeff of most particles increased with the increases in pollution levels and relative humidity, yet they varied differently for different types of particles, highlighting the impacts of aging and formation processes on the changes of particle density and mixing state.

Significant Reductions in Secondary Aerosols after the Three-Year Action Plan in Beijing Summer.

Air quality in China has continuously improved during the Three-Year Action Plan (2018-2020); however, the changes in aerosol composition, properties, and sources in Beijing summer remain poorly understood. Here, we conducted real-time measurements of aerosol composition in five summers from 2018 to 2022 along with WRF-Community Multiscale Air Quality simulations to characterize the changes in aerosol chemistry and the roles of meteorology and emission reductions. Largely different from winter, secondary inorganic aerosol and photochemical-related secondary organic aerosol (SOA) showed significant decreases by 55-67% in summer, and the most decreases occurred in 2021. Comparatively, the decreases in the primary aerosol species and gaseous precursors were comparably small. While decreased atmospheric oxidation capacity as indicated by ozone changes played an important role in changing SOA composition, the large decrease in aerosol liquid water and small increase in particle acidity were critical for nitrate changes by decreasing gas-particle partitioning substantially (∼28%). Analysis of meteorological influences demonstrated clear and similar transitions in aerosol composition and formation mechanisms at a relative humidity of 50-60% in five summers. Model simulations revealed that emission controls played the decisive role in reducing sulfate, primary OA, and anthropogenic SOA during the Three-Year Action Plan, while meteorology affected more nitrate and biogenic SOA.

Roles of Regional Transport and Vertical Mixing in Aerosol Pollution in Shanghai Over the COVID‐19 Lockdown Period Observed Above Urban Canopy

AbstractRegional transport and vertical mixing are important for aerosol pollution of megacities, but their roles are often challenging to assess via ground observations. In this study, we measured aerosol chemical composition simultaneously above urban canopy (Shanghai Tower, 609 m), a site representative of aerosols from regional scale, and a nearby ground site and investigated the roles of regional transport and vertical mixing over 2020 COVID‐19 lockdown period (1 January to 18 April), when local emissions were first drastically reduced and then recovered. During the lockdown period, regional transport was the major source of most aerosol species (organics, NO3−, SO42−, and NH4+) at both heights according to the high correlation coefficients (R = 0.81–0.87) between both heights. Correlation coefficients and vertical ratios (609 m/ground) of most aerosol components showed similar diurnal variations with the evolution of planetary boundary layer height, indicating the role of vertical mixing in aerosol pollution. Moreover, the concentrations of aerosol components at 609 m generally preceded those at ground level by 1–2 hr, indicating that aerosols were first transported at upper boundary layer, and then were mixed downwards to ground level. At 609 m, highly oxidized oxygenated organic aerosol (OOA; a surrogate of secondary organic aerosol (SOA)) dominated in organic aerosol (≥75%). The high correlations (R = 0.96) between OOA and hydrocarbon‐like organic aerosol (HOA; a surrogate of primary organic aerosol (POA)) at 609 m indicated that they originated similarly from regional transport. This study highlights the importance of regional joint prevention and control of pollutant emissions and observation above urban canopy.

A Multiphase Sampling and Analytical Approach for Investigating Airborne PFAS Transmission

The authors evaluated a sampling and analytical system to measure differing modes of atmospheric per- and polyfluorinated substances (PFAS) transmission. The sampling was conducted with the FM4 sampling module, which features particulate collection followed by polyurethane foam sorption followed by activated carbon disc adsorption, a configuration designed to capture PFAS-laden particulate matter (one-micron to ten-microns), as well as aerosol, volatile, and ionic PFAS species in a single sampling event. Individual sampling media fractions were subsequently analyzed by gas chromatography coupled with tandem quadrupole mass spectrometry (GC–MS/MS) and liquid chromatography coupled with tandem quadrupole mass spectrometry (LC–MS/MS) for 33 ionic and 20 neutral PFAS species. A multi-day field sampling event conducted in an outdoor urban environment demonstrated that the system was able to capture and differentiate a number of airborne PFAS species. The study showed that simultaneous, multi-phase sample collection coupled with GC–MS/MS and LC–MS/MS analysis can be a useful approach to further elucidate the mode and manner of atmospheric PFAS transmission.

