Isoprene Epoxydiols Research Articles

Molecular dynamics investigation of IEPOX chemical behavior at the interface and in the bulk phase of acidic aerosols

Isoprene epoxydiol (IEPOX) is an important reactive gas-phase intermediate produced by the photooxidation of isoprene under low NOx conditions, playing a key role in the formation of secondary organic aerosols (SOA). Previous studies have mostly focused on the liquid-phase reactions of IEPOX within aerosols; however, interfacial heterogeneous chemical reactions are equally important in SOA formation. This study systematically explores the reaction mechanisms of IEPOX at the acidic aerosol interface and in the bulk phase using classical molecular dynamics (MD) and ab initio molecular dynamics simulations (AIMD). The study found that the free energy of IEPOX at the aerosol interface significantly decreases, indicating that interfacial heterogeneous chemical reactions are indispensable for the formation of IEPOX-derived SOA. The research reveals the formation pathways of 2-methyltetrols (2-MTO) and 1,3,4-trihydroxy-3-methylbutan-2-yl sulfates (2-MTOOS), finding that the protonation of the epoxy O atom and the cleavage of the C–O bond are the rate-controlling steps, while the nucleophilic addition is a spontaneous process. Through multiple sets of simulations, it was observed that the formation frequency of 2-MTO at the acidic aerosol interface and in the bulk phase reached 53.8%, significantly higher than the 30.8% of 2-MTOOS, which is consistent with field observation data. Additionally, through metadynamics (MTD) simulations, it was suggested that IEPOX could undergoes acid-catalyzed ring-opening reactions at the interface, potentially followed by the transfer of H atoms from primary alcohols into the aerosol, leading to the possible formation of the intermediate product 3-methylbut-3-ene-1,2,4-triol (one of the proposed structures of C5-alkene triols). These findings provide new insights into the formation mechanism of IEPOX-derived SOA and offer a scientific basis for future studies on their physicochemical properties and atmospheric fate.

Development of a multiphase chemical mechanism to improve secondary organic aerosol formation in CAABA/MECCA (version 4.7.0)

Abstract. During the last few decades, the impact of multiphase chemistry on secondary organic aerosols (SOAs) has been demonstrated to be the key to explaining laboratory experiments and field measurements. However, global atmospheric models still show large biases when simulating atmospheric observations of organic aerosols (OAs). Major reasons for the model errors are the use of simplified chemistry schemes of the gas-phase oxidation of vapours and the parameterization of heterogeneous surface reactions. The photochemical oxidation of anthropogenic and biogenic volatile organic compounds (VOCs) leads to products that either produce new SOA or are taken up by existing aqueous media like cloud droplets and deliquescent aerosols. After partitioning, aqueous-phase processing results in polyols, organosulfates, and other products with a high molar mass and oxygen content. In this work, we introduce the formation of new low-volatility organic compounds (LVOCs) to the multiphase chemistry box model CAABA/MECCA. Most notable are the additions of the SOA precursors, limonene and n-alkanes (5 to 8 C atoms), and a semi-explicit chemical mechanism for the formation of LVOCs from isoprene oxidation in the gas and aqueous phases. Moreover, Henry's law solubility constants and their temperature dependences are estimated for the partitioning of organic molecules to the aqueous phase. Box model simulations indicate that the new chemical scheme predicts the enhanced formation of LVOCs, which are known for being precursor species to SOAs. As expected, the model predicts that LVOCs are positively correlated to temperature but negatively correlated to NOx levels. However, the aqueous-phase processing of isoprene epoxydiols (IEPOX) displays a more complex dependence on these two key variables. Semi-quantitative comparison with observations from the SOAS campaign suggests that the model may overestimate methylbutane-1,2,3,4-tetrol (MeBuTETROL) from IEPOX. Further application of the mechanism in the modelling of two chamber experiments, one in which limonene is consumed by ozone and one in which isoprene is consumed by NO3 shows a sufficient agreement with experimental results within model limitations. The extensions in CAABA/MECCA are transferred to the 3D atmospheric model MESSy for a comprehensive evaluation of the impact of aqueous- and/or aerosol-phase chemistry on SOA at a global scale in a follow-up study.

