Organic aerosol (OA) particles constitute a substantial fraction of submicron particulate mass in the atmosphere and play a critical role in climate system. OA undergoes dynamic aging processes in the atmosphere, with photolytic aging induced by ultraviolet solar irradiance being an important yet poorly characterized mechanism. Knowledge gaps persist regarding the role of volatility transformations during photolytic aging on the OA mass decay kinetics and the evolution of climate-relevant properties, such as hygroscopicity, hindering the model evaluation of OA spatiotemporal distributions and atmospheric budgets. In this study, we conduct isothermal photolytic aging experiments on both laboratory-generated secondary organic aerosols and ambient-collected particles from urban Atlanta, utilizing a high-sensitivity Quartz Crystal Microbalance. Our results reveal that photolytic aging reduces 40-66% of the low-volatility OA mass with lifetimes ranging from 8 to 200 h under solar irradiance, and 44-92% of the photolytic mass loss is through slow evaporation of semi- or intermediate-volatile products, kinetically limited by their volatility. We observe up to ±50% changes in OA hygroscopicity with the transformation of fresh OA to photorecalcitrant low-volatility products, associated with changes in oxygen-to-carbon ratio and molecular weight. A kinetic model incorporating photolytic volatility transformation provides a cohesive explanation for the observed photolysis-induced changes in mass, volatility, and hygroscopicity. Our results can help constrain model representation of the dynamic evolutions of mass and climate-relevant properties during photolytic aging processes of the ambient OA, improving our understanding of OA atmospheric behavior and climate impact.

Nanoplastic particles (NPPs) are emerging anthropogenic pollutants identified from urban to remote areas. Characterizing the spatial and temporal distribution, process, and cloud-forming potential of atmospheric NPPs improves understanding of their environmental processes and climate impacts. This study provides the spatial and temporal distribution of several types of nanoplastic particles in the Houston area, including polystryene (PS), polyethylene (PE), polyethylene terephthalate (PET), and Polyvinyl chloride (PVC), showing an average concentration ranging from tens to hundreds of nanogram per cubic meter, with high spatial variability.In addition, we also presented the first quantified heterogeneous reaction rate and lifetimes of polystyrene (PS) NPPs against common atmospheric oxidants. The atomized PS NPPs were introduced to a Potential Aerosol Mass (PAM) oxidation flow reactor with ·OH exposure of 0 to 1.5 × 1012 molecule cm-3 s, equivalent to atmospheric exposure from 0 to 18 days, assuming ambient ·OH concentration of 1 × 106 cm-3. The decay of the PS mass concentration was quantified by monitoring tracer ions, C6H6+ (m/z 78) and C8H8+ (m/z 104), using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The pseudo-first-order rate constant of PS particles reacting with ·OH, kOH, was determined to be (3.2 × 0.7) × 10-13 cm3 molecule-1 s-1, equivalent to a half-lifetime of a few hours to ~80 days in the atmosphere, depending on particle sizes and hydroxyl radical concentrations. The hygroscopicity of 100 nm PS NPPs at different ·OH exposure levels was quantified using a cloud condensation nuclei counter (CCNC), showing a two-fold increase of hygroscopicity parameter upon 27 days of atmospheric photo-oxidation.Overall, the above results suggest that atmospheric processes can be an important part of the total plastic cycle in the environmental systems, faciliating both short range and long range transport of plastic globally.

