Showing papers by "Ulrich Platt published in 2002"

Impact of nitrous acid photolysis on the total hydroxyl radical budget during the Limitation of Oxidant Production/Pianura Padana Produzione di Ozono study in Milan

Abstract: [1] The photolysis of nitrous acid (HONO) in the early morning hours is believed to be a significant source of hydroxyl radicals (OH), the most important daytime oxidizing species. Although the importance of this mechanism has been recognized for many years, no accurate experimental quantification is available. Here we present measurements of HONO, NO2, SO2, O3 and HCHO by Differential Optical Absorption Spectroscopy (DOAS) during the Limitation of Oxidant Production/Pianura Padana Produzione di Ozono (LOOP/PIPAPO) study in May–June 1998 in Milan, Italy. The concentration of NO and J(NO2)/J(HONO) were simultaneously monitored by in situ monitors. The photolysis frequencies of HCHO and O3 were determined with a radiative transfer model. High nocturnal HONO mixing ratios of up to 4.4 ppb were regularly observed. Elevated daytime HONO levels during cloudy periods show that the formation of HONO proceeds after sunrise and therefore also represents a source of hydroxyl radicals throughout the day. Averaged over 24 hours, HCHO photolysis is the most important source of OH in Milan, followed by either ozone or HONO photolysis. Our observations indicate that on certain days the OH production from HONO can be even more important than that from ozone photolysis. The diurnal variation of the different OH formation mechanisms shows that HONO photolysis is by far the most important source in the early hours of the morning, and can be as large as and even surpass the total OH production at noon.

OH-initiated oxidation of benzene

Abstract: The present work represents a continuation of part I of this series of papers, in which we investigated the phenol yields in the OH-initiated oxidation of benzene under conditions of low to moderate concentrations of NOx, to elevated NOx levels. The products of the OH-initiated oxidation of benzene in 700–760 Torr of N2/O2 diluent at 297 ± 4 K were investigated in 3 different photochemical reaction chambers. In situ spectroscopic techniques were employed for the detection of products, and the initial concentrations of benzene, NOx, and O2 were widely varied (by factors of 6300, 1500, and 13, respectively). In contrast to results from previous studies, a pronounced dependence of the product distribution on the NOx concentration was observed. The phenol yield decreases from approximately 50–60% in the presence of low concentrations ( 10 000 ppb) NOx concentrations. In the presence of high concentrations of NOx, the phenol yield increases with increasing O2 partial pressure. The rate constant of the reaction of hydroxycyclohexadienyl peroxyl radicals with NO was determined to be (1.7 ± 0.6) × 10−11 cm3 molecule−1 s−1. This reaction leads to the formation of E,E-2,4-hexadienedial as the main identifiable product (29 ± 16%). The reaction of the hydroxycyclohexadienyl radical with NO2 gave phenol (5.9 ± 3.4%) and E,E-2,4-hexadienedial (3.4 ± 1.9%), no other products could be identified. The residual FTIR product spectra indicate the formation of unknown nitrates or other nitrogen-containing species in high yield. The results from the present work also show that experimental studies aimed at establishing/verifying chemical mechanisms for aromatic hydrocarbons must be performed using NOx levels which are representative of those found in the atmosphere.

Intercontinental transport of nitrogen oxide pollution plumes

Abstract: . We describe the first satellite observation of intercontinental transport of nitrogen oxides emitted by power plants, verified by simulations with a particle tracer model. The analysis of such episodes shows that anthropogenic NOx plumes may influence the atmospheric chemistry thousands of kilometers away from its origin, as well as the ocean they traverse due to nitrogen fertilization. This kind of monitoring became possible by applying an improved algorithm to extract the tropospheric fraction of NO2 from the spectral data coming from the GOME instrument. As an example we show the observation of NO2 in the time period 4--14 May, 1998, from the South African Plateau to Australia which was possible due to favourable weather conditions during that time period which availed the satellite measurement. This episode was also simulated with the Lagrangian particle dispersion model FLEXPART which uses NOx emissions taken from an inventory for industrial emissions in South Africa and is driven with analyses from the European Centre for Medium-Range Weather Forecasts. Additionally lightning emissions were taken into account by utilizing Lightning Imaging Sensor data. Lightning was found to contribute probably not more than 25% of the resulting concentrations. Both, the measured and simulated emission plume show matching patterns while traversing the Indian Ocean to Australia and show great resemblance to the aerosol and CO2 transport observed by Piketh et al. (2000).

