Anthropogenic emissions and land use changes have modified atmospheric aerosol concentrations and size distributions over time. Understanding preindustrial conditions and changes in organic aerosol due to anthropogenic activities is important because these features (1) influence estimates of aerosol radiative forcing and (2) can confound estimates of the historical response of climate to increases in greenhouse gases. Secondary organic aerosol (SOA), formed in the atmosphere by oxidation of organic gases, represents a major fraction of global submicron-sized atmospheric organic aerosol. Over the past decade, significant advances in understanding SOA properties and formation mechanisms have occurred through measurements, yet current climate models typically do not comprehensively include all important processes. This review summarizes some of the important developments during the past decade in understanding SOA formation. We highlight the importance of some processes that influence the growth of SOA particles to sizes relevant for clouds and radiative forcing, including formation of extremely low volatility organics in the gas phase, acid-catalyzed multiphase chemistry of isoprene epoxydiols, particle-phase oligomerization, and physical properties such as volatility and viscosity. Several SOA processes highlighted in this review are complex and interdependent and have nonlinear effects on the properties, formation, and evolution of SOA. Current global models neglect this complexity and nonlinearity and thus are less likely to accurately predict the climate forcing of SOA and project future climate sensitivity to greenhouse gases. Efforts are also needed to rank the most influential processes and nonlinear process-related interactions, so that these processes can be accurately represented in atmospheric chemistry-climate models.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2016RG000540

Recent advances in understanding secondary organic aerosol: Implications for global climate forcing

. The chemistry of isoprene degradation in the Master Chemical Mechanism (MCM) has been systematically refined and updated to reflect recent advances in understanding, with these updates appearing in the latest version, MCM v3.3.1. The complete isoprene degradation mechanism in MCM v3.3.1 consists of 1926 reactions of 602 closed shell and free radical species, which treat the chemistry initiated by reaction with OH radicals, NO3 radicals and ozone (O3). A detailed overview of the updates is provided, within the context of reported kinetic and mechanistic information. The revisions mainly relate to the OH-initiated chemistry, which tends to dominate under atmospheric conditions, although these include updates to the chemistry of some products that are also generated from the O3- and NO3-initiated oxidation. The revisions have impacts in a number of key areas, including HOx recycling, NOx recycling and the formation of species reported to play a role in SOA (secondary organic aerosol)-formation mechanisms. The performance of the MCM v3.3.1 isoprene mechanism has been compared with those of earlier versions (MCM v3.1 and MCM v3.2) over a range of relevant conditions, using a box model of the tropical forested boundary layer. The results of these calculations are presented and discussed and are used to illustrate the impacts of the mechanistic updates in MCM v3.3.1.

/pdf/the-mcm-v3-3-1-degradation-scheme-for-isoprene-3n8apu6wog.pdf

The MCM v3.3.1 degradation scheme for isoprene

Isoprene carries approximately half of the flux of non-methane volatile organic carbon emitted to the atmosphere by the biosphere. Accurate representation of its oxidation rate and products is essential for quantifying its influence on the abundance of the hydroxyl radical (OH), nitrogen oxide free radicals (NOx), ozone (O3), and, via the formation of highly oxygenated compounds, aerosol. We present a review of recent laboratory and theoretical studies of the oxidation pathways of isoprene initiated by addition of OH, O3, the nitrate radical (NO3), and the chlorine atom. From this review, a recommendation for a nearly complete gas-phase oxidation mechanism of isoprene and its major products is developed. The mechanism is compiled with the aims of providing an accurate representation of the flow of carbon while allowing quantification of the impact of isoprene emissions on HOx and NOx free radical concentrations and of the yields of products known to be involved in condensed-phase processes. Finally, a sim...

Gas-Phase Reactions of Isoprene and Its Major Oxidation Products.