The importance of nitrate radicals (NO3) as an atmospheric oxidant is well-established. For decades, laboratory studies of multiphase NO3 chemistry have used the same methods - either NO2 + O3 reactions or N2O5 thermal decomposition - to generate NO3 as it occurs in the atmosphere. These methods, however, come with limitations, especially for N2O5, which must be produced and stored under cold and dry conditions until its use. Recently, we developed a new photolytic source of gas-phase NO3 by irradiating aqueous solutions of ceric ammonium nitrate and nitric acid. In this study, we adapted the method to maintain stable NO3 concentrations for over 24 h. We applied the method in laboratory oxidation flow reactor (OFR) experiments to measure the yield and chemical composition of oxygenated volatile organic compounds (OVOCs) and secondary organic aerosol (SOA) formed from NO3 oxidation of volatile organic compounds (VOCs) emitted by biogenic sources (isoprene, beta-pinene, limonene, and beta-caryophyllene) and biomass burning sources (phenol, guaiacol, and syringol). SOA yields and elemental ratios were typically within a factor of 2 and 10%, respectively, of those obtained in studies using conventional NO3 sources. Maximum SOA yields obtained in our studies ranged from 0.02 (isoprene/NO3) to 0.96 (beta-caryophyllene/NO3). The highest SOA oxygen-to-carbon ratios (O/C) ranged from 0.48 (beta-caryophyllene/NO3) to 1.61 (syringol/NO3). Additionally, we characterized novel condensed-phase oxidation products from syringol/NO3 reactions. Overall, the use of irradiated aqueous cerium nitrate as a source of gas-phase NO3 may enable more widespread studies of NO3-initiated oxidative aging, which has been less explored compared to that of hydroxyl radical chemistry.

Long-term measurements of the composition and mass concentration of particulate matter (PM) are important for source apportionment, epidemiological studies, and trends in atmospheric chemistry. The Aerosol Chemical Speciation Monitor (ACSM) has been widely used for in situ, real time measurements of PM. However, ACSMs provide unit mass resolution data, meaning isobaric ions (same unit mass, different exact m/z) cannot be separated, which can impact detection limits and separation and identification of different organic ions (e.g., C2H3O+ vs C3H7+). Here, we present a new Time-of-Flight ACSM with eXtended resolution (TOF-ACSM-X). With a mass resolving power of similar to 2000 m/Delta m, the TOF-ACSM-X enables higher-resolution, multi-peak fitting of individual ions compared to the two other existing ACSM models, namely the Time-of-Flight ACSM (TOF-ACSM) and quadrupole ACSM (Q-ACSM). This improved resolution leads to a factor of 25 improvement in ammonium detection limits, from similar to 0.200 to similar to 0.008 mu g m-3 (TOF-ACSM versus TOF-ACSM-X, respectively), for 10-minute integration times, allows for elemental analysis (O/C and H/C) of organic aerosol, and enables improved mass spectral separation of the CH2O+ and NO+ signals at m/z = 30 for improved quantification of organic and inorganic particle nitrate. Comparisons of the TOF-ACSM-X with ambient measurements from two separate instruments show that the TOF-ACSM-X agrees quantitatively and that the TOF-ACSM-X provides unconstrained positive matrix factorization results for the organic aerosol that would not be possible with the unit mass resolution TOF-ACSM. Finally, we are now recommending a more direct and unifying calculation of nitrate mass concentration for both AMS and ACSM ionization efficiency calibrations.

Abstract Aqueous‐phase uptake and processing of water‐soluble organic compounds can promote secondary organic aerosol (SOA) production. We evaluated the contributions of aqueous‐phase chemistry to summertime urban SOA at two sites in New York City. The relative role of aqueous‐phase processing varied with chemical and environmental conditions, with evident daytime SOA enhancements (e.g., >1 μg/m3) during periods with relative humidities (RH) exceeding 65% and often higher temperatures. Oxygenated organic aerosol (OOA) production was also sensitive to secondary inorganic aerosols, in part through their influence on aerosol liquid water. On average, high‐RH periods exhibited a 69% increase in less‐oxidized OOA production in Queens, NY. These enhancements coincided with southerly backward trajectories and greater inorganic aerosol concentrations, yet showed substantial intra‐city variability between Queens and Manhattan. The observed aqueous‐phase SOA production, even with historically low sulfate and nitrate aerosol loadings, highlights both opportunities and challenges for continued reductions in summertime PM2.5 in urban communities.

