Evaluating evidence for Cl sources and oxidation chemistry in a coastal, urban environment

Abstract. The role of chlorine atoms (Cl) in atmospheric oxidation was traditionally thought to be limited to the marine boundary layer, where they are produced through heterogeneous reactions involving sea salt. However, recent observation of photolytic Cl precursors (ClNO2 and Cl2) formed from anthropogenic pollution has expanded the potential importance of Cl to include coastal and continental urban areas. Measurements of ClNO2 in Los Angeles during CalNex showed it to be an important primary (first generation) radical source. Ratios of volatile organic compounds (VOCs) have been proposed as a sensitive method to quantify Cl oxidation, but have shown little evidence for a significant role of Cl outside of the Arctic. We used a box model with the Master Chemical Mechanism (MCM v3.2) chemistry scheme, constrained by observations in Los Angeles, to examine the Cl-sensitivity of the most commonly used VOC ratios (i-butane, n-butane, and propane) as a function of NOx and secondary radical production. Model results indicated these and faster reacting VOC tracer ratios could not detect the influence of Cl unless the sustained ratio of OH to Cl was below 200. However, the model results also show that secondary (second generation) OH production resulting from Cl oxidation of VOCs is strongly influenced by NOx, and that this effect can obscure the importance of Cl as a primary oxidant. Calculated concentrations of Cl showed a maximum in mid-morning due to a photolytic source from ClNO2 and loss primarily to reactions with VOCs. The OH to Cl ratio was below 200 for approximately three hours in the morning, but Cl oxidation was not evident from the measured ratios of VOCs. Instead, model simulations show that secondary OH production causes VOC ratios to follow the values expected for OH oxidation despite the significant input of primary Cl from ClNO2 photolysis in the morning. Despite the prevalence of secondary OH as an oxidant in Los Angeles, Cl may play an important role in tropospheric chemistry. The reactivity of Cl in Los Angeles during CalNex was more than an order of magnitude larger than that of OH. In addition, because of its reactivity toward different classes of VOCs and its greater propensity to participate in chain propagation rather than sink reactions, Cl atoms have a different impact on regional atmospheric oxidation than do OH radicals.

Download Full-text

Chlorine as a primary radical: evaluation of methods to understand its role in initiation of oxidative cycles

Atmospheric Chemistry and Physics ◽

10.5194/acp-14-3427-2014 ◽

2014 ◽

Vol 14 (7) ◽

pp. 3427-3440 ◽

Cited By ~ 51

Author(s):

C. J. Young ◽

R. A. Washenfelder ◽

P. M. Edwards ◽

D. D. Parrish ◽

J. B. Gilman ◽

...

Keyword(s):

Los Angeles ◽

Urban Areas ◽

Marine Boundary Layer ◽

Recent Observation ◽

Anthropogenic Pollution ◽

Heterogeneous Reactions ◽

Chemical Mechanism ◽

Production Model ◽

Primary Radical

Abstract. The role of chlorine atoms (Cl) in atmospheric oxidation has been traditionally thought to be limited to the marine boundary layer, where they are produced through heterogeneous reactions involving sea salt. However, recent observation of photolytic Cl precursors (ClNO2 and Cl2) formed from anthropogenic pollution has expanded the potential importance of Cl to include coastal and continental urban areas. Measurements of ClNO2 in Los Angeles during CalNex (California Nexus – Research at the Nexus of Air Quality and Climate Change) showed it to be an important primary (first generation) radical source. Evolution of ratios of volatile organic compounds (VOCs) has been proposed as a method to quantify Cl oxidation, but we find no evidence from this approach for a significant role of Cl oxidation in Los Angeles. We use a box model with the Master Chemical Mechanism (MCM v3.2) chemistry scheme, constrained by observations in Los Angeles, to examine the Cl sensitivity of commonly used VOC ratios as a function of NOx and secondary radical production. Model results indicate VOC tracer ratios could not detect the influence of Cl unless the ratio of [OH] to [Cl] was less than 200 for at least a day. However, the model results also show that secondary (second generation) OH production resulting from Cl oxidation of VOCs is strongly influenced by NOx, and that this effect obscures the importance of Cl as a primary oxidant. Calculated concentrations of Cl showed a maximum in mid-morning due to a photolytic source from ClNO2 and loss primarily to reactions with VOCs. The [OH] to [Cl] ratio was below 200 for approximately 3 h in the morning, but Cl oxidation was not evident from the measured ratios of VOCs. Instead, model simulations show that secondary OH production causes VOC ratio evolution to follow that expected for OH oxidation, despite the significant input of primary Cl from ClNO2 photolysis in the morning. Even though OH is by far the dominant oxidant in Los Angeles, Cl atoms do play an important role in photochemistry there, constituting 9% of the primary radical source. Furthermore, Cl–VOC reactivity differs from that of OH, being more than an order of magnitude larger and dominated by VOCs, such as alkanes, that are less reactive toward OH. Primary Cl is also slightly more effective as a radical source than primary OH due to its greater propensity to initiate radical propagation chains via VOC reactions relative to chain termination via reaction with nitrogen oxides.

