Gas-phase low volatility organic compounds (LVOC), produced from oxidation of isoprene 4-hydroxy-3-hydroperoxide (4,3-ISOPOOH) under low-NO conditions, were observed during the FIXCIT chamber study. Decreases in LVOC directly correspond to appearance and growth in secondary organic aerosol (SOA) of consistent elemental composition, indicating that LVOC condense (at OA below 1 μg m–3). This represents the first simultaneous measurement of condensing low volatility species from isoprene oxidation in both the gas and particle phases. The SOA formation in this study is separate from previously described isoprene epoxydiol (IEPOX) uptake. Assigning all condensing LVOC signals to 4,3-ISOPOOH oxidation in the chamber study implies a wall-loss corrected non-IEPOX SOA mass yield of ∼4%. By contrast to monoterpene oxidation, in which extremely low volatility VOC (ELVOC) constitute the organic aerosol, in the isoprene system LVOC with saturation concentrations from 10–2 to 10 μg m–3 are the main constituents. These LVOC may be important for the growth of nanoparticles in environments with low OA concentrations. LVOC observed in the chamber were also observed in the atmosphere during SOAS-2013 in the Southeastern United States, with the expected diurnal cycle. This previously uncharacterized aerosol formation pathway could account for ∼5.0 Tg yr–1 of SOA production, or 3.3% of global SOA.

We use a large laboratory, modeling, and field dataset to investigate the isoprene + O3 reaction, with the goal of better understanding the fates of the C1 and C4 Criegee intermediates in the atmosphere.

S1.0 CIMS Sensitivities CIMS sensitivities to the oxidation products were determined in multiple ways. Hydroxyacetone and glycolaldehyde are commercially available and were quantified gravimetrically and by Fourier Transform Infrared Spectroscopy (FT-IR) for CIMS calibration.1 Uncalibrated compounds (glycolic acid and all products identified by m/z) were assigned a generic CIMS sensitivity of 2.5×10 -4 ncts /pptv, and are considered accurate to within a factor of 2. Here, normalized counts (ncts) represent the counts observed at the analyte m/z divided by the reagent ion counts. OH + ISOPOOH à ISOPOOH-OH à ProductsIn Figures Figure S1. The different reaction pathways for the reaction between (1,2)-ISOPOOH and OH radical. Figure S2. The different reaction pathways for the reaction between (4,3)-ISOPOOH and OH radical. 11,12 The general reaction scheme is shown in Figure S3. Figure S3. The reaction scheme as used in the MESMER model (Only an illustration, not the energetically correct picture of the reactions). The ISOPOOH-OH complexes are different for each of the reaction pathways even though they are given with the same energy at this figure. S5 S6 S7In our Mesmer modeling the Lennard-Jones (L-J) parameters of the bath gas were chosen to be a nitrogen gas resembling the atmospheric gas ISOPOOH + OH ISOPOOH-OH complex TS TS TSAbstraction trans-Add1 cis-Add1Add2 trans-IEPOX cis-IEPOX S9We have preformed a sensitivity test of Mesmer input parameters. In our sensitivity test we used three collisional activation/deactivation energies of 50, 100 and 200 cm -1 and two different grain sizes of 25 and 50 cm -1 . We did not observe any significant changes in the reaction rate constants (only changes of a few percent). We have also tested the system with different sizes of grain span, e.g., 10 kT, 20 kT, 30 kT, 40 kT and 50 kT. If a grain span of 30 kT or higher is used, the reaction rate constants do not change. We have therefore used a grain size of 30 kT.The reaction rate constants are sensitive to the choice of the Arrhenius pre-exponential factor (A). Each reaction pathway is a separate Mesmer calculation (See Figure S4 and S5 for the individual reaction pathways) -we have not coupled between the reactions in the fitting of the Arrhenius pre-exponential factor (A). We treat the pre-exponential factor as temperature independent and it is varied between 1.0×10 -12 and 2.0×10 -10 cm 3 molecule -1 s -1 . We use nine different Arrhenius pre-exponential factors to calculate the rates. Three of the factors are from the three reactions of n-butane, 3-methyl-3-butene-1-ol and 1-butene with OH. [14][15][16] The total reaction rate constants (OH +ISOPOOH → Products) of the (1,2)-ISOPOOH and (4,3)-ISOPOOH systems are shown in Table S3 and Table S4, respectively. S3.3 (1,2)-ISOPOOHFor the (1,2)-ISOPOOH + OH reactions, the absolute rate constants of all the different reaction pathways increase with an increase in the Arrhenius pre-exponential factor, and the relative yields (in %) of the reaction pathways also change.The yiel...

With a large global emission rate and high reactivity, isoprene has a profound effect upon atmospheric chemistry and composition. The atmospheric pathways by which isoprene converts to secondary organic aerosol (SOA) and how anthropogenic pollutants such as nitrogen oxides and sulfur affect this process are subjects of intense research because particles affect Earth's climate and local air quality. In the absence of both nitrogen oxides and reactive aqueous seed particles, we measure SOA mass yields from isoprene photochemical oxidation of up to 15%, which are factors of 2 or more higher than those typically used in coupled chemistry climate models. SOA yield is initially constant with the addition of increasing amounts of nitric oxide (NO) but then sharply decreases for input concentrations above 50 ppbv. Online measurements of aerosol molecular composition show that the fate of second-generation RO2 radicals is key to understanding the efficient SOA formation and the NOx-dependent yields described here and in the literature. These insights allow for improved quantitative estimates of SOA formation in the preindustrial atmosphere and in biogenic-rich regions with limited anthropogenic impacts and suggest that a more-complex representation of NOx-dependent SOA yields may be important in models.

