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The budget of reactive nitrogen species, which play a central role in atmospheric chemistry (e.g. in photochemical O3 production), is poorly understood in forested regions. In this study, through observations of NO, NO2 , NOy , and O3 in the Rambouillet Forest near Paris, France, we have examined nighttime processes controlling NOx in an anthropogenically impacted forest environment. The O3 mixing ratios displayed a strong diel profile at the site that was driven by a variable but generally rapid deposition to soil and foliar surfaces. The O3 diel profile was strongly influenced by relative humidity and temperature inversion. Only when the O3 mixing ratio was sufficiently low (and thus the NO lifetime sufficiently long) were sustained NO peaks observed above the instrumental detection limit, enabling the derivation of average NO emission rates of ∼1.4 ppbvh-1 from the soil. Observations of the lack of increase in NO2 at night, despite a significant production rate from the reaction of NO with O3 , enabled an effective lifetime of NO2 of ∼0.5 –3 h to be derived. As the loss of NO2 was not compensated for by the formation of gas- or particle-phase reactive nitrogen species, it was presumably either driven by deposition to soil and foliar surfaces or any products formed were themselves short-lived with respect to deposition. By comparison, the daytime lifetime of NO2 with respect to loss by reaction with OH is about 1 d. Our results indicate that the nighttime deposition of NO2 is a major sink of boundary layer NOx in this temperate forest environment. [ABSTRACT FROM AUTHOR]

We present direct measurements of biogenic volatile organic compound (BVOC)-induced nitrate radical (NO3) reactivity (kVOC) through the diel cycle in the suburban, temperate forest of Rambouillet near Paris (France). The data were obtained in a 6-week summer period in 2022 as part of the Atmospheric ChemistRy Of the Suburban foreSt (ACROSS) campaign. kVOC was measured in a small (700 m2) clearing mainly at a height of 5.5 m above ground level but also at 40 m (for 5 d and nights). At nighttime, mean values (and 25th–75th percentile ranges) of knightVOC(5.5m) = (0.24-0.06+0.32) s-1 and knightVOC(40m) = (0.016-0.007+0.018) s-1 indicate a significant vertical gradient and low NO3 reactivity above the canopy, whereas knightVOC (5.5 m) showed peak values of up to 2 s-1 close to the ground. The strong vertical gradient in NO3 reactivity could be confirmed by measurements between 0 and 24 m on one particular night characterized by a strong temperature inversion and is a result of the decoupling of air masses aloft from the ground- and canopy-level sources of BVOCs (and nitric oxide, NO). No strong vertical gradient was observed in the mean daytime NO3 reactivity, with kdayVOC(5.5m) = (0.12±0.04) s-1 for the entire campaign and kdayVOC(40m) = (0.07±0.02) s-1 during the 5 d period. Within the clearing, the fractional contribution of VOCs to the total NO3 loss rate coefficient (ktot , determined by photolysis, reaction with NO and VOCs) was 80 %–90 % during the night and ∼ 50 % during the day. In terms of chemical losses of α -pinene below canopy height in the clearing, we find that at nighttime hydroxyl radicals (OH) and ozone (O3) dominate, with NO3 contributing "only" 17 %, which decreases further to 8.5 % during the day. Based on measured OH, measured O3 , and calculated NO3 concentrations, the chemical lifetime of BVOCs at noon is about 1 h and is likely to be longer than timescales of transport out of the canopy (typically of the order of minutes), thus significantly reducing the importance of daytime in-canopy processing. Clearly, in forested regions where sufficient nitric oxide and nitrogen dioxide (NOx) is available, the role of NO3 and OH as initiators of BVOC oxidation is not strictly limited to nighttime and daytime, respectively, as often implied in e.g. atmospheric chemistry textbooks. [ABSTRACT FROM AUTHOR]