Application of advanced causal analyses to identify processes governing secondary organic aerosols

Understanding how different physical and chemical atmospheric processes affect the formation of fine particles has been a persistent challenge. Inferring causal relations between the various measured features affecting the formation of secondary organic aerosol (SOA) particles is complicated since correlations between variables do not necessarily imply causality. Here, we apply a state-of-the-art information transfer measure coupled with the Koopman operator framework to infer causal relations between isoprene epoxydiol SOA (IEPOX-SOA) and different chemistry and meteorological variables derived from detailed regional model predictions over the Amazon rainforest. IEPOX-SOA represents one of the most complex SOA formation pathways and is formed by the interactions between natural biogenic isoprene emissions and anthropogenic emissions affecting sulfate, acidity and particle water. Since the regional model captures the known relations of IEPOX-SOA with different chemistry and meteorological features, their simulated time series implicitly include their causal relations. We show that our causal model successfully infers the known major causal relations between total particle phase 2-methyl tetrols (the dominant component of IEPOX-SOA over the Amazon) and input features. We provide the first proof of concept that the application of our causal model better identifies causal relations compared to correlation and random forest analyses performed over the same dataset. Our work has tremendous implications, as our methodology of causal discovery could be used to identify unknown processes and features affecting fine particles and atmospheric chemistry in the Earth’s atmosphere.

Applying a Phase-Separation Parameterization in Modeling Secondary Organic Aerosol Formation from Acid-Driven Reactive Uptake of Isoprene Epoxydiols under Humid Conditions.

Secondary organic aerosol (SOA) from acid-driven reactive uptake of isoprene epoxydiols (IEPOX) contributes up to 40% of organic aerosol (OA) mass in fine particulate matter. Previous work showed that IEPOX substantially converts particulate inorganic sulfates to surface-active organosulfates (OSs). This decreases aerosol acidity and creates a viscous organic-rich shell that poses as a diffusion barrier, inhibiting additional reactive uptake of IEPOX. To account for this "self-limiting" effect, we developed a phase-separation box model to evaluate parameterizations of IEPOX reactive uptake against time-resolved chamber measurements of IEPOX-SOA tracers, including 2-methyltetrols (2-MT) and methyltetrol sulfates (MTS), at ~ 50% relative humidity. The phase-separation model was most sensitive to the mass accommodation coefficient, IEPOX diffusivity in the organic shell, and ratio of the third-order reaction rate constants forming 2-MT and MTS ( ). In particular, had to be lower than 0.1 to bring model predictions of 2-MT and MTS in closer agreement with chamber measurements; prior studies reported values larger than 0.71. The model-derived rate constants favor more particulate MTS formation due to 2-MT likely off-gassing at ambient-relevant OA loadings. Incorporating this parametrization into chemical transport models is expected to predict lower IEPOX-SOA mass and volatility due to the predominance of OSs.

Modeling Novel Aqueous Particle and Cloud Chemistry Processes of Biomass Burning Phenols and Their Potential to Form Secondary Organic Aerosols.

