Evaporation-Induced Transformations in Volatile Chemical Product-Derived Secondary Organic Aerosols: Browning Effects and Alterations in Oxidative Reactivity.

Volatile chemical products (VCPs) are increasingly recognized as significant sources of volatile organic compounds (VOCs) in urban atmospheres, potentially serving as key precursors for secondary organic aerosol (SOA) formation. This study investigates the formation and physicochemical transformations of VCP-derived SOA, produced through ozonolysis of VOCs evaporated from a representative room deodorant air freshener, focusing on the effects of aerosol evaporation on its molecular composition, light absorption properties, and reactive oxygen species (ROS) generation. Following aerosol evaporation, solutes become concentrated, accelerating reactions within the aerosol matrix that lead to a 42% reduction in peroxide content and noticeable browning of the SOA. This process occurs most effectively at moderate relative humidity (∼40%), reaching a maximum solute concentration before aerosol solidification. Molecular characterization reveals that evaporating VCP-derived SOA produces highly conjugated nitrogen-containing products from interactions between existing or transformed carbonyl compounds and reduced nitrogen species, likely acting as chromophores responsible for the observed brownish coloration. Additionally, the reactivity of VCP-derived SOA was elucidated through heterogeneous oxidation of sulfur dioxide (SO2), which revealed enhanced photosensitized sulfate production upon drying. Direct measurements of ROS, including singlet oxygen (1O2), superoxide (O2•-), and hydroxyl radicals (•OH), showed higher abundances in dried versus undried SOA samples under light exposure. Our findings underscore that drying significantly alters the physicochemical properties of VCP-derived SOA, impacting their roles in atmospheric chemistry and radiative balance.

Aqueous-Phase Photoreactions of Mixed Aromatic Carbonyl Photosensitizers Yield More Oxygenated, Oxidized, and less Light-Absorbing Secondary Organic Aerosol (SOA) than Single Systems.

Aromatic carbonyls have been mainly probed as photosensitizers for aqueous secondary organic aerosol (aqSOA) and light-absorbing organic aerosol (i.e., brown carbon or BrC) formation, but due to their organic nature, they can also undergo oxidation to form aqSOA and BrC. However, photochemical transformations of aromatic carbonyl photosensitizers, particularly in multicomponent systems, are understudied. This study explored aqSOA formation from the irradiation of aromatic carbonyl photosensitizers in mixed and single systems under cloud/fog conditions. Mixed systems consisting of phenolic carbonyls only (VL + ActSyr + SyrAld: vanillin [VL] + acetosyringone [ActSyr] + syringaldehyde [SyrAld]) and another composed of both nonphenolic and phenolic carbonyls (DMB + ActSyr + SyrAld: 3,4-dimethoxybenzaldehyde [DMB], a nonphenolic carbonyl, + ActSyr + SyrAld) were compared to single systems of VL (VL*) and DMB (DMB*), respectively. In mixed systems, the shorter lifetimes of VL and DMB indicate their diminished capacity to trigger the oxidation of other organic compounds (e.g., guaiacol [GUA], a noncarbonyl phenol). In contrast to the slow decay and minimal photoenhancement for DMB*, the rapid photodegradation and significant photoenhancement for VL* indicate efficient direct photosensitized oxidation (i.e., self-photosensitization). Relative to single systems, the increased oxidant availability promoted functionalization in VL + ActSyr + SyrAld and accelerated the conversion of early generation aqSOA in DMB + ActSyr + SyrAld. Moreover, the increased availability of oxidizable substrates countered by stronger oxidative capacity limited the contribution of mixed systems to aqSOA light absorption. This suggests a weaker radiative effect of BrC from mixed photosensitizer systems than BrC from single photosensitizer systems. Furthermore, more oxygenated and oxidized aqSOA was observed with increasing complexity of the reaction systems (e.g., VL* < VL + ActSyr + SyrAld < VL + ActSyr + SyrAld + GUA). This work offers new insights into aqSOA formation by emphasizing the dual role of organic photosensitizers as oxidant sources and oxidizable substrates.

Fostering a Holistic Understanding of the Full Volatility Spectrum of Organic Compounds from Benzene Series Precursors through Mechanistic Modeling.