Free radicals and fast photochemistry during BERLIOZ

Abstract: The free radicals OH, HO2, RO2, and NO3 are known to be the driving force for most chemical processes in the atmosphere. Since the low concentration of the above radicals makes measurements particularly difficult, only relatively few direct measurements of free radical concentrations have been reported to date. We present a comprehensive set of simultaneous radical measurements performed by Laser Induced Fluorescence (LIF), Matrix Isolation — Electron spin Resonance (MI-ESR), Peroxy Radical Chemical Amplification (PERCA), and Differential Optical Absorption Spectroscopy (DOAS) during the BERLIner OZonexperiment (BERLIOZ) during July and August of 1998 near Berlin, Germany. Most of the above radical species were measured by more than one technique and an intercomparison gave good agreement. This data set offered the possibility to study and quantify the role of each radical at a rural, semi-polluted site in the continental boundary layer and to investigate interconnections and dependencies among these free radicals. In general (box) modelled diurnal profiles of the different radicals reproduced the measurements quite well, however measured absolute levels are frequently lower than model predictions. These discrepancies point to disturbing deficiencies in our understanding of the chemical system in urban air masses. In addition considerable night-time peroxy radical production related to VOC reactions with NO3 and O3 could be quantified.

Comparison of measurements and model calculations of stratospheric bromine monoxide

Abstract: Ground-based zenith sky UV-visible measurements of stratospheric bromine monoxide (BrO) slant column densities are compared with simulations from the SLIMCAT three-dimensional chemical transport model. The observations have been obtained from a network of 11 sites, covering high and midlatitudes of both hemispheres. This data set gives for the first time a near-global picture of the distribution of stratospheric BrO from ground-based observations and is used to test our current understanding of stratospheric bromine chemistry. In order to allow a direct comparison between observations and model calculations, a radiative transfer model has been coupled to the chemical model to calculate simulated slant column densities. The model reproduces the observations in general very well. The absolute amount of the BrO slant columns is consistent with a total stratospheric bromine loading of 20 ± 4 ppt for the period 1998-2000, in agreement with previous estimates. The seasonal and latitudinal variations of BrO are well reproduced by the model. In particular, the good agreement between the observed and modeled diurnal variation provides strong evidence that the BrO-related bromine chemistry is correctly modeled. A discrepancy between observed and modeled BrO at high latitudes during events of chlorine activation can be resolved by increasing the rate constant for the reaction BrO + ClO → BrCl + O 2 to the upper limit of current recommendations. However, other possible causes of the discrepancy at high latitudes cannot be ruled out.

Verification of the Contribution of Vehicular Traffic to the Total NMVOC Emissions in Germany and the Importance of the NO3 Chemistry in the City Air

Abstract: In 1997 and 1998 several field campaigns for monitoring non-methane volatile organic compounds (NMVOCs) and nitrogen oxides (NO x ) were carried out in a road traffic tunnel and in the city center of Wuppertal, Germany. C2–C10 aliphatic and aromatic hydrocarbons were monitored using a compact GC instrument. DOAS White and long path systems were used to measure aromatic hydrocarbons and oxygenated aromatic compounds. A formaldehyde monitor was used to measure formaldehyde. Chemiluminescence NO analysers with NO2 converter were used for measuring NO and NO2. The high mixing ratios of the NMVOCs observed in the road traffic tunnel, especially 2.9 ppbv phenol, 1.5 ppbv para-cresol and 4.4 ppbv benzaldehyde, in comparison with the measured background concentration clearly indicate that these compounds were directly emitted from road traffic. Para-Cresol was for the first time selectively detected as primary pollutant from traffic. From the measured data a NMVOC profile of the tunnel air and the city air, normalised to benzene (ppbC/ppbC), was derived. For most compounds the observed city air NMVOC profile is almost identical with that obtained in the traffic tunnel. Since benzene originates mainly from road traffic emission, the comparison of the normalised emission ratios indicate that the road traffic emissions in Wuppertal have still the largest impact on the city air composition, which is in contrast to the German emission inventory. In both NMVOC profiles, aromatic compounds have remarkably large contributions of more than 40 ppbC%. In addtion, total NMVOC/NO x ratios from 0.6 up to 3.0 ppbC/ppb in the traffic tunnel air and 3.4 ± 0.5 in the city air of Wuppertal were obtained. From the observed para-cresol/toluene and ortho-cresol/toluene ratios in the city air, evidence was found that also during daytime NO3 radical reactions play an important role in urban air.