At fixed aerosol acidity, we recently demonstrated that dimers in isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) can heterogeneously react with hydroxyl radical (·OH) at faster rates than monomers. Aerosol acidity influences this aging process by enhancing the formation of oligomers in freshly generated IEPOX-SOA. Therefore, we systematically examined the role of aerosol acidity on kinetics and products resulting from heterogeneous ·OH oxidation of freshly generated IEPOX-SOA. IEPOX reacted with inorganic sulfate aerosol of varying initial pH (0.5, 1.5, and 2.5) in a steady-state smog chamber to yield a constant source of freshly generated IEPOX-SOA, which was aged in an oxidation flow reactor for 0-22 equiv days of atmospheric ·OH exposure. Molecular-level chemical analyses revealed that the most acidic sulfate aerosol (pH 0.5) formed the largest oligomeric mass fraction, causing the slowest IEPOX-SOA mass decay with aging. Reactive uptake coefficients of ·OH (γOH) were 0.24 ± 0.06, 0.40 ± 0.05, and 0.49 ± 0.20 for IEPOX-SOA generated at pH 0.5, 1.5, and 2.5, respectively. IEPOX-SOA became more liquid-like for pH 1.5 and 2.5, while exhibiting an irregular pattern for pH 0.5 with aging. Using kinetic and physicochemical data derived for a single aerosol pH in atmospheric models could inaccurately predict the fate of the IEPOX-SOA.

The oxidation flow reactor (OFR) has been widely used to simulate secondary organic aerosol (SOA) formation in laboratory and field studies. OH exposure (OHexp), representing the extent of hydroxyl (OH) radical oxidation and normally expressed as the product of OH concentration and residence time in the OFR, is important in assessing the oxidation chemistry in SOA formation. Several models have been developed to quantify the OHexp in OFRs, and empirical equations have been proposed to parameterize OHexp. Practically, the empirical equations and the associated parameters are derived under atmospheric relevant conditions (i.e., external OH reactivity) with limited variations in calibration conditions, such as residence time, water vapor mixing ratio, and ozone (O3) concentration. Whether the equations or parameters derived under limited sets of calibration conditions can accurately predict the OHexp under dynamically changing experimental conditions with large variations (i.e., extremely high external OH reactivity) in real applications remains uncertain. In this study, we conducted 62 sets of experiments (416 data points) under a wide range of experimental conditions to evaluate the scope of the application of the empirical equations to estimate OHexp. Sensitivity tests were also conducted to obtain a minimum number of data points, which is necessary for generating the fitting parameters. We showed that, for the OFR185 mode (185 nm lamps with internal O3 generation), except for external OH reactivity, the parameters obtained within a narrow range of calibration conditions can be extended to estimate the OHexp when the experiments are in wider ranges of conditions. For example, parameters derived within a narrow water vapor mixing ratio range (0.49 %–0.99 %, corresponding to 15.1 %–30.8 % of relative humidity at 101.325 kPa and 298 K) can be extended to estimate the OHexp under the entire range of water vapor mixing ratios (0.49 %–2.76 %, equivalent to 15.1 %–85.7 % of relative humidity under identical conditions). However, the parameters obtained when the external OH reactivity is below 23 s−1 could not be used to reproduce the OHexp under the entire range of external OH reactivity (4–204 s−1). For the OFR254 mode (254 nm lamps with external O3 generation), all parameters obtained within a narrow range of conditions can be used to estimate OHexp accurately when experimental conditions are extended. Additionally, when using the OFR254 mode, lamp voltages that are too low should be avoided, as they will generally result in large deviations in the estimations of OHexp from empirical equations. Regardless of whether the OFR185 or OFR254 mode is used, at least 20–30 data points from sulfur dioxide (SO2) or carbon monoxide (CO) decay with varying conditions are required to fit a set of empirical parameters that can accurately estimate OHexp. Caution should be exercised to use fitted parameters from low external OH reactivity to high ones, for instance, those from direct emissions such as vehicular exhaust and biomass burning.

Atmospheric aerosols can be composed of a wide variety of organic and inorganic chemicals, which can dramatically affect the environmental impact of the particles. Understanding the chemical composition of aerosol assists in understanding the sources and fate of these emissions. Additionally, as volatile chemical products (VCPs) have surpassed vehicle emissions in urban areas, there is a need to understand the changing composition of urban aerosol and how it affects aerosol loadings and formation. Here, I present a simple tool called the Vaporization Inlet for Aerosols (VIA) to vaporize ambient aerosol to measure the chemical composition of the resulting gas-phase products with a time-of-flight chemical ionization mass spectrometer. The laboratory results from the VIA are compared to those obtained from the previously characterized Filter Inlet for Gases and Aerosols (FIGAERO). Both inlets have different benefits in terms of temporal resolution, gas-phase comparisons, ease of use, and long-term operation, which will be discussed. Finally, the thermal decomposition products of organic molecules in the VIA is compared to the FIGAERO to understand the chemical formulas detected by the mass spectrometer.