A comprehensive understanding of the full volatility spectrum of organic oxidation products from the benzene series precursors is important to quantify the air quality and climate effects of secondary organic aerosol (SOA) and new particle formation (NPF). However, current models fail to capture the full volatility spectrum due to the absence of important reaction pathways. Here, we develop a novel unified model framework, the integrated two-dimensional volatility basis set (I2D-VBS), to simulate the full volatility spectrum of products from benzene series precursors by simultaneously representing first-generational oxidation, multigenerational aging, autoxidation, dimerization, nitrate formation, etc. The model successfully reproduces the volatility and O/C distributions of oxygenated organic molecules (OOMs) as well as the concentrations and the O/C of SOA over wide-ranging experimental conditions. In typical urban environments, autoxidation and multigenerational oxidation are the two main pathways for the formation of OOMs and SOA with similar contributions, but autoxidation contributes more to low-volatility products. NOx can reduce about two-thirds of OOMs and SOA, and most of the extremely low-volatility products compared to clean conditions, by suppressing dimerization and autoxidation. The I2D-VBS facilitates a holistic understanding of full volatility product formation, which helps fill the large gap in the predictions of organic NPF, particle growth, and SOA formation.

Sulfate Formation by Photosensitization in Mixed Incense Burning-Sodium Chloride Particles: Effects of RH, Light Intensity, and Aerosol Aging.

Elevated particulate sulfate concentrations have been frequently observed in coastal areas when air masses are influenced by continental emissions, especially combustion sources like biomass burning. We studied the SO2 uptake by laboratory-generated droplets containing incense smoke extracts and sodium chloride (IS-NaCl) under irradiation and found enhanced sulfate production over pure NaCl droplets, attributable to photosensitization induced by constituents in IS. Low relative humidity and high light intensity facilitated sulfate formation and increased the SO2 uptake coefficient by IS-NaCl particles. Aging of the IS particles further enhanced sulfate production, attributable to the enhanced secondary oxidant production promoted by increased proportions of nitrogen-containing CHN and oxygen- and nitrogen-containing CHON species under light and air. Experiments using model compounds of syringaldehyde, pyrazine, and 4-nitroguaiacol verified the enhancements of CHN and CHON species in sulfate formation. This work provides experimental evidence of enhanced sulfate production in laboratory-generated IS-NaCl droplets via enhanced secondary oxidant production triggered by photosensitization in multiphase oxidation processes under light and air. Our results can shed light on the possible interactions between sea salt and biomass burning aerosols in enhancing sulfate production.

Concluding remarks: Faraday Discussion on air quality in megacities.

Megacities are metropolitan areas with populations over 10 million, and many of them are facing significant global environmental challenges such as air pollution. Intense economic and human activities in megacities result in air pollution emissions, inducing high levels of air pollutants in the atmosphere that harm human health, cause regional haze and acid deposition, damage crops, influence regional air quality, and contribute to climate change. Since the Great London Smog and the first recognized episode of Los Angeles photochemical smog seventy years ago, substantial progress has been achieved in improving the scientific understanding of air pollution and in developing emissions reduction technologies and control measures. However, much remains to be understood about the complex processes of atmospheric transport and reaction mechanisms; the formation and evolution of secondary particles, especially those containing organic species; and the influence of emerging emissions sources and changing climate on air quality and health. Molina (DOI: ) has provided an excellent overview of the sources of emissions in megacities, atmospheric physicochemical processes, air quality trends and management, and the impacts on health and climate for the introductory lecture of this Faraday Discussion.

Production of Formate via Oxidation of Glyoxal Promoted by Particulate Nitrate Photolysis.

