Oxidized and Unsaturated: Key Organic Aerosol Traits Associated with Cellular Reactive Oxygen Species Production in the Southeastern United States

Exposure to ambient fine particulate matter (PM2.5) is associated with millions of premature deaths annually. Oxidative stress through overproduction of reactive oxygen species (ROS) is a possible mechanism for PM2.5-induced health effects. Organic aerosol (OA) is a dominant component of PM2.5 worldwide, yet its role in PM2.5 toxicity is poorly understood due to its chemical complexity. Here, through integrated cellular ROS measurements and detailed multi-instrument chemical characterization of PM in urban southeastern United States, we show that oxygenated OA (OOA), especially more-oxidized OOA, is the main OA type associated with cellular ROS production. We further reveal that highly unsaturated species containing carbon-oxygen double bonds and aromatic rings in OOA are major contributors to cellular ROS production. These results highlight the key chemical features of ambient OA driving its toxicity. As more-oxidized OOA is ubiquitous and abundant in the atmosphere, this emphasizes the need to understand its sources and chemical processing when formulating effective strategies to mitigate PM2.5 health impacts.

Overview of ICARUS-A Curated, Open Access, Online Repository for Atmospheric Simulation Chamber Data

Atmospheric simulation chambers continue to be indispensable tools for research in the atmospheric sciences. Insights from chamber studies are integrated into atmospheric chemical transport models, which are used for science-informed policy decisions. However, a centralized data management and access infrastructure for their scientific products had not been available in the United States and many parts of the world. ICARUS (Integrated Chamber Atmospheric data Repository for Unified Science) is an open access, searchable, web-based infrastructure for storing, sharing, discovering, and utilizing atmospheric chamber data [https://icarus.ucdavis.edu]. ICARUS has two parts: a data intake portal and a search and discovery portal. Data in ICARUS are curated, uniform, interactive, indexed on popular search engines, mirrored by other repositories, version-tracked, vocabulary-controlled, and citable. ICARUS hosts both legacy data and new data in compliance with open access data mandates. Targeted data discovery is available based on key experimental parameters, including organic reactants and mixtures that are managed using the PubChem chemical database, oxidant information, nitrogen oxide (NOx) content, alkylperoxy radical (RO₂) fate, seed particle information, environmental conditions, and reaction categories. A discipline-specific repository such as ICARUS with high amounts of metadata works to support the evaluation and revision of atmospheric model mechanisms, intercomparison of data and models, and the development of new model frameworks that can have more predictive power in the current and future atmosphere. The open accessibility and interactive nature of ICARUS data may also be useful for teaching, data mining, and training machine learning models.

Low-Molecular-Weight Carboxylic Acids in the Southeastern U.S.: Formation, Partitioning, and Implications for Organic Aerosol Aging

While carboxylic acids are important components in both particle and gas phases in the atmosphere, their sources and partitioning are not fully understood. In this study, we present real-time measurements of both particle- and gas-phase concentrations for five of the most common and abundant low-molecular-weight carboxylic acids (LMWCA) in a rural region in the southeastern U.S. in Fall 2016. Through comparison with secondary organic aerosol (SOA) tracers, we find that isoprene was the most important local precursor for all five LMWCA but via different pathways. We propose that monocarboxylic acids (formic and acetic acids) were mainly formed through gas-phase photochemical reactions, while dicarboxylic acids (oxalic, malonic, and succinic acids) were predominantly from aqueous processing. Unexpectedly high concentrations of particle-phase formic and acetic acids (in the form of formate and acetate, respectively) were observed and likely the components of long-range transport organic aerosol (OA), decoupled from their gas-phase counterparts. In addition, an extraordinarily strong correlation (R² = 0.90) was observed between a particulate LMWCA and aged SOA, which we tentatively attribute to boundary layer dynamics.

Particle Size Distribution Dynamics Can Help Constrain the Phase State of Secondary Organic Aerosol

Particle phase state is a property of atmospheric aerosols that has important implications for the formation, evolution, and gas/particle partitioning of secondary organic aerosol (SOA). In this work, we use a size-resolved chemistry and microphysics model (Statistical Oxidation Model coupled to the TwO Moment Aerosol Sectional (SOM-TOMAS)), updated to include an explicit treatment of particle phase state, to constrain the bulk diffusion coefficient (D_b) of SOA produced from α-pinene ozonolysis. By leveraging data from laboratory experiments performed in the absence of a seed and under dry conditions, we find that the D_b for SOA can be constrained ((1-7) × 10^-15 cm² s^-1 in these experiments) by simultaneously reproducing the time-varying SOA mass concentrations and the evolution of the particle size distribution. Another version of our model that used the predicted SOA composition to calculate the glass-transition temperature, viscosity, and, ultimately, D_b (∼10^-15 cm² s^-1) of the SOA was able to reproduce the mass and size distribution measurements when we included oligomer formation (oligomers accounted for about a fifth of the SOA mass). Our work highlights the potential of a size-resolved SOA model to constrain the particle phase state of SOA using historical measurements of the evolution of the particle size distribution.

Effects of Molecular-Level Compositional Variability in Organic Aerosol on Phase State and Thermodynamic Mixing Behavior

The molecular-level composition and structure of organic aerosol (OA) affect its chemical/physical properties, transformations, and impacts. Here, we use the molecular-level chemical composition of functionalized OA from three diverse field sites to evaluate the effect of molecular-level compositional variability on OA phase state and thermodynamic mixing favorability. For these ambient sites, modeled aerosol phase state ranges from liquid to semisolid. The observed variability in OA composition has some effect on resulting phase state, but other factors like the presence of inorganic ions, aerosol liquid water, and internal versus external mixing with water are determining factors in whether these particles exist as liquids, semisolids, or solids. Organic molecular composition plays a more important role in determining phase state for phase-separated (verus well-mixed) systems. Similarly, despite the observed OA compositional differences, the thermodynamic mixing favorability for OA samples with aerosol liquid water, isoprene oxidation products, or monoterpene oxidation products remains fairly consistent within each campaign. Mixing of filter-sampled OA and isoprene or monoterpene oxidation products is often favorable in both seasons, while mixing with water is generally unfavorable.