Download Full-text

A fast stratospheric chemistry solver: the E4CHEM submodel for the atmospheric chemistry global circulation model EMAC

Geoscientific Model Development ◽

10.5194/gmd-3-321-2010 ◽

2010 ◽

Vol 3 (1) ◽

pp. 321-328 ◽

Cited By ~ 2

Author(s):

A. J. G. Baumgaertner ◽

P. Jöckel ◽

B. Steil ◽

H. Tost ◽

R. Sander

Keyword(s):

Atmospheric Chemistry ◽

General Circulation ◽

Circulation Model ◽

Reaction Rates ◽

Heterogeneous Reactions ◽

Tropospheric Chemistry ◽

Box Model ◽

Chemical Mechanism ◽

Global Circulation Model ◽

Stratospheric Chemistry

Abstract. The atmospheric chemistry general circulation model ECHAM5/MESSy (EMAC) and the atmospheric chemistry box model CAABA are extended by a computationally very efficient submodel for atmospheric chemistry, E4CHEM. It focuses on stratospheric chemistry but also includes background tropospheric chemistry. It is based on the chemistry of MAECHAM4-CHEM and is intended to serve as a simple and fast alternative to the flexible but also computationally more demanding submodel MECCA. In a model setup with E4CHEM, EMAC is now also suitable for simulations of longer time scales. The reaction mechanism contains basic O3, CH4, CO, HOx, NOx, and ClOx gas phase chemistry. In addition, E4CHEM includes optional fast routines for heterogeneous reactions on sulphate aerosols and polar stratospheric clouds (substituting the existing submodels PSC and HETCHEM), and scavenging (substituting the existing submodel SCAV). We describe the implementation of E4CHEM into the MESSy structure of CAABA and EMAC. For some species the steady state in the box model differs by up to 100% when compared to results from CAABA/MECCA due to different reaction rates. After an update of the reaction rates in E4CHEM the mixing ratios in both boxmodel and 3-D model simulations are in satisfactory agreement with the results from a simulation where MECCA with a similar chemistry scheme was employed. Finally, a comparison against a simulation with a more complex and already evaluated chemical mechanism is presented in order to discuss shortcomings associated with the simplification of the chemical mechanism.

Download Full-text

Evaluation of 1,3,5 trimethylbenzene degradation in the detailed tropospheric chemistry mechanism, MCMv3.1, using environmental chamber data

Atmospheric Chemistry and Physics ◽

10.5194/acp-8-6453-2008 ◽

2008 ◽

Vol 8 (21) ◽

pp. 6453-6468 ◽

Cited By ~ 39

Author(s):

A. Metzger ◽

J. Dommen ◽

K. Gaeggeler ◽

J. Duplissy ◽

A. S. H. Prevot ◽

...

Keyword(s):

Degradation Mechanism ◽

Reactive Systems ◽

Tropospheric Chemistry ◽

Chemical Mechanism ◽

Oh Radicals ◽

Environmental Chamber ◽

Photo Oxidation ◽

Trimethyl Benzene ◽

Using Data ◽

Paul Scherrer

Abstract. The degradation mechanism of 1,3,5-trimethyl- benzene (TMB) as implemented in the Master Chemical Mechanism version 3.1 (MCM) was evaluated using data from the environmental chamber at the Paul Scherrer Institute. The results show that the MCM provides a consistent description of the photo-oxidation of TMB/NOx mixtures for a range of conditions. In all cases the agreement between the measurement and the simulation decreases with decreasing VOC-NOx ratio and in addition with increasing precursor concentration. A significant underestimation of the decay rate of TMB and thus underestimation of reactivity in the system, consistent with results from previous appraisals of the MCM, was observed. Much higher nitrous acid (HONO) concentrations compared to simulations and expected from chamber characterization experiments were measured during these smog chamber experiments. A light induced NO2 to HONO conversion at the chamber walls is suggested to occur. This photo-enhanced NO2 to HONO conversion with subsequent HONO photolysis enhances the reactivity of the system. After the implementation of this reaction in the model it describes the decay of TMB properly. Nevertheless, the model still over-predicts ozone at a later stage of the experiment. This can be attributed to a too slow removal of NO2. It is also shown that this photo-enhanced HONO formation is not restricted to TMB photo-oxidation but also occurs in other chemical systems (e.g. α-pinene). However, the influence of HONO as a source of OH radicals is less important in these more reactive systems and therefore the importance of the HONO chemistry is less obvious.