Particulate nitrate photolysis can produce oxidants (i.e., OH, NO2, and NO2-/HNO2) in aqueous droplets and may play a potential role in increased atmospheric oxidative capacity. Our earlier works have reported on the SO2 oxidation promoted by nitrate photolysis to produce sulfate. Here, we used glyoxal as a model precursor to examine the role of particulate nitrate photolysis in the formation of secondary organic aerosol (SOA) from particle-phase oxidation of glyoxal by OH radicals. Particles containing sodium nitrate and glyoxal were irradiated at 300 nm. Interestingly, typical oxidation products of oxalic acid, glyoxylic acid, and higher-molecular-weight products reported in the literature were not found in the photooxidation process of glyoxal during nitrate photolysis in the particle phase. Instead, formic acid/formate production was found as the main oxidation product. At glyoxal concentration higher than 3 M, we found that the formic acid/formate production rate increases significantly with increasing glyoxal concentration. Such results suggest that oxidation of glyoxal at high concentrations by OH radicals produced from nitrate photolysis in aqueous particles may not contribute significantly to SOA formation since formic acid is a volatile species. Furthermore, recent predictions of formic acid/formate concentration from the most advanced chemical models are lower than ambient observations at both the ground level and high altitude. The present study reveals a new insight into the production of formic acid/formate as well as a sink of glyoxal in the atmosphere, which may partially narrow the gap between model predictions and field measurements in both species.

Emissions and Secondary Formation of Air Pollutants from Modern Heavy-Duty Trucks in Real-World Traffic-Chemical Characteristics Using On-Line Mass Spectrometry.

Complying with stricter emissions standards, a new generation of heavy-duty trucks (HDTs) has gradually increased its market share and now accounts for a large percentage of on-road mileage. The potential to improve air quality depends on an actual reduction in both emissions and subsequent formation of secondary pollutants. In this study, the emissions in real-world traffic from Euro VI-compliant HDTs were compared to those from older classes, represented by Euro V, using high-resolution time-of-flight chemical ionization mass spectrometry. Gas-phase primary emissions of several hundred species were observed for 70 HDTs. Furthermore, the particle phase and secondary pollutant formation (gas and particle phase) were evaluated for a number of HDTs. The reduction in primary emission factors (EFs) was evident (∼90%) and in line with a reduction of 28-97% for the typical regulated pollutants. Secondary production of most gas- and particle-phase compounds, for example, nitric acid, organic acids, and carbonyls, after photochemical aging in an oxidation flow reactor exceeded the primary emissions (EFAged/EFFresh ratio ≥2). Byproducts from urea-selective catalytic reduction systems had both primary and secondary sources. A non-negative matrix factorization analysis highlighted the issue of vehicle maintenance as a remaining concern. However, the adoption of Euro VI has a significant positive effect on emissions in real-world traffic and should be considered in, for example, urban air quality assessments.

Enhanced Nitrite Production from the Aqueous Photolysis of Nitrate in the Presence of Vanillic Acid and Implications for the Roles of Light-Absorbing Organics.

A prominent source of hydroxyl radicals (•OH), nitrous acid (HONO) plays a key role in tropospheric chemistry. Apart from direct emission, HONO (or its conjugate base nitrite, NO2-) can be formed secondarily in the atmosphere. Yet, how secondary HONO forms requires elucidation, especially for heterogeneous processes involving numerous organic compounds in atmospheric aerosols. We investigated nitrite production from aqueous photolysis of nitrate for a range of conditions (pH, organic compound, nitrate concentration, and cation). Upon adding small oxygenates such as ethanol, n-butanol, or formate as •OH scavengers, the average intrinsic quantum yield of nitrite [Φ(NO2-)] was 0.75 ± 0.15%. With near-UV-light-absorbing vanillic acid (VA), however, the effective Φ(NO2-) was strongly pH-dependent, reaching 8.0 ± 2.1% at a pH of 8 and 1.5 ± 0.39% at a more atmospherically relevant pH of 5. Our results suggest that brown carbon (BrC) may greatly enhance the nitrite production from the aqueous nitrate photolysis through photosensitizing reactions, where the triplet excited state of BrC may generate solvated electrons, which reduce nitrate to NO2 for further conversion to nitrite. This photosensitization process by BrC chromophores during nitrate photolysis under mildly acidic conditions may partly explain the missing HONO in urban environments.

Nitrate Photolysis in Mixed Sucrose-Nitrate-Sulfate Particles at Different Relative Humidities.