New Insights Into Scavenging Effect of Aerosol Species During Summer Rainfall Process in Beijing

AbstractPrecipitation is a critical factor in changing aerosol life cycle, yet its impact on aerosol species with different properties in megacities remains unclear. Here we characterized the changes of PM2.5 aerosol species during rainfall processes in five summers (2018–2022) in Beijing using highly time‐resolved measurements of aerosol chemical speciation monitor along with precipitation. Average pattern of 233 rainfall processes showed that over 30% decreases in aerosol species before rain start were due to the increased wind speed, and the subsequent decreases were caused by the combined effects of precipitation and winds with an average scavenging of 62%–100% in 1 hr. We also observed very different responses of aerosol species to precipitation depending on intensities, duration and formation mechanisms. During the rainfall processes, aerosol composition showed decreased contributions of organics and sulfate, particularly from late night to morning, while increased contributions of nitrate and chloride due to enhanced gas‐particle partitioning associated with the increase of relative humidity and the decrease of temperature. The scavenging rates of aerosol species significantly increased as the increase of rainfall intensity (>5 mm hr−1) and duration (>4 hr). However, the scavenging effect of light rainfall was negligible although the cumulative contribution was ∼50% due to high frequency, and even caused increases in nitrate and chloride. The case analysis of aerosol evolution during weak and heavy rainfall events further illustrated the dual impacts of precipitation on aerosol species through wet scavenging and secondary formation.

Aging of biomass burning emissions in the Indo-Gangetic Plain outflow: Implications for black carbon light-absorption enhancement

Recent studies worldwide have demonstrated that coating on atmospheric black carbon (BC) aerosol enhances its absorption potential. However, studies quantifying the light-absorption enhancement (Eabs) of BC are extremely limited over the Indian region. The northwestern Indo-Gangetic Plain (IGP) region witnesses annual episodes of substantial crop-residue burning during the post-monsoon season (PoMS), which has been attracting wide attention due to its impact on regional air quality and aerosol characteristics. Chemical aging of these biomass burning (BB) emissions occurs during its long-range transport across the IGP by the weak westerlies, and such atmospheric processing affects their chemical, optical, and radiative properties over downwind locations. This study examines the impact of atmospheric aging of biomass burning emissions on BC light-absorption enhancement (Eabs) using extensive, simultaneous measurements of single particle refractory BC and near-real-time mass spectrometry measurements of non-refractory submicron aerosols from an IGP outflow site. It was observed that organics were the dominant submicron aerosol species, and positive matrix factorization analysis of the organics mass spectra suggested that a large fraction of organics (64%) was secondary in nature. The BC particles in the IGP outflow (mass median diameter ∼ 0.18 ± 0.01 μm) were found to be thickly coated with volume-weighted bulk relative coating thickness in the range of 1.3–1.8. The evolution of biomass-burning organic aerosols (BBOA) indicated the presence of significant amounts of aged BBOA during the PoMS. Further, the oxidation products from the evolution of BB emissions contributed to enhanced coatings (∼ 30–70%) on BC. Mie core-shell theory calculations were then used to assess the changes in BC light absorption characteristics due to the coating of non-refractory mass on its cores. It was found that thick coatings on BC led to significant light-absorption enhancement (Eabs) (bulk Eabs in the range of 1.14–1.94 at 550 nm, depending on the extent of coating). Increased Eabs values of ≥1.8 were seen when the coating on BC exceeded 60% of its core size. The modeled bulk Eabs at 550 nm depicted a better association with highly oxidized secondary organic aerosols (SOA) than sulfate among the secondary aerosol species, suggesting an SOA-dominant regime. The present findings suggest that the critical impact of the aging of BC and co-emitted species from widespread BB emissions in the IGP outflow should be considered when evaluating the regional radiative implications.