Download Full-text

A fast stratospheric chemistry solver: the E4CHEM submodel for the atmospheric chemistry global circulation model EMAC

Geoscientific Model Development Discussions ◽

10.5194/gmdd-3-181-2010 ◽

2010 ◽

Vol 3 (1) ◽

pp. 181-200

Author(s):

A. J. G. Baumgaertner ◽

P. Jöckel ◽

B. Steil ◽

H. Tost ◽

R. Sander

Keyword(s):

Atmospheric Chemistry ◽

General Circulation ◽

Circulation Model ◽

Reaction Rates ◽

Heterogeneous Reactions ◽

Tropospheric Chemistry ◽

Box Model ◽

Chemical Mechanism ◽

Global Circulation Model ◽

Stratospheric Chemistry

Abstract. The atmospheric chemistry general circulation model ECHAM5/MESSy (EMAC) and the atmospheric chemistry box model CAABA are extended by a computationally very efficient submodel for atmospheric chemistry, E4CHEM. It focuses on stratospheric chemistry but also includes background tropospheric chemistry. It is based on the chemistry of MAECHAM4-CHEM and is intended to serve as a simple and fast alternative to the flexible but also computationally more demanding submodel MECCA. In a model setup with E4CHEM, EMAC is now also suitable for simulations of longer time scales. The reaction mechanism contains basic O3, CH4, CO, HOx, NOx and ClOx gas phase chemistry. In addition, E4CHEM includes optional fast routines for heterogeneous reactions on sulphate aerosols and polar stratospheric clouds (substituting the existing submodels PSC and HETCHEM), and scavenging (substituting the existing submodel SCAV). We describe the implementation of E4CHEM into the MESSy structure of CAABA and EMAC. For some species the steady state in the box model differs by up to 100% when compared to results from CAABA/MECCA due to different reaction rates. After an update of the reaction rates in E4CHEM the mixing ratios in both boxmodel and 3-D model simulations are in satisfactory agreement with the results from a simulation where MECCA with a similar chemistry scheme was employed. Finally, a comparison against a simulation with a more complex and already evaluated chemical mechanism is presented in order to discuss shortcomings associated with the simplification of the chemical mechanism.

Download Full-text

The potential importance of frost flowers, recycling on snow, and open leads for Ozone Depletion Events

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-7-4521-2007 ◽

2007 ◽

Vol 7 (2) ◽

pp. 4521-4595 ◽

Cited By ~ 7

Author(s):

M. Piot ◽

R. von Glasow

Keyword(s):