Cellular and Acellular Assays for Measuring Oxidative Stress Induced by Ambient and Laboratory-Generated Aerosols

INTRODUCTION

Many studies have established associations between exposure to air pollution, or atmospheric particulate matter (PM), and adverse health effects. An increasing array of studies have suggested oxidative stress as a possible mechanism by which PM-induced health effects arise, and as a result, many chemical and cellular assays have been developed to study PM-induced oxidant production. Although significant progress has been made in recent years, there are still many gaps in this area of research that have not been addressed. Many prior studies have focused on the aerosol of primary origin (e.g., the aerosol emitted from combustion engines) although the aerosol formed from the oxidation of volatile species, secondary organic aerosol (SOA), has been shown to be the predominant type of aerosol even in urban areas. Current SOA health studies are limited in number, and as such, the health effects of SOA are poorly characterized. Also, there is a lack of perspective in terms of the relative toxicities of different SOA systems. Additionally, although chemical assays have identified some SOA constituents associated with adverse health endpoints, the applicability of these results to cellular responses has not been well established.

SPECIFIC AIMS

The overall objective of this study was to better understand the oxidative properties of different types and components of PM mixtures (especially SOA) through systematic laboratory chamber experiments and ambient field studies. The study had four specific aims.

1 To develop a cellular assay optimized for measuring reactive oxygen and nitrogen species (ROS/RNS) production resulting from PM exposure and to identify a robust parameter that could represent ROS/RNS levels for comparison with different endpoints.

2 To identify ambient PM components associated with ROS/RNS production and evaluate whether results from chemical assays represented cellular responses in terms of ROS/RNS production.

3 To investigate and provide perspective on the relative toxicities of SOA formed from common biogenic and anthropogenic precursors under different conditions (e.g., humidity, nitrogen oxides [NOx], and redox-active metals) and identify bulk aerosol properties associated with cellular responses.

4 To investigate the effects of photochemical aging on aerosol toxicity.

METHODS

Ambient PM samples were collected from urban and rural sites in the greater Atlanta area as part of the Southeastern Center for Air Pollution and Epidemiology (SCAPE) study between June 2012 and October 2013. The concentrations of water-soluble species (e.g., water-soluble organic carbon [WSOC], brown carbon [Br C], and metals) were characterized using a variety of instruments. Samples for this study were chosen to span the observed range of dithiothreitol (DTT) activities.

Laboratory studies were conducted in the Georgia Tech Environmental Chamber (GTEC) facility in order to generate SOA under well-controlled photooxidation conditions. Precursors of biogenic origin (isoprene, α-pinene, and β-caryophyllene) and anthropogenic origin (pentadecane, m-xylene, and naphthalene) were oxidized under various formation conditions (dry vs. humid, NOx, and ammonium sulfate vs. iron sulfate seed particles) to produce SOA of differing chemical composition and mass loading. For the naphthalene system, a series of experiments were conducted with different initial hydrocarbon concentrations to produce aerosols with various degree of oxidation. A suite of instruments was utilized to monitor gas- and particle-phase species. Bulk aerosol properties (e.g., O:C, H:C, and N:C ratios) were measured using a high-resolution time-of-flight aerosol mass spectrometer. Filter samples were collected for chemical oxidative potential and cellular measurements. For the naphthalene system, multiple filter samples were collected over the course of a single experiment to collect aerosols of different photochemical aging.

For all filter samples, chemical oxidative potentials were determined for water-soluble extracts using a semiautomated DTT assay system. Murine alveolar macrophages and neonatal rat ventricular myocytes were also exposed to PM samples extracted in cell culture medium to investigate cellular responses. ROS/RNS production was detected using the intracellular ROS/RNS probe, carboxy-2',7'-dichlorodihydrofluorescein diacetate (carboxy-H2DCFA), whereas cellular metabolic activity was assessed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). Finally, cytokine production, that is, secreted levels of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), were measured post-exposure using an enzyme-linked immunosorbent assay (ELISA). To identify PM constituents associated with oxidative properties, linear regressions between oxidative properties (cellular responses or DTT activity) and aerosol composition (metals, elemental ratios, etc.) were evaluated using Pearson's correlation coefficient, where the significance was determined using multiple imputation and evaluated using a 95% confidence interval.

RESULTS

We optimized several parameters for the ROS/RNS assay, including cell density (2 × 104 cells/well for macrophages and 3.33 × 104 cells/well for cardiomyocytes), probe concentration (10 µM), and sample incubation time (24 hours). Results from both ambient and laboratory-generated aerosols demonstrate that ROS/RNS production was highly dose-dependent and nonlinear with respect to PM dose. Of the dose-response metrics investigated in this study (maximum response, dose at which the response is 10% above the baseline [threshold], dose at which 50% of the response is attained [EC50], rate at which the maximum response is attained [Hill slope], and area under the dose-response curve [AUC]), we found that the AUC was the most robust parameter whose informativeness did not depend on dose range.

A positive, significant correlation was observed between ROS/RNS production as represented by AUC and chemical oxidative potential as measured by DTT for ambient samples collected in summer. Conversely, a relatively constant AUC was observed for ambient samples collected in winter regardless of the corresponding DTT activity. We also identified several PM constituents (WSOC, BrC, iron, and titanium) that were significantly correlated with AUC for summer samples. The strong correlation between organic species and ROS/RNS production highlights a need to understand the contribution of organic aerosols to PM-induced health effects. No significant correlations were observed for other ROS/RNS metrics or PM constituents, and no spatial trends were observed.

For laboratory-generated aerosol, precursor identity influenced oxidative potentials significantly, with isoprene and naphthalene SOA having the lowest and highest DTT activities, respectively. Both precursor identity and formation condition significantly influenced inflammatory responses induced by SOA exposure, and several response patterns were identified for SOA precursors whose photooxidation products share similar carbon-chain length and functionalities. The presence of iron sulfate seed particles did not have an apparent effect on oxidative potentials; however, a higher level of ROS/RNS production was observed for all SOA formed in the presence of iron sulfate compared with ammonium sulfate. We also identified a significant positive correlation between ROS/RNS production and average carbon oxidation state, a bulk aerosol property. It may therefore be possible to roughly estimate ROS/RNS production using this property, which is readily obtainable. This correlation may have significant implications as aerosols have an atmospheric lifetime of a week, during which average carbon oxidation state increases because of atmospheric photochemical aging. Our results suggest that aerosols might become more toxic as they age in the atmosphere. Finally, in the context of ambient samples, laboratory-generated SOA induced comparable or higher levels of ROS/RNS. Oxidative potentials for all laboratory SOA systems, with the exception of naphthalene (which was higher), were all comparable with oxidative potentials observed in ambient samples.

