scholarly journals Formaldehyde production from isoprene oxidation across NO<sub><i>x</i></sub> regimes

2015 ◽  
Vol 15 (21) ◽  
pp. 31587-31620 ◽  
Author(s):  
G. M. Wolfe ◽  
J. Kaiser ◽  
T. F. Hanisco ◽  
F. N. Keutsch ◽  
J. A. de Gouw ◽  
...  
Keyword(s):  
First Generation ◽  
Transport Model ◽  
Peroxy Radical ◽  
Oxidation Products ◽  
Peroxy Radicals ◽  
Chemical Transport Model ◽  
Mixing Ratios ◽  
Urban Rural ◽  
Isoprene Oxidation ◽  
Southeast Us

Abstract. The chemical link between isoprene and formaldehyde (HCHO) is a strong, non-linear function of NOx (= NO + NO2). This relationship is a linchpin for top-down isoprene emission inventory verification from orbital HCHO column observations. It is also a benchmark for overall mechanism performance with regard to VOC oxidation. Using a comprehensive suite of airborne in situ observations over the Southeast US, we quantify HCHO production across the urban-rural spectrum. Analysis of isoprene and its major first-generation oxidation products allows us to define both a "prompt" yield of HCHO (molecules of HCHO produced per molecule of freshly-emitted isoprene) and the background HCHO mixing ratio (from oxidation of longer-lived hydrocarbons). Over the range of observed NOx values (roughly 0.1–2 ppbv), the prompt yield increases by a factor of 3 (from 0.3 to 0.9 ppbv ppbv−1), while background HCHO increases by more than a factor of 2 (from 1.5 to 3.3 ppbv). We apply the same method to evaluate the performance of both a global chemical transport model (AM3) and a measurement-constrained 0-D chemical box model. Both models reproduce the NOx dependence of the prompt HCHO yield, illustrating that models with updated isoprene oxidation mechanisms can adequately capture the link between HCHO and recent isoprene emissions. On the other hand, both models under-estimate background HCHO mixing ratios, suggesting missing HCHO precursors, inadequate representation of later-generation isoprene degradation and/or under-estimated hydroxyl radical concentrations. Moreover, we find that the total organic peroxy radical production rate is essentially independent of NOx, as the increase in oxidizing capacity with NOx is largely balanced by a decrease in VOC reactivity. Thus, the observed NOx dependence of HCHO mainly reflects the changing fate of organic peroxy radicals.

scholarly journals Formaldehyde production from isoprene oxidation across NO<sub><i>x</i></sub> regimes

2016 ◽  
Vol 16 (4) ◽  
pp. 2597-2610 ◽  
Author(s):  
G. M. Wolfe ◽  
J. Kaiser ◽  
T. F. Hanisco ◽  
F. N. Keutsch ◽  
J. A. de Gouw ◽  
...  
Keyword(s):  
Transport Model ◽  
Model Simulation ◽  
Nonlinear Function ◽  
Peroxy Radical ◽  
Fold Increase ◽  
Oxidation Products ◽  
Box Model ◽  
Chemical Transport Model ◽  
Mixing Ratios ◽  
Isoprene Oxidation

Abstract. The chemical link between isoprene and formaldehyde (HCHO) is a strong, nonlinear function of NOx (i.e., NO + NO2). This relationship is a linchpin for top-down isoprene emission inventory verification from orbital HCHO column observations. It is also a benchmark for overall photochemical mechanism performance with regard to VOC oxidation. Using a comprehensive suite of airborne in situ observations over the southeast US, we quantify HCHO production across the urban–rural spectrum. Analysis of isoprene and its major first-generation oxidation products allows us to define both a "prompt" yield of HCHO (molecules of HCHO produced per molecule of freshly emitted isoprene) and the background HCHO mixing ratio (from oxidation of longer-lived hydrocarbons). Over the range of observed NOx values (roughly 0.1–2 ppbv), the prompt yield increases by a factor of 3 (from 0.3 to 0.9 ppbv ppbv−1), while background HCHO increases by a factor of 2 (from 1.6 to 3.3 ppbv). We apply the same method to evaluate the performance of both a global chemical transport model (AM3) and a measurement-constrained 0-D steady-state box model. Both models reproduce the NOx dependence of the prompt HCHO yield, illustrating that models with updated isoprene oxidation mechanisms can adequately capture the link between HCHO and recent isoprene emissions. On the other hand, both models underestimate background HCHO mixing ratios, suggesting missing HCHO precursors, inadequate representation of later-generation isoprene degradation and/or underestimated hydroxyl radical concentrations. Detailed process rates from the box model simulation demonstrate a 3-fold increase in HCHO production across the range of observed NOx values, driven by a 100 % increase in OH and a 40 % increase in branching of organic peroxy radical reactions to produce HCHO.