Gas Phase ◽

Ozone Depletion ◽

Aerosol Deposition ◽

Heterogeneous Reactions ◽

The Arctic ◽

Acid Uptake ◽

Mixing Ratios ◽

Haze Pollution ◽

Frost Flowers

Abstract. We present model studies with the one-dimensional model MISTRA to investigate the potential role of frost flowers, recycling on snow, and open leads in the depletion of tropospheric ozone in the Arctic spring. In our model, we assumed frost flower aerosols to be the major source of bromine. We show that a major ozone depletion event can be satisfactorily reproduced only if the recycling on snow of deposited bromine into gas phase bromine is assumed. In the model, this cycling is more efficient than the bromine explosion process and maintains sufficiently high levels of bromine to deplete ozone down to few nmol mol−1 within four days. We assessed the influence of different surface combinations (open lead/frost flowers) on the chemistry in the model. Results showed noticeable modifications affecting the composition of aerosols and the deposition velocities. A model run with a series of coupled frost flower fields and open leads, separated by large areas of snow, showed results comparable with field observations. In addition, we studied the effects of modified temperature of either the frost flower field or the ambient airmass. A warmer frost flower field increases the relative humidity and the aerosol deposition rate. The deposition/re-emission process gains in importance, inducing more reactive bromine in the gas phase, and a stronger ozone depletion. A decrease of 1 K in airmass temperature shows in our model that the aerosol uptake capacities of all gas phase species substantially increases, leading to enhanced uptake of acids from the gas phase. Consequently, the so-called bromine explosion accelerated and O3 mixing ratios decreased. In our model representation, variations in wind speed affected the aerosol source function and influenced the amount of bromine in the atmosphere and thus the ozone depletion strength. Recent studies have suggested the important role of the precipitation of calcium carbonate (CaCO3) out of the brine layer for the possible acidification of the liquid phase by acid uptake. Our investigation showed that this precipitation is a crucial process for the timing of the bromine explosion in aerosols. Nevertheless, model runs with either 50% precipitation or complete precipitation displayed a relatively weak difference in ozone mixing ratios after four simulated days. By considering conditions typical for "Arctic Haze" pollution events at the start of the run we obtained a low pH in frost flower aerosols due to a greater mixing ratio of SO2, and a strong recycling efficiency via large aerosol number concentration. The aerosol acidification during a haze event most likely intensifies the ozone depletion strength and occurrence. The comparison between our modeled deposition on snow and sampled snow at Barrow (Alaska) shows that approximately 75% of deposited bromine may be re-emitted into the gas phase as Br2/BrCl. Among several non-halogen fluxes from the snow, model simulations showed that only HONO affects the chemistry. Finally, we investigated the release of Br2 potentially produced by heterogeneous reactions directly on frost flowers. In this case, we obtained unrealistic results of aerosol compositions and deposition rates on snow compared to observations in the Arctic.

Download Full-text

Observationally constrained modeling of atmospheric oxidation capacity and photochemical reactivity in Shanghai, China

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-1217-2020 ◽

2020 ◽

Vol 20 (3) ◽

pp. 1217-1232 ◽

Cited By ~ 3

Author(s):

Jian Zhu ◽

Shanshan Wang ◽

Hongli Wang ◽

Shengao Jing ◽

Shengrong Lou ◽

...

Keyword(s):

Air Quality ◽

Chain Length ◽

Ambient Air ◽

Chemical Mechanism ◽

Atmospheric Oxidation ◽

Oh Radicals ◽

Ozone Pollution ◽

Oxidation Capacity ◽

High Level ◽

Ozone Levels

Abstract. An observation-based model coupled to the Master Chemical Mechanism (V3.3.1) and constrained by a full suite of observations was developed to study atmospheric oxidation capacity (AOC), OH reactivity, OH chain length and HOx (=OH+HO2) budget for three different ozone (O3) concentration levels in Shanghai, China. Five months of observations from 1 May to 30 September 2018 showed that the air quality level is lightly polluted or worse (Ambient Air Quality Index, AQI, of > 100) for 12 d, of which ozone is the primary pollutant for 10 d, indicating ozone pollution was the main air quality challenge in Shanghai during summer of 2018. The levels of ozone and its precursors, as well as meteorological parameters, revealed the significant differences among different ozone levels, indicating that the high level of precursors is the precondition of ozone pollution, and strong radiation is an essential driving force. By increasing the input JNO2 value by 40 %, the simulated O3 level increased by 30 %–40 % correspondingly under the same level of precursors. The simulation results show that AOC, dominated by reactions involving OH radicals during the daytime, has a positive correlation with ozone levels. The reactions with non-methane volatile organic compounds (NMVOCs; 30 %–36 %), carbon monoxide (CO; 26 %–31 %) and nitrogen dioxide (NO2; 21 %–29 %) dominated the OH reactivity under different ozone levels in Shanghai. Among the NMVOCs, alkenes and oxygenated VOCs (OVOCs) played a key role in OH reactivity, defined as the inverse of the OH lifetime. A longer OH chain length was found in clean conditions primarily due to low NO2 in the atmosphere. The high level of radical precursors (e.g., O3, HONO and OVOCs) promotes the production and cycling of HOx, and the daytime HOx primary source shifted from HONO photolysis in the morning to O3 photolysis in the afternoon. For the sinks of radicals, the reaction with NO2 dominated radical termination during the morning rush hour, while the reactions of radical–radical also contributed to the sinks of HOx in the afternoon. Furthermore, the top four species contributing to ozone formation potential (OFP) were HCHO, toluene, ethylene and m/p-xylene. The concentration ratio (∼23 %) of these four species to total NMVOCs is not proportional to their contribution (∼55 %) to OFP, implying that controlling key VOC species emission is more effective than limiting the total concentration of VOC in preventing and controlling ozone pollution.