Parameterized Yields of Semivolatile Products from Isoprene Oxidation under Different NO x Levels: Impacts of Chemical Aging and Wall-Loss of Reactive Gases

We developed a parametrizable box model to empirically derive the yields of semivolatile products from VOC oxidation using chamber measurements, while explicitly accounting for the multigenerational chemical aging processes (such as the gas-phase fragmentation and functionalization and aerosol-phase oligomerization and photolysis) under different NO _x levels and the loss of particles and gases to chamber walls. Using the oxidation of isoprene as an example, we showed that the assumptions regarding the NO _x-sensitive, multigenerational aging processes of VOC oxidation products have large impacts on the parametrized product yields and SOA formation. We derived sets of semivolatile product yields from isoprene oxidation under different NO _x levels. However, we stress that these product yields must be used in conjunction with the corresponding multigenerational aging schemes in chemical transport models. As more mechanistic insights regarding SOA formation from VOC oxidation emerge, our box model can be expanded to include more explicit chemical aging processes and help ultimately bridge the gap between the process-based understanding of SOA formation from VOC oxidation and the bulk-yield parametrizations used in chemical transport models.

Aerosol Vacuum-Assisted Plasma Ionization (Aero-VaPI) Coupled to Ion Mobility-Mass Spectrometry

In this communication, we report on the real-time analysis of organic aerosol particles by Vacuum-assisted Plasma Ionization-Mass Spectrometry (Aero-VaPI-MS) using a home-built VaPI ion source coupled to a Synapt G2-S HDMS ion mobility-mass spectrometry (IM-MS) system. Standards of organic molecules of interest in prebiotic chemistry were used to generate aerosols. Monocaprin and decanoic acid aerosol particles were successfully detected in both the positive and negative ion modes, respectively. A complex aerosol mixture of different sizes of polymers of L-malic acid was also examined through ion mobility (IM) separations, resulting in the detection of polymers of up to eight monomeric units. This noncommercial plasma ion source is proposed as a low cost alternative to other plasma ionization platforms used for aerosol analysis, and a higher-performance alternative to more traditional aerosol mass spectrometers. VaPI provides robust online ionization of organics in aerosols without extensive ion activation, with the coupling to IM-MS providing higher peak capacity and excellent mass accuracy. Graphical Abstract ᅟ.

Coupling of organic and inorganic aerosol systems and the effect on gas-particle partitioning in the southeastern US

Several models were used to describe the partitioning of ammonia, water, and organic compounds between the gas and particle phases for conditions in the southeastern US during summer 2013. Existing equilibrium models and frameworks were found to be sufficient, although additional improvements in terms of estimating pure-species vapor pressures are needed. Thermodynamic model predictions were consistent, to first order, with a molar ratio of ammonium to sulfate of approximately 1.6 to 1.8 (ratio of ammonium to 2× sulfate, RN/2S ≈ 0.8 to 0.9) with approximately 70% of total ammonia and ammonium (NH x ) in the particle. Southeastern Aerosol Research and Characterization Network (SEARCH) gas and aerosol and Southern Oxidant and Aerosol Study (SOAS) Monitor for AeRosols and Gases in Ambient air (MARGA) aerosol measurements were consistent with these conditions. CMAQv5.2 regional chemical transport model predictions did not reflect these conditions due to a factor of 3 overestimate of the nonvolatile cations. In addition, gas-phase ammonia was overestimated in the CMAQ model leading to an even lower fraction of total ammonia in the particle. Chemical Speciation Network (CSN) and aerosol mass spectrometer (AMS) measurements indicated less ammonium per sulfate than SEARCH and MARGA measurements and were inconsistent with thermodynamic model predictions. Organic compounds were predicted to be present to some extent in the same phase as inorganic constituents, modifying their activity and resulting in a decrease in [H+]air (H+ in μgm-3 air), increase in ammonia partitioning to the gas phase, and increase in pH compared to complete organic vs. inorganic liquid-liquid phase separation. In addition, accounting for nonideal mixing modified the pH such that a fully interactive inorganic-organic system had a pH roughly 0.7 units higher than predicted using traditional methods (pH = 1.5 vs. 0.7). Particle-phase interactions of organic and inorganic compounds were found to increase partitioning towards the particle phase (vs. gas phase) for highly oxygenated (O : C≥0.6) compounds including several isoprene-derived tracers as well as levoglu-cosan but decrease particle-phase partitioning for low O: C, monoterpene-derived species.

Regional Similarities and NOx-related Increases in Biogenic Secondary Organic Aerosol in Summertime Southeastern U.S

During the 2013 Southern Oxidant and Aerosol Study, Fourier Transform Infrared Spectroscopy (FTIR) and Aerosol Mass Spectrometer (AMS) measurements of submicron mass were collected at Look Rock (LRK), Tennessee, and Centreville (CTR), Alabama. Carbon monoxide and submicron sulfate and organic mass concentrations were 15-60% higher at CTR than at LRK but their time series had moderate correlations (r~0.5). However, NOx had no correlation (r=0.08) between the two sites with nighttime-to-early-morning peaks 3~10 times higher at CTR than at LRK. Organic mass (OM) sources identified by FTIR Positive Matrix Factorization (PMF) had three very similar factors at both sites: Fossil Fuel Combustion (FFC) related organic aerosols, Mixed Organic Aerosols (MOA), and Biogenic Organic Aerosols (BOA). The BOA spectrum from FTIR is similar (cosine similarity > 0.6) to that of lab-generated particle mass from the photochemical oxidation of both isoprene and monoterpenes under high NOx conditions from chamber experiments. The BOA mass fraction was highest during the night at CTR but in the afternoon at LRK. AMS PMF resulted in two similar pairs of factors at both sites and a third nighttime NOx-related factor (33% of OM) at CTR but a daytime nitrate-related factor (28% of OM) at LRK. NOx was correlated with BOA and LO-OOA for NOx concentrations higher than 1 ppb at both sites, producing 0.5 ± 0.1 μg m-3 for CTR-LO-OOA and 1.0 ± 0.3 μg m-3 for CTR-BOA above 1 ppb additional biogenic OM for each 1 ppb increase of NOx.