Atmospheric particles can be viscous. The limitation in diffusion impedes the mass transfer of oxidants from the gas phase to the particle phase and hinders multiphase oxidation processes. On the other hand, nitrate photolysis has been found to be effective in producing oxidants such as OH radicals within the particles. Whether nitrate photolysis can effectively proceed in viscous particles and how it may affect the physicochemical properties of the particle have not been much explored. In this study, we investigated particulate nitrate photolysis in mixed sucrose-nitrate-sulfate particles as surrogates of atmospheric viscous particles containing organic and inorganic components as a function of relative humidity (RH) and the molar fraction of sucrose to the total solute (FSU) with an in situ micro-Raman system. Sucrose suppressed nitrate crystallization, and high photolysis rate constants (∼10-5 s-1) were found, irrespective of the RH. For FSU = 0.5 and 0.33 particles under irradiation at 30% RH, we observed morphological changes from droplets to the formation of inclusions and then likely "hollow" semisolid particles, which did not show Raman signal at central locations. Together with the phase states of inorganics indicated by the full width at half-maxima (FWHM), images with bulged surfaces, and size increase of the particles in optical microscopic imaging, we inferred that the hindered diffusion of gaseous products (i.e., NOx, NOy) from nitrate photolysis is a likely reason for the morphological changes. Atmospheric implications of these results are also presented.

Multiphase Photochemistry of Iron-Chloride Containing Particles as a Source of Aqueous Chlorine Radicals and Its Effect on Sulfate Production.

Photolysis of iron chlorides is a well-known photolytic source of Cl• in environmental waters. However, the role of particulate chlorine radicals (Cl• and Cl2•-) in their multiphase oxidative potential has been much less explored. Herein, we examine the effect of Cl•/Cl2•- produced from photolysis of particulate iron chlorides on atmospheric multiphase oxidation. As a model system, experiments on multiphase oxidation of SO2 by Cl•/Cl2•- were performed. Fast sulfate production from SO2 oxidation was observed with reactive uptake coefficients of ∼10-5, comparable to the values necessary for explaining the observations in the haze events in China. The experimental and modeling results found a good positive correlation between the uptake coefficient, γSO2, and the Cl• production rate, d[Cl•]/dt, as γSO2 = 5.3 × 10-6 × log(d[Cl•]/dt) + 4.9 × 10-5. When commonly found particulate dicarboxylic acids (oxalic acid or malonic acid) were added, sulfate production was delayed due to the competition of Fe3+ between chloride and the dicarboxylic acid for its complexation at the initial stage. After the delay, comparable sulfate production was observed. The present study highlights the importance of photochemistry of particulate iron chlorides in multiphase oxidation processes in the atmosphere.

Enhanced Sulfate Production by Nitrate Photolysis in the Presence of Halide Ions in Atmospheric Particles.

Heterogeneous oxidation of SO2 is an effective production pathway of sulfate in the atmosphere. We recently reported a novel pathway for the heterogeneous oxidation of SO2 by in-particle oxidants (OH, NO2, and NO2-/HNO2) produced from particulate nitrate photolysis (Environ. Sci. Technol. 2019, 53, 8757-8766). Particulate nitrate is often found to coexist with chloride and other halide ions, especially in aged sea-salt aerosols and combustion aerosols. Reactive uptake experiments of SO2 with UV-irradiated nitrate particles showed that sulfate production rates were enhanced by a factor of 1.4, 1.3, and 2.0 in the presence of Cl-, Br-, and I-, respectively, compared to those in the absence of halide ions. The larger sulfate production was attributed to enhanced nitrate photolysis promoted by the increased incomplete solvation of nitrate at the air-particle interface due to the presence of surface-active halide ions. Modeling results based on the experimental data showed that the nitrate photolysis rate constants increased by a factor of 2.0, 1.7, and 3.7 in the presence of Cl-, Br-, and I-, respectively. A linear relation was found between the nitrate photolysis rate constant, jNO3-, and the initial molar ratio of Cl- to NO3-, [Cl-]0/[NO3-]0, as jNO3- = 9.7 × 10-5[Cl-]0/[NO3-]0 + 1.9 × 10-5 at [Cl-]0/[NO3-]0 below 0.2. The present study demonstrates that the presence of halide ions enhances sulfate production produced during particulate nitrate photolysis and provides insights into the enhanced formation of in-particle oxidants that may increase atmospheric oxidative capacity.

Relative Humidity History Affects Hygroscopicity of Mixed Particles of Glyoxal and Reduced Nitrogenous Species.

