OH and RO 2 radicals at Dome C (East Antarctica): first observations and assessment of photochemical budget

International audienceMeasurements of OH and total peroxy RO 2 (HO 2 + organic peroxy) radicals were performed in December 2011/January 2012 at the Dome C Concordia station (East Antarctica, 75.1˚S / 123.3˚E) in the frame of the Oxi-dant Production over Antarctic Land and its Export (OPALE) project. The goal of these first on the East Antarctica plateau radical measurements was to estimate the oxidative capacity and assess the role of snow emissions on the radical budget in this part of Antarctica. The OH concentration levels were found to be in general similar to those observed at South Pole. However, based on the analysis of the OH sources and sinks derived from the available measurements of NO x , HONO, HCHO, H 2 O 2 and others, it has been concluded that, in contrast to South Pole, the photolysis of HONO is the major OH source at Dome C site. The role of HONO as the major source of OH is also supported by an excellent correlation of OH with the production rate of OH from the HONO photolysis. The observed diurnal profiles of OH and RO 2 are discussed in relation with boundary dynamics and the variability of photolysis and snow emissions rates

Study of the unknown HONO daytime source at a European suburban site during the MEGAPOLI summer and winter field campaigns

International audienceNitrous acid measurements were carried out during the MEGAPOLI summer and winter field campaigns at SIRTA observatory in Paris surroundings. Highly variable HONO levels were observed during the campaigns, ranging from 10 ppt to 500 ppt in summer and from 10 ppt to 1.7 ppb in winter. Significant HONO mixing ratios have also been measured during daytime hours, comprised between some tenth of ppt and 200 ppt for the summer campaign and between few ppt and 1 ppb for the winter campaign. Ancillary measurements, such as NOx , O3 , photolysis frequencies, meteorological parameters (pressure, temperature, relative humidity , wind speed and wind direction), black carbon concentration , total aerosol surface area, boundary layer height and soil moisture, were conducted during both campaigns. In addition, for the summer period, OH radical measurements were made with a CIMS (Chemical Ionisation Mass Spectrometer). This large dataset has been used to investigate the HONO budget in a suburban environment. To do so, calculations of HONO concentrations using PhotoStationary State (PSS) approach have been performed, for daytime hours. The comparison of these calculations with measured HONO concentrations revealed an underestimation of the calculations making evident a missing source term for both campaigns. This unknown HONO source exhibits a bell-shaped like average diurnal profile with a maximum around noon of approximately 0.7 ppb h−1 and 0.25 ppb h−1 , during summer and winter respectively. This source is the main HONO source during daytime hours for both campaigns. In both cases, this source shows a slight positive correlation with J (NO2) and the product between J (NO2) and soil moisture. This original approach had, thus, indicated that this missing source is photolytic and might be heterogeneous occurring at ground surface and involving water content available on the ground. Published by Copernicus Publications on behalf of the European Geosciences Union. 2806 V. Michoud et al.: Study of the unknown HONO daytime sourc

Pressure and temperature dependence of ethyl nitrate formation in the C₂H₅O₂ + NO reaction

International audienceThe branching ratio β = k1b/k1a for the formation of ethyl nitrate, C2H5ONO2, in the gas-phase C2H5O2 + NO reaction, C2H5O2 + NO → C2H5O + NO2 (1a), C2H5O2 + NO → C2H5ONO2 (1b), was determined over the pressure and temperature ranges 100−600 Torr and 223−298 K, respectively, using a turbulent flow reactor coupled with a chemical ionization mass spectrometer. At 298 K the C2H5ONO2 yield was found to increase linearly with pressure from about 0.7% at 100 Torr to about 3% at 600 Torr. At each pressure, the branching ratio of C2H5ONO2 formation increases with the decrease of temperature. The following parametrization equation has been derived in the pressure and temperature ranges of the study: β(P,T) (%) = (3.88 × 10−3*P (Torr) + 0.365)*(1 + 1500(1/T − 1/298)). The atmospheric implication of the results obtained is briefly discussed, in particular the impact of β on the evolution of ethyl nitrate in urban plumes

Chemical ionisation mass spectrometer for measurements of OH and Peroxy radical concentrations in moderately polluted atmospheres

International audienceA 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 NO3--, 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 NO3-- 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

Influence of Water on the H2SO4 Yield from the Ozonolysis of 2,3-dimethyl-butene (TME) in Presence of SO2