Molecular-Size-Separated Brown Carbon Absorption for Biomass-Burning Aerosol at Multiple Field Sites

Biomass burning is a known source of brown carbon aerosol in the atmosphere. We collected filter samples of biomass-burning emissions at three locations in Canada and the United States with transport times of 10 h to >3 days. We analyzed the samples with size-exclusion chromatography coupled to molecular absorbance spectroscopy to determine absorbance as a function of molecular size. The majority of absorption was due to molecules >500 Da, and these contributed an increasing fraction of absorption as the biomass-burning aerosol aged. This suggests that the smallest molecular weight fraction is more susceptible to processes that lead to reduced light absorption, while larger-molecular-weight species may represent recalcitrant brown carbon. We calculate that these large-molecular-weight species are composed of more than 20 carbons with as few as two oxygens and would be classified as extremely low volatility organic compounds (ELVOCs).

Derivation of Hydroperoxyl Radical Levels at an Urban Site via Measurement of Pernitric Acid by Iodide Chemical Ionization Mass Spectrometry

Hydroperoxyl radical (HO₂) is a key species to atmospheric chemistry. At warm temperatures, the HO₂ and NO₂ come to a rapid steady state with pernitric acid (HO₂NO₂). This paper presents the derivation of HO₂ from observations of HO₂NO₂ and NO₂ in metropolitan Atlanta, US, in winter 2014 and summer 2015. HO₂ was observed to have a diurnal cycle with morning concentrations suppressed by high NO from the traffic. At night, derived HO₂ levels were nonzero and exhibited correlations with O₃ and NO₃, consistent with previous studies that ozonolysis and oxidation by NO₃ are sources of nighttime HO₂. Measured and model calculated HO₂ were in reasonable agreement: Without the constraint of measured HO₂NO₂, the model reproduced HO₂ with a model-to-observed ratio (M/O) of 1.27 (r = 0.54) for winter, 2014, and 0.70 (r = 0.80) for summer, 2015. Adding measured HO₂NO₂ as a constraint, the model predicted HO₂ with M/O = 1.13 (r = 0.77) for winter 2014 and 0.90 (r = 0.97) for summer 2015. These results demonstrate the feasibility of deriving HO₂ from HO₂NO₂ measurements in warm regions where HO₂NO₂ has a short lifetime.

Semivolatile POA and parameterized total combustion SOA in CMAQv5.2: impacts on source strength and partitioning

Mounting evidence from field and laboratory observations coupled with atmospheric model analyses shows that primary combustion emissions of organic compounds dynamically partition between the vapor and particulate phases, especially as near-source emissions dilute and cool to ambient conditions. The most recent version of the Community Multiscale Air Quality model version 5.2 (CMAQv5.2) accounts for the semivolatile partitioning and gas-phase aging of these primary organic aerosol (POA) compounds consistent with experimentally derived parameterizations. We also include a new surrogate species, potential secondary organic aerosol from combustion emissions (pcSOA), which provides a representation of the secondary organic aerosol (SOA) from anthropogenic combustion sources that could be missing from current chemical transport model predictions. The reasons for this missing mass likely include the following: (1) unspeciated semivolatile and intermediate volatility organic compound (SVOC and IVOC, respectively) emissions missing from current inventories, (2) multigenerational aging of organic vapor products from known SOA precursors (e.g., toluene, alkanes), (3) underestimation of SOA yields due to vapor wall losses in smog chamber experiments, and (4) reversible organic compounds-water interactions and/or aqueous-phase processing of known organic vapor emissions. CMAQ predicts the spatially averaged contribution of pcSOA to OA surface concentrations in the continental United States to be 38.6 and 23.6 % in the 2011 winter and summer, respectively. Whereas many past modeling studies focused on a particular measurement campaign, season, location, or model configuration, we endeavor to evaluate the model and important uncertain parameters with a comprehensive set of United States-based model runs using multiple horizontal scales (4 and 12 km), gas-phase chemical mechanisms, and seasons and years. The model with representation of semivolatile POA improves predictions of hourly OA observations over the traditional nonvolatile model at sites during field campaigns in southern California (CalNex, May-June 2010), northern California (CARES, June 2010), the southeast US (SOAS, June 2013; SEARCH, January and July, 2011). Model improvements manifest better correlations (e.g., the correlation coefficient at Pasadena at night increases from 0.38 to 0.62) and reductions in underprediction during the photochemically active afternoon period (e.g., bias at Pasadena from -5.62 to -2.42 μg m^-3). Daily averaged predictions of observations at routine-monitoring networks from simulations over the continental US (CONUS) in 2011 show modest improvement during winter, with mean biases reducing from 1.14 to 0.73μg m^-3, but less change in the summer when the decreases from POA evaporation were similar to the magnitude of added SOA mass. Because the model-performance improvement realized by including the relatively simple pcSOA approach is similar to that of more-complicated parameterizations of OA formation and aging, we recommend caution when applying these more-complicated approaches as they currently rely on numerous uncertain parameters. The pcSOA parameters optimized for performance at the southern and northern California sites lead to higher OA formation than is observed in the CONUS evaluation. This may be due to any of the following: variations in real pcSOA in different regions or time periods, too-high concentrations of other OA sources in the model that are important over the larger domain, or other model issues such as loss processes. This discrepancy is likely regionally and temporally dependent and driven by interferences from factors like varying emissions and chemical regimes.

Semivolatile POA and parameterized total combustion SOA in CMAQv5.2: impacts on source strength and partitioning