Phenols emitted from biomass burning contribute significantly to secondary organic aerosol (SOA) formation through the partitioning of semivolatile products formed from gas-phase chemistry and multiphase chemistry in aerosol liquid water and clouds. The aqueous-phase SOA (aqSOA) formed via hydroxyl radical (•OH), singlet molecular oxygen (1O2*), and triplet excited states of organic compounds (3C*), which oxidize dissolved phenols in the aqueous phase, might play a significant role in the evolution of organic aerosol (OA). However, a quantitative and predictive understanding of aqSOA has been challenging. Here, we develop a stand-alone box model to investigate the formation of SOA from gas-phase •OH chemistry and aqSOA formed by the dissolution of phenols followed by their aqueous-phase reactions with •OH, 1O2*, and 3C* in cloud droplets and aerosol liquid water. We investigate four phenolic compounds, i.e., phenol, guaiacol, syringol, and guaiacyl acetone (GA), which represent some of the key potential sources of aqSOA from biomass burning in clouds. For the same initial precursor organic gas that dissolves in aerosol/cloud liquid water and subsequently reacts with aqueous phase oxidants, we predict that the aqSOA formation potential (defined as aqSOA formed per unit dissolved organic gas concentration) of these phenols is higher than that of isoprene-epoxydiol (IEPOX), a well-known aqSOA precursor. Cloud droplets can dissolve a broader range of soluble phenols compared to aqueous aerosols, since the liquid water contents of aerosols are orders of magnitude smaller than cloud droplets. Our simulations suggest that highly soluble and reactive multifunctional phenols like GA would predominantly undergo cloud chemistry within cloud layers, while gas-phase chemistry is likely to be more important for less soluble phenols. But in the absence of clouds, the condensation of low-volatility products from gas-phase oxidation followed by their reversible partitioning to organic aerosols dominates SOA formation, while the SOA formed through aqueous aerosol chemistry increases with relative humidity (RH), approaching 40% of the sum of gas and aqueous aerosol chemistry at 95% RH for GA. Our model developments of biomass-burning phenols and their aqueous chemistry can be readily implemented in regional and global atmospheric chemistry models to investigate the aqueous aerosol and cloud chemistry of biomass-burning organic gases in the atmosphere.

Glass Transition Temperatures of Organic Mixtures from Isoprene Epoxydiol-Derived Secondary Organic Aerosol.

The phase states and glass transition temperatures (Tg) of secondary organic aerosol (SOA) particles are important to resolve for understanding the formation, growth, and fate of SOA as well as their cloud formation properties. Currently, there is a limited understanding of how Tg changes with the composition of organic and inorganic components of atmospheric aerosol. Using broadband dielectric spectroscopy, we measured the Tg of organic mixtures containing isoprene epoxydiol (IEPOX)-derived SOA components, including 2-methyltetrols (2-MT), 2-methyltetrol-sulfate (2-MTS), and 3-methyltetrol-sulfate (3-MTS). The results demonstrate that the Tg of mixtures depends on their composition. The Kwei equation, a modified Gordon-Taylor equation with an added quadratic term and a fitting parameter representing strong intermolecular interactions, provides a good fit for the Tg-composition relationship of complex mixtures. By combining Raman spectroscopy with geometry optimization simulations obtained using density functional theory, we demonstrate that the non-linear deviation of Tg as a function of composition may be caused by changes in the extent of hydrogen bonding in the mixture.

Physics informed deep neural network embedded in a chemical transport model for the Amazon rainforest

Secondary organic aerosols (SOA) are fine particles in the atmosphere, which interact with clouds, radiation and affect the Earth’s energy budget. SOA formation involves chemistry in gas phase, aqueous aerosols, and clouds. Simulating these chemical processes involve solving a stiff set of differential equations, which are computationally expensive steps for three-dimensional chemical transport models. Deep neural networks (DNNs) are universal function approximators that could be used to represent the complex nonlinear changes in aerosol physical and chemical processes; however, key challenges such as generalizability to extended time periods, preservation of mass balance, simulating sparse model outputs, and maintaining physical constraints have limited their use in atmospheric chemistry. Here, we develop an approach of using a physics-informed DNN that overcomes previous such challenges and demonstrates its applicability for the chemical formation processes of isoprene epoxydiol SOA (IEPOX-SOA) over the Amazon rainforest. The DNN is trained with data generated by simulating IEPOX-SOA over the entire atmospheric column, using the Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem). The trained DNN is then embedded within WRF-Chem to replace the computationally expensive default solver of IEPOX-SOA formation. The trained DNN predictions generalizes well with the default model simulation of the IEPOX-SOA mass concentrations and its size distribution (20 size bins) over several days of simulations in both dry and wet seasons. The embedded DNN reduces the computational expense of WRF-Chem by a factor of 2. Our approach shows promise in terms of application to other computationally expensive chemistry solvers in climate models.