scholarly journals Sensitivity to grid resolution in the ability of a chemical transport model to simulate observed oxidant chemistry under high-isoprene conditions

2016 ◽  
Author(s):  
Karen Yu ◽  
Daniel J. Jacob ◽  
Jenny A. Fisher ◽  
Patrick S. Kim ◽  
Eloise A. Marais ◽  
...  
Keyword(s):  
Negative Correlation ◽  
Transport Model ◽  
Chemical Transport ◽  
Grid Resolution ◽  
Cumulative Probability ◽  
Peroxy Radicals ◽  
Chemical Transport Model ◽  
Coarse Resolution ◽  
Isoprene Oxidation ◽  
Southeast Us

Abstract. Formation of ozone and organic aerosol in continental atmospheres depends on whether isoprene emitted by vegetation is oxidized by the high-NOx pathway (where peroxy radicals react with NO) or by low-NOx pathways (where peroxy radicals react by alternate channels, mostly with HO2). We used mixed layer observations from the SEAC4RS aircraft campaign over the Southeast US to test the ability of the GEOS-Chem chemical transport model at different grid resolutions (0.25° × 0.3125°, 2° × 2.5°, 4° × 5°) to simulate this chemistry under high-isoprene, variable-NOx conditions. Observations of isoprene and NOx over the Southeast US show a negative correlation, reflecting in part the spatial segregation of emissions; this negative correlation is captured in the model at 0.25° × 0.3125° resolution but not at coarser resolutions. As a result, less isoprene oxidation takes place by the high-NOx pathway in the model at 0.25° × 0.3125° resolution (54 %) than at coarser resolution (59 %). The cumulative probability distribution functions (CDFs) of NOx, isoprene, and ozone concentrations show little difference across model resolutions and good agreement with observations, while formaldehyde is overestimated at coarse resolution because excessive isoprene oxidation takes place by the high-NOx pathway (which has high formaldehyde yield). Correlations of simulated vs. observed concentrations do not improve with grid resolution because finer modes of variability are intrinsically more difficult to capture. Higher model resolution leads to decreased conversion of NOx to organic nitrates and increased conversion to nitric acid, with total reactive nitrogen oxides (NOy) changing little across model resolutions. In the lower free troposphere, model output is similarly insensitive to grid resolution, indicating that the effect on export of ozone and NOx is small. The overall low sensitivity of modeled concentrations to grid resolution implies that coarse resolution is adequate when modeling regional boundary layer chemistry for global applications.

scholarly journals Sensitivity to grid resolution in the ability of a chemical transport model to simulate observed oxidant chemistry under high-isoprene conditions

2016 ◽  
Vol 16 (7) ◽  
pp. 4369-4378 ◽  
Author(s):  
Karen Yu ◽  
Daniel J. Jacob ◽  
Jenny A. Fisher ◽  
Patrick S. Kim ◽  
Eloise A. Marais ◽  
...  
Keyword(s):  
Negative Correlation ◽  
Transport Model ◽  
Chemical Transport ◽  
Grid Resolution ◽  
Peroxy Radicals ◽  
Chemical Transport Model ◽  
Coarse Resolution ◽  
Isoprene Oxidation ◽  
Southeast Us ◽  
Good Agreement

Abstract. Formation of ozone and organic aerosol in continental atmospheres depends on whether isoprene emitted by vegetation is oxidized by the high-NOx pathway (where peroxy radicals react with NO) or by low-NOx pathways (where peroxy radicals react by alternate channels, mostly with HO2). We used mixed layer observations from the SEAC4RS aircraft campaign over the Southeast US to test the ability of the GEOS-Chem chemical transport model at different grid resolutions (0.25°  ×  0.3125°, 2°  ×  2.5°, 4°  ×  5°) to simulate this chemistry under high-isoprene, variable-NOx conditions. Observations of isoprene and NOx over the Southeast US show a negative correlation, reflecting the spatial segregation of emissions; this negative correlation is captured in the model at 0.25°  ×  0.3125° resolution but not at coarser resolutions. As a result, less isoprene oxidation takes place by the high-NOx pathway in the model at 0.25°  ×  0.3125° resolution (54 %) than at coarser resolution (59 %). The cumulative probability distribution functions (CDFs) of NOx, isoprene, and ozone concentrations show little difference across model resolutions and good agreement with observations, while formaldehyde is overestimated at coarse resolution because excessive isoprene oxidation takes place by the high-NOx pathway with high formaldehyde yield. The good agreement of simulated and observed concentration variances implies that smaller-scale non-linearities (urban and power plant plumes) are not important on the regional scale. Correlations of simulated vs. observed concentrations do not improve with grid resolution because finer modes of variability are intrinsically more difficult to capture. Higher model resolution leads to decreased conversion of NOx to organic nitrates and increased conversion to nitric acid, with total reactive nitrogen oxides (NOy) changing little across model resolutions. Model concentrations in the lower free troposphere are also insensitive to grid resolution. The overall low sensitivity of modeled concentrations to grid resolution implies that coarse resolution is adequate when modeling continental boundary layer chemistry for global applications.