Mounting evidence from field and laboratory observations coupled with atmospheric model analyses shows that primary combustion emissions of organic compounds dynamically partition between the vapor and particulate phases, especially as near-source emissions dilute and cool to ambient conditions. The most recent version of the Community Multiscale Air Quality model version 5.2 (CMAQv5.2) accounts for the semivolatile partitioning and gas-phase aging of these primary organic aerosol (POA) compounds consistent with experimentally derived parameterizations. We also include a new surrogate species, potential secondary organic aerosol from combustion emissions (pcSOA), which provides a representation of the secondary organic aerosol (SOA) from anthropogenic combustion sources that could be missing from current chemical transport model predictions. The reasons for this missing mass likely include the following: (1) unspeciated semivolatile and intermediate volatility organic compound (SVOC and IVOC, respectively) emissions missing from current inventories, (2) multigenerational aging of organic vapor products from known SOA precursors (e.g., toluene, alkanes), (3) underestimation of SOA yields due to vapor wall losses in smog chamber experiments, and (4) reversible organic compounds-water interactions and/or aqueous-phase processing of known organic vapor emissions. CMAQ predicts the spatially averaged contribution of pcSOA to OA surface concentrations in the continental United States to be 38.6 and 23.6 % in the 2011 winter and summer, respectively. Whereas many past modeling studies focused on a particular measurement campaign, season, location, or model configuration, we endeavor to evaluate the model and important uncertain parameters with a comprehensive set of United States-based model runs using multiple horizontal scales (4 and 12 km), gas-phase chemical mechanisms, and seasons and years. The model with representation of semivolatile POA improves predictions of hourly OA observations over the traditional nonvolatile model at sites during field campaigns in southern California (CalNex, May-June 2010), northern California (CARES, June 2010), the southeast US (SOAS, June 2013; SEARCH, January and July, 2011). Model improvements manifest better correlations (e.g., the correlation coefficient at Pasadena at night increases from 0.38 to 0.62) and reductions in underprediction during the photochemically active afternoon period (e.g., bias at Pasadena from -5.62 to -2.42 μg m^-3). Daily averaged predictions of observations at routine-monitoring networks from simulations over the continental US (CONUS) in 2011 show modest improvement during winter, with mean biases reducing from 1.14 to 0.73μg m^-3, but less change in the summer when the decreases from POA evaporation were similar to the magnitude of added SOA mass. Because the model-performance improvement realized by including the relatively simple pcSOA approach is similar to that of more-complicated parameterizations of OA formation and aging, we recommend caution when applying these more-complicated approaches as they currently rely on numerous uncertain parameters. The pcSOA parameters optimized for performance at the southern and northern California sites lead to higher OA formation than is observed in the CONUS evaluation. This may be due to any of the following: variations in real pcSOA in different regions or time periods, too-high concentrations of other OA sources in the model that are important over the larger domain, or other model issues such as loss processes. This discrepancy is likely regionally and temporally dependent and driven by interferences from factors like varying emissions and chemical regimes.

On the implications of aerosol liquid water and phase separation for organic aerosol mass

Organic compounds and liquid water are major aerosol constituents in the southeast United States (SE US). Water associated with inorganic constituents (inorganic water) can contribute to the partitioning medium for organic aerosol when relative humidities or organic matter to organic carbon (OM/OC) ratios are high such that separation relative humidities (SRH) are below the ambient relative humidity (RH). As OM/OC ratios in the SE US are often between 1.8 and 2.2, organic aerosol experiences both mixing with inorganic water and separation from it. Regional chemical transport model simulations including inorganic water (but excluding water uptake by organic compounds) in the partitioning medium for secondary organic aerosol (SOA) when RH > SRH led to increased SOA concentrations,· particularly at night. Water uptake to the organic phase resulted in even greater SOA concentrations as a result of a positive feedback in which water uptake increased SOA, which further increased aerosol water and organic aerosol. Aerosol properties· such as the OM/OC and hygroscopicity parameter (κorg), were captured well by the model compared with measurements during the Southern Oxidant and Aerosol Study (SOAS) 2013. Organic nitrates from monoterpene oxidation were predicted to be the least water-soluble semivolatile species in the model, but most biogenically derived semivolatile species in the Community Multiscale Air Quality (CMAQ) model were highly water soluble and expected to contribute to water-soluble organic carbon (WSOC). Organic aerosol and SOA precursors were abundant at night, but additional improvements in daytime organic aerosol are needed to close the model-measurement gap. When taking into account deviations from ideality, including both inorganic (when RH > SRH) and organic water in the organic partitioning medium reduced the mean bias in SOA for routine monitoring networks and improved model performance compared to observations from SOAS. Property updates from this work will be released in CMAQ v5.2.

Modeling the Current and Future Roles of Particulate Organic Nitrates in the Southeastern United States

Organic nitrates are an important aerosol constituent in locations where biogenic hydrocarbon emissions mix with anthropogenic NOx sources. While regional and global chemical transport models may include a representation of organic aerosol from monoterpene reactions with nitrate radicals (the primary source of particle-phase organic nitrates in the Southeast United States), secondary organic aerosol (SOA) models can underestimate yields. Furthermore, SOA parametrizations do not explicitly take into account organic nitrate compounds produced in the gas phase. In this work, we developed a coupled gas and aerosol system to describe the formation and subsequent aerosol-phase partitioning of organic nitrates from isoprene and monoterpenes with a focus on the Southeast United States. The concentrations of organic aerosol and gas-phase organic nitrates were improved when particulate organic nitrates were assumed to undergo rapid (τ = 3 h) pseudohydrolysis resulting in nitric acid and nonvolatile secondary organic aerosol. In addition, up to 60% of less oxidized-oxygenated organic aerosol (LO-OOA) could be accounted for via organic nitrate mediated chemistry during the Southern Oxidants and Aerosol Study (SOAS). A 25% reduction in nitrogen oxide (NO + NO2) emissions was predicted to cause a 9% reduction in organic aerosol for June 2013 SOAS conditions at Centreville, Alabama.

Enhanced light absorption by mixed source black and brown carbon particles in UK winter

Black carbon (BC) and light-absorbing organic carbon (brown carbon, BrC) play key roles in warming the atmosphere, but the magnitude of their effects remains highly uncertain. Theoretical modelling and laboratory experiments demonstrate that coatings on BC can enhance BC's light absorption, therefore many climate models simply assume enhanced BC absorption by a factor of ∼1.5. However, recent field observations show negligible absorption enhancement, implying models may overestimate BC's warming. Here we report direct evidence of substantial field-measured BC absorption enhancement, with the magnitude strongly depending on BC coating amount. Increases in BC coating result from a combination of changing sources and photochemical aging processes. When the influence of BrC is accounted for, observationally constrained model calculations of the BC absorption enhancement can be reconciled with the observations. We conclude that the influence of coatings on BC absorption should be treated as a source and regionally specific parameter in climate models.

Evaluation of one-dimensional and two-dimensional volatility basis sets in simulating the aging of secondary organic aerosol with smog-chamber experiments

We evaluate the one-dimensional volatility basis set (1D-VBS) and two-dimensional volatility basis set (2D-VBS) in simulating the aging of SOA derived from toluene and α-pinene against smog-chamber experiments. If we simulate the first-generation products with empirical chamber fits and the subsequent aging chemistry with a 1D-VBS or a 2D-VBS, the models mostly overestimate the SOA concentrations in the toluene oxidation experiments. This is because the empirical chamber fits include both first-generation oxidation and aging; simulating aging in addition to this results in double counting of the initial aging effects. If the first-generation oxidation is treated explicitly, the base-case 2D-VBS underestimates the SOA concentrations and O:C increase of the toluene oxidation experiments; it generally underestimates the SOA concentrations and overestimates the O:C increase of the α-pinene experiments. With the first-generation oxidation treated explicitly, we could modify the 2D-VBS configuration individually for toluene and α-pinene to achieve good model-measurement agreement. However, we are unable to simulate the oxidation of both toluene and α-pinene with the same 2D-VBS configuration. We suggest that future models should implement parallel layers for anthropogenic (aromatic) and biogenic precursors, and that more modeling studies and laboratory research be done to optimize the "best-guess" parameters for each layer.