AMORE-Isoprene v1.0: a new reduced mechanism for gas-phase isoprene oxidation

Abstract. Gas-phase oxidation of isoprene by ozone (O3) and the hydroxyl (OH) and nitrate (NO3) radicals significantly impacts tropospheric oxidant levels and secondary organic aerosol formation. The most comprehensive and up-to-date chemical mechanism for isoprene oxidation consists of several hundred species and over 800 reactions. Therefore, the computational expense of including the entire mechanism in large-scale atmospheric chemical transport models is usually prohibitive, and most models employ reduced isoprene mechanisms ranging in size from ∼ 10 to ∼ 200 species. We have developed a new reduced isoprene oxidation mechanism using a directed-graph path-based automated model reduction approach, with minimal manual adjustment of the output mechanism. The approach takes as inputs a full isoprene oxidation mechanism, the environmental parameter space, and a list of priority species which are protected from elimination during the reduction process. Our reduced mechanism, AMORE-Isoprene (where AMORE stands for Automated Model Reduction), consists of 12 species which are unique to the isoprene mechanism as well as 22 reactions. We demonstrate its performance in a box model in comparison with experimental data from the literature and other current isoprene oxidation mechanisms. AMORE-Isoprene's performance with respect to predicting the time evolution of isoprene oxidation products, including isoprene epoxydiols (IEPOX) and formaldehyde, is favorable compared with other similarly sized mechanisms. When AMORE-Isoprene is included in the Community Regional Atmospheric Chemistry Multiphase Mechanism 1.0 (CRACMM1AMORE) in the Community Multiscale Air Quality Model (CMAQ, v5.3.3), O3 and formaldehyde agreement with Environmental Protection Agency (EPA) Air Quality System observations is improved. O3 bias is reduced by 3.4 ppb under daytime conditions for O3 concentrations over 50 ppb. Formaldehyde bias is reduced by 0.26 ppb on average for all formaldehyde measurements compared with the base CRACMM1. There was no significant change in computation time between CRACMM1AMORE and the base CRACMM. AMORE-Isoprene shows a 35 % improvement in agreement between simulated IEPOX concentrations and chamber data over the base CRACMM1 mechanism when compared in the Framework for 0-D Atmospheric Modeling (F0AM) box model framework. This work demonstrates a new highly reduced isoprene mechanism and shows the potential value of automated model reduction for complex reaction systems.

Identification and quantification of IEPOX in ambient aerosols, using electron and chemical ionization sources GC/MS as their trimethylsilyl ethers, and using H-NMR

Isoprene is the most abundant non-methane hydrocarbon (NMHC) emitted by vegetation, and the precursor that makes the greatest contribution to secondary organic aerosols (SOA) in the troposphere. Atmospheric oxidation of isoprene produces a series of reactive intermediates, isoprene epoxydiols (IEPOX). The reactive uptake of IEPOX is the significant formation pathway of atmospheric SOA. In this work, four isomers of IEPOX were synthesized, derivatized by silylation reagents, and measured by gas chromatography/mass spectrometry (GC/MS). The electron-impact (EI) and methane chemical ionization (CI) sources mass spectra of trimethylsilyl ester (TMS) derivatives of IEPOX isomers were obtained and the fragmentation behaviors of the derivatives were examined. Moreover, the hydrogen nuclear magnetic resonance (H-NMR) spectra of IEPOX isomers were also obtained and the peak intensities in the H-NMR spectra were analyzed. Based on the standard spectra, IEPOX isomers were identified in ambient PM2.5 aerosols in the Gongga Mountain (China). The peak sequence of TMS derivatives of IEPOX isomers in GC/MS chromatogram was δ4-IEPOX, δ1-IEPOX, cis-β-IEPOX and trans-β-IEPOX. The isomers with the highest concentrations were δ1-IEPOX (threo- and erythro-). The mass ratios of IEPOX to 2-methyltetrols were 0.02–6.0 and the concentrations of IEPOX were 0.8–41.6 ng/m3 in the PM2.5 aerosols. The current study verified the core roles of IEPOX as active intermediates in photo oxidation of isoprene in ambient atmosphere.