Organic species in the atmosphere originate from a wide range of sources and processes. While real time chemical ionization mass spectrometry (CIMS) has improved our capability to characterize individual organic species in the atmosphere, the selectivity of CIMS reagent ions can limit the range of species that can be measured. In this work the need to detect a broader range of species with a single CIMS instrument is addressed. A fast-switching bipolar time-of-flight CIMS that switches between four different reagent ions, including positive and negative ions, is demonstrated. The performance and utility of this instrument is demonstrated by measurements obtained on board a ship in Antarctica during the PolarChange field campaign and from New York City during the AEROMMA campaign. During both campaigns the instrument cycled through iodide (I-), benzene (C6H6+), and acetone dimer ((C3H6O)2H+) reagent ions at a 2 second data acquisition rate per cycle. In the case of PolarChange, this combination of ions enabled simultaneous detection of trends in primary marine biological emissions such as dimethyl sulfide, nucleating species such as ammonia and methyl amine, and acids, such as nitric acid. During AEROMMA, the fast bipolar switching capability enabled Eddy Correlation measurements of primary biogenic and urban emissions (i.e. monoterpenes and aromatics), secondary products of atmospheric oxidation (i.e. highly oxidized organics and organic nitrates), and reduced nitrogen species. Preliminary results from this dataset, including positive matrix analyses of the combined multi-reagent ion datasets, are discussed. Simultaneous gas and aerosol composition measurements obtained by coupling this mass spectrometer with aerosol inlets are also described.

Nanoplastic particles (NPPs) are emerging anthropogenic pollutants and have been detected in urban, rural, and remote areas. Characterizing the lifetime, fate, and cloud-forming potential of atmospheric NPPs improves our understanding of their environmental processes and climate impacts. This study provides the first quantified heterogeneous reaction rate and lifetime of polystyrene (PS) NPPs against common atmospheric oxidants. The atomized PS NPPs were introduced to a Potential Aerosol Mass (PAM) oxidation flow reactor with ·OH exposure of 0 to 1.5 × 1012 molecules cm-3 s, equivalent to atmospheric exposure from 0 to 18 days, assuming an ambient ·OH concentration of 1 × 106 cm-3. The decay of the PS mass concentration was quantified by monitoring tracer ions, C6H6+ (m/z 78) and C8H8+ (m/z 104), by using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The pseudo-first-order rate constant of PS particles reacting with ·OH, kOH, was determined to be (3.2 ± 0.7) × 10-13 cm3 molecule-1 s-1, equivalent to a half-lifetime of a few hours to ∼80 days in the atmosphere, depending on particle sizes and hydroxyl radical concentrations. The hygroscopicity of 100 nm PS NPPs at different ·OH exposure levels was quantified using a cloud condensation nuclei counter (CCNC), showing a twofold increase of hygroscopicity parameter upon 27 days of atmospheric photooxidation.

Oxidation flow reactors (OFRs) have been extensively utilized to examine the formation of secondary organic aerosol (SOA). However, the UV lamps typically employed to initiate the photochemistry in OFRs can result in an elevated reactor temperature when their implications are not thoroughly evaluated. In this study, we conducted a comprehensive investigation into the temperature distribution within an Aerodyne potential aerosol mass OFR (PAM-OFR) and then examined the subsequent effects on flow and chemistry due to lamp heating. A lamp-induced temperature increase was observed, which was a function of lamp-driving voltage, number of lamps, lamp types, OFR residence time, and positions within the PAM-OFR. Under typical PAM-OFR operational conditions (e.g., < 5 d of equivalent atmospheric OH exposure under low-NOx conditions), the temperature increase typically ranged from 1–5 °C. Under extreme (but less frequently encountered) conditions, the heating could reach up to 15 °C. The influences of the increased temperature over ambient conditions on the flow distribution, gas, and condensed-phase chemistry within PAM-OFR were evaluated. Our findings indicate that the increase in temperature altered the flow field, resulting in a diminished tail on the residence time distribution and corresponding oxidant exposure due to faster recirculation. According to simulation results from a radical chemistry box model, the variation in absolute oxidant concentration within PAM-OFR due to temperature increase was minimal (< 5 %). The temperature influences on seed organic aerosol (OA) and newly formed secondary OA were also investigated, suggesting that an increase in temperature can impact the yield, size, and oxidation levels of representative biogenic and anthropogenic SOA types. Recommendations for temperature-dependent SOA yield corrections and PAM-OFR operating protocols that mitigate lamp-induced temperature enhancement and fluctuations are presented. We recommend blowing air around the reactor's exterior with fans during PAM-OFR experiments to minimize the temperature increase within PAM-OFR. Temperature increases are substantially lower for OFRs utilizing less powerful lamps compared to the Aerodyne version.