Shortwave absorption by wildfire smoke dominated by dark brown carbon

Wildfires emit large amounts of black carbon and light-absorbing organic carbon, known as brown carbon, into the atmosphere. These particles perturb Earth’s radiation budget through absorption of incoming shortwave radiation. It is generally thought that brown carbon loses its absorptivity after emission in the atmosphere due to sunlight-driven photochemical bleaching. Consequently, the atmospheric warming effect exerted by brown carbon remains highly variable and poorly represented in climate models compared with that of the relatively nonreactive black carbon. Given that wildfires are predicted to increase globally in the coming decades, it is increasingly important to quantify these radiative impacts. Here we present measurements of ensemble-scale and particle-scale shortwave absorption in smoke plumes from wildfires in the western United States. We find that a type of dark brown carbon contributes three-quarters of the short visible light absorption and half of the long visible light absorption. This strongly absorbing organic aerosol species is water insoluble, resists daytime photobleaching and increases in absorptivity with night-time atmospheric processing. Our findings suggest that parameterizations of brown carbon in climate models need to be revised to improve the estimation of smoke aerosol radiative forcing and associated warming.

Large differences of highly oxygenated organic molecules (HOMs) and low-volatile species in secondary organic aerosols (SOAs) formed from ozonolysis of β-pinene and limonene

Abstract. Secondary organic aerosols (SOAs) play a key role in climate change and public health. However, the oxidation state and volatility of SOAs are still not well understood. Here, we investigated the highly oxygenated organic molecules (HOMs) in SOAs formed from ozonolysis of β-pinene and limonene. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used to characterize HOMs in aerosol filter samples, and a scanning mobility particle sizer (SMPS) was used to measure the concentration and size distribution of SOA particles. The relative abundance of HOMs (i.e., ratio of summed mass spectrometry peak intensity of HOMs to totally identified organic compounds) in limonene SOA was 14 %–20 %, higher than in β-pinene SOA (3 %–13 %), exhibiting different trends with increasing ozone concentrations. β-pinene oxidation-derived HOMs exhibit higher yield at high ozone concentration, accompanied by substantial formation of ultra-low-volatile organic compounds (ULVOCs). Limonene oxidation-derived HOMs exhibit higher yield at moderate ozone concentrations, with semi-, low-, and extremely low-volatile organic compounds (SVOCs, LOVCs, and ELVOCs) play a major role. Combined experimental evidence and theoretical analysis indicate that oxygen-increasing-based peroxy radical chemistry is a plausible mechanism for the formation of oxygenated organic compounds with 10 carbon atoms. Our findings show that HOMs and low-volatile species in β-pinene and limonene SOA are largely different. The ozone concentration-driven SOA formation and evolution mechanism for monoterpenes is suggested to be considered in future climate or exposure risk models, which may enable more accurate air quality prediction and management.

Does prognostic seeding along flight tracks produce the desired effects of cirrus cloud thinning?

Abstract. To date the climate intervention (CI) proposal of cirrus cloud thinning (CCT) was only assessed in general circulation models (GCMs) using a globally uniform distribution of artificial ice nucleating particles (INPs). In this study, we made the first attempt using the ECHAM–HAM (Hamburg Aerosol Module) GCM to simulate CCT using a fully prognostic cirrus seeding aerosol species. Seeding particles were assumed to be made of bismuth triiodide and were emitted into the atmosphere following aircraft emissions of black carbon (soot). This new approach drastically reduced the number concentration of seeding particles available as INPs in our cirrus ice nucleation sub-model compared to the globally uniform approach. As a result, we found that in order to achieve a significant signal we needed to reduce the assumed radius of emitted seeding particles by an order of magnitude to 0.01 µm and scale the mass emissions of seeding particles by at least a factor of 100 or 1000. This latter scaling factor led to a large net top-of-atmosphere (TOA) warming effect of 5.9 W m−2. This warming effect was a clear response to overseeding with a large concentration of seeding particles (>105 L−1 in the Northern Hemisphere) that was most evident in the tropics. Due to this undesired effect, in a second series of simulations we avoided seeding the tropics by restricting emissions to only the Northern Hemisphere (NH) during winter. We also found a small and insignificant effect, or overseeding, which for the extreme case was reduced compared to the global aircraft emission scenario (2.2 W m−2). Ice crystal radius anomalies were not what we expected, with the largest reduction in size found for the case with a mass scaling factor of 10 instead of the extreme, ×1000, scenario. We attributed this peculiar behavior to the differences in the competition between different seeding particle concentrations and background particles. Finally, we also found that seeding with such large concentrations increased the albedo effect of mixed-phase clouds in the NH due to less efficient cloud droplet consumption, consistent with previous findings from our model. Overall, however, based on this study it is recommended to pause further modeling efforts of CCT unless more observational-based evidence of aerosol–ice-cloud interactions indicates favorable conditions for producing the desired outcome of this CI proposal.