Derivation of tropospheric NO3 profiles using off‐axis differential optical absorption spectroscopy measurements during sunrise and comparison with simulations

Abstract: [1] Early morning vertical profiles of tropospheric NO3 were derived by spectroscopy of scattered sunlight in off-axis geometry during sunrise. The measurements were carried out in the urban area of Heidelberg, Germany (April, July, and August 1999). The retrieval algorithm is based on the nitrate radical's rapid photolysis during sunrise, radiative transfer, as well photochemical modeling. We derived NO3 near-ground concentrations of typically (0.2–18) × 107 cm−3, concentrations above 3 km of (5–50) × 107 cm−3, and a NO3 maximum at a height of ∼350 m with concentrations of (100–900) × 107 cm−3. Assuming the case of very high terpene levels even at higher altitudes, which is very unlikely for the atmospheric conditions during the measurements, we obtain a different mathematical solution of our inversion problem yielding the NO3 maximum at altitudes of ∼2.5 km. The enriched layer was found to hold the bulk of the tropospheric nighttime NO3. The retrieved profile confirms earlier suggestions that ground-level measurements may be significantly underestimating the oxidative capacity of the boundary layer under stable nocturnal conditions. The NO3 layer is probably formed as a result of the vertical profiles of the NO3 educts (NO2 and O3), with NO2 concentrations falling off more slowly with height than the NO3 scavengers, e.g., anthropogenic NO as well as volatile organic compounds emitted at ground level. Independently from these measurements model simulations with a comprehensive three-dimensional model system were performed for an area, which includes the measurement site. A pronounced maximum (3.3 × 109 cm−3) of the nocturnal NO3 concentration was found at ∼250 m above the surface, i.e., at the top of the nocturnal stable boundary layer. The average NO3 concentration close to the surface was 5 × 107 cm−3. The only significant difference between the observations and the model results was a steeper decrease of the NO3 concentration above the maximum of the observed profiles.

Temperature dependence of the NO3 loss frequency: A new indicator for the contribution of NO3 to the oxidation of monoterpenes and NOx removal in the atmosphere

Abstract: [1] Nitrate radicals are known to act as important oxidizing agents for many volatile organic compounds (VOCs) (especially terpenes and dimethyl sulfide (DMS)) and NOx in the nighttime boundary layer. The quantification of these oxidation rates so far ideally implied the measurement of all species reacting with this radical and was therefore restricted to short-term intensive campaigns. In this paper we present a new method to quantify the seasonal variation of the contribution of NO3 to the atmospheric oxidation capacity and the removal of nitrogen oxides. The indicator is based on the different temperature dependences of the NO3 loss processes, which arise from the temperature dependences of kinetic constants and of the monoterpene (or DMS) emission rate. The application of this indicator is simple: It is sufficient to measure ambient concentration time series of NO3, NO2, O3, and temperature (the data should cover a time period of preferably several months). The three concentrations can be readily measured, e.g., using a Differential Optical Absorption Spectroscopy system. The new indicator was applied to data of long-term observations at a maritime (Rugen, in 1994) and a continental site (Lindenberg, in 1998) in Germany. At the latter site the seasonal variation of the VOC oxidation and NOx removal (conversion rate of NOx to HNO3) rate was studied: During August a diurnal average of the NO3-induced monoterpene oxidation rate of 9 × 105 molecules cm−3 s−1 was determined, which is 3 times higher than during March. The production rate of HNO3/NO3− was found to have its minimum during the summer time, while it is a factor of 10 higher during March, averaging 6.3 × 105 cm−3 s−1 from March to September 1998 during a full diurnal cycle.

Continuous monitoring of the high and persistent chlorine activation during the Arctic winter 1999/2000 by the GOME instrument on ERS‐2

Abstract: [1] Measurements of OClO total column amounts during the cold Arctic winter 1999/2000 retrieved from observations by the satellite instrument Global Ozone Monitoring Experiment (GOME) are presented. OClO is formed as a minor product of the reaction of BrO + ClO and thus serves as an indicator for a stratospheric chlorine activation. As a result of the good spatial and temporal coverage of GOME, it is possible to follow the temporal development of the stratospheric chlorine activation during the winter and spring on a daily basis. An initial weak chlorine activation was observed during mid November, shortly after stratospheric temperatures were sufficiently low that formation of polar stratospheric clouds resulted. Strong chlorine activation started around 22 December, when PSC formation was possible over a large altitude range. Chlorine activation was significant and large until the beginning of March, peaking mid February. In the middle of March the chlorine activation steeply decreased and ended around 20 March, when the polar vortex broke up. The duration and the magnitude of the chlorine activation in the Arctic winter 1999/2000 were higher than during all previous Arctic winters since the launch of the GOME instrument in April 1995.