Organosulfate Formation in Proxies for Aged Sea Spray Aerosol: Reactive Uptake of Isoprene Epoxydiols to Acidic Sodium Sulfate

Organosulfate Formation in Proxies for Aged Sea Spray Aerosol: Reactive Uptake of Isoprene Epoxydiols to Acidic Sodium Sulfate

Isoprene Epoxydiol-Derived Sulfated and Nonsulfated Oligomers Suppress Particulate Mass Loss during Oxidative Aging of Secondary Organic Aerosol.

Acid-driven multiphase chemistry of isoprene epoxydiols (IEPOX) with inorganic sulfate aerosols contributes substantially to secondary organic aerosol (SOA) formation, which constitutes a large mass fraction of atmospheric fine particulate matter (PM2.5). However, the atmospheric chemical sinks of freshly generated IEPOX-SOA particles remain unclear. We examined the role of heterogeneous oxidation of freshly generated IEPOX-SOA particles by gas-phase hydroxyl radical (•OH) under dark conditions as one potential atmospheric sink. After 4 h of gas-phase •OH exposure (∼3 × 108 molecules cm-3), chemical changes in smog chamber-generated IEPOX-SOA particles were assessed by hydrophilic interaction liquid chromatography coupled with electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). A comparison of the molecular-level compositional changes in IEPOX-SOA particles during aging with or without •OH revealed that decomposition of oligomers by heterogeneous •OH oxidation acts as a sink for •OH and maintains a reservoir of low-volatility compounds, including monomeric sulfate esters and oligomer fragments. We propose tentative structures and formation mechanisms for previously uncharacterized SOA constituents in PM2.5. Our results suggest that this •OH-driven renewal of low-volatility products may extend the atmospheric lifetimes of particle-phase IEPOX-SOA by slowing the production of low-molecular weight, high-volatility organic fragments and likely contributes to the large quantities of 2-methyltetrols and methyltetrol sulfates reported in PM2.5.

Fate of Oxygenated Volatile Organic Compounds in the Yangtze River Delta Region: Source Contributions and Impacts on the Atmospheric Oxidation Capacity.

The Community Multiscale Air Quality model (CMAQv5.2) was implemented to investigate the sources and sinks of oxygenated volatile organic compounds (OVOCs) during a high O3 and high PM2.5 season in the Yangtze River Delta (YRD) region, based on constraints from observations. The model tends to overpredict non-oxygenated VOCs and underpredict OVOCs, which has been improved with adjusted emissions of all VOCs. The OVOCs in the YRD are dominated by ketones, aldehydes, and alcohols. Ketones and aldehydes mainly originate from direct emissions and secondary formation in the northern YRD, and primarily originate from secondary formation in the southern part influenced by biogenic emissions. The concentration of secondary organic aerosols (SOA) produced by OVOCs is 0.5-1.5 μg/m3, with 40-80% originated from organic nitrates, 20-70% originated from dicarbonyls, and 0-20% originated from isoprene epoxydiols. The influences of OVOCs on the atmospheric oxidation capacity are dominated by the OH• pathway during the day (∼350%) and by the NO3• pathway at night (∼150%). Consequently, O3 is enhanced by 30-70% in the YRD. Aerosols are also enhanced by 50-140%, 20-80%, and ∼20% for SOA, nitrate, and sulfate, respectively.