Changing patterns in the highly contributing aerosol types/species across the globe in the past two decades

With the rapidly changing aerosol emissions due to the increase in urbanization, energy consumption, population density, and industrialization in the past two decades across the globe, there is an evolution of different chemical properties of aerosols that are yet not quantified properly. Therefore, a rigorous attempt is made in this study to obtain the long-term changing patterns in the contribution of different aerosol types/species, to the total aerosol loading. This study is carried out only over those regions exhibiting either increasing or decreasing trends in the aerosol optical depth (AOD) parameter on a global scale. Applying the multivariate linear regression trend analysis on Modern-Era Retrospective Analysis for Research and Application version 2 (MERRA-2) aerosol species dataset obtained between 2001 and 2020, we found that despite the overall statistically significant decrease in total columnar AOD trend values over North-Eastern America, and Eastern and Central China regions, an increase in the dust and organic carbon aerosols is observed, respectively. As the uneven vertical distribution of aerosols can alter the direct radiative effects, the extinction profiles of different aerosol types obtained using Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) dataset between 2006 and 2020, are further partitioned, for the first time, based on their presence in different altitudes (i.e., within the atmospheric boundary layer and free-troposphere) as well as measurement timing (i.e., daytime and night-time) regimes. The detailed analysis showed that there exists an overall higher contribution of aerosols persisting in the free troposphere region which in turn can have a long-term effect on climate due to their higher residence time, particularly absorbing aerosols. As the trends are mostly associated with the changes in energy use, regional regulatory policies, and/or changing background meteorology conditions, therefore this study also elaborates on the effectiveness of these factors with the changes obtained in different aerosol species/types over the region.

The classification of atmospheric hydrometeors and aerosols from the EarthCARE radar and lidar: the A-TC, C-TC and AC-TC products

Abstract. The EarthCARE mission aims to probe the Earth's atmosphere by measuring cloud and aerosol profiles using its active instruments, the Cloud Profiling Radar (CPR) and ATmospheric LIDar (ATLID). The correct identification of hydrometeors and aerosols from atmospheric profiles is an important step in retrieving the properties of clouds, aerosols and precipitation. Ambiguities in the nature of atmospheric targets can be removed using the synergy of collocated radar and lidar measurements, which is based on the complementary spectral response of radar and lidar relative to atmospheric targets present in the profiles. The instruments are sensitive to different parts of the particle size distribution and provide independent but overlapping information in optical and microwave wavelengths. ATLID is sensitive to aerosols and small cloud particles, and CPR is sensitive to large ice particles, snowflakes and raindrops. It is therefore possible to better classify atmospheric targets when collocated radar and lidar measurements exist compared to using a single instrument. The cloud phase, precipitation and aerosol type within the column sampled by the two instruments can then be identified. ATLID-CPR target classification (AC-TC) is the product created for this purpose by combining the ATLID target classification (A-TC) and CPR target classification (C-TC). AC-TC is crucial for the subsequent synergistic retrieval of cloud, aerosol and precipitation properties. AC-TC builds upon previous target classifications using CloudSat and CALIPSO synergy while providing richer target classification using the enhanced capabilities of EarthCARE's instruments, specifically CPR's Doppler velocity measurements to distinguish snow and rimed snow from ice clouds and ATLID's lidar ratio measurements to objectively discriminate between different aerosol species and optically thin ice clouds. In this paper, we first describe how the single-instrument A-TC and C-TC products are derived from ATLID and CPR measurements. Then the AC-TC product, which combines the A-TC and C-TC classifications using a synergistic decision matrix, is presented. Simulated EarthCARE observations based on combined cloud-resolving and aerosol model data are used to test the processors generating the target classifications. Finally, the target classifications are evaluated by quantifying the fractions of ice and snow, liquid clouds, rain, and aerosols in the atmosphere that can be successfully identified by each instrument and their synergy. We show that radar–lidar synergy helps better detect ice and snow, with ATLID detecting radiatively important optically thin cirrus and cloud tops, while CPR penetrates most deep and highly concentrated ice clouds. The detection of rain and drizzle is entirely due to C-TC, while that of liquid clouds and aerosols is due to A-TC. The evaluation also shows that simple assumptions can be made to compensate for when the instruments are obscured by extinction (ATLID) or surface clutter and multiple scattering (CPR); this allows for the recovery of the majority of liquid cloud not detected by the active instruments.