scholarly journals Glyoxal yield from isoprene oxidation and relation to formaldehyde: chemical mechanism, constraints from SENEX aircraft observations, and interpretation of OMI satellite data

2016 ◽  
Author(s):  
Christopher Chan Miller ◽  
Daniel J. Jacob ◽  
Eloise A. Marais ◽  
Karen Yu ◽  
Katherine R. Travis ◽  
...  
Keyword(s):  
Transport Model ◽  
Peroxy Radical ◽  
Satellite Observations ◽  
Chemical Mechanism ◽  
Chemical Transport Model ◽  
Formation Pathway ◽  
Isoprene Emission ◽  
Aircraft Observations ◽  
Isoprene Oxidation ◽  
Southeast Us

Abstract. Glyoxal (CHOCHO) is produced in the atmosphere by oxidation of volatile organic compounds (VOCs). It is measurable from space by solar backscatter along with formaldehyde (HCHO), another oxidation product of VOCs. Isoprene emitted by vegetation is the dominant source of CHOCHO and HCHO in most of the world. We use aircraft observations of CHOCHO and HCHO from the SENEX campaign over the Southeast US in summer 2013 to better understand the time-dependent yields from isoprene oxidation, their dependences on nitrogen oxides (NOx ≡ NO + NO2), the behaviour of the CHOCHO-HCHO relationship, the quality of OMI satellite observations, and the implications for using satellite CHOCHO observations as constraints on isoprene emission. We simulate the SENEX and OMI observations with the GEOS-Chem chemical transport model featuring a new chemical mechanism for CHOCHO formation from isoprene. The mechanism includes prompt CHOCHO formation under low-NOx conditions following the isomerization of the isoprene peroxy radical (ISOPO2). The SENEX observations provide support for this prompt CHOCHO formation pathway, and are generally consistent with the GEOS-Chem mechanism. Boundary layer CHOCHO and HCHO are strongly correlated in the observations and the model, with some departure under low-NOx conditions due to prompt CHOCHO formation. SENEX vertical profiles indicate a free tropospheric CHOCHO background that is absent from the model. The OMI CHOCHO data provide some support for this free tropospheric background and show Southeast US enhancements consistent with the isoprene source but a factor of 2 too low. Part of this OMI bias is due to excessive surface reflectivities assumed in the retrieval. The OMI CHOCHO and HCHO seasonal data over the Southeast US are tightly correlated and provide redundant proxies of isoprene emission. Higher temporal resolution in future geostationary satellite observations may enable detection of the prompt CHOCHO production under low-NOx conditions apparent in the SENEX data.

scholarly journals Glyoxal yield from isoprene oxidation and relation to formaldehyde: chemical mechanism, constraints from SENEX aircraft observations, and interpretation of OMI satellite data

2017 ◽  
Vol 17 (14) ◽  
pp. 8725-8738 ◽  
Author(s):  
Christopher Chan Miller ◽  
Daniel J. Jacob ◽  
Eloise A. Marais ◽  
Karen Yu ◽  
Katherine R. Travis ◽  
...  
Keyword(s):  
Transport Model ◽  
Peroxy Radical ◽  
Satellite Observations ◽  
Chemical Mechanism ◽  
Chemical Transport Model ◽  
Formation Pathway ◽  
Earth Observing System ◽  
Aircraft Observations ◽  
Isoprene Oxidation ◽  
Southeast Us