As part of the summer 2022 NYC-METS (New York City metropolitan Measurements of Emissions and TransformationS) campaign and the ASCENT (Atmospheric Science and Chemistry mEasurement NeTwork) observational network, speciated particulate matter was measured in real time in Manhattan and Queens, NY, with additional gas-phase measurements. Largely due to observed reductions in inorganic sulfate aerosol components over the 21st century, summertime aerosol composition in NYC has become predominantly organic (80-83%). Organic aerosol source apportionment via positive matrix factorization showed that this is dominated by secondary production as oxygenated organic aerosol (OOA) source factors comprised 73-76% of OA. Primary factors, including cooking-related organic aerosol (COA) and hydrocarbon-like organic aerosol (HOA) comprised minor fractions of OA, only 13-15% and 10-11%, respectively. The two sites presented considerable spatiotemporal variations in OA source factor concentrations despite similar average PM2.5 concentrations. The less- and more-oxidized OOA factors exhibited clear temperature dependences at both sites with increased concentrations and greater degrees of oxidation at higher temperatures, including during a heatwave. With strong temperature sensitivity and minimal changes in summertime concentrations since 2001, secondary OA poses a particular challenge for air quality policy in NYC that will very likely be exacerbated by continued climate change and extreme heat events.

Terrestrial vegetation emits complex mixtures of highly reactive biogenic volatile organic compounds (BVOCs) that contribute to secondary organic aerosol (SOA) formation. The aerosol chemistry of many BVOCs remains unexplored. Oxygenated monoterpenes are one class of BVOCs that comprise a large proportion of woody shrub emission profiles and can also be induced after stress exposure. In this work, SOA was generated from the photooxidation of oxygenated monoterpenes in an oxidation flow reactor and compared to SOA generated from a reference cyclic terpene-alpha-pinene. The oxygenated terpenes used as SOA precursors included camphor (C10H16O), borneol (C10H18O), 1,8-cineole (C10H18O), and bornyl acetate (C12H20O2). Results show that SOA mass yields from oxygenated terpenes were usually greater than or equal to alpha-pinene except for bornyl acetate, which had the lowest yields. SOA composition measured offline with liquid chromatography high resolution mass spectrometry (UHPLC-ESI-HRMS) was used to compare the different SOA types. Additionally, the composition of SOA generated from emissions of two coastal sage shrubs, with emissions dominated by oxygenated terpenes, was compared with SOA formed from single component standards using aerosol mass spectrometry. The composition of 1,8-cineole SOA was most dissimilar from the other types of SOA. Additionally, SOA generated from real plant emissions of two different species was more similar in composition to one another than to the SOA generated from the individual components, suggesting non-linearity of the chemistry of BVOC mixtures. These results highlight the importance of biogenic SOA studies using complex mixtures that are more representative of real plant emissions.

