Isoprene photochemistry over the Amazon rainforest

Atmospheric Chemistry and Physics, 2009

Isoprene represents the single most important reactive hydrocarbon for atmospheric chemistry in the tropical atmosphere. It plays a central role in global and regional atmospheric chemistry and possible climate feedbacks. Photooxidation of primary hydrocarbons (e.g. isoprene) leads to the formation of oxygenated VOCs (OVOCs). The evolution of these intermediates affects the oxidative capacity of the atmosphere (by reacting with OH) and can contribute to secondary aerosol formation, a poorly understood process. An accurate and quantitative understanding of VOC oxidation processes is needed for model simulations of regional air quality and global climate. Based on field measurements conducted during the Amazonian Aerosol Characterization Experiment (AMAZE-08) we show that the production of certain OVOCs (e.g. hydroxyacetone) from isoprene photooxidation in the lower atmosphere is significantly underpredicted by standard chemistry schemes. Recently reported fast secondary production could explain 50% of the observed discrepancy with the remaining part possibly produced via a novel primary production channel, which has been proposed theoretically. The observations of OVOCs are also used to test a recently proposed HO x recycling mechanism via degradation of isoprene peroxy radicals. If generalized our observations suggest that prompt photochemical formation of OVOCs and other uncertainties in VOC oxidation schemes could result in uncertainties of modelled OH reactivity, potentially explaining a fraction of the missing OH sink over forests which has previously been largely attributed to a missing source of primary biogenic VOCs.

Science advances, 2018

Nitrogen oxides (NO ) emitted from human activities are believed to regulate the atmospheric oxidation capacity of the troposphere. However, observational evidence is limited for the low-to-median NO concentrations prevalent outside of polluted regions. Directly measuring oxidation capacity, represented primarily by hydroxyl radicals (OH), is challenging, and the span in NO concentrations at a single observation site is often not wide. Concentrations of isoprene and its photo-oxidation products were used to infer the equivalent noontime OH concentrations. The fetch at an observation site in central Amazonia experienced varied contributions from background regional air, urban pollution, and biomass burning. The afternoon concentrations of reactive nitrogen oxides (NO ), indicative of NO exposure during the preceding few hours, spanned from 0.3 to 3.5 parts per billion. Accompanying the increase of NO concentration, the inferred equivalent noontime OH concentrations increased by at le...

Atmospheric Chemistry and Physics Discussions, 2015

This study explores our ability to simulate the atmospheric chemistry stemming from isoprene emissions in pristine and polluted regions of the Amazon basin. We confront two atmospheric chemistry models – a global, Eulerian chemistry-climate model (UM-UKCA) and a trajectory-based Lagrangian model (CiTTyCAT) – with recent airborne measurements of atmospheric composition above the Amazon made during the SAMBBA campaign of 2012. The simulations with the two models prove relatively insensitive to the chemical mechanism employed; we explore one based on the Mainz Isoprene Mechanism, and an updated one that includes changes to the chemistry of first generation isoprene nitrates (ISON) and the regeneration of hydroxyl radicals via the formation of hydroperoxy-aldehydes (HPALDS) from hydroperoxy radicals (ISO<sub>2</sub>). In the Lagrangian model, the impact of increasing the spatial resolution of trace gas emissions employed from 3.75° × 2.5° to 0.1° × 0.1° varies fr...

2001

Airborne measurements of volatile organic compounds (VOC) were performed over the tropical rainforest in Surinam (0-12 km altitude, 2 • -7 • N, 54 • -58 • W) using the proton transfer reaction mass spectrometry (PTR-MS) technique, which allows online monitoring of compounds like isoprene, its oxidation products methyl vinyl ketone, methacrolein, tentatively identified hydroxyisoprene-hydroperoxides, and several other organic compounds. Isoprene volume mixing ratios (VMR) varied from below the detection limit at the highest altitudes to about 7 nmol/mol in the planetary boundary layer shortly before sunset. Correlations between isoprene and its product compounds were made for different times of day and altitudes, with the isoprene-hydroperoxides showing the highest correlation. Model calculated mixing ratios of the isoprene oxidation products using a detailed hydrocarbon oxidation mechanism, as well as the intercomparison measurement with air samples collected during the flights in canisters and later analysed with a GC-FID, showed good agreement with the PTR-MS measurements, in particular at the higher mixing ratios.