Initial pH Governs Secondary Organic Aerosol Phase State and Morphology after Uptake of Isoprene Epoxydiols (IEPOX).

Aerosol acidity increases secondary organic aerosol (SOA) formed from the reactive uptake of isoprene-derived epoxydiols (IEPOX) by enhancing condensed-phase reactions within sulfate-containing submicron particles, leading to low-volatility organic products. However, the link between the initial aerosol acidity and the resulting physicochemical properties of IEPOX-derived SOA remains uncertain. Herein, we show distinct differences in the morphology, phase state, and chemical composition of individual organic-inorganic mixed particles after IEPOX uptake to ammonium sulfate particles with different initial atmospherically relevant acidities (pH = 1, 3, and 5). Physicochemical properties were characterized via atomic force microscopy coupled with photothermal infrared spectroscopy (AFM-PTIR) and Raman microspectroscopy. Compared to less acidic particles (pH 3 and 5), reactive uptake of IEPOX to the most acidic particles (pH 1) resulted in 50% more organosulfate formation, clearer phase separation (core-shell), and more irregularly shaped morphologies, suggesting that the organic phase transitioned to semisolid or solid. This study highlights that initial aerosol acidity may govern the subsequent aerosol physicochemical properties, such as viscosity and morphology, following the multiphase chemical reactions of IEPOX. These results can be used in future studies to improve model parameterizations of SOA formation from IEPOX and its properties, toward the goal of bridging predictions and atmospheric observations.

Novel Application of Machine Learning Techniques for Rapid Source Apportionment of Aerosol Mass Spectrometer Datasets

We apply machine learning approaches sparse multinomial logistic regression to classify aerosol mass spectrometer (AMS) unit mass resolution (UMR) data followed by an ensemble regression technique for source apportionment of organic aerosols (OA). The classifier was trained on 60 well characterized laboratory and positive matrix factorization (PMF) deconvolved reference spectra to identify eight OA types. These include four laboratory-derived secondary organic aerosol (SOA) spectra, which include isoprene photooxidation SOA, isoprene epoxydiols (IEPOX) SOA, a monoterpene SOA type that includes α-pinene and β-pinene SOA, and aromatic SOA from oxidation of naphthalene and m-xylene precursors, as well as PMF deconvolved spectra for three primary organic aerosol (POA) types, namely, hydrocarbon-like organic aerosol (HOA), biomass burning organic aerosol (BBOA), and cooking OA (COA), and a more oxidized oxygenated OA type (MO-OOA). A 5-fold cross-validation strategy, repeated 10 times, was used to assess the classifier’s performance. The classifier had high classification accuracy for COA, aromatic SOA, and isoprene SOA spectra but incorrectly classified ∼9% by number of MO-OOA spectra as BBOA, 12% of BBOA spectra as HOA (and vice versa), and 18% of IEPOX-SOA spectra as aromatic SOA. Next, an ensemble regression model was trained on an artificially generated dataset consisting of mixtures of different OA types to assess its ability to predict fractional mass abundances from classification probabilities of various OA species obtained from the multinomial logistic regression classifier trained on the reference spectra. Ultimately, the proposed approach was applied for source apportionment of aircraft-based AMS measurements of OA UMR spectra during the HI-SCALE field campaign. On two representative days (May 6th and 18th, 2016), the algorithm determined that ∼50−60% of OA by mass was MO-OOA, which represented a highly aged organic aerosol mixture from different sources. On both days, BBOA was determined to contribute less than 10% to OA by mass. However, on May 18th, the aromatic SOA fraction was higher compared to that on May 6th. The proposed approach is capable of rapidly analyzing AMS data in real time, making it suitable for applications where rapid source apportionment of AMS OA spectra is desirable.