We present direct measurements of BVOC-induced nitrate radical (NO3) reactivity (kVOC) through the diel cycle in the suburban, temperate forest of Rambouillet near Paris (France). The data were obtained in a six-week summer period in 2022 as part of the ACROSS campaign (Atmospheric ChemistRy Of the Suburban foreSt). kVOC was measured in a small (700 m²) clearing mainly at a height of 5.5 m above ground level, but also at 40 m (for 5 days/nights). At nighttime, mean values of kVOCnight (5.5 m) = (0.24 ± 0.27) s-1 and kVOCnight (40 m) = (0.016 ± 0.007) s-1 indicate a significant vertical gradient and low NO3 reactivity above the canopy, whereas kVOCnight (5.5 m) showed peak values of up to 2 s-1 close to the ground. The strong vertical gradient in NO3 reactivity could be confirmed by measurements between 0 and 24 m on one particular night characterised by a strong temperature inversion, and is a result of the decoupling of air masses aloft from the ground- and canopy-level sources of BVOCs (and NO). No strong vertical gradient was observed in the mean daytime NO3 reactivity with kVOCnight(5.5 m) = (0.12 ± 0.04) s-1 for the entire campaign and kVOCnight(40 m) = (0.07 ± 0.02) s-1 during the 5-day period. Within the clearing, the fractional contribution of VOCs to the total NO3 loss rate (LNO3, determined by photolysis, reaction with NO and VOCs) was 80-90 % during the night and ~50 % during the day. In terms of chemical losses of α-pinene below canopy height in the clearing, we find that at nighttime OH and O3 dominate with NO3 contributing "only" 17 %, which decreases further to 8.5 % during the day. Based on OH, O3 and NO3 concentrations, the chemical lifetime of BVOCs at noon is about one hour and is likely to be longer than timescales of transport out of the canopy (typically in the order of minutes), thus significantly reducing the importance of daytime, in-canopy processing. Clearly, in forested regions where sufficient NOX is available, the role of NO3 and OH as initiators of BVOC oxidation are not strictly limited to the night and to the day, respectively, as often implied in e.g. atmospheric chemistry text-books. [ABSTRACT FROM AUTHOR]

Through observations of NO, NO2, NOY and O3 in the Rambouillet forest near Paris, France, (as part of the ACROSS campaign, 2022) we have gained insight into nighttime processes controlling NOX in an anthropogenically impacted forest environment. O3 mixing ratios displayed a strong diel profile at the site, which was driven by a variable but generally rapid deposition to soil and foliar surfaces. The O3 diel profile was strongly influenced by relative humidity, which impacted the surface resistance to uptake, and temperature inversion, which influenced the rate of entrainment of O3 from above the canopy. Only when the O3 mixing ratio was sufficiently low (and thus the NO lifetime sufficiently long), were sustained NO peaks observed above the instrumental detection limit, enabling derivation of average NO emission rates from the soil of -1.4 ppbv h-1. Observations of the lack of increase in NO2 at night, despite a significant production rate from the reaction of NO with O3, enabled an effective lifetime of NO2 of -0.5-3 h to be derived. As the loss of NO2 was not compensated by the formation of gas- or particle-phase reactive nitrogen species it was presumably driven by deposition to soil and foliar surfaces, or any products formed were themselves short-lived with respect to deposition. By comparison, the daytime lifetime of NO2 with respect to loss by reaction with OH is about 1 day. We conclude that the nighttime deposition of NO2 is a major sink of boundary layer NOX in this temperate forest environment. [ABSTRACT FROM AUTHOR]

Through observations of NO, NO2, NOY and O3 in the Rambouillet forest near Paris, France, (as part of the ACROSS campaign, 2022) we have gained insight into nighttime processes controlling NOX in an anthropogenically impacted forest environment. O3 mixing ratios displayed a strong diel profile at the site, which was driven by a variable but generally rapid deposition to soil and foliar surfaces. The O3 diel profile was strongly influenced by relative humidity, which impacted the surface resistance to uptake, and temperature inversion, which influenced the rate of entrainment of O3 from above the canopy. Only when the O3 mixing ratio was sufficiently low (and thus the NO lifetime sufficiently long), were sustained NO peaks observed above the instrumental detection limit, enabling derivation of average NO emission rates from the soil of ~1.4 ppbv h‑1. Observations of the lack of increase in NO2 at night, despite a significant production rate from the reaction of NO with O3, enabled an effective lifetime of NO2 of ⁓0.5–3 h to be derived. As the loss of NO2 was not compensated by the formation of gas- or particle-phase reactive nitrogen species it was presumably driven by deposition to soil and foliar surfaces, or any products formed were themselves short-lived with respect to deposition. By comparison, the daytime lifetime of NO2 with respect to loss by reaction with OH is about 1 day. We conclude that the nighttime deposition of NO2 is a major sink of boundary layer NOX in this temperate forest environment. [ABSTRACT FROM AUTHOR]