Atmospheric Chemistry and Physics, 2016

The emission, dispersion, and photochemistry of isoprene (C 5 H 8) and related chemical species in the convective boundary layer (CBL) during sunlit daytime were studied over a mixed forest in the southeastern United States by combining ground-based and aircraft observations. Fluxes of isoprene and monoterpenes were quantified at the top of the forest canopy using a high-resolution proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS). Snapshot (∼ 2 min sampling duration) vertical profiles of isoprene, methyl vinyl ketone (MVK) + methacrolein (MACR), and monoterpenes were collected from aircraft every hour in the CBL (100-1000 m). Both ground-based and airborne collected volatile organic compound (VOC) data are used to constrain the initial conditions of a mixed-layer chemistry model (MXLCH), which is applied to examine the chemical evolution of the O 3-NO x-HO x-VOC system and how it is affected by boundary layer dynamics in the CBL. The chemical loss rate of isoprene (∼ 1 h) is similar to the turbulent mixing timescale (0.1-0.5 h), which indicates that isoprene concentrations are equally dependent on both photooxidation and boundary layer dynamics. Analysis of a modelderived concentration budget suggests that diurnal evolution of isoprene inside the CBL is mainly controlled by surface emissions and chemical loss; the diurnal evolution Published by Copernicus Publications on behalf of the European Geosciences Union. 7726 L. Su et al.: Isoprene photooxidation in the southeastern US of O 3 is dominated by entrainment. The NO to HO 2 ratio (NO : HO 2) is used as an indicator of anthropogenic impact on the CBL chemical composition and spans a wide range (1-163). The fate of hydroxyl-substituted isoprene peroxyl radical (HOC 5 H 8 OO q ; ISOPOO) is strongly affected by NO : HO 2 , shifting from NO-dominant to NO-HO 2-balanced conditions from early morning to noontime. This chemical regime change is reflected in the diurnal evolution of isoprene hydroxynitrates (ISOPN) and isoprene hydroxy hydroperoxides (ISOPOOH).

Atmospheric Chemistry and Physics Discussions, 2015

The emission, dispersion and photochemistry of isoprene (C<sub>5</sub>H<sub>8</sub>) and related chemical species in the convective boundary layer (CBL) during sunlit daytime was studied over a mixed forest in the Southeast United States by combining ground-based and aircraft observations. Fluxes of isoprene and monoterpenes were quantified at the top of the forest canopy using a high resolution Proton Transfer Reaction Time of Flight Mass Spectrometer (PTR-TOF-MS). Snapshot (~ 2 min sampling duration) vertical profiles of isoprene, methyl vinyl ketone (MVK) + methacrolein (MACR), and monoterpenes were collected from aircraft every hour in the CBL (100–1000 m). Both ground-based and airborne collected volatile organic compound (VOC) data are used to constrain the initial conditions of a mixed layer chemistry model (MXLCH), which is applied to examine the chemical evolution of the O<sub>3</sub>-NO<sub><i>x</i></sub>-HO<su...

Atmospheric Chemistry and Physics, 2007

We estimated the isoprene and monoterpene source strengths of a pristine tropical forest north of Manaus in the central Amazon Basin using three different micrometeorological flux measurement approaches. During the early dry season campaign of the Cooperative LBA Airborne Regional Experiment (LBA-CLAIRE-2001), a towerbased surface layer gradient (SLG) technique was applied simultaneously with a relaxed eddy accumulation (REA) system. Airborne measurements of vertical profiles within and above the convective boundary layer (CBL) were used to estimate fluxes on a landscape scale by application of the mixed layer gradient (MLG) technique. The mean daytime fluxes of organic carbon measured by REA were 2.1 mg C m −2 h −1 for isoprene, 0.20 mg C m −2 h −1 for αpinene, and 0.39 mg C m −2 h −1 for the sum of monoterpenes. These values are in reasonable agreement with fluxes determined with the SLG approach, which exhibited a higher scatter, as expected for the complex terrain investigated. The observed VOC fluxes are in good agreement with simulations using a single-column chemistry and climate model (SCM).