Download Full-text

Technical note: Conversion of isoprene hydroxy hydroperoxides (ISOPOOHs) on metal environmental simulation chamber walls

Atmospheric Chemistry and Physics ◽

10.5194/acp-17-4053-2017 ◽

2017 ◽

Vol 17 (6) ◽

pp. 4053-4062 ◽

Cited By ~ 4

Author(s):

Anne-Kathrin Bernhammer ◽

Martin Breitenlechner ◽

Frank N. Keutsch ◽

Armin Hansel

Keyword(s):

Gas Phase ◽

Technical Note ◽

Vinyl Ketone ◽

Heterogeneous Reactions ◽

Methyl Vinyl Ketone ◽

Oxidation Products ◽

Chemical Mechanism ◽

Stainless Steel Surface ◽

Methyl Vinyl

Abstract. Sources and sinks of isoprene oxidation products from low-NOx isoprene chemistry have been studied at the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber with a custom-built selective reagent ion time-of-flight mass spectrometer (SRI-ToF-MS), which allows quantitative measurement of isoprene hydroxy hydroperoxides (ISOPOOHs). The measured concentrations of the main oxidation products were compared to chemical box model simulations based on the Leeds Master Chemical Mechanism (MCM) v3.3. The modeled ISOPOOH concentrations are a factor of 20 higher than the observed concentrations, and methyl vinyl ketone (MVK) and methacrolein (MACR) concentrations are up to a factor of 2 lower compared to observations, despite the artifact-free detection method. Addition of catalytic conversion of 1,2-ISOPOOH and 4,3-ISOPOOH to methyl vinyl ketone (MVK) and methacrolein (MACR) on the stainless-steel surface of the chamber to the chemical mechanism resolves the discrepancy between model predictions and observation. This suggests that isoprene chemistry in a metal chamber under low-NOx conditions cannot be described by a pure gas phase model alone. Biases in the measurement of ISOPOOH, MVK, and MACR can be caused not only intra-instrumentally but also by the general experimental setup. The work described here extends the role of heterogeneous reactions affecting gas phase composition and properties from instrumental surfaces, described previously, to general experimental setups. The role of such conversion reactions on real environmental surfaces is yet to be explored.

Download Full-text

Evaluation of 1,3,5 trimethylbenzene degradation in the detailed tropospheric chemistry mechanism, MCMv3.1, using environmental chamber data

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-8-11567-2008 ◽

2008 ◽

Vol 8 (3) ◽

pp. 11567-11607 ◽

Cited By ~ 2

Author(s):

A. Metzger ◽

J. Dommen ◽

K. Gaeggeler ◽

J. Duplissy ◽

A. S. H. Prevot ◽

...

Keyword(s):

Degradation Mechanism ◽

Reactive Systems ◽

Tropospheric Chemistry ◽

Chemical Mechanism ◽

Oh Radicals ◽

Environmental Chamber ◽

Chemical Systems ◽

Photo Oxidation ◽

Using Data ◽

Paul Scherrer

Abstract. The degradation mechanism of 1,3,5-trimethylbenzene (TMB) as implemented in the Master Chemical Mechanism version 3.1 (MCM) was evaluated using data from the environmental chamber at the Paul Scherrer Institute. The results show that the MCM provides a consistent description of the photo-oxidation of TMB/NOx mixtures for a range of conditions. In all cases the agreement between the measurement and the simulation decreases with decreasing VOC-NOx ratio and in addition with increasing precursor concentration. A significant underestimation of the decay rate of TMB and thus underestimation of reactivity in the system, consistent with results from previous appraisals of the MCM, was observed. Much higher nitrous acid (HONO) concentrations compared to simulations and expected from chamber characterization experiments were measured during these smog chamber experiments. A light induced NO2 to HONO conversion at the chamber walls is suggested to occur. This photo-enhanced NO2 to HONO conversion with subsequent HONO photolysis enhances the reactivity of the system. After the implementation of this reaction in the model it describes the decay of TMB properly. Nevertheless, the model still over-predicts ozone at a later stage of the experiment. This can be attributed to a too slow removal of NO2. It is also shown that this photo-enhanced HONO formation is not restricted to TMB photo-oxidation but also occurs in other chemical systems (e.g. α-pinene). However, the influence of HONO as a source of OH radicals is less important in these more reactive systems and therefore the importance of the HONO chemistry is less obvious.