Morphology and Viscosity Changes after Reactive Uptake of Isoprene Epoxydiols in Submicrometer Phase Separated Particles with Secondary Organic Aerosol Formed from Different Volatile Organic Compounds

Morphology and Viscosity Changes after Reactive Uptake of Isoprene Epoxydiols in Submicrometer Phase Separated Particles with Secondary Organic Aerosol Formed from Different Volatile Organic Compounds

Observational Insights into Isoprene Secondary Organic Aerosol Formation through the Epoxide Pathway at Three Urban Sites from Northern to Southern China.

Isoprene is the most abundant precursor of global secondary organic aerosol (SOA). The epoxide pathway plays a critical role in isoprene SOA (iSOA) formation, in which isoprene epoxydiols (IEPOX) and/or hydroxymethyl-methyl-α-lactone (HMML) can react with nucleophilic sulfate and water producing isoprene-derived organosulfates (iOSs) and oxygen-containing tracers (iOTs), respectively. This process is complicated and highly influenced by anthropogenic emissions, especially in the polluted urban atmospheres. In this study, we took a 1-year measurement of the paired iOSs and iOTs formed through the IEPOX and HMML pathways at the three urban sites from northern to southern China. The annual average concentrations of iSOA products at the three sites ranged from 14.6 to 36.5 ng m-3. We found that the nucleophilic-addition reaction of isoprene epoxides with water dominated over that with sulfate in the polluted urban air. A simple set of reaction rate constant could not fully describe iOS and iOT formation everywhere. We also found that the IEPOX pathway was dominant over the HMML pathway over urban regions. Using the kinetic data of IEPOX to estimate the reaction parameters of HMML will cause significant underestimation in the importance of HMML pathway. All these findings provide insights into iSOA formation over polluted areas.

Emissions, chemistry or bidirectional surface transfer? Gas phase formic acid dynamics in the atmosphere

Organic acids are among the many secondary components produced from organic compounds that can impact particulate matter composition and aerosol and cloud water acidity. The most abundant gas phase organic acid in the atmosphere is formic acid, which has been observed in concentrations in excess of 2.5 ppbV in rural areas. However, atmospheric model simulations of formic acid are typically biased low, potentially due to biases in their direct emissions and/or chemistry. In this study, we used the Community Multiscale Air Quality (CMAQ) to simulate gas phase formic acid in Yorkville, Georgia during an intensive campaign that lasted from September to October 2016. Similar to previous studies, the diurnal trend of simulated formic acid did not match observations, being biased low with a dissimilar diel profile. Potential reasons for mismatch were investigated. Additional gas phase reactions of isoprene and monoterpenes suggested by recent studies were added, leading to a small increase in acid concentrations, but having little impact on the diel profile. Surface reaction between OH and isoprene epoxydiols (IEPOX) was simulated, increasing the simulated concentration of formic acid by up to 150%; though the resulting simulated diurnal profile did not match the observed. Examination of the diel profile found that formic acid concentrations began to rise early in the morning before suspected precursors (e.g., isoprene, IEPOX and methanediol) and the hydroxyl radical, and that concentrations decreased rapidly in the afternoon. The rapid, early morning rise could be reproduced with increased emissions. Given the long lifetime of formic acid, and that the condensed phase formate concentrations are small, the evening decrease is explained mainly by rapid dry deposition. This suggests that formic acid underwent bidirectional deposition/emission, e.g., depositing rapidly as dew formed at night, with subsequent re-emission during the following day as the dew evaporated. This also led to observed decreases in concentrations with height.

Tight Coupling of Surface and In-Plant Biochemistry and Convection Governs Key Fine Particulate Components over the Amazon Rainforest