Effects of NOx on the volatility of secondary organic aerosol from isoprene photooxidation

The effects of NOx on the volatility of the secondary organic aerosol (SOA) formed from isoprene photooxidation are investigated in environmental chamber experiments. Two types of experiments are performed. In HO2-dominant experiments, organic peroxy radicals (RO2) primarily react with HO2. In mixed experiments, RO2 reacts through multiple pathways, including with NO, NO2, and HO2. The volatility and oxidation state of isoprene SOA are sensitive to and exhibit a nonlinear dependence on NOx levels. Depending on the NOx levels, the SOA formed in mixed experiments can be of similar or lower volatility compared to that formed in HO2-dominant experiments. The dependence of SOA yield, volatility, and oxidation state on the NOx level likely arises from gas-phase RO2 chemistry and succeeding particle-phase oligomerization reactions. The NOx level also plays a strong role in SOA aging. While the volatility of SOA in mixed experiments does not change substantially over time, SOA becomes less volatile and more oxidized as oxidation progresses in HO2-dominant experiments.

Contribution of nitrated phenols to wood burning brown carbon light absorption in Detling, United Kingdom during winter time

We show for the first time quantitative online measurements of five nitrated phenol (NP) compounds in ambient air (nitrophenol C6H5NO3, methylnitrophenol C7H7NO3, nitrocatechol C6H5NO4, methylnitrocatechol C7H7NO4, and dinitrophenol C6H4N2O5) measured with a micro-orifice volatilization impactor (MOVI) high-resolution chemical ionization mass spectrometer in Detling, United Kingdom during January-February, 2012. NPs absorb radiation in the near-ultraviolet (UV) range of the electromagnetic spectrum and thus are potential components of poorly characterized light-absorbing organic matter ("brown carbon") which can affect the climate and air quality. Total NP concentrations varied between less than 1 and 98 ng m(-3), with a mean value of 20 ng m(-3). We conclude that NPs measured in Detling have a significant contribution from biomass burning with an estimated emission factor of 0.2 ng (ppb CO)(-1). Particle light absorption measurements by a seven-wavelength aethalometer in the near-UV (370 nm) and literature values of molecular absorption cross sections are used to estimate the contribution of NP to wood burning brown carbon UV light absorption. We show that these five NPs are potentially important contributors to absorption at 370 nm measured by an aethalometer and account for 4 ± 2% of UV light absorption by brown carbon. They can thus affect atmospheric radiative transfer and photochemistry and with that climate and air quality.

Understanding atmospheric organic aerosols via factor analysis of aerosol mass spectrometry: a review

Organic species are an important but poorly characterized constituent of airborne particulate matter. A quantitative understanding of the organic fraction of particles (organic aerosol, OA) is necessary to reduce some of the largest uncertainties that confound the assessment of the radiative forcing of climate and air quality management policies. In recent years, aerosol mass spectrometry has been increasingly relied upon for highly time-resolved characterization of OA chemistry and for elucidation of aerosol sources and lifecycle processes. Aerodyne aerosol mass spectrometers (AMS) are particularly widely used, because of their ability to quantitatively characterize the size-resolved composition of submicron particles (PM₁). AMS report the bulk composition and temporal variations of OA in the form of ensemble mass spectra (MS) acquired over short time intervals. Because each MS represents the linear superposition of the spectra of individual components weighed by their concentrations, multivariate factor analysis of the MS matrix has proved effective at retrieving OA factors that offer a quantitative and simplified description of the thousands of individual organic species. The sum of the factors accounts for nearly 100% of the OA mass and each individual factor typically corresponds to a large group of OA constituents with similar chemical composition and temporal behavior that are characteristic of different sources and/or atmospheric processes. The application of this technique in aerosol mass spectrometry has grown rapidly in the last six years. Here we review multivariate factor analysis techniques applied to AMS and other aerosol mass spectrometers, and summarize key findings from field observations. Results that provide valuable information about aerosol sources and, in particular, secondary OA evolution on regional and global scales are highlighted. Advanced methods, for example a-priori constraints on factor mass spectra and the application of factor analysis to combined aerosol and gas phase data are discussed. Integrated analysis of worldwide OA factors is used to present a holistic regional and global description of OA. Finally, different ways in which OA factors can constrain global and regional models are discussed.

Real-time methods for estimating organic component mass concentrations from aerosol mass spectrometer data

We use results from positive matrix factorization (PMF) analysis of 15 urban aerosol mass spectrometer (AMS) data sets to derive simple methods for estimating major organic aerosol (OA) component concentrations in real time. PMF analysis extracts mass spectral (MS) profiles and mass concentrations for key OA components such as hydrocarbon-like OA (HOA), oxygenated OA (OOA), low-volatility OOA (LV-OOA), semivolatile OOA (SV-OOA), and biomass burning OA (BBOA). The variability in the component MS across all sites is characterized and used to derive standard profiles for real-time estimation of component concentrations. Two methods for obtaining first-order estimates of the HOA and OOA mass concentrations are evaluated. The first approach is the tracer m/z method, in which the HOA and OOA concentrations are estimated from m/z 57 and m/z 44 as follows: HOA ∼ 13.4 × (C(57) - 0.1 × C(44)) and OOA ∼ 6.6 × C(44), where C(i) is the equivalent mass concentration of tracer ion m/z i. The second approach uses a chemical mass balance (CMB) method in which standard HOA and OOA profiles are used as a priori information for calculating their mass concentrations. The HOA and OOA mass concentrations obtained from the first-order estimates are evaluated by comparing with the corresponding PMF results for each site. Both methods reproduce the HOA and OOA concentrations to within ∼30% of the results from detailed PMF analysis at most sites, with the CMB method being slightly better. For hybrid CMB methods, we find that fixing the LV-OOA spectrum and not constraining the other spectra produces the best results.