Download Full-text

Non-OH chemistry in oxidation flow reactors for the study of atmospheric chemistry systematically examined by modeling

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-15-23543-2015 ◽

2015 ◽

Vol 15 (17) ◽

pp. 23543-23586 ◽

Cited By ~ 4

Author(s):

Z. Peng ◽

D. A. Day ◽

A. M. Ortega ◽

B. B. Palm ◽

W. W. Hu ◽

...

Keyword(s):

Los Angeles ◽

Atmospheric Chemistry ◽

Uv Light ◽

Field Studies ◽

Tropospheric Chemistry ◽

Oh Radicals ◽

Relative Importance ◽

Flow Reactors ◽

Wide Range ◽

The Impact

Abstract. Oxidation flow reactors (OFRs) using low-pressure Hg lamp emission at 185 and 254 nm produce OH radicals efficiently and are widely used in atmospheric chemistry and other fields. However, knowledge of detailed OFR chemistry is limited, allowing speculation in the literature about whether some non-OH reactants, including several not relevant for tropospheric chemistry, may play an important role in these OFRs. These non-OH reactants are UV radiation, O(1D), O(3P), and O3. In this study, we investigate the relative importance of other reactants to OH for the fate of reactant species in OFR under a wide range of conditions via box modeling. The relative importance of non-OH species is less sensitive to UV light intensity than to relative humidity (RH) and external OH reactivity (OHRext), as both non-OH reactants and OH scale roughly proportional to UV intensity. We show that for field studies in forested regions and also the urban area of Los Angeles, reactants of atmospheric interest are predominantly consumed by OH. We find that O(1D), O(3P), and O3 have relative contributions to VOC consumption that are similar or lower than in the troposphere. The impact of O atoms can be neglected under most conditions in both OFR and troposphere. Under "pathological OFR conditions" of low RH and/or high OHRext, the importance of non-OH reactants is enhanced because OH is suppressed. Some biogenics can have substantial destructions by O3, and photolysis at non-tropospheric wavelengths (185 and 254 nm) may also play a significant role in the degradation of some aromatics under pathological conditions. Working under low O2 with the OFR185 mode allows OH to completely dominate over O3 reactions even for the biogenic species most reactive with O3. Non-tropospheric VOC photolysis may have been a problem in some laboratory and source studies, but can be avoided or lessened in future studies by diluting source emissions and working at lower precursor concentrations in lab studies, and by humidification. SOA photolysis is shown to be insignificant for most functional groups, except for nitrates and especially aromatics, which may be photolyzed at high UV flux settings. Our work further establishes the OFR's usefulness as a tool to study atmospheric chemistry and enables better experiment design and interpretation, as well as improved future reactor design.

Download Full-text

Technical Note: Conversion of Isoprene Hydroxy Hydroperoxides (ISOPOOH) on Metal Environmental Simulation Chamber Walls

10.5194/acp-2016-816 ◽

2016 ◽

Author(s):

Anne-Kathrin Bernhammer ◽

Martin Breitenlechner ◽

Frank N. Keutsch ◽

Armin Hansel ◽

Keyword(s):

Gas Phase ◽

Technical Note ◽

Heterogeneous Reactions ◽

Oxidation Products ◽

Chemical Mechanism ◽

Stainless Steel Surface ◽

Sources And Sinks ◽

Environmental Surfaces ◽

Isoprene Oxidation

Abstract. Sources and sinks of isoprene oxidation products from low NOx isoprene chemistry have been studied at the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber with a custom-built selective reagent ion time of flight mass spectrometer (SRI-ToF-MS), which allows quantitative measurement of isoprene hydroxy hydroperoxides (ISOPOOH). The measured concentrations of the main oxidation products were compared to chemical box model simulations based on the Leeds Master Chemical Mechanism (MCM) v3.3. The modelled ISOPOOH concentrations are by a factor of 20 higher than the observed and methyl vinyl ketone (MVK) and methacrolein (MACR) concentrations are by a factor of up to 2 lower compared to observations, despite the artifact-free detection method. Addition of catalytic conversion of 1,2-ISOPOOH and 4,3-ISOPOOH to MVK and MACR on the stainless steel surface of the chamber to the chemical mechanism resolves the discrepancy between model predictions and observation. This suggests that isoprene chemistry in a metal chamber under low NOx conditions cannot be described by a pure gas phase model alone. Biases in the measurement of ISOPOOH, MVK and MACR can not only be caused intra-instrumentally but also by the general experimental setup. The work described here extends the role of heterogeneous reactions affection gas phase composition and properties from instrumental surfaces, described previously, to general experimental setups. The role of such conversion reactions on real environmental surfaces is yet to be explored.

Download Full-text