An Advanced Cloud Product For The Interpretation of Tropospheric Data From Gome and Sciamachy

Abstract: Introduction This study focuses on the determination of cloud properties (e.g. effective cloud fraction and average cloud to height) from satellite observations and on the quantification of the corresponding cloud influence on tropospheric trace gas products derived from satellites. The investigations first concentrate on GOME and will later also be applied to SCIAMACHY on ENVISAT. The most dominant effects of clouds are (a) that they shield the atmosphere below the cloud cover and (b) that their albedo is typically significantly larger compared to the earth’s surface. Because of these effects the determination of quantitative tropospheric trace gas products depends strongly on the knowledge of cloud properties. Already existing cloud algorithms are based on spatially resolved intensity measurements (Polarisation measurement devices, PMD, see ESA (1995)) as well as on the determination of the optical depth of the O2-A-band (see e.g. Kuce and Chance (1994), Kjoelemeijer and Stammes, 1999). However, both quantities show important shortcomings, especially over snow and ice surfaces, which strongly limit their applicability.

The Impact of Halogen Chemistry on the Oxidation Capacity of the Troposphere

Abstract: The role of reactive halogen species in the destruction of stratospheric ozone is well known and largely understood [e.g. Solomon 1990] and it is clear that reactive halogen species (RHS = X, X2, XY, XO, HOX, where X, Y denotes CI, Br and possibly I) contribute considerably to the loss of stratospheric ozone. Only in recent years has it become clear that RHS can also playa significant role in the troposphere. For instance, recently in the tropospheric boundary layer (BL) significant amounts of BrO and I0 could be directly determined by Differential Optical Absorption Spectroscopy (DOAS), also indirect evidence for CI and Br atoms were found under certain conditions.These observations were made at a variety of sites (see Table 1): (1) Bromine monoxide was found in the Arctic and Antarctic troposphere by ground based and satellite observations (see Table 1 for references). These episodes of BL BrO were only found in springtime. It could be shown that BrO (at levels up to 30 ppt) is always connected to episodes of BL ozone destruction [e.g. Platt and Lehrer 1995]. (2) Measurements by chemical amplification [Perner et al. 1999], DOAS [Tuckermann et al. 1997], and Hydrocarbon Clock data [e.g. Jobson et al. 1994] suggest CIO levels in the ppt range in the Arctic BL. (3) In addition 10 and 010 was found to reach several ppt at coastal sites in Ireland and at the Canary Islands (see Table 1), and also in the Arctic [Honninger and Platt 2001] and Antarctica [Friess et al. 2001]. Model calculation suggest that 03 destruction due to these I0 levels is comparable to the classic photochemical 03 loss in the tropospheric boundary layer [Stutz et al. 1999]. (4) Even higher BrO mixing ratios (up to 90 ppt) were found in the Dead Sea basin [Hebestreit et al. 1999, Matveev et al. 2001], where also complete BL ozone destruction is associated with episodes of high BrO. Also at other salt lakes e.g. near Salt Lake City, USA, elevated levels of BrO (up to 6 ppt) and also CIO (around 15 ppt) were reported [Stutz et al. 2001]. (5) Recently evidence is accumulating that there exist significant amounts of BrO (of the order of 1–2 ppt) also in the free troposphere. Measurements of the BrO column density from the ground [e.g. Eisinger et al. 1997, Kreher et al. 1997, Otten et al. 1998, FrieB et al. 1999] and from satellite [Richter et al. 1998, Wagner and Platt 1998, Wagner et al. 2001] consistently yield higher values than obtained by integrating stratospheric in-situ profiles or profiles from balloon-borne spectroscopic measurements [e.g. Harder et al. 2000, Ferlemann et al. 1998]. Additional evidence comes from the analysis of diurnal profiles of the BrO — ‘slant column’ density (SCD). BrO SCD data measured by ground based ZSL-DOAS fit model calculations of the SCD much better if a tropospheric (vertical) BrO column density of (0.5 – 2)⊙1013 cm−2 is assumed corresponding to 1–2 ppt BrO throughout the troposphere [Friess et al. 1999, van Roozendael et al. 2001]. The remarkable similarity in the tropospheric residual at various measurement sites suggests that BrO could be a rather ubiquitous constituent of the global troposphere.