This review aims to provide a comprehensive examination of oxidation flow reactor (OFR) studies and their applications in both laboratory and field investigations. OFRs play a crucial role in understanding secondary organic aerosol (SOA) formation and aging processes in the atmosphere. By evaluating the advancements and limitations of OFR technology, this review seeks to identify key research directions and challenges for future studies in atmospheric chemistry and air quality research. In recent years, OFR has emerged as an encouraging alternative to smog chambers for SOA study. The high oxidative capacity and short residence time of OFR enable its wide application in both laboratory and field studies. Research utilizing OFR has uncovered the critical role of semi-volatile and intermediate-volatility organic compounds (S/IVOCs) in the formation of SOA from various sources, including vehicle emissions, biomass burning, cooking activities, and non-traditional emissions such as volatile chemical products. Notably, field studies have observed considerable variability in the SOA formation potential across different environments globally, generally showing higher formation potential in urban areas compared to rural and forest regions. OFR studies have significantly advanced our understanding of SOA formation and aging processes, identifying key precursors, evaluating influencing factors, and quantifying SOA formation potential. However, challenges remain in unraveling detailed mechanisms due to the complexity of SOA sources and properties. Future OFR research should focus on innovations in OFR design, study non-traditional emissions, conduct long-term field observations, develop standardized calibration procedures, and establish SOA yield parameterization schemes for S/IVOCs.

Particulate organic nitrate (pON) can be a major part of secondary organic aerosol (SOA) and is commonly quantified by indirect means from aerosol mass spectrometer (AMS) data. However, pON quantification remains challenging. Here, we set out to quantify and characterize pON in the boreal forest, through direct field observations at Station for Measuring Ecosystem Atmosphere Relationships (SMEAR) II in Hyytiälä, Finland, and targeted single-precursor laboratory studies. We utilized a long time-of-flight AMS (LToF-AMS) for aerosol chemical characterization, with a particular focus to identify C x H y O z N+ ("CHON+") fragments. We estimate that during springtime at SMEAR II, pON (including both the organic and nitrate part) accounts for ∼10% of the particle mass concentration (calculated by the NO+/NO2+ method) and originates mainly from the NO3 radical oxidation of biogenic volatile organic compounds. The majority of the background nitrate aerosol measured is organic. The CHON+ fragment analysis was largely unsuccessful at SMEAR II, mainly due to low concentrations of the few detected fragments. However, our findings may be useful at other sites as we identified 80 unique CHON+ fragments from the laboratory measurements of SOA formed from NO3 radical oxidation of three pON precursors (β-pinene, limonene, and guaiacol). Finally, we noted a significant effect on ion identification during the LToF-AMS high-resolution data processing, resulting in too many ions being fit, depending on whether tungsten ions (W+) were used in the peak width determination. Although this phenomenon may be instrument-specific, we encourage all (LTOF-) AMS users to investigate this effect on their instrument to reduce the possibility of incorrect identifications.

Data of photochemical aging of cooking and burning emissions

Heterogeneous hydroxyl radical ((OH)-O-center dot) oxidation is an important aging process for isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) that alters its chemical composition. It was recently demonstrated that heterogeneous (OH)-O-center dot oxidation can age single-component particulate methyltetrol sulfates (MTSs), causing similar to 55% of the SOA mass loss. However, our most recent study of freshly generated IEPOX-SOA particulate mixtures suggests that the lifetime of the complete IEPOX-SOA mixture against heterogeneous (OH)-O-center dot oxidation can be prolonged through the fragmentation of higher-order oligomers. Published studies suggest that the heterogeneous (OH)-O-center dot oxidation of IEPOX-SOA could affect the organic atmospheric aerosol budget at varying rates, depending on aerosol chemical composition. However, heterogeneous (OH)-O-center dot oxidation kinetics for the full IEPOX-SOA particulate mixture have not been reported. Here, we exposed freshly generated IEPOX-SOA particles to heterogeneous oxidation by (OH)-O-center dot under humid conditions (relative humidity similar to 57%) for 0-15 atmospheric-equivalent days of aging and derived an effective heterogeneous (OH)-O-center dot rate coefficient (k(OH)) of 2.64 +/- 0.4 x 10(-13) cm(3) molecules(-1) s(-1). While similar to 44% of particulate organic mass of nonoxidized IEPOX-SOA was consumed over the entire 15 day aging period, only <7% was consumed during the initial 10 aging days. By molecular-level chemical analysis, we determined oligomers were consumed at a faster rate (by a factor of 2-4) than monomers. Analysis of aerosol physicochemical properties shows that IEPOX-SOA has a core-shell morphology, and the shell becomes thinner with (OH)-O-center dot oxidation. In summary, this study demonstrates that heterogeneous (OH)-O-center dot oxidation of IEPOX-SOA particles is a dynamic process in which aerosol chemical composition and physicochemical properties play important roles.