Abstract. Glyoxal (CHOCHO) is produced in the atmosphere by the oxidation of volatile organic compounds (VOCs). Like formaldehyde (HCHO), another VOC oxidation product, it is measurable from space by solar backscatter. Isoprene emitted by vegetation is the dominant source of CHOCHO and HCHO in most of the world. We use aircraft observations of CHOCHO and HCHO from the SENEX campaign over the southeast US in summer 2013 to better understand the CHOCHO time-dependent yield from isoprene oxidation, its dependence on nitrogen oxides (NOx  ≡  NO + NO2), the behavior of the CHOCHO–HCHO relationship, the quality of OMI CHOCHO satellite observations, and the implications for using CHOCHO observations from space as constraints on isoprene emissions. We simulate the SENEX and OMI observations with the Goddard Earth Observing System chemical transport model (GEOS-Chem) featuring a new chemical mechanism for CHOCHO formation from isoprene. The mechanism includes prompt CHOCHO formation under low-NOx conditions following the isomerization of the isoprene peroxy radical (ISOPO2). The SENEX observations provide support for this prompt CHOCHO formation pathway, and are generally consistent with the GEOS-Chem mechanism. Boundary layer CHOCHO and HCHO are strongly correlated in the observations and the model, with some departure under low-NOx conditions due to prompt CHOCHO formation. SENEX vertical profiles indicate a free-tropospheric CHOCHO background that is absent from the model. The OMI CHOCHO data provide some support for this free-tropospheric background and show southeast US enhancements consistent with the isoprene source but a factor of 2 too low. Part of this OMI bias is due to excessive surface reflectivities assumed in the retrieval. The OMI CHOCHO and HCHO seasonal data over the southeast US are tightly correlated and provide redundant proxies of isoprene emissions. Higher temporal resolution in future geostationary satellite observations may enable detection of the prompt CHOCHO production under low-NOx conditions apparent in the SENEX data.

scholarly journals Global simulations of monoterpene-derived peroxy radical fates and the distributions of highly oxygenated organic molecules (HOM) and accretion products

2021 ◽  
Author(s):  
Ruochong Xu ◽  
Joel A. Thornton ◽  
Ben H. Lee ◽  
Yanxu Zhang ◽  
Lyatt Jaeglé ◽  
...  
Keyword(s):  
Rate Constants ◽  
First Generation ◽  
Transport Model ◽  
Peroxy Radical ◽  
Organic Aerosol ◽  
Laboratory Studies ◽  
Chemical Transport Model ◽  
Cross Reactions ◽  
Formation Kinetics ◽  
The Impact

Abstract. We evaluate monoterpene-derived peroxy radical (MT-RO2) unimolecular autoxidation and self and cross reactions with other RO2 in the GEOS-Chem global chemical transport model. Formation of associated highly oxygenated organic molecule (HOM) and accretion products are tracked in competition with other bimolecular reactions. Autoxidation is the dominant fate up to 6–8 km for first-generation MT-RO2 which can undergo unimolecular H-shifts. Reaction with NO can be a more common fate for H-shift rate constants < 0.1 s−1 or at altitudes higher than 8 km due to the imposed Arrhenius temperature dependence of unimolecular H-shifts. For MT-derived HOM-RO2, generated by multi-step autoxidation of first-generation MT-RO2, reaction with other RO2 is predicted to be the major fate throughout most of the boreal and tropical forested regions, while reaction with NO dominates in temperate and subtropical forests of the Northern Hemisphere. The newly added reactions result in ~4 % global average decrease of HO2 and RO2 mainly due to faster self-/cross-reactions of MT-RO2, but the impact upon HO2/OH/NOx abundances is only important in the planetary boundary layer (PBL) over portions of tropical forests. Within the bounds of formation kinetics and HOM photochemical lifetime constraints from laboratory studies, predicted HOM concentrations in MT-rich regions and seasons reach 10 % or even exceed total organic aerosol as predicted by the standard GEOS-Chem model. Comparisons to observations reveal large uncertainties remain for key reaction parameters and processes, especially the photochemical lifetime of HOM and associated accretion products. Using the highest reported yields and H-shift rate constants of MT-RO2 that undergo autoxidation, HOM concentrations tend to exceed the limited set of observations. Similarly, we infer that RO2 cross reactions rate constants near the gas-kinetic limit with accretion product branching greater than ~0.25 are inconsistent with total organic aerosol unless there is rapid decomposition of accretion products, the accretion products have saturation vapor concentrations > > 1 μg m−3, or modeled MT emission rates are overestimated. This work suggests further observations and laboratory studies related to MT-RO2 derived HOM and gas-phase accretion product formation kinetics, and especially their atmospheric fate, such as gas-particle partitioning, multi-phase chemistry, and net SOA formation, are needed.

