Differences in BVOC oxidation and SOA formation above and below the forest canopy

Gas-phase biogenic volatile organic compounds (BVOCs) are oxidized in the troposphere to produce secondary pollutants such as ozone (O3), organic nitrates (RONO2), and secondary organic aerosol (SOA). Two coupled zero-dimensional models have been used to investigate differences in oxidation and SOA production from isoprene and α-pinene, especially with respect to the nitrate radical (NO3), above and below a forest canopy in rural Michigan. In both modeled environments (above and below the canopy), NO3 mixing ratios are relatively small (< 0.5 pptv); however, daytime (08:00–20:00 LT) mixing ratios below the canopy are 2 to 3 times larger than those above. As a result of this difference, NO3 contributes 12 % of total daytime α-pinene oxidation below the canopy while only contributing 4 % above. Increasing background pollutant levels to simulate a more polluted suburban or peri-urban forest environment increases the average contribution of NO3 to daytime below-canopy α-pinene oxidation to 32 %. Gas-phase RONO2 produced through NO3 oxidation undergoes net transport upward from the below-canopy environment during the day, and this transport contributes up to 30 % of total NO3-derived RONO2 production above the canopy in the morning (∼ 07:00). Modeled SOA mass loadings above and below the canopy ultimately differ by less than 0.5 µg m−3, and extremely low-volatility organic compounds dominate SOA composition. Lower temperatures below the canopy cause increased partitioning of semi-volatile gas-phase products to the particle phase and up to 35 % larger SOA mass loadings of these products relative to above the canopy in the model. Including transport between above- and below-canopy environments increases above-canopy NO3-derived α-pinene RONO2 SOA mass by as much as 45 %, suggesting that below-canopy chemical processes substantially influence above-canopy SOA mass loadings, especially with regard to monoterpene-derived RONO2

Validity and limitations of simple reaction kinetics to calculate concentrations of organic compounds from ion counts in PTR-MS

In September 2017, we conducted a proton-transfer-reaction mass-spectrometry (PTR-MS) intercomparison campaign at the CESAR observatory, a rural site in the central Netherlands near the village of Cabauw. Nine research groups deployed a total of 11 instruments covering a wide range of instrument types and performance. We applied a new calibration method based on fast injection of a gas standard through a sample loop. This approach allows calibrations on timescales of seconds, and within a few minutes an automated sequence can be run allowing one to retrieve diagnostic parameters that indicate the performance status. We developed a method to retrieve the mass-dependent transmission from the fast calibrations, which is an essential characteristic of PTR-MS instruments, limiting the potential to calculate concentrations based on counting statistics and simple reaction kinetics in the reactor/drift tube. Our measurements show that PTR-MS instruments follow the simple reaction kinetics if operated in the standard range for pressures and temperature of the reaction chamber (i.e. 1-4 mbar, 30-120 degrees, respectively), as well as a reduced field strength E/N in the range of 100-160 Td. If artefacts can be ruled out, it becomes possible to quantify the signals of uncalibrated organics with accuracies better than +/- 30 %. The simple reaction kinetics approach produces less accurate results at E/N levels below 100 Td, because significant fractions of primary ions form water hydronium clusters. Deprotonation through reactive collisions of protonated organics with water molecules needs to be considered when the collision energy is a substantial fraction of the exoergicity of the proton transfer reaction and/or if protonated organics undergo many collisions with water molecules.Peer reviewe

Measurement of interferences associated with the detection of the hydroperoxy radical in the atmosphere using laser-induced fluorescence

One technique used to measure concentrations of the hydroperoxy radical (HO$_2$) in the atmosphere involves chemically converting it to OH by addition of NO and subsequent detection of OH. However, some organic peroxy radicals (RO$_2$) can also be rapidly converted to HO2 (and subsequently OH) in the presence of NO, interfering with measurements of ambient HO$_2$ radical concentrations. This interference must be characterized for each instrument to determine to what extent various RO$_2$ radicals interfere with measurements of HO$_2$ and to assess the impact of this interference on past measurements. The efficiency of RO$_2$-to-HO$_2$ conversion for the Indiana University laser-induced fluorescence– fluorescence assay by gas expansion (IU-FAGE) instrument was measured for a variety of RO$_2$ radicals. Known quantities of OH and HO$_2$ radicals were produced from the photolysis of water vapor at 184.9 nm, and RO$_2$ radicals were produced by the reaction of several volatile organic compounds (VOCs) with OH. The conversion efficiency of RO$_2$ radicals to HO$_2$ was measured when NO was added to the sampling cell for conditions employed during several previous field campaigns. For these conditions, approximately 80 % of alkene-derived RO$_2$ radicals and 20 % of alkane-derived RO2 radicals were converted to HO$_2$. Based on these measurements, interferences from various RO$_2$ radicals contributed to approximately 35 % of the measured HO$_2$ signal during the Mexico City Metropolitan Area (MCMA) 2006 campaign (MCMA-2006), where the measured VOCs consisted of a mixture of saturated and unsaturated species. However, this interference can contribute more significantly to the measured HO$_2$ signal in forested environments dominated by unsaturated biogenic emissions such as isoprene