Highly Time-Resolved Apportionment of Carbonaceous Aerosols from Wildfire Using the TC-BC Method: Camp Fire 2018 Case Study.

The Camp Fire was one of California's deadliest and most destructive wildfires, and its widespread smoke threatened human health over a large area in Northern California in November 2018. To analyze the Camp Fire influence on air quality on a 200 km distant site in Berkeley, highly time-resolved total carbon (TC), black carbon (BC), and organic carbon (OC) were measured using the Carbonaceous Aerosol Speciation System (CASS, Aerosol Magee Scientific), comprising two instruments, a Total Carbon Analyzer TCA08 in tandem with an Aethalometer AE33. During the period when the air quality was affected by wildfire smoke, the BC concentrations increased four times above the typical air pollution level presented in Berkeley before and after the event, and the OC increased approximately ten times. High-time-resolution measurements allow us to study the aging of OC and investigate how the characteristics of carbonaceous aerosols evolve over the course of the fire event. A higher fraction of secondary carbonaceous aerosols was observed in the later phase of the fire. At the same time, the amount of light-absorbing organic aerosol (brown carbon) declined with time.

Modeling of Organic Aerosol in Seoul Using CMAQ with AERO7

The Community Multiscale Air Quality (CMAQ) model with the 7th generation aerosol module (AERO7) was employed to simulate organic aerosol (OA) in Seoul, Korea, for the year 2016. The goal of the present study includes the 1-year simulation of OA using WRF-CMAQ with recently EPA-developed AERO7 with pcVOC (potential VOC from combustion) scale factor revision and analysis of the seasonal behavior of OA surrogate species in Seoul. The AERO7, the most recent version of the aerosol module of the CMAQ model, includes a new secondary organic aerosol (SOA) species, pcSOA (potential SOA from combustion), to resolve the inherent under-prediction problem of OA. The AERO7 classified OA into three groups: primary organic aerosol (POA), anthropogenic SOA (ASOA), and biogenic SOA (BSOA). Each OA group was further classified into 6~15 individual OA surrogate species according to volatility and oxygen content to model the aging of OA and the formation of SOA. The hourly emissions of POA and SOA precursors were compiled and fed into the CMAQ to successfully simulate seasonal variations of OA compositions and ambient organic-matter to organic-carbon ratios (OM/OC). The model simulation showed that the POA and ASOA were major organic groups in the cool months (from November to March) while BSOA was a major organic group in the warm months (from April to October) in Seoul. The simulated OM/OCs ranged from 1.5~2.1 in Seoul, which agreed well with AMS measurements in Seoul in May 2016.