Reaction of stabilized Criegee intermediates (SCIs) with SO 2 was proposed as an additional pathway of gaseous sulfuric acid (H 2 SO 4) formation in the atmosphere, supplementary to the conventional mechanism of H 2 SO 4 production by oxidation of SO 2 in reaction with OH radicals. However, because of a large uncertainty in mechanism and rate coefficients for the atmospheric formation and loss reactions of different SCIs, the importance of this additional source is not well established. In this work, we present an estimation of the role of SCIs in H 2 SO 4 formation at a western Mediterranean (Cape Corsica) remote site, where comprehensive field observations including gas-phase H 2 SO 4 , OH radicals, SO 2 , volatile organic compounds (VOCs) and aerosol size distribution measurements were performed in July–August 2013 as a part of the project ChArMEx (Chemistry-Aerosols Mediterranean Experiment). The measurement site was under strong influence of local emissions of biogenic volatile organic compounds, including monoterpenes and isoprene generating SCIs in reactions with ozone, and, hence, presenting an additional source of H 2 SO 4 via SO 2 oxidation by the SCIs. Assuming the validity of a steady state between H 2 SO 4 production and its loss by condensation on existing aerosol particles with a unity accommodation coefficient, about 90 % of the H 2 SO 4 formation during the day could be explained by the reaction of SO 2 with OH. During the night the oxidation of SO 2 by OH radicals was found to contribute only about 10 % to the H 2 SO 4 formation. The accuracy of the derived values for the contribution of OH + SO 2 reaction to the H 2 SO 4 formation is limited mostly by a large, at present factor of 2, uncertainty in the OH + SO 2 reaction rate coefficient. The contribution of the SO 2 oxidation by SCIs to the H 2 SO 4 formation was evaluated using available measurements of unsaturated VOCs and steady-state SCI concentrations estimated by adopting rate coefficients for SCI reactions based on structure–activity relationships (SARs). The estimated concentration of the sum of SCIs was in the range of (1–3) × 10 3 molec. cm -3. During the day the reaction of SCIs with SO 2 was found to account for about 10 % and during the night for about 40 % of the H 2 SO 4 production, closing the H 2 SO 4 budget during the day but leaving unexplained about 50 % of the H 2 SO 4 formation during the night. Despite large uncertainties in used kinetic parameters, these results indicate that the SO 2 oxidation by SCIs may represent an important H 2 SO 4 source in VOC-rich environments, especially during nighttime. [ABSTRACT FROM AUTHOR]

Reaction of stabilized Criegee Intermediates (SCIs) with SO2 was proposed as an additional pathway of gaseous sulfuric acid (H2SO4) formation in the atmosphere, supplementary to the conventional mechanism of H2SO4 production by oxidation of SO2 in reaction with OH radicals. However, because of a large uncertainty in mechanism and rate coefficients for the atmospheric formation and loss reactions of different SCIs, the importance of this additional source is not well established. In this work, we present an estimation of the role of SCIs in H2SO4 formation at a western Mediterranean (Cape Corsica) remote site, where a comprehensive field observations including gas phase H2SO4, OH radicals, SO2, volatile organic compounds (VOCs) and aerosol size distribution measurements have been performed in July-August 2013 as a part of the project ChArMEx. The measurement site was under strong influence of local emissions of biogenic volatile organic compounds including monoterpenes and isoprene generating SCIs in reactions with ozone and, hence, presenting an additional source of H2SO4 via SO2 oxidation by the SCIs. Assuming the validity of a steady state between H2SO4 production and its loss by condensation on existing aerosol particles with a unity accommodation coefficient, about 90 % of the H2SO4 formation during the day could be explained by the reaction of SO2 with OH. During the night the oxidation of SO2 by OH radicals was found to contribute only about 10 % to the H2SO4 formation. The accuracy of the derived values for the contribution of OH+SO2 reaction to the H2SO4 formation is limited mostly by a large, presently of a factor of 2, uncertainty in OH+SO2 reaction rate coefficient. The contribution of the SO2 oxidation by SCIs to the H2SO4 formation was evaluated using available measurements of unsaturated VOCs and steady state SCIs concentrations estimated by adopting rate coefficients for SCIs reactions based on structure-activity relationships (SARs). The estimated concentration of the sum of SCIs was in the range of (1-3) × 103 molecule cm-3. During the day the reaction of SCIs with SO2 was found to account for about 10 % and during the night for about 40 % of the H2SO4 production, closing the H2SO4 budget during the day but leaving unexplained about 50 % of the H2SO4 formation during the night. Despite large uncertainties in used kinetic parameters, these results indicate that the SO2 oxidation by SCIs may represent an important H2SO4 source in VOCs-rich environments, especially during night-time. [ABSTRACT FROM AUTHOR]