The relative humidity (RH) history that manifests the cycling of dehydration (water evaporation) and hydration (water uptake) may affect particle-phase reactions, products from which have strong influences on the physical properties and thus climatic effects of atmospheric particles. Using single-trapped particles, we show herein hygroscopic growths of mixed particles with reactive species undergoing three types of RH cycles, simulating different degrees of particle-phase reactions in the atmosphere. The reactive species are the widely known α-dicarbonyl glyoxal (GLY), and five reduced nitrogenous species, ammonium sulfate (AS), glycine (GC), l-alanine (AL), dimethylamine (DMA), and diethylamine (DEA). The results showed that the mixed particles after reactions generally had altered efflorescence relative humidity (ERH) and deliquescence relative humidity (DRH) values and reduced hygroscopic growths at moderately high RH (>80%) conditions. For example, with an additional slow drying step, the mean mass growth factors at 90% RH during dehydration dropped from 2.56 to 2.02 for GC/GLY mixed particles and from 2.45 to 1.23 for AL/GLY mixed particles. The reduced hygroscopicity with more RH cycling will thus lead to less efficient light scattering of the mixed particles, thereby resulting in less cooling and exacerbating direct heating due to light absorption by the products formed.

Heterogeneous Oxidation of SO2 in Sulfate Production during Nitrate Photolysis at 300 nm: Effect of pH, Relative Humidity, Irradiation Intensity, and the Presence of Organic Compounds.

Heterogeneous oxidation of SO2 is one of the promising mechanisms to account for high loading of sulfate during severe haze periods in China. Our earlier work reported on the SO2 oxidation by OH and NO2 produced during 250 nm nitrate photolysis (Environ. Sci. Technol. Lett. 2019, 6, 86-91). Here, we extend that work to examine sulfate production during nitrate photolysis at 300 nm irradiation, which can additionally generate NO2- or HNO2, N(III). Flow cell/in situ Raman experiments showed that the reactive uptake coefficient of SO2, γSO2, can be expressed as γSO2 = 1.64 × pNO3-, where pNO3- is the nitrate photolysis rate in the range of (1.0-8.0) × 10-5 M s-1. Our kinetic model with the pNO3- predicts that N(III) is the main contributor to the SO2 oxidation, followed by NO2 contribution. Furthermore, the addition of OH scavengers (e.g., glyoxal or oxalic acid) does not suppress the sulfate production because of the reduced N(III)-consuming reactions and the high particle pH sustained by their presence. Our calculations illustrate that under characteristic haze conditions, the nitrate photolysis mechanism can produce sulfate at ∼1 µg m-3 h-1 at pH 4-6 and pNO3- = 10-5 M s-1. The present study highlights the importance of in-particle nitrate photolysis in heterogeneous oxidation of SO2 by reactive nitrogen (NO2-/HNO2 and NO2) under atmospherically relevant actinic irradiation. However, the nitrate photolysis rate constant needs to be better constrained for ambient aerosols.

Secondary Organic Aerosol Formation from Urban Roadside Air in Hong Kong.

Motor vehicle emissions are an important but poorly constrained source of secondary organic aerosol (SOA). Here, we investigated in situ SOA formation from urban roadside air in Hong Kong during winter time using an oxidation flow reactor (OFR), with equivalent atmospheric oxidation ranging from several hours to several days. The campaign-average mass enhancement of OA, nitrate, sulfate, and ammonium upon OFR aging was 7.0, 7.2, 0.8, and 2.6 µg m-3, respectively. To investigate the sources of SOA formation potential, we performed multilinear regression analysis between measured peak SOA concentrations from OFR and the concentrations of toluene that represent motor vehicle emissions and cooking OA from positive matrix factorization (PMF) analysis of ambient OA. Traffic-related SOA precursors contributed 92.3%, 92.4%, and 83.1% to the total SOA formation potential during morning rush hours, noon and early afternoon, and evening meal time, respectively. The SOA production factor (PF) was approximately 5.2 times of primary OA (POA) emission factor (EF) and the secondary particulate matter (PM) PF was approximately 2.6 times of primary particles EF. This study highlights the potential benefit of reducing secondary PM production from motor vehicle emissions in mitigating PM pollutions.