Isoprene photooxidation is a major driver of atmospheric chemistry over forested regions. Isoprene reacts with hydroxyl radicals (OH) and molecular oxygen to produce isoprene peroxy radicals (ISOPOO). These radicals can react with hydroperoxyl radicals (HO 2 ) to dominantly produce hydroxyhydroperoxides (ISOPOOH). They can also react with nitric oxide (NO) to largely produce methyl vinyl ketone (MVK) and methacrolein (MACR). Unimolecular isomerization and bimolecular reactions with organic peroxy radicals are also possible. There is uncertainty about the relative importance of each of these pathways in the atmosphere and possible changes because of anthropogenic pollution. Herein, measurements of ISOPOOH and MVK + MACR concentrations are reported over the central region of the Amazon basin during the wet season. The research site, downwind of an urban region, intercepted both background and polluted air masses during the GoAmazon2014/5 Experiment. Under background conditions, the confidence interval for the ratio of the ISOPOOH concentration to that of MVK + MACR spanned 0.4-0.6. This result implies a ratio of the reaction rate of ISOPOO with HO 2 to that with NO of approximately unity. A value of unity is significantly smaller than simulated at present by global chemical transport models for this important, nominally low-NO, forested region of Earth. Under polluted conditions, when the concentrations of reactive nitrogen compounds were high (>1 ppb), ISOPOOH concentrations dropped below the instrumental detection limit (<60 ppt). This abrupt shift in isoprene photooxidation, sparked by human activities, speaks to ongoing and possible future changes in the photochemistry active over the Amazon rainforest.isoprene photochemistry | Amazon | organic hydroperoxides

Atmospheric volatile organic compound (VOC) oxidation mechanisms under pristine (rural/remote) and urban (anthropogenically-influenced) conditions follow distinct pathways due to large differences in nitrogen oxide (NO x ) concentrations. These two pathways lead to products that have different chemical and physical properties and reactivity. Under pristine conditions, isoprene hydroxy hydroperoxides (ISOPOOHs) are the dominant first-generation isoprene oxidation products. Utilizing authentic ISOPOOH standards, we demonstrate that two of the most commonly used methods of measuring VOC oxidation products (i.e., gas chromatography and proton transfer reaction mass spectrometry) observe these hydroperoxides as their equivalent high-NO isoprene oxidation products -methyl vinyl ketone (MVK) and methacrolein (MACR). This interference has led to an observational bias affecting our understanding of global atmospheric processes. Considering these artifacts will help close the gap on discrepancies regarding the identity and fate of reactive organic carbon, revise our understanding of surface-atmosphere exchange of reactive carbon and SOA formation, and improve our understanding of atmospheric oxidative capacity.

Abstract. We present measurements of secondary organic aerosol (SOA) formation from isoprene photochemical oxidation in an environmental simulation chamber at a variety of oxidant conditions and using dry neutral seed particles to suppress acid-catalyzed multiphase chemistry. A high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) utilizing iodide-adduct ionization coupled to the Filter Inlet for Gases and Aerosols (FIGAERO) allowed for simultaneous online sampling of the gas and particle composition. Under high-HO2 and low-NO conditions, highly oxygenated (O : C  ≥  1) C5 compounds were major components (∼ 50 %) of SOA. The SOA composition and effective volatility evolved both as a function of time and as a function of input NO concentrations. Organic nitrates increased in both the gas and particle phases as input NO increased, but the dominant non-nitrate particle-phase components monotonically decreased. We use comparisons of measured and predicted gas-particle partitioning of individual components to assess the validity of literature-based group-contribution methods for estimating saturation vapor concentrations. While there is evidence for equilibrium partitioning being achieved on the chamber residence timescale (5.2 h) for some individual components, significant errors in group-contribution methods are revealed. In addition, > 30 % of the SOA mass, detected as low-molecular-weight semivolatile compounds, cannot be reconciled with equilibrium partitioning. These compounds desorb from the FIGAERO at unexpectedly high temperatures given their molecular composition, which is indicative of thermal decomposition of effectively lower-volatility components such as larger molecular weight oligomers.

The oxidation of sulfur dioxide (SO2) by peroxides leads to the formation of sulfate in cloudwater, contributing to particulate matter (PM) formation. The reaction with hydrogen peroxide (H2O2) is considered to be the main cloud oxidation pathway. Previous studies have examined the oxidation of SO2 in cloudwater by small organic peroxides with one functional group; however, oxidation by multifunctional organic hydroperoxides, which are expected to have higher water solubility and reactivity, has not been examined. We investigate the aqueous oxidation of SO2 by the two main isomers of isoprene hydroxyl hydroperoxide (ISOPOOH), the primary low-NO x isoprene oxidation products in the atmosphere. Having large Henry’s law constants and being among the most abundant multifunctional hydroperoxides, they are among the most important organic hydroperoxides present in clouds. The pH dependence of the reactions was investigated at cloud relevant pH of 3–6, and the results reveal their importance compared to the oxidation of SO2 via H2O2. Model simulations in GEOS-Chem, updated with the chemistry described herein, highlight the importance of these pathways for sulfate formation in regions with high isoprene emissions and low-NO x atmospheric conditions, especially if they maintain significant SO2 emissions.