scholarly journals α-pinene photooxidation under controlled chemical conditions – Part 1: Gas-phase composition in low- and high-NO<sub>x</sub> environments

2012 ◽  
Vol 12 (14) ◽  
pp. 6489-6504 ◽  
Author(s):  
N. C. Eddingsaas ◽  
C. L. Loza ◽  
L. D. Yee ◽  
J. H. Seinfeld ◽  
P. O. Wennberg
Keyword(s):  
Gas Phase ◽  
Carboxylic Acids ◽  
First Generation ◽  
Peroxy Radical ◽  
Organic Aerosol ◽  
Oxidation Products ◽  
Peroxy Radicals ◽  
Alkoxy Radical ◽  
Alkyl Peroxy Radicals ◽  
Chemical Conditions

Abstract. The OH oxidation of α-pinene under both low- and high-NOx environments was studied in the Caltech atmospheric chambers. Ozone was kept low to ensure OH was the oxidant. The initial α-pinene concentration was 20–50 ppb to ensure that the dominant peroxy radical pathway under low-NOx conditions is reaction with HO2, produced from reaction of OH with H2O2, and under high-NOx conditions, reactions with NO. Here we present the gas-phase results observed. Under low-NOx conditions the main first generation oxidation products are a number of α-pinene hydroxy hydroperoxides and pinonaldehyde, accounting for over 40% of the yield. In all, 65–75% of the carbon can be accounted for in the gas phase; this excludes first-generation products that enter the particle phase. We suggest that pinonaldehyde forms from RO2 + HO2 through an alkoxy radical channel that regenerates OH, a mechanism typically associated with acyl peroxy radicals, not alkyl peroxy radicals. The OH oxidation and photolysis of α-pinene hydroxy hydroperoxides leads to further production of pinonaldehyde, resulting in total pinonaldehyde yield from low-NOx OH oxidation of ~33%. The low-NOx OH oxidation of pinonaldehyde produces a number of carboxylic acids and peroxyacids known to be important secondary organic aerosol components. Under high-NOx conditions, pinonaldehyde was also found to be the major first-generation OH oxidation product. The high-NOx OH oxidation of pinonaldehyde did not produce carboxylic acids and peroxyacids. A number of organonitrates and peroxyacyl nitrates are observed and identified from α-pinene and pinonaldehyde.

scholarly journals Ozone production in remote oceanic and industrial areas derived from ship based measurements of peroxy radicals during TexAQS 2006

2011 ◽  
Vol 11 (6) ◽  
pp. 2471-2485 ◽  
Author(s):  
R. Sommariva ◽  
S. S. Brown ◽  
J. M. Roberts ◽  
D. M. Brookes ◽  
A. E. Parker ◽  
...  
Keyword(s):  
Gulf Coast ◽  
Peroxy Radical ◽  
Oxidation Products ◽  
Ozone Production ◽  
Ozone Formation ◽  
Peroxy Radicals ◽  
Industrial Areas ◽  
Mixing Ratios ◽  
Photolysis Rates ◽  
Houston Ship Channel

Abstract. During the Texas Air Quality Study II (TexAQS 2006) campaign, a PEroxy Radical Chemical Amplifier (PERCA) was deployed on the NOAA research vessel R/V Brown to measure total peroxy radicals (HO2+Σ RO2). Day-time mixing ratios of HO2+Σ RO2 between 25 and 110 ppt were observed throughout the study area – the Houston/Galveston region and the Gulf coast of the US – and analyzed in relation to measurements of nitrogen oxides, volatile organic compounds (VOC) and photolysis rates to assess radical sources and sinks in the region. The measurements of HO2+Σ RO2 were used to calculate the in-situ net photochemical formation of ozone. Measured median values ranged from 0.6 ppb/h in clean oceanic air masses up to several tens of ppb/h in the most polluted industrial areas. The results are consistent with previous studies and generally agree with observations made during the previous TexAQS 2000 field campaign. The net photochemical ozone formation rates determined at Barbours Cut, a site immediately south of the Houston Ship Channel, were analyzed in relation to local wind direction and VOC reactivity to understand the relationship between ozone formation and local VOC emissions. The measurements of HO2+Σ RO2 made during the R/V Brown TexAQS 2006 cruise indicate that ozone formation is NOx-limited in the Houston/Galveston region and influenced by highly reactive hydrocarbons, especially alkenes from urban and industrial sources and their photo-oxidation products, such as formaldehyde.