Simulating secondary organic aerosol in a regional air quality model using the statistical oxidation model - Part 3: Assessing the influence of semi-volatile and intermediate-volatility organic compounds and NOx_x

Semi-volatile and intermediate-volatility organic compounds (SVOCs and IVOCs) from anthropogenic sources are likely to be important precursors of secondary organic aerosol (SOA) in urban airsheds, yet their treatment in most models is based on limited and obsolete data or completely missing. Additionally, gas-phase oxidation of organic precursors to form SOA is influenced by the presence of nitric oxide (NO), but this influence is poorly constrained in chemical transport models. In this work, we updated the organic aerosol model in the UCD/CIT (University of California at Davis/California Institute of Technology) chemical transport model to include (i) a semi-volatile and reactive treatment of primary organic aerosol (POA), (ii) emissions and SOA formation from IVOCs, (iii) the NO$_x$ influence on SOA formation, and (iv) SOA parameterizations for SVOCs and IVOCs that are corrected for vapor wall loss artifacts during chamber experiments. All updates were implemented in the statistical oxidation model (SOM) that simulates the oxidation chemistry, thermodynamics, and gas–particle partitioning of organic aerosol (OA). Model treatment of POA, SVOCs, and IVOCs was based on an interpretation of a comprehensive set of source measurements available up to the year 2016 and resolved broadly by source type. The NO$_x$ influence on SOA formation was calculated offline based on measured and modeled VOC:NO$_x$ ratios. Finally, the SOA formation from all organic precursors (including SVOCs and IVOCs) was modeled based on recently derived parameterizations that accounted for vapor wall loss artifacts in chamber experiments. The updated model was used to simulate a 2-week summer episode over southern California at a model resolution of 8 km. When combustion-related POA was treated as semi-volatile, modeled POA mass concentrations were reduced by 15 %–40 % in the urban areas in southern California but were still too high when compared against “hydrocarbon-like organic aerosol” factor measurements made at Riverside, CA, during the Study of Organic Aerosols at Riverside (SOAR-1) campaign of 2005. Treating all POA (except that from marine sources) to be semi-volatile, similar to diesel exhaust POA, resulted in a larger reduction in POA mass concentrations and allowed for a better model–measurement comparison at Riverside, but this scenario is unlikely to be realistic since this assumes that POA from sources such as road and construction dust are semi-volatile too. Model predictions suggested that both SVOCs (evaporated POA vapors) and IVOCs did not contribute as much as other anthropogenic precursors (e.g., alkanes, aromatics) to SOA mass concentrations in the urban areas (< 5 % and < 15 % of the total SOA respectively) as the timescales for SOA production appeared to be shorter than the timescales for transport out of the urban airshed. Comparisons of modeled IVOC concentrations with measurements of anthropogenic SOA precursors in southern California seemed to imply that IVOC emissions were underpredicted in our updated model by a factor of 2. Correcting for the vapor wall loss artifact in chamber experiments enhanced SOA mass concentrations although the enhancement was precursor-dependent as well as NO$_x$-dependent. Accounting for the influence of NO$_x$ using the VOC:NO$_x$ ratios resulted in better predictions of OA mass concentrations in rural/remote environments but still underpredicted OA mass concentrations in urban environments. The updated model's performance against measurements combined with the results from the sensitivity simulations suggests that the OA mass concentrations in southern California are constrained within a factor of 2. Finally, simulations performed for the year 2035 showed that, despite reductions in VOC and NO$_x$ emissions in the future, SOA mass concentrations may be higher than in the year 2005, primarily from increased hydroxyl radical (OH) concentrations due to lower ambient NO$_2$ concentrations