Chemical composition of secondary organic aerosol formed from the photooxidation of isoprene

Recent work in our laboratory has shown that the photooxidation of isoprene (2-methyl-1,3-butadiene, C(5)H(8)) leads to the formation of secondary organic aerosol (SOA). In the current study, the chemical composition of SOA from the photooxidation of isoprene over the full range of NO(x) conditions is investigated through a series of controlled laboratory chamber experiments. SOA composition is studied using a wide range of experimental techniques: electrospray ionization-mass spectrometry, matrix-assisted laser desorption ionization-mass spectrometry, high-resolution mass spectrometry, online aerosol mass spectrometry, gas chromatography/mass spectrometry, and an iodometric-spectroscopic method. Oligomerization was observed to be an important SOA formation pathway in all cases; however, the nature of the oligomers depends strongly on the NO(x) level, with acidic products formed under high-NO(x) conditions only. We present, to our knowledge, the first evidence of particle-phase esterification reactions in SOA, where the further oxidation of the isoprene oxidation product methacrolein under high-NO(x) conditions produces polyesters involving 2-methylglyceric acid as a key monomeric unit. These oligomers comprise approximately 22-34% of the high-NO(x) SOA mass. Under low-NO(x) conditions, organic peroxides contribute significantly to the low-NO(x) SOA mass (approximately 61% when SOA forms by nucleation and approximately 25-30% in the presence of seed particles). The contribution of organic peroxides in the SOA decreases with time, indicating photochemical aging. Hemiacetal dimers are found to form from C(5) alkene triols and 2-methyltetrols under low-NO(x) conditions; these compounds are also found in aerosol collected from the Amazonian rainforest, demonstrating the atmospheric relevance of these low-NO(x) chamber experiments.

Reactions of semivolatile organics and their effects on secondary organic aerosol formation

Secondary organic aerosol (SOA) constitutes a significant fraction of total atmospheric particulate loading, but there is evidence that SOA yields based on laboratory studies may underestimate atmospheric SOA. Here we present chamber data on SOA growth from the photooxidation of aromatic hydrocarbons, finding that SOA yields are systematically lower when inorganic seed particles are not initially present. This indicates that concentrations of semivolatile oxidation products are influenced by processes beyond gas-particle partitioning, such as chemical reactions and/or loss to chamber walls. Predictions of a kinetic model in which semivolatile compounds may undergo reactions in both the gas and particle phases in addition to partitioning are qualitatively consistent with the observed seed effect, as well as with a number of other recently observed features of SOA formation chemistry. The behavior arises from a kinetic competition between uptake to the particle phase and reactive loss of the semivolatile product. It is shown that when hydrocarbons react in the absence of preexisting organic aerosol, such loss processes may lead to measured SOA yields lower than would occur under atmospheric conditions. These results underscore the need to conduct studies of SOA formation in the presence of atmospherically relevant aerosol loadings.

Evidence for organosulfates in secondary organic aerosol

Recent work has shown that particle-phase reactions contribute to the formation of secondary organic aerosol (SOA), with enhancements of SOA yields in the presence of acidic seed aerosol. In this study, the chemical composition of SOA from the photooxidations of alpha-pinene and isoprene, in the presence or absence of sulfate seed aerosol, is investigated through a series of controlled chamber experiments in two separate laboratories. By using electrospray ionization-mass spectrometry, sulfate esters in SOA produced in laboratory photooxidation experiments are identified for the first time. Sulfate esters are found to account for a larger fraction of the SOA mass when the acidity of seed aerosol is increased, a result consistent with aerosol acidity increasing SOA formation. Many of the isoprene and alpha-pinene sulfate esters identified in these chamber experiments are also found in ambient aerosol collected at several locations in the southeastern U.S. It is likely that this pathway is important for other biogenic terpenes, and may be important in the formation of humic-like substances (HULIS) in ambient aerosol.

Characterization of 2-methylglyceric acid oligomers in secondary organic aerosol formed from the photooxidation of isoprene using trimethylsilylation and gas chromatography/ion trap mass spectrometry

In the present work, we have characterized in detail the chemical structures of secondary organic aerosol (SOA) components that were generated in a smog chamber and result from the photooxidation of isoprene under high-NO(x) conditions typical for a polluted atmosphere. Isoprene high-NO(x) SOA contains 2-methylglyceric acid (2-MG) and oligoester derivatives thereof. Trimethylsilylation, in combination with capillary gas chromatography (GC)/ion trap mass spectrometry (MS) and detailed interpretation of the MS data, allowed structural characterization the polar oxygenated compounds present in isoprene SOA up to 2-MG trimers. GC separation was achieved between 2-MG linear and branched dimers or trimers, as well as between the 2-MG linear dimer and isomeric mono-acetate derivatives thereof. The electron ionization (EI) spectra of the trimethylsilyl derivatives contain a wealth of structural information, including information about the molecular weight (MW), oligoester linkages, terminal carboxylic and hydroxymethyl groups, and esterification sites. Only part of this information can be achieved with a soft ionization technique such as electrospray (ESI) in combination with collision-induced dissociation (CID). The methane chemical ionization (CI) data were used to obtain supporting MW information. Interesting EI spectral differences were observed between the trimethylsilyl derivatives of 2-MG linear and branched dimers or trimers and between 2-MG linear dimer mono-acetate isomers.

Contribution of first- versus second-generation products to secondary organic aerosols formed in the oxidation of biogenic hydrocarbons

Biogenic hydrocarbons emitted by vegetation are important contributors to secondary organic aerosol (SOA), but the aerosol formation mechanisms are incompletely understood. In this study, the formation of aerosols and gas-phase products from the ozonolysis and photooxidation of a series of biogenic hydrocarbons (isoprene, 8 monoterpenes, 4 sesquiterpenes, and 3 oxygenated terpenes) are examined. By comparing aerosol growth (measured by Differential Mobility Analyzers, DMAs) and gas-phase concentrations (monitored by a Proton Transfer Reaction Mass Spectrometer, PTR-MS), we study the general mechanisms of SOA formation. Aerosol growth data are presented in terms of a "growth curve", a plot of aerosol mass formed versus the amount of hydrocarbon reacted. From the shapes of the growth curves, it is found that all the hydrocarbons studied can be classified into two groups based entirely on the number of double bonds of the hydrocarbon, regardless of the reaction systems (ozonolysis or photooxidation) and the types of hydrocarbons studied: compounds with only one double bond and compounds with more than one double bond. For compounds with only one double bond, the first oxidation step is rate-limiting, and aerosols are formed mainly from low volatility first-generation oxidation products; whereas for compounds with more than one double bond, the second oxidation step may also be rate-limiting and second-generation products contribute substantially to SOA growth. This behavior is characterized by a vertical section in the growth curve, in which continued aerosol growth is observed even after all the parent hydrocarbon is consumed.