scholarly journals Long-term total OH reactivity measurements in a boreal forest

2019 ◽  
Author(s):  
Arnaud P. Praplan ◽  
Toni Tykkä ◽  
Dean Chen ◽  
Michael Boy ◽  
Ditte Taipale ◽  
...  
Keyword(s):  
Boreal Forest ◽  
Transport Model ◽  
Inorganic Compounds ◽  
Data Availability ◽  
Oxidation Products ◽  
Forest Site ◽  
Chemical Transport Model ◽  
Missing Measurements ◽  
Boundary Layer Height ◽  
Mixing Ratios

Abstract. Total hydroxyl radical (OH) reactivity measurements were conducted at the second Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II), a boreal forest site located in Hyytiälä, Finland, from April to July 2016. The measured values were compared with OH reactivity calculated from a combination of data from the routine trace gas measurements (station mast) as well as online and offline analysis with gas chromatography coupled to mass spectrometry (GC-MS) and offline liquid chromatography. Up to 104 compounds, mostly Volatile Organic Compounds (VOCs) and oxidised VOCs, but also inorganic compounds, were included in the analysis, even though the data availability for each compound varied with time. The averaged experimental total OH reactivity increased from April to June (from 5.3 to 11.3 s−1) and decreased in July (8.8 s−1) due to different environmental conditions during the measurement days. In general, the total OH reactivity increased in late-afternoon and is high at night. It decreases in the morning and is low during the day, following the pattern of mixing ratios due to change of the boundary layer height. The missing reactivity fraction (defined as the different between measured and calculated OH reactivity) was found to be high. Several reasons that can explain the missing reactivity are discussed in detail such as (1) missing measurements due to technical issues, (2) not measuring oxidation compounds of detected biogenic VOCs, (3) missing important reactive compounds or classes of compounds with the available measurements. In order to test the second hypothesis, a one-dimensional chemical transport model (SOSAA) has been used to estimate the amount of unmeasured oxidation products and their expected contribution to the reactivity for three different short periods in April, May, and July. However, only a small fraction (

Chamber Studies of NO3 reactivity during the oxidation of isoprene

2020 ◽  
Author(s):  
Patrick Dewald ◽  
Justin Shenolikar ◽  
Nils Friedrich ◽  
Franz Rohrer ◽  
Ralf Tillmann ◽  
...  
Keyword(s):  
Oxidation Products ◽  
Research Centre ◽  
Biogenic Emissions ◽  
Organic Nitrates ◽  
Peroxy Radicals ◽  
Experimental Conditions ◽  
Mixing Ratios ◽  
Volatile Organic ◽  
Isoprene Oxidation ◽  
Chamber Studies

&lt;p&gt;Isoprene is the major volatile organic compound that is released into the environment via biogenic emissions and its oxidation can result in formation of secondary organic aerosol (SOA). Although isoprene emission occurs mainly at daytime, it can accumulate at nighttime and be oxidized by the nitrate radical (NO&lt;sub&gt;3&lt;/sub&gt;) to form organic nitrates that can partition to the particle phase. A detailed understanding of the reaction between isoprene and NO&lt;sub&gt;3&lt;/sub&gt; is thus required to predict its role in e.g. NO&lt;sub&gt;X&lt;/sub&gt; lifetimes and SOA formation.&lt;/p&gt;&lt;p&gt;The reaction between NO&lt;sub&gt;3&lt;/sub&gt; and isoprene was investigated under varying experimental conditions (high or low RO&lt;sub&gt;2&lt;/sub&gt;/HO&lt;sub&gt;2&lt;/sub&gt;, temperature, humidity, seed aerosols) during the NO3ISOP campaign at the atmospheric simulation chamber SAPHIR of the research centre in J&amp;#252;lich (Germany). Direct measurement of the NO&lt;sub&gt;3&lt;/sub&gt; reactivity was carried out with means of a flowtube coupled to a cavity-ring-down spectroscopy (FT-CRDS) setup which enabled the evolution of the NO&lt;sub&gt;3&lt;/sub&gt; lifetime during the isoprene oxidation process to be monitored.&lt;/p&gt;&lt;p&gt;By comparing direct NO&lt;sub&gt;3&lt;/sub&gt; reactivity measurements with those calculated from VOC mixing ratios and those calculated from a stationary-state analysis we identify the contributions of isoprene, secondary oxidation products and peroxy radicals to NO&lt;sub&gt;3&lt;/sub&gt; losses.&lt;/p&gt;

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