Data associated with "Simulating secondary organic aerosol in a regional air quality model using the statistical oxidation model – Part 3: Assessing the influence of semi-volatile and intermediate-volatility organic compounds and NOx"

The dataset includes model predictions of gas- and particle-phase organic compounds from the UCD/CIT model and measurements over southern California. The dataset is limited to the information presented in the figures.Semi-volatile and intermediate-volatility organic compounds (SVOCs and IVOCs) from anthropogenic sources are likely to be important precursors of secondary organic aerosol (SOA) in urban airsheds, yet their treatment in most models is based on limited and obsolete data or completely missing. Additionally, gas-phase oxidation of organic precursors to form SOA is influenced by the presence of nitric oxide (NO), but this influence is poorly constrained in chemical transport models. In this work, we updated the organic aerosol model in the UCD/CIT (University of California at Davis and California Institute of Technology) chemical transport model to include (i) a semi-volatile and reactive treatment of primary organic aerosol (POA), (ii) emissions and SOA formation from IVOCs, (iii) the NOx influence on SOA formation, and (iv) SOA parameterizations for SVOCs and IVOCs that are corrected for vapor wall loss artifacts during chamber experiments. All updates were implemented in the statistical oxidation model (SOM) that simulates the oxidation chemistry, thermodynamics, and gas–particle partitioning of organic aerosol (OA). Model treatment of POA, SVOCs, and IVOCs was based on an interpretation of a comprehensive set of source measurements available up to the year 2016 and resolved broadly by source type. The NOx influence on SOA formation was calculated offline based on measured and modeled VOC:NOx ratios. Finally, the SOA formation from all organic precursors (including SVOCs and IVOCs) was modeled based on recently derived parameterizations that accounted for vapor wall loss artifacts in chamber experiments. The updated model was used to simulate a 2-week summer 30 episode over southern California at a model resolution of 8 km. When combustion-related POA was treated as semi-volatile, modeled POA mass concentrations were reduced by 15%-40% in the urban areas in southern California but were still too high when compared against "hydrocarbon-like organic aerosol" factor measurements made at Riverside, CA, during the Study of Organic Aerosols at Riverside (SOAR-1) campaign of 2005. Treating all POA (except that from marine sources) to be semi-volatile, similar to diesel exhaust POA, resulted in a larger reduction in POA mass concentrations and allowed for a better model–measurement comparison at Riverside, but this scenario is unlikely to be realistic since this assumes that POA from sources such as road and construction dust are semi-volatile too. Model predictions suggested that both SVOCs (evaporated POA vapors) and IVOCs did not contribute as much as other anthropogenic precursors (e.g., alkanes, aromatics) to SOA mass concentrations in the urban areas (<5% and <15% of the total SOA respectively) as the timescales for SOA production appeared to be shorter than the timescales for transport out of the urban airshed. Comparisons of modeled IVOC concentrations with measurements of anthropogenic SOA precursors in southern California seemed to imply that IVOC emissions were underpredicted in our updated model by a factor of 2. Correcting for the vapor wall loss artifact in chamber experiments enhanced SOA mass concentrations although the enhancement was precursor-dependent as well as NOx-dependent. Accounting for the influence of NOx using the VOC:NOx ratios resulted in better predictions of OA mass concentrations in rural/remote environments but still underpredicted OA mass concentrations in urban environments. The updated model's performance against measurements combined with the results from the sensitivity simulations suggests that the OA mass concentrations in southern California are constrained within a factor of 2. Finally, simulations performed for the year 2035 showed that, despite reductions in VOC and NOx emissions in the future, SOA mass concentrations may be higher than in the year 2005, primarily from increased hydroxyl radical (OH) concentrations due to lower ambient NO2 concentrations.Ali Akherati and Shantanu H. Jathar were partially supported by the National Oceanic and Atmospheric Administration (NA17OAR4310003). Jose L. Jimenez was supported by the Environmental Protection Agency (EPA) STAR program (83587701-0). Stephen M. Griffith, Sebastien Dusanter, Philip S. Stevens, and Christopher D. Cappa were supported by the National Science Foundation (AGS-0612738, AGS-1104880, and AGS-1523500)

The influence of organic and inorganic gases during New Particle Formation (NPF) events at the Mediterranean remote site of ERSA in Cape-Corsica during the summer of 2013.