. Ozone pollution in the Southeast US involves complex chemistry driven by emissions of anthropogenic nitrogen oxide radicals (NOx  ≡  NO + NO2) and biogenic isoprene. Model estimates of surface ozone concentrations tend to be biased high in the region and this is of concern for designing effective emission control strategies to meet air quality standards. We use detailed chemical observations from the SEAC4RS aircraft campaign in August and September 2013, interpreted with the GEOS-Chem chemical transport model at 0.25°  ×  0.3125° horizontal resolution, to better understand the factors controlling surface ozone in the Southeast US. We find that the National Emission Inventory (NEI) for NOx from the US Environmental Protection Agency (EPA) is too high. This finding is based on SEAC4RS observations of NOx and its oxidation products, surface network observations of nitrate wet deposition fluxes, and OMI satellite observations of tropospheric NO2 columns. Our results indicate that NEI NOx emissions from mobile and industrial sources must be reduced by 30–60 %, dependent on the assumption of the contribution by soil NOx emissions. Upper-tropospheric NO2 from lightning makes a large contribution to satellite observations of tropospheric NO2 that must be accounted for when using these data to estimate surface NOx emissions. We find that only half of isoprene oxidation proceeds by the high-NOx pathway to produce ozone; this fraction is only moderately sensitive to changes in NOx emissions because isoprene and NOx emissions are spatially segregated. GEOS-Chem with reduced NOx emissions provides an unbiased simulation of ozone observations from the aircraft and reproduces the observed ozone production efficiency in the boundary layer as derived from a regression of ozone and NOx oxidation products. However, the model is still biased high by 6 ± 14 ppb relative to observed surface ozone in the Southeast US. Ozonesondes launched during midday hours show a 7 ppb ozone decrease from 1.5 km to the surface that GEOS-Chem does not capture. This bias may reflect a combination of excessive vertical mixing and net ozone production in the model boundary layer.

/pdf/why-do-models-overestimate-surface-ozone-in-the-southeast-1myfduc4d1.pdf

Why do models overestimate surface ozone in the Southeast United States

Proton-transfer-reaction mass spectrometry (PTR-MS) has been widely used to study the emissions, distributions, and chemical evolution of volatile organic compounds (VOCs) in the atmosphere. The applications of PTR-MS have greatly promoted understanding of VOC sources and their roles in air-quality issues. In the past two decades, many new mass spectrometric techniques have been applied in PTR-MS instruments, and the performance of PTR-MS has improved significantly. This Review summarizes these developments and recent applications of PTR-MS in the atmospheric sciences. We discuss the latest instrument development and characterization work on PTR-MS instruments, including the use of time-of-flight mass analyzers and new types of ion guiding interfaces. Here we review what has been learned about the specificity of different product ion signals for important atmospheric VOCs. We present some of the recent highlights of VOC research using PTR-MS including new observations in urban air, biomass-burning plumes,...

Proton-Transfer-Reaction Mass Spectrometry: Applications in Atmospheric Sciences

. The photo-oxidation chemistry of isoprene (ISOP; C 5 H 8 ) was studied in a continuous-flow chamber under conditions such that the reactions of the isoprene-derived peroxyl radicals (RO 2 ) were dominated by the hydroperoxyl (HO 2 ) pathway. A proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) with switchable H 3 O + and NO + reagent ions was used for product analysis. The products methyl vinyl ketone (MVK; C 4 H 6 O) and methacrolein (MACR; C 4 H 6 O) were differentiated using NO + reagent ions. The MVK and MACR yields via the HO 2 pathway were (3.8 ± 1.3)% and (2.5 ± 0.9)%, respectively, at +25 °C and 2 pathway implies concomitant production of hydroxyl ((6.3 ± 2.1)%) and hydroperoxyl ((6.3 ± 2.1)%) radicals, meaning a HO x recycling of (12.6 ± 4.2)% given that HO 2 was both a reactant and product. Other isoprene oxidation products, believed to be mostly organic hydroperoxides, also contributed to the ion intensity at the same mass-to-charge ( m/z ) ratios as the MVK and MACR product ions for HO 2 -dominant conditions. These products were selectively removed from the gas phase by placement of a cold trap (−40 °C) inline prior to the PTR-TOF-MS. When incorporated into regional and global chemical transport models, the yields of MVK and MACR and the concomitant HO x recycling reported in this study can improve the accuracy of the simulation of the HO 2 reaction pathway of isoprene, which is believed to be the fate of approximately half of atmospherically produced isoprene-derived peroxy radicals on a global scale.

/pdf/production-of-methyl-vinyl-ketone-and-methacrolein-via-the-2salsqoaa8.pdf

I. S. Herdlinger-Blatt

Papers

Production of methyl vinyl ketone and methacrolein via the hydroperoxyl pathway of isoprene oxidation