International audienceThe influence of the water vapor content on the yield of H2SO4 from the ozonolysis of 2,3-dimethyl-butene (TME) in presence of SO2 was studied using laminar flow reactor coupled with Chemical Ionisation Mass Spectrometer (CIMS) for the H2SO4 monitoring within the range of H2O from 10 ppmv to 3×104 ppmv at different concentrations of TME, O3, SO2. The observed dependences of the H2SO4 yield on H2O concentration can be interpreted by assuming two different paths of the H2SO4 formation: 1) via the formation of SO3 in the reaction of Stabilized Criegee Intermediate (SCI) with SO2 (2a) followed by the reaction of SO3 with H2O (3) and 2) via the formation of stabilized secondary ozonide (SOZ) (2b) producing H2SO4 in the reaction with H2O (4a) in competition with the SOZ decomposition to other products (5): O3+TME => (CH3)2COO (1) (CH3)2COO + SO2 => SO3 (2a) => SOZ (2b) SO3 + H2O => H2SO4 (3) SOZ + H2O => H2SO4 (or SO3) (4a) SOZ + M => products (5) The yield of the SCI, SOZ and the rates of the SCI and SOZ decomposition relative to their reactions with SO2 and H2O, respectively, were estimated from the dependencies of the H2SO4 yield on the concentrations of the reactants

HNO3 Forming Channel of the HO2 + NO Reaction as a Function of Pressure and Temperature in the Ranges of 72-600 Torr and 223-323 K

A high-pressure turbulent flow reactor coupled with a chemical ionization mass-spectrometer was used to determine the branching ratio of the HO2 + NO reaction: HO2 + NO OH + NO2 (1a), HO2 + NO HNO3 (1b). The branching ratio, = k1b/k1a, was derived from the measurements of "chemically amplified" concentrations of the NO2 and HNO3 products in the presence of O2 and CO. The pressure and temperature dependence of was determined in the pressure range of 72-600 Torr of N2 carrier gas between 323 and 223 K. At each pressure, the branching ratio was found to increase with the decrease of temperature, the increase becoming less pronounced with the increase of pressure. In the 298-223 K range, the data could be fitted by the expression: (T,P) = (530 ± 10)/T(K) + (6.4 ± 1.3) Å~ 10-4P(Torr) - (1.73 ± 0.07), giving 0.5% near the Earth's surface (298 K, 760 Torr) and 0.8% in the tropopause region (220 K, 200 Torr). The atmospheric implication of these results is briefly discussed

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Influence of Water on the H2SO4 Yield from the Ozonolysis of 2,3-dimethyl-butene (TME) in Presence of SO2

International audienceThe influence of the water vapor content on the yield of H2SO4 from the ozonolysis of 2,3-dimethyl-butene (TME) in presence of SO2 was studied using laminar flow reactor coupled with Chemical Ionisation Mass Spectrometer (CIMS) for the H2SO4 monitoring within the range of H2O from 10 ppmv to 3×104 ppmv at different concentrations of TME, O3, SO2. The observed dependences of the H2SO4 yield on H2O concentration can be interpreted by assuming two different paths of the H2SO4 formation: 1) via the formation of SO3 in the reaction of Stabilized Criegee Intermediate (SCI) with SO2 (2a) followed by the reaction of SO3 with H2O (3) and 2) via the formation of stabilized secondary ozonide (SOZ) (2b) producing H2SO4 in the reaction with H2O (4a) in competition with the SOZ decomposition to other products (5): O3+TME => (CH3)2COO (1) (CH3)2COO + SO2 => SO3 (2a) => SOZ (2b) SO3 + H2O => H2SO4 (3) SOZ + H2O => H2SO4 (or SO3) (4a) SOZ + M => products (5) The yield of the SCI, SOZ and the rates of the SCI and SOZ decomposition relative to their reactions with SO2 and H2O, respectively, were estimated from the dependencies of the H2SO4 yield on the concentrations of the reactants

Pressure Dependence of Iso-Propyl Nitrate Formation in the i-C₃H₇O₂ + NO Reaction

International audienceThe branching ratio β = k1b/k1a for the formation of iso-propyl nitrate, i-C3H7ONO2, in the gas-phase reaction of i-C3H7O2 with NO, i-C3H7O2 + NO → i-C3H7O + NO2 (1a), i-C3H7O2 + NO → i-C3H7ONO2 (1b), was determined over the pressure range 55-500 Torr at 298 K using a high-pressure turbulent flow reactor coupled with a chemical ionization quadrupole mass-spectrometer. The β coefficient was found to increase linearly with pressure from about 0.6% at 55 Torr to about 3% at 500 Torr. The atmospheric implication of the results obtained is briefly discussed

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