International audienceAs part of the CHARMEX (Chemistry Aerosol Mediterranean Experiments) project, more than one hundred organic and inorganic gaseous compounds were measured in the summer of 2013 at the Mediterranean remote site of ERSA in Cape-Corsica. During this period, New Particle formation (NPF) events were identified from July 31th to august 2nd when air masses originated from the North-eastern sector (Southern Europe). The results were compared to a non-NPF event from July 21th to July 23rd for which the same wind sectors were identified. They showed that the particles number [10-20 nm] measured by SMPS (Scanning Mobility Particle Sizer) were more correlated with carbon monoxide (CO) during non-NPF events indicating an influence of more polluted and more aged air masses (residence time of CO of 60 days). Sulfuric acid (H2SO4) and sulfur dioxide do not show a significant influence in the formation of nucleation events. On the other hand, biogenic Volatile Organic Compounds (BVOCs) such as isoprene, and mono-terpenes as well as their oxidation products (e.g. MACR+MVK, MTOP) showed good correlation during NPF-events in the range of (r from 0.45 to 0.59) higher than the ones reported during non-NPF events (0.11-0.34) highlighting the importance of these BVOCs on NPF days. The comparison of measured vs calculated reactivity (Zannoni et al, 2016) showed that during NPF-events, the missing part of OH reactivity was higher. It indicates that unmeasured species like sesquiterpenes, organo-nitrates, or oxygenated compounds may play a significant role in such events

The influence of organic and inorganic gases during New Particle Formation (NPF) events at the Mediterranean remote site of ERSA in Cape-Corsica during the summer of 2013.

International audienceAs part of the CHARMEX (Chemistry Aerosol Mediterranean Experiments) project, more than one hundred organic and inorganic gaseous compounds were measured in the summer of 2013 at the Mediterranean remote site of ERSA in Cape-Corsica. During this period, New Particle formation (NPF) events were identified from July 31th to august 2nd when air masses originated from the North-eastern sector (Southern Europe). The results were compared to a non-NPF event from July 21th to July 23rd for which the same wind sectors were identified. They showed that the particles number [10-20 nm] measured by SMPS (Scanning Mobility Particle Sizer) were more correlated with carbon monoxide (CO) during non-NPF events indicating an influence of more polluted and more aged air masses (residence time of CO of 60 days). Sulfuric acid (H2SO4) and sulfur dioxide do not show a significant influence in the formation of nucleation events. On the other hand, biogenic Volatile Organic Compounds (BVOCs) such as isoprene, and mono-terpenes as well as their oxidation products (e.g. MACR+MVK, MTOP) showed good correlation during NPF-events in the range of (r from 0.45 to 0.59) higher than the ones reported during non-NPF events (0.11-0.34) highlighting the importance of these BVOCs on NPF days. The comparison of measured vs calculated reactivity (Zannoni et al, 2016) showed that during NPF-events, the missing part of OH reactivity was higher. It indicates that unmeasured species like sesquiterpenes, organo-nitrates, or oxygenated compounds may play a significant role in such events

Atmospheric Chemistry linked to HO x radicals of a Suburban Forest during the ACROSS summer Field Campaign

International audienceParis, one of the largest European megacities, transports pollution to different surrounding areas depending on the variation of the wind direction associated with specific meteorological conditions. The relatively unique situation of this isolated megacity from other urban areas make it a suitable location to study the impact of urban emissions on the chemistry of close biogenic environments such as forests and vice versa. In order to investigate this influence, the ACROSS (Atmospheric ChemistRy Of the Suburban foreSt) field campaign was performed during summer 2022, with a measurement site located in the Rambouillet forest. The combination of the data provided during this field campaign from different research groups (such as measurements of VOCs, inorganic species, particle concentration and composition, …) will allow a better understanding of the influence of mixing anthropogenic urban or oceanic air masses, leading to different NO concentrations, with biogenic forestry emissions on the oxidation of tropospheric VOCs. This will ultimately help improving this chemistry within atmospheric models. The UL-FAGE instrument was deployed during the ACROSS campaign, where different types of measurements were performed: OH, HO2, ROx radical quantification at the ground level and OH reactivity. The OH reactivity was alternatively measured at two different levels: below (ground level) and above the forest canopy (top of a 40 m tower). Clear stratification was observed during the night with a higher OH reactivity at the ground level than above the canopy. Comparison between the measured and the calculated OH reactivity allows to identify the diurnal missing reactivity at both levels. Preliminary results of the OH reactivity and the radical quantification will be presented