Atmospheric Chemistry and Physics, 2009

Tropical forests are a strong source of biogenic volatile organic compounds (BVOCs) to the atmosphere which can potentially impact the atmospheric oxidation capacity. Here we present airborne and ground-based BVOC measurements representative for the long dry season covering a large area of the northern Amazonian rainforest (6-3 • N, 50-59 • W). The measurements were conducted during the October 2005 GABRIEL (Guyanas Atmosphere-Biosphere exchange and Radicals Intensive Experiment with the Learjet) campaign. The vertical (35 m to 10 km) and diurnal (09:00-16:00) profiles of isoprene, its oxidation products methacrolein and methyl vinyl ketone and methanol and acetone, measured by PTR-MS (Proton Transfer Reaction Mass Spectrometry), have been used to empirically estimate their emission fluxes from the forest canopy on a regional scale. The mixed layer isoprene emission flux, inferred from the airborne measurements above 300 m, is 5.7 mg isoprene m −2 h −1 after compensating for chemistry and ∼6.9 mg isoprene m −2 h −1 taking detrainment into account. This surface flux is in general agreement with previous tropical forest studies. Inferred methanol and acetone emission fluxes are 0.5 mg methanol m −2 h −1 and 0.35 mg acetone m −2 h −1 , respectively. The BVOC measurements were compared with fluxes and mixing ratios simulated with

Atmospheric Environment (1967), 1984

Ah&act-A chemical mechanism describing the oxidation of isoprene by OH and 0s in the troposphere has been developed and incorporated into a one-dimensional steady-state photochemical model. Calculations have been performed for continental conditions at two latitudes, 15"N and 45"N. At 45"N latitude, the effects of anthropogenic hydrocarbon emissions on the vertical profiles of NO, (NO + N02) and HNO, overshadow the effects of isoprene emissions; at lS"N, the reduced anthropogenic emissions and the increased isoprene emissions produce increases of 26 and 4% in the column contents of NO, and HNOs, respectively. The integrated columns of CO at 45"N and 15"N latitude increased by 1Oand 31 yO, respectively, when isoprene chemistry was included, but these increases are much smaller than those that would have been obtained ifall the carbon in isoprene had been photochemically transformed into CO. It appears that the fate of a significant quantity of isoprene is in the formation of longer carbon-chain (R > CHs) oxygenated organic species. The longer carbon-chain alkylhydroperoxides and alkylperoxyacids appear to be important free radical sinks in the tropics given the lower NO, concentrations generally found in the tropics.

Atmospheric Chemistry and Physics, 2011

Biogenic volatile organic compounds (BVOCs) such as isoprene constitute a large proportion of the global atmospheric oxidant sink. Their reactions in the atmosphere contribute to processes such as ozone production and secondary organic aerosol formation. However, over the tropical rainforest, where 50% of the global emissions of 5 BVOCs are believed to occur, atmospheric chemistry models have been unable to simultaneously simulate the measured daytime concentration of isoprene and that of its principal oxidant, hydroxyl (OH). One reason for this model-measurement discrepancy may be incomplete mixing of isoprene within the convective boundary layer, leading to patchiness or segregation in isoprene and OH mixing ratios and average concentra-10 20 the spectrum of isoprene concentrations measured, to estimate segregation intensity of isoprene and OH from high-frequency isoprene time series. The method successfully reproduces the only directly observed segregation. The effective rate constant reduction for the reaction of isoprene and OH over a South-East Asian rainforest is calculated to be typically <15%. This estimate is not sensitive to heterogeneities in 25 NO at this remote site, unless they are correlated with those of isoprene, or to OHrecycling schemes in the isoprene oxidation mechanism, unless the recycling happens in the first reaction step. Segregation alone is therefore unlikely to be the sole cause of 18198 Abstract 18201 Abstract 25 ered to be ∼125 m above the forest canopy, taking into account that the measurement tower is sited on a hill . Abstract i.e. the number of instances in which that compound is registered at the detector each second. This is later converted into a mixing ratio as described in Langford et al. Abstract co-variance between isoprene and OH. A coupled LES-chemistry model would attempt 18204 Abstract ACPD Abstract ACPD Abstract ACPD Abstract 20 Abstract a Lagrangian box model using field measurements from EASE (Eastern Atlantic Summer Abstract 18220 Abstract 18221 Abstract Influence of variations in isoprene concentration