Solubility Considerations for Cloud Condensation Nuclei (CCN) Activity Analysis of Pure and Mixed Black Carbon Species

Black carbon (BC) is an aerosol that is released into the atmosphere due to the incomplete burning of biomass and can affect the climate directly or indirectly. BC commonly mixes with other primary or secondary aerosols to undergo aging, thereby changing its radiative properties and cloud condensation nuclei (CCN) activity. The composition of aged BC species in the atmosphere is difficult to measure with high confidence, so their associated CCN activity can be uncertain. In this work, the CCN activity analysis of BC was performed using laboratory measurements of proxy aged BC species. Vulcan XC72R carbon black was used as the representative of BC, and three structural isomers of benzenedicarboxylic acid─phthalic acid (PTA), isophthalic acid (IPTA), and terephthalic acid (TPTA)─were mixed with BC to generate three different proxies of aged BC species. Most studies related to CCN activity analysis of BC aerosols use the traditional Köhler theory or an adsorption theory (such as the Frenkel-Halsey-Hill adsorption theory). PTA, IPTA, and TPTA fall in the sparingly water-soluble range and therefore do not fully obey either of the aforementioned theories. Consequently, a novel hybrid activity model (HAM) was used for the CCN activity analysis of the BC mixtures studied in this work. HAM combines the features of adsorption theory via the adsorption isotherm with the features of Köhler theory by incorporating solubility partitioning. The results in this work showed that HAM improves the representation of CCN activity of pure and mixed BC aerosol species with high certainty, evident from generally better goodness of fit, R2 > 0.9. This work implies that the hygroscopicity parameterization based on HAM captures the size-dependent variability in the CCN activity of the pure and aged BC species.

Comparison of the simulated outcomes of aerosol–cloud interaction by a meteorological model with and without an interactive chemistry module

With the general rise of computational capacities and the continued efforts to improve computational efficiencies, more and more studies have been utilizing state-of-the-art atmospheric models that enable cloud-resolving simulations over a global domain. Microphysical processes inside clouds, however, are on a scale much smaller than that of a cloud itself, and therefore resolving clouds in a model is not equivalent to resolving cloud microphysical processes. When aerosol–cloud interaction (ACI) is studied, chemistry models enable the prognostic calculations for chemical species, including aerosols, which can perturb cloud microphysics and eventually impact clouds and climate. The large drawback of these models is the high computational cost required for tracking chemical species in space and time that may not be affordable in some studies. As a result, some studies have used non-chemistry models with prescribed cloud droplet number concentrations n_c and compared multiple simulations with different n_c to assess the impacts of varying aerosol concentrations on clouds. In this study we assess whether the same or similar ACI can be simulated when the aerosol number is increased in a chemistry model and when n_c is raised in a non-chemistry model. A case study has been conducted over the Maritime Continent in September 2015 when an extremely large number of aerosols were observed due to fires under a dry condition brought by a strong El Niño. The comparison of the simulations by the chemistry and non-chemistry models shows that the aerosol-driven enhancement of rainfall in the chemistry simulations was not present in the non-chemistry simulations, even with prescribed n_c with certain spatial variation based on the chemistry runs. Therefore, simulated ACI may largely differ depending on the manner by which an increase or decrease of aerosols is represented in a model. The result suggests the further need for powerful computational capabilities and the pursuit of a rigorous means to incorporate aerosol species in a non-chemistry model.

Seasonal evolution of aerosol loading and its vertical distribution in northeastern China from long-term satellite observations and model reanalysis

The vertical structure of aerosols plays an important role in regional air pollution and climate change. Using long-term data on aerosol optical properties obtained through satellite observations (MODIS/Terra, MODIS/Aqua, CALIOP) and reanalysis products (CAMS) during 2007–2021, we analysed the seasonal evolution and vertical distribution of aerosol loading in northeastern China (NEC). We also analysed the seasonal variation trends of aerosol optical depth (AOD) of aerosol species. The AODs during spring and summer were high and distributed in Liaoning (LN) with an average value of approximately 0.6, which was higher in central Jilin and southern Heilongjiang. Sulfate AOD (AOD_SO4) and organic matter AOD (AOD_OM) in NEC were higher during spring and summer (approximately 0.2), and the maximum value of dust AOD (AOD_DU) was approximately 0.1 obtained for western LN. During spring, the aerosol extinction coefficient (AEC) for NEC was high at heights below 2.0 km in LN, and its value was maximum (approximately 0.16 km−1) near the ground in LN during winter. The AEC was concentrated within 0–1 km during winter and higher within 1–3 km during spring and summer in NEC. During 2007–2021, AOD exhibited a decreasing trend, with higher average AOD during spring and summer and lower average AOD during autumn and winter. The seasonal trend of AOD in NEC exhibited a decreasing trend. BC_AOD showed the largest decline in spring, followed by DU_AOD and OM_AOD, and SO4_AOD showed a significant decline in summer. This paper presents accurate information on regional types and vertical distribution of aerosols. This information can be used for model improvement and satellite validation.