Peroxy radical measurements by ethane - nitric oxide chemical amplification and laser-induced fluorescence during the IRRONIC field campaign in a forest in Indiana

Peroxy radicals were measured in a mixed deciduous forest atmosphere in Bloomington, Indiana, USA, during the Indiana Radical, Reactivity and Ozone Production Intercomparison (IRRONIC) during the summer of 2015. Total peroxy radicals ([XO$_2$]≡[HO$_2$]+Σ[RO$_2$]) were measured by a newly developed technique involving chemical amplification using nitric oxide (NO) and ethane (C$_2$H$_6$) followed by NO$_2$ detection by cavity-attenuated phase-shift spectroscopy (hereinafter referred to as ECHAMP – Ethane CHemical AMPlifier). The sum of hydroperoxy radicals (HO$_2$) and a portion of organic peroxy radicals ([HO$_2^∗$]=[HO$_2$]+$Σ_{αi}$[RiO$_2$], 0<α<1) was measured by the Indiana University (IU) laser-induced fluorescence–fluorescence assay by gas expansion instrument (LIF-FAGE). Additional collocated measurements include concentrations of NO, NO$_2$, O$_3$, and a wide range of volatile organic compounds (VOCs) and meteorological parameters. XO$_2$ concentrations measured by ECHAMP peaked between 13:00 and 16:00 local time (LT), with campaign average concentrations of 41±15 ppt (1σ) at 14:00 LT. Daytime concentrations of isoprene averaged 3.6±1.9 ppb (1σ), whereas average concentrations of NO$_x$ ([NO] + [NO$_2$]) and toluene were 1.2 and 0.1 ppb, respectively, indicating a low impact from anthropogenic emissions at this site. We compared ambient measurements from both instruments and conducted a calibration source comparison. For the calibration comparison, the ECHAMP instrument, which is primarily calibrated with an acetone photolysis method, sampled the output of the LIF-FAGE calibration source which is based on the water vapor photolysis method and, for these comparisons, generated a 50 %–50 % mixture of HO$_2$ and either butane or isoprene-derived RO$_2$. A bivariate fit of the data yields the relation [XO$_2$]$_{\textrm{ECHAMP}}$=(0.88±0.02;[HO$_2$ ]+[RO$_2$ ])_{\textrm{IU_cal}}+(6.6±4.5) ppt. This level of agreement is within the combined analytical uncertainties for the two instruments' calibration methods. A linear fit of the daytime (09:00–22:00 LT) 30 min averaged [XO$_2$] ambient data with the 1 min averaged [HO$_2^∗$] data (one point per 30 min) yields the relation [XO$_2$]=(1.08±0.05)[HO$_2^∗$]−(1.4±0.3). Day-to-day variability in the [XO$_2$]/[HO$_2^∗$] ratio was observed. The lowest [XO$_2$ ]/[HO$_2^∗$] ratios between 13:00 and 16:00 LT were 0.8 on 13 and 18 July, whereas the highest ratios of 1.1 to 1.3 were observed on 24 and 25 July – the same 2 d on which the highest concentrations of isoprene and ozone were observed. Although the exact composition of the peroxy radicals during IRRONIC is not known, zero-dimensional photochemical modeling of the IRRONIC dataset using two versions of the Regional Atmospheric Chemistry Mechanism (RACM2 and RACM2 -LIM1) and the Master Chemical Mechanism (MCM 3.2 and MCM 3.3.1) all predict afternoon [XO$_2$]/[HO$_2^∗$] ratios of between 1.2 and 1.5. Differences between the observed ambient [XO$_2$]/[HO$_2^∗$] ratio and that predicted with the 0-D modeling can be attributed to deficiencies in the model, errors in one of the two measurement techniques, or both. Time periods in which the ambient ratio was less than 1 are definitely caused by measurement errors (including calibration differences), as such ratios are not physically meaningful. Although these comparison results are encouraging and demonstrate the viability in using the new ECHAMP technique for field measurements of peroxy radicals, further research investigating the overall accuracy of the measurements and possible interferences from both methods is warranted