Secondary organic aerosol formation from isoprene photooxidation

Recent work has shown that the atmospheric oxidation of isoprene (2-methyl-1,3-butadiene, C5H8) leads to the formation of secondary organic aerosol (SOA). In this study, the mechanism of SOA formation by isoprene photooxidation is comprehensively investigated, by measurements of SOA yields over a range of experimental conditions, namely isoprene and NOx concentrations. Hydrogen peroxide is used as the radical precursor, substantially constraining the observed gas-phase chemistry; all oxidation is dominated by the OH radical, and organic peroxy radicals (RO2) react only with HO2 (formed in the OH + H2O2 reaction) or NO concentrations, including NOx-free conditions. At high NOx, yields are found to decrease substantially with increasing [NOx], indicating the importance of RO2 chemistry in SOA formation. Under low-NOx conditions, SOA mass is observed to decay rapidly, a result of chemical reactions of semivolatile SOA components, most likely organic hydroperoxides.

Measurements of secondary organic aerosol from oxidation of cycloalkenes, terpenes, and m-xylene using an Aerodyne aerosol mass spectrometer

The Aerodyne aerosol mass spectrometer (AMS) was used to characterize physical and chemical properties of secondary organic aerosol (SOA) formed during ozonolysis of cycloalkenes and biogenic hydrocarbons and photo-oxidation of m-xylene. Comparison of mass and volume distributions from the AMS and differential mobility analyzers yielded estimates of "effective" density of the SOA in the range of 0.64-1.45 g/cm3, depending on the particular system. Increased contribution of the fragment at m/z 44, C02+ ion fragment of oxygenated organics, and higher "delta" values, based on ion series analysis of the mass spectra, in nucleation experiments of cycloalkenes suggest greater contribution of more oxygenated molecules to the SOA as compared to those formed under seeded experiments. Dominant negative "delta" values of SOA formed during ozonolysis of biogenics indicates the presence of terpene derivative structures or cyclic or unsaturated oxygenated compounds in the SOA. Evidence of acid-catalyzed heterogeneous chemistry, characterized by greater contribution of higher molecular weight fragments to the SOA and corresponding changes in "delta" patterns, is observed in the ozonolysis of alpha-pinene. Mass spectra of SOA formed during photooxidation of m-xylene exhibit features consistent with the presence of furandione compounds and nitro organics. This study demonstrates that mixtures of SOA compounds produced from similar precursors result in broadly similar AMS mass spectra. Thus, fragmentation patterns observed for biogenic versus anthropogenic SOA may be useful in determining the sources of ambient SOA.

Particle phase acidity and oligomer formation in secondary organic aerosol

A series of controlled laboratory experiments are carried out in dual Teflon chambers to examine the presence of oligomers in secondary organic aerosols (SOA) from hydrocarbon ozonolysis as well as to explore the effect of particle phase acidity on SOA formation. In all seven hydrocarbon systems studied (i.e., alpha-pinene, cyclohexene, 1-methyl cyclopentene, cycloheptene, 1-methyl cyclohexene, cyclooctene, and terpinolene), oligomers with MW from 250 to 1600 are present in the SOA formed, both in the absence and presence of seed particles and regardless of the seed particle acidity. These oligomers are comparable to, and in some cases, exceed the low molecular weight species (MW < 250) in ion intensities in the ion trap mass spectra, suggesting they may comprise a substantial fraction of the total aerosol mass. It is possible that oligomers are widely present in atmospheric organic aerosols, formed through acid- or base-catalyzed heterogeneous reactions. In addition, as the seed particle acidity increases, larger oligomers are formed more abundantly in the SOA; consequently, the overall SOA yield also increases. This explicit effect of particle phase acidity on the composition and yield of SOA may have important climatic consequences and need to be considered in relevant models.

Organ-bath studies using the irreversible beta-adrenoreceptor antagonist bromoacetylalprenololmenthane (BAAM)

1. The effects of the irreversible beta-adrenoreceptor antagonist bromoacetylalprenololmenthane (BAAM) were studied in isolated cardiac and uterine preparations from guinea-pigs and rats and in guinea-pig ileal preparations. 2. In the presence of BAAM (0.1-10 microM) concentration-effect curves to (-)-isoprenaline were shifted to the right in a concentration-dependent manner in all cardiac and uterine tissues. Maximum responses to (-)-isoprenaline were unaffected by BAAM except in guinea-pig left atrial and in some guinea-pig uterine preparations; however, the reductions in the maximum responses were not concentration-dependent. The mean pKB values for BAAM in guinea-pig left atria, right atria, rat whole atria and rat uterus were 7.26, 7.24, 6.84 and 7.90 respectively. 3. In guinea-pig ileal preparations, BAAM (0.1-30 microM) relaxed contractions induced by K+, histamine and acetylcholine in a non-beta-adrenoreceptor-related manner since relaxant responses were unaffected by propranolol (0.5 microM). In other tissues higher concentrations of BAAM (30-100 microM) elicited atrial standstill and depressed K+-induced contractions in uterine smooth muscle. 4. Treatment of tissues with BAAM (10-100 microM) followed by extensive wash-out increased the EC50 values for (-)-isoprenaline 21- to 83-fold. The maximum response to the catecholamine was unaffected by BAAM except in guinea-pig left atrial preparations following treatment with 100 microM BAAM. At these concentrations BAAM markedly increased the effective refractory period. 5. Concentration-effect curves for the partial agonist, oxymethylene-isoprenaline (OM-ISO) were shifted to the right 12- to 355-fold after pretreatment of tissues with BAAM (10-30 microM) followed by wash-out. The maximum response to OM-ISO was unaltered in guinea-pig and rat uteri and was reduced to a similar degree as observed with (-)-isoprenaline in guinea-pig left atria. 6. In general, the non-selective beta-adrenoreceptor antagonist BAAM depressed maximum responses to beta-adrenoreceptor agonists only in cardiac preparations and at concentrations which elicited depressant activity. On the basis of the present study, BAAM does not appear to be a suitable irreversible beta-adrenoreceptor antagonist for use in organ bath experiments.