Combining unique high-altitude aircraft measurements and detailed regional model simulations, we show that in-plant biochemistry plays a central but previously unidentified role in fine particulate-forming processes and atmosphere–biosphere–climate interactions over the Amazon rainforest. Isoprene epoxydiol secondary organic aerosols (IEPOX-SOA) are key components of sub-micrometer aerosol particle mass throughout the troposphere over the Amazon rainforest and are traditionally thought to form by multiphase chemical pathways. Here, we show that these pathways are strongly inhibited by the solid thermodynamic phase state of aerosol particles and lack of particle and cloud liquid water in the upper troposphere. Strong diffusion limitations within organic aerosol coatings prevailing at low temperatures and low relative humidity in the upper troposphere strongly inhibit the reactive uptake of IEPOX to inorganic aerosols. We find that direct emissions of 2-methyltetrol gases formed by in-plant biochemical oxidation and/or oxidation of deposited IEPOX gases on the surfaces of soils and leaves and their transport by cloud updrafts followed by their condensation at low temperatures could explain over 90% of the IEPOX-SOA mass concentrations in the upper troposphere. Our simulations indicate that even near the surface, direct emissions of 2-methyltetrol gases represent a ubiquitous, but previously unaccounted for, source of IEPOX-SOA. Our results provide compelling evidence for new pathways related to land surface–aerosol–cloud interactions that have not been considered previously.

Evaluation of Biogenic Organic Aerosols in the Amazon Rainforest Using WRF‐Chem With MOSAIC

AbstractBiogenic secondary organic aerosol (BSOA) accounts for 70%–80% of submicron aerosol over the Amazon rainforest. However, uncertainties with regard to the chemical formation pathway, mass budget, and radiative impact of BSOA in the region still remain high. To address the issue, we used a regional chemistry transport model (WRF‐Chem‐MOSAIC) to evaluate the budget of BSOA in the Amazon forest during the wet season (March) of 2014 when the air mass was clean from influences from anthropogenic and biomass burning emissions. We found that the default model underestimated the surface concentration of submicron organic aerosol (OA) by 57% at a ground measurement site (T3). We subsequently updated the WRF‐Chem‐MOSAIC model by including SOA formation from isoprene epoxydiols (IEPOX‐SOA) and SOA aging schemes. The updated model reproduced the observed submicron OA concentration well with an overestimation of only 2.5%. With the updated model, we will show that IEPOX‐SOA accounts for 23% of surface submicron OA averaged over the entire Amazon rainforest. The modeled surface SOA (smaller than 2.5 μm in diameter) concentration in the Amazon region was about a factor of 14 larger than the global mean. Finally, a sensitivity study was conducted which showed that convective plumes can remove more than 40% of submicron OA below 5 km in the Amazon region. Overall, our study highlights the importance of IEPOX chemistry as well as the convective removal processes in the global SOA budget.

Observational insights into the compound environmental effect for 2-methyltetrols formation under humid ambient conditions

Laboratory experiments suggest acid-catalyzed aqueous-phase production can promote the formation of isoprene SOA, i.e., 2-methyltetrols. In this study we use ambient observations of the 2-methyltetrols along with other chemical measurements, as well as meteorological factors to investigate the relative importance of environmental influence for isoprene epoxydiols (IEPOX) SOA formation under atmospheric humidity conditions. The 2-Methyltetrols revealed good relationships with temperature and total solar radiation, but were weakly correlated with aerosol acidity and SO42−. EC-scaled 2-methyltetrols were observed to vary in a narrow pH range (1.5–2.0), indicating aerosol acidity was not a limiting factor for 2-methyltetrols formation. High values of 2-methyltetrols were consistently observed at high total solar radiation, the strong dependence of total solar radiation demonstrated that photochemical processes dominated 2-methyltetrols formation in humid environments. Although 2-methyltetrols can be enhanced by acid-catalyzed aqueous-phase reactions, it is not sufficient to compensate the synchronously weakened photochemical activity influence, leading to an obvious net decrease in the formation of 2-methyltetrols in the ambient. Moreover, aerosol droplet acidity was reduced under high liquid water content (LWC) condition, subsequently diminishing the enhancement of SOA formation by acidity. Overall, our results highlight that the environmental impact factors are highly variable and interplay, influencing the production of 2-methyltetrols, and suggest that the formation pathway of 2-methyltetrols is insensitive to aerosol acidity but dominated by photochemical production process in humid environments.