Insights into the compositional differences of PM1 and PM2.5 from aerosol mass spectrometer measurements in Beijing, China

Aerosol mass spectrometers (AMS) have been widely used for characterization of submicron aerosol (PM1) species in the world. However, the chemical differences between PM1 and fine particles (PM2.5) in megacities are poorly understood. Here a PM1 high-resolution AMS with standard vaporizer (SV) and a PM2.5 time-of-flight aerosol chemical speciation monitor (ToF-ACSM) with capture vaporizer (CV) were deployed simultaneously in summer and winter in Beijing, China, and the chemical differences of non-refractory aerosol species between PM1 and PM2.5 were characterized. Our results showed that aerosol species in PM2.5 were overall highly correlated with those in PM1 (r2 > 0.8), yet the PM1/PM2.5 ratios varied largely for different aerosol species. Nitrate existed dominantly in PM1 (80–90%), while a considerable fraction of sulfate (30–45%) was observed in the range of 1–2.5 μm (PM1–2.5). Comparatively, PM1 organics showed higher fractions in PM2.5 in summer than winter (∼70%). The changes in PM1/PM2.5 ratios of secondary inorganic aerosol (SIA) depended strongly on relative humidity (RH) in summer with considerable decreases as the increase of RH, while such a RH dependence of PM1/PM2.5 ratios was not significant in winter due to the absence of high RH data. We further compared the source apportionment results of organic aerosol (OA) between PM1 and PM2.5. Similar to previous studies, the SV AMS reported ubiquitously higher primary OA than the CV ToF-ACSM, nearly by a factor of 2 for cooking OA, and the difference in secondary OA composition can be substantial, particularly in winter. With the simultaneous measurements of AMS and ACSM, a parameterized relationship between oxygen-to-carbon (O/C) ratio and CV f44 (fraction of m/z 44 in OA) in summer and winter was developed, which has a potential implication for studying the oxidation and hygroscopicity properties of OA considering the rapid increases in CV ToF-ACSM measurements worldwide.

Analysis of amino acid enantiomers in ambient aerosols: Effects and removal of coexistent aerosol matrix

Amino acids (AAs) including D- and L- enantiomers are a group of organic nitrogen species in ambient aerosol. Due to the low abundances of AAs (level of ng/m3) and the matrix effects by coexistent components, it is challenging to quantify AA enantiomers in ambient aerosols especially under pollution conditions. In this study, we present an optimized method for analyzing AA enantiomers in atmospheric aerosol samples including a pretreatment process and the detection by high performance liquid chromatography coupled to a fluorescence detector (HPLC-FLD). Matrix effects caused by coexistent chemicals on AA enantiomers analysis in ambient aerosol samples were investigated especially for those collected in pollution episodes. The results revealed that the determination of AA enantiomers is significantly affected by the coexistent organic carbon (as a proxy of organic matter) and water-soluble ion of NH4+. To remove the matrix effects, we applied a pretreatment using the solid phase extraction column coupled with alkaline adjustment to sample extract. After pretreatment, 18 AAs including 6 pairs of D- and L-enantiomers (i.e., leucine, isoleucine, valine, alanine, serine, and aspartic acid) can be successfully separated and quantified in aerosol samples by HPLC-FLD. The recoveries are in the range of 67%-106%. This method was successfully applied to the urban aerosol samples from pollution and non-pollution periods for AA enantiomers determination. We suggest that the concentrations of D-AAs and the ratio of D-AA/L-AA are indicative of the contribution of bacterial sources and the influence of biomass burning.