Real time analysis of lead-containing atmospheric particles in Guangzhou during wintertime using single particle aerosol mass spectrometry.

The toxic effects of lead on human health and the environment have long been a focus of research. To explore sources of lead in Guangzhou, China, we investigated atmospheric lead-containing particles (LCPs) during wintertime using a single particle aerosol mass spectrometer (SPAMS). Based on mass spectral features, LCPs were classified into eight major particle types, including Pb-Cl and Pb-Cl-Li (coal combustion and waste incineration), Pb-Cl-EC and Pb-Cl-OC (diesel trucks and coal combustion), Pb-Cl-Fe (iron and steel industry), Pb-Cl-AlSi (dust), Pb-Sec (secondary formation), and Pb-Cl-Zn (industrial process); these sources (in parentheses) were identified by comparing atmospheric LCP mass spectra with authentic Pb emission source mass spectra. Sampling periods with LCP number fractions (NFs) more than three times the average LCP NF (APF = 4.35%) and below the APF were defined as high LCP NF periods (HLFPs: H1, H3, and H5) and low LCP NF APF periods (LLFPs: L2 and L4), respectively. Diurnal patterns and high Pb-Sec content during LLFPs indicate that photochemical activity and heterogeneous reactions may have controlled Pb-Sec particle formation. The inverse Pb-Cl and Pb-Sec particle diurnal trends during LLFPs suggest the replacement of Cl by sulfate and nitrate. On average over the five periods, ~ 76% of the LCPs likely arose from coal combustion and/or waste incineration, which were dominant sources during all five periods, followed by diesel trucks during LLFPs and iron- and steel-related sources during HLFPs; HLFP LCPs arose mainly from primary emissions. These results can be used to more efficiently control Pb emission sources and prevent harm to human and environmental health from Pb toxicity.

Reactive Uptake of Glyoxal by Ammonium-Containing Salt Particles as a Function of Relative Humidity.

Reactions between dissolved ammonia and carbonyls, which form light-absorbing species in atmospheric particles, can be accelerated by actively removing water from the reaction system. Here, we examine the effects of relative humidity (RH) on the reactive uptake of glyoxal (Gly) by aqueous particles of ammonium sulfate (AS), ammonium bisulfate, sodium sulfate, magnesium sulfate, ammonium nitrate (AN), and sodium nitrate. In situ Raman analysis was used to quantify particle-phase Gly and a colored product, 2,2'-biimidazole (BI), as a function of uptake time. Overall, the Gly uptake rate increases with decreasing RH, reflecting the "salting-in" effect. The BI formation rate increases significantly with decreasing RH or aerosol liquid water (ALW). Compared to that at 75% RH, the BI formation rate is enhanced by factors of 29 at 60% RH and 330 at 45% RH for AS particles and 65 at 60% RH, 210 at 45% RH, and 460 at 30% RH for AN particles. These enhancement factors are much larger than those estimated from increased reactant concentrations due to decreases in RH and ALW alone. We postulate that the reduction in ALW at low RH increases the Gly uptake rate via the "salting-in" effect and the BI formation rate by facilitating dehydration reactions.

Formation and Evolution of aqSOA from Aqueous-Phase Reactions of Phenolic Carbonyls: Comparison between Ammonium Sulfate and Ammonium Nitrate Solutions.

We investigate the effects of sulfate and nitrate on the formation and evolution of secondary organic aerosol formed in the aqueous phase (aqSOA) from photooxidation of two phenolic carbonyls emitted from wood burning. AqSOA was formed efficiently from the photooxidation of both syringaldehyde (C9H10O4) and acetosyringone (C10H12O4) in ammonium sulfate and ammonium nitrate solutions, with mass yields ranging from 30% to 120%. Positive matrix factorization on the organic mass spectra acquired by an Aerosol Mass Spectrometer revealed a combination of functionalization, oligomerization, and fragmentation processes in the chemical evolution of aqSOA. Functionalization and oligomerization dominated in the first 4 h of reaction, with phenolic oligomers and their derivatives significantly contributing to aqSOA formation; and oxidation of the first-generation products led to an abundance of oxygenated ring-opening products. Degradation rates of syringaldehyde and acetosyringone in nitrate solutions were 1.5 and 3.5 times faster than rates in sulfate solutions, and aqSOA yields in nitrate experiments are twice as high as in sulfate experiments. Nitrate likely promoted the reactions because it is a photolytic source of OH radicals, while sulfate is not, highlighting the importance of aerosol-phase nitrate in the formation of aqSOA by facilitating the photooxidation of organic precursors.