Methylglyoxal (MGLY) is an important atmospheric α-dicarbonyl species for which photolysis acts as a significant source of peroxy radicals, contributing to the oxidizing capacity of the atmosphere and, as such, the formation of secondary pollutants such as organic aerosols and ozone. However, despite its importance, only a few techniques exhibit time resolutions and detection limits that are suitable for atmospheric measurements. This study presents the first field measurements of MGLY by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) performed during the ChArMEx SOP2 field campaign. This campaign took place at a Mediterranean site characterized by intense biogenic emissions and low levels of anthropogenic trace gases. Concomitant measurements of MGLY were performed using the 2,4-dinitrophenylhydrazine (DNPH) derivatization technique and high performance liquid chromatography (HPLC) with UV detection. PTR-ToF-MS and DNPH-HPLC measurements were compared to determine whether these techniques can perform reliable measurements of MGLY. Ambient time series revealed levels of MGLY ranging from 28 to 365 pptv, with a clear diurnal cycle due to elevated concentrations of primary biogenic species during the daytime, and its oxidation led to large production rates of MGLY. A scatter plot of the PTR-ToF-MS and DNPH-HPLC measurements indicates a reasonable correlation (R² = 0:48) but a slope significantly lower than unity (0:58±0:05) and a significant intercept of 88:3±8:0 pptv. A careful investigation of the differences between the two techniques suggests that this disagreement is not due to spectrometric interferences from H3O+(H2O)3 or methyl ethyl ketone (or butanal) detected at m/z 73.050 and m/z 73.065, respectively, which are close to the MGLY m/z of 73.029. The differences are more likely due to uncorrected sampling artifacts such as overestimated collection efficiency or loss of MGLY into the sampling line for the DNPH-HPLC technique or unknown isobaric interfering compounds such as acrylic acid and propanediol for the PTR-ToF-MS. Calculations of MGLY loss rates with respect to OH oxidation and direct photolysis indicate similar contributions for these two loss pathways. [ABSTRACT FROM AUTHOR]

Methylglyoxal (MGLY) is an important atmospheric α-dicarbonyl species whose photolysis acts as a significant source of peroxy radicals, contributing to the oxidizing capacity of the atmosphere and, as such, the formation of secondary pollutants such as organic aerosols and ozone. However, despite its importance, only a few techniques exhibit time resolutions and detection limits that are suitable for atmospheric measurements. This study present, to the best of our knowledge, the first measurements of ambient MGLY using Proton Transfer Reaction-Time of Flight Mass Spectrometry (PTR-ToFMS). These measurements were performed during the ChArMEx SOP2 field campaign. This campaign took place at a Mediterranean site characterized by intense biogenic emissions and low levels of anthropogenic trace gases. Concomitant measurements of MGLY were performed using the 2,4-dinitrophenylhydrazine (DNPH) derivatization technique and High Performance Liquide Chromatography (HPLC) with UV detection. PTR-ToFMS and DNPH-HPLC measurements were compared to determine whether these techniques can perform reliable measurements of MGLY. Ambient time series revealed levels of MGLY ranging from 28-365 pptv, with a clear diurnal cycle due to elevated concentrations of primary biogenic species during daytime, whose oxidation led to large production rates of MGLY. A scatter plot of the PTR-ToFMS and DNPH-HPLC measurements indicates a reasonable correlation (R2 = 0.48) but a slope significantly lower than unity (0.58 ± 0.05) and a significant intercept of 88.3 ± 8.0 pptv. A careful investigation of the differences between the two techniques suggests that this disagreement is not due to spectrometric interferences from H3O+(H2O)3, MEK (or butanal) detected at m/z 73.050 and m/z 73.065, respectively, which are close to the MGLY m/z of 73.029. The differences are more likely due to uncorrected sampling artefacts such as overestimated collection efficiency or loss of MGLY into the sampling line for the DNPH-HPLC technique or unknown isobaric interfering compounds such as acrylic acid and propanediol for the PTR-ToFMS. Calculations of MGLY loss rates with respect to OH-oxidation and direct photolysis indicate similar contributions for these two loss pathways. [ABSTRACT FROM AUTHOR]