Global Survey of Antibiotic Resistance Genes in Air.

Despite its emerging significant public health concern, the presence of antibiotic resistance genes (ARGs) in urban air has not received significant attention. Here, we profiled relative abundances (as a fraction, normalized by 16S rRNA gene) of 30 ARG subtypes resistant to seven common classes of antibiotics, which are quinolones, ß-lactams, macrolides, tetracyclines, sulfonamides, aminoglycosides, and vancomycins, in ambient total particulate matter (PM) using a novel protocol across 19 world cities. In addition, their longitudinal changes in PM2.5 samples in Xi'an, China as an example were also studied. Geographically, the ARGs were detected to vary by nearly 100-fold in their abundances, for example, from 0.07 (Bandung, Indonesia) to 5.6 (San Francisco, USA). The ß-lactam resistance gene blaTEM was found to be most abundant, seconded by quinolone resistance gene qepA; and their corresponding relative abundances have increased by 178% and 26%, respectively, from 2004 to 2014 in Xi'an. Independent of cities, gene network analysis indicates that airborne ARGs were differentially contributed by bacterial taxa. Results here reveal that urban air is being polluted by ARGs, and different cities are challenged with varying health risks associated with airborne ARG exposure. This work highlights the threat of urban airborne transmission of ARGs and the need of redefining our current air quality standards in terms with public health.

Size-resolved effective density of submicron particles during summertime in the rural atmosphere of Beijing, China.

Particle density is an important physical property of atmospheric particles. The information on high time-resolution size-resolved particle density is essential for understanding the atmospheric physical and chemical aging processes of aerosols particles. In the present study, a centrifugal particle mass analyzer (CPMA) combined with a differential mobility analyzer (DMA) was deployed to determine the size-resolved effective density of 50 to 350nm particles at a rural site of Beijing during summer 2016. The measured particle effective densities decreased with increasing particle sizes and ranged from 1.43 to 1.55g/cm3, on average. The effective particle density distributions were dominated by a mode peaked at around 1.5g/cm3 for 50 to 350nm particles. Extra modes with peaks at 1.0, 0.8, and 0.6g/cm3 for 150, 240, and 350nm particles, which might be freshly emitted soot particles, were observed during intensive primary emissions episodes. The particle effective densities showed a diurnal variation pattern, with higher values during daytime. A case study showed that the effective density of Aitken mode particles during the new particle formation (NPF) event decreased considerably, indicating the significant contribution of organics to new particle growth.

Reactive Uptake of Dimethylamine by Ammonium Sulfate and Ammonium Sulfate-Sucrose Mixed Particles.

Short-chain alkyl amines can undergo gas-to-particle partitioning via reactive uptake by ammonium salts, whose phases have been thought to largely influence the extent of amine uptake. Previous studies mainly focused on particles of single ammonium salt at either dry or wet conditions without any addition of organic compounds. Here we report the uptake of dimethylamine (DMA) by ammonium sulfate (AS) and AS-sucrose mixed particles at different relative humidities (RHs) using an electrodynamic balance coupled with in situ Raman spectroscopy. DMA is selected as a representative of short-chain alkyl amines, and sucrose is used as a surrogate of viscous and hydrophilic organics. Effective DMA uptake was observed for most cases, except for the water-limiting scenario at <5% RH and the formation of an ultraviscous sucrose coating at 10% RH and below. DMA uptake coefficients (γ) were estimated using the particle mass measurements during DMA uptake. Addition of sucrose can increase γ by absorbing water or inhibiting AS crystallization and decrease γ by elevating the particle viscosity and forming a coating layer. DMA uptake can be facilitated for crystalline AS or retarded for aqueous AS with hydrophilic viscous organics (e.g., secondary organic material formed via the oxidation of biogenic volatile organic compounds) present in aerosol particles.