Surface ozone has been measured since 2004 at the coastal East Antarctic site of Dumont d'Urville (DDU), and since 2007 at the Concordia station located on the high East Antarctic plateau. This paper discusses long-term changes, seasonal and diurnal cycles, as well as inter-annual summer variability observed at these two East Antarctic sites. At Concordia, near-surface ozone data were complemented by balloon soundings and compared to similar measurements done at the South Pole. The DDU record is compared to those obtained at the coastal site of Syowa, also located in East Antarctica, as well as the coastal sites of Neumayer and Halley, both located on the coast of the Weddell Sea in West Antarctica. Surface ozone mixing ratios exhibit very similar seasonal cycles at Concordia and the South Pole. However, in summer the diurnal cycle of ozone is different at the two sites with a drop of ozone in the afternoon at Concordia but not at the South Pole. The vertical distribution of ozone above the snow surface also differs. When present, the ozone-rich layer located near the ground is better mixed and deeper at Concordia (up to 400 m) than at the South Pole during sunlight hours. These differences are related to different solar radiation and wind regimes encountered at these two inland sites. DDU appears to be the coastal site where the impact of the late winter/spring bromine chemistry is the weakest, but where the impact of elevated ozone levels caused by NOx snow emissions from the high Antarctic plateau is the highest. The highest impact of the bromine chemistry is seen at Halley and Neumayer, and to a lesser extent at Syowa. These three sites are only weakly impacted by the NOx chemistry and the net ozone production occurring on the high Antarctic plateau. The differences in late winter/spring are attributed to the abundance of sea ice offshore from the sites, whereas those in summer are related to the topography of East Antarctica that promotes the katabatic flow bringing oxidant-rich inland air masses to the site. There appears to be a decreasing change in summer surface ozone at the two East Antarctic sites of Concordia and DDU over the most recent period (2004-2014 and 2007-2014). Further research, including continued monitoring, is needed at these two sites to better separate the effect of synoptic transport from possible change of NOx snow emissions in response to recovery of the stratospheric ozone layer leading to penetration of more UV radiation to the surface. [ABSTRACT FROM AUTHOR]

Variations in the stable oxygen isotope composition of atmospheric nitrate act as novel tools for studying oxidative processes taking place in the troposphere. They provide both qualitative and quantitative constraints on the pathways determining the fate of atmospheric nitrogen oxides (NO + NO2 = NOx). The unique and distinctive 17O excess (Δ17O = δ17O - 0.52 × δ18O) of ozone, which is transferred to NOx via oxidation, is a particularly useful isotopic fingerprint in studies of NOx transformations. Constraining the propagation of 17O excess within the NOx cycle is critical in polar areas, where there exists the possibility of extending atmospheric investigations to the glacial-interglacial timescale using deep ice core records of nitrate. Here we present measurements of the comprehensive isotopic composition of atmospheric nitrate collected at Dome C (East Antarctic Plateau) during the austral summer of 2011/2012. Nitrate isotope analysis has been here combined for the first time with key precursors involved in nitrate production (NOx, O3, OH, HO2, RO2, etc.) and direct observations of the transferrable Δ17O of surface ozone, which was measured at Dome C throughout 2012 using our recently developed analytical approach. Assuming that nitrate is mainly produced in Antarctica in summer through the OH + NO2 pathway and using concurrent measurements of OH and NO2, we calculated a Δ17O signature for nitrate on the order of (21-22 ± 3) ‰. These values are lower than the measured values that ranged between 27 and 31 ‰. This discrepancy between expected and observed Δ17O(NO3-) values suggests the existence of an unknown process that contributes significantly to the atmospheric nitrate budget over this East Antarctic region. However, systematic errors or false isotopic balance transfer functions are not totally excluded. [ABSTRACT FROM AUTHOR]

A new version of an atmospheric pressure chemical ionisation mass spectrometer has been developed for ground based in situ atmospheric measurements of OH and total peroxy (HO2 + organic peroxy) radicals. Based on the previously developed principle of chemical conversion of OH radicals to H2SO4 in reaction with SO2 and detection of H2SO4 using an ion molecule reaction with NO, the new instrument is equipped with a turbulent chemical conversion reactor allowing for measurements in moderately polluted atmosphere at NO concentrations up to several ppb. Unlike other similar devices, where the primary NO ions are produced using radioactive ion sources, the new instrument is equipped with a specially developed corona discharge ion source. According to laboratory measurements, the overall accuracy and detection limits are estimated to be, respectively, 25% and 2 × 105 molecule cm-3 for OH and 30% and 1 × 105 molecule cm-3 for HO2 at 10 min integration times. The detection limit for measurements of OH radicals under polluted conditions is 5 × 105 molecules cm-3 at 10 min integration times. Examples of ambient air measurements during a field campaign near Paris in July 2007 are presented demonstrating the capability of the new instrument, although with reduced performance due to the employment of non isotopic SO2. [ABSTRACT FROM AUTHOR]