Catalysed self-cleaning of air on the earth's surface

Generally, it is assumed that UV-light, high temperature or reactive molecules like O₃ and OH are needed to activate gas reactions in air. In consequence, the catalytic activity on natural materials such as sand and soil on the earth's surface is assumed to be insignificant. We have measured O₂-dissociation rates on natural quartz sand at 40˚C and compared these with O₂-dissociation rates near 500˚C on materials with well-known catalytic activity. In terms of probabilities for dissociation of impinging O₂-molecules the measured rates are in the 10⁻¹²–10⁻⁴ range. We have also measured dissociation rates of H₂ and N₂, water-formation from H₂ and O₂ mixtures, exchange of N between N₂, NO _x and a breakdown of HNO₃, NO₂ and CH₄ on natural quartz sand at 40˚C. The measured rates together with an effective global land area have been used to estimate the impact of thermodynamically driven reactions on the earth's surface on the global atmospheric budgets of H₂, NO₂ and CH₄. The experimental data on natural quartz sand together with data from equilibrium calculations of air suggest that an expected increase in anthropogenic supply of air pollutants, such as NO _x or other “reactive” nitrogen compounds, hydrogen and methane, will be counter-acted by catalysis on the earth's surface. On the other hand, at Polar Regions and boreal forests where the “reactive” nitrogen concentration is below equilibrium, the same catalytic effect activates formation of bio-available nitrogen compounds from N₂, O₂ and H₂O.     

Molecular Line Parameters for the “MASTER” (Millimeter Wave Acquisitions for Stratosphere/Troposphere Exchange Research) Database

In order to investigate the upper troposphere/lower stratosphere (UTLS) region of the earth's atmosphere, ESA/ESTEC (European space agency) is considering the opportunity to develop the spaceborne limb sounding millimeter sensor “MASTER” (millimeter wave acquisitions for stratosphere/troposphere exchange research). This instrument is part of the “atmospheric composition explorer for chemistry and climate interactions” (ACECHEM) project. In addition, ESA/ESTEC is developing the “MARSCHALS” (millimeter-wave airborne receiver for spectroscopic characterization of atmospheric limb sounding) airborne instrument which will demonstrate the feasibility of MASTER. The present paper describes the line-by-line database which was generated in order to meet at best the needs of the MASTER (or MARSCHALS) instrument. The linelist involves line positions, line intensities, line broadening and line shift parameters in the 294–305, 316–325, 342–348, 497–506 and 624–626 GHz spectral microwindows. This database was first generated for the target molecules for MASTER (H₂O, O₃, N₂O, CO, O₂, HNO₃, HCl, ClO, CH₃Cl, BrO). In addition, ten additional molecules (SO₂, NO₂, OCS, H₂CO, HOCl, HCN, H₂O₂, COF₂, HO₂ and HOBr) had also to be considered in the database as “possible interfering species” for the retrieval of the target molecules of MASTER. The line parameters were derived, depending on their estimated accuracy, (i) from a combination of spectral parameters included in the JPL and HITRAN catalogs (ii) from data taken into the literature or (iii) using data obtained through experimental measurements (and/or) calculations performed during the present study.     

Aqueous Phase Reaction of HNO4: The Impact on Tropospheric Chemistry

We use a global atmospheric chemistry transport model to study the possible influence of aqueous phase reactions of peroxynitric acid (HNO₄) on the concentrations and budgets of NO_x, SO_x, O₃ and H₂O₂. Laboratory studies have shown that the aqueous reaction of HNO_4aq withHSO^– _3aq, and the uni-molecular decomposition of the NO₄ ^– anion to form NO₂ ^– (nitrite) occur on a time scale of about a second. Despite a substantial contribution of the reaction of HSO^– _3aq with HNO_4aq to the overall in-cloud conversion of SO₂ to SO₄ ^2–, a simultaneous decrease of other oxidants (most notably H₂O₂) more than compensated the increase in SO₄ ^2– production. The strongest influence of heterogeneous HNO₄ chemistry was found in the boundary layer, where calculated monthly average ozone concentrations were reduced between 2% to 10% andchanges of H₂O₂ between –20% to +10%compared to a simulation which ignores this reaction. Furthermore, SO₂ was increased by 10% to 20% and SO₄ ^2–depleted by up to 10%. Since the resolution of our global model does not enable a detailed comparison with measurements in polluted regions, it is not possible to verify whether considering heterogeneous HNO₄ reactions results in a substantial improvement of atmospheric chemistry transport models. However, the conversion of HNO₄ in the aqueous phase seems to be efficient enough to warrant further laboratory investigations and more detailed model studies on this topic.     

北京城区冬春季过氧化氢浓度变化特征

大气过氧化氢（H₂O₂）是一种重要的光化学产物,也是硫酸盐气溶胶生成及降水酸化过程的关键氧化剂。然而,我国对H₂O₂的观测研究较少,尤其对雾霾期间H₂O₂浓度变化特征认识不足。该文介绍了冬春时段（2016年12月-2017年4月）在北京城区中国气象局的H₂O₂观测结果,并结合同期O₃,PAN,NO_X,PM_2.5等污染物和气象要素观测数据,分析H₂O₂浓度变化特征与影响因素。观测结果表明:观测期间H₂O₂体积混合比（简称为浓度）为（0.65±0.59）×10-9,其中,春季浓度（0.83±0.67）×10-9高于冬季浓度（0.51±0.47）×10-9;H₂O₂平均日变化基本呈现单峰特征,峰值出现在18:00-21:00,比其他地区峰值出现稍晚,并滞后于O₃峰值时间4~7 h;相对湿度对H₂O₂日峰值时间和浓度水平有影响,小于55%时日峰值出现于18:00-24:00,平均峰值浓度1.52×10-9;大于65%时日峰值出现于11:00-16:00,日峰值浓度均小于1×10-9。H₂O₂,O₃和PAN虽然同属光化学产物,但在不同污染状况下浓度水平和变化趋势差异明显;H₂O₂清洁日峰值浓度高于污染日,但11:00-15:00污染日浓度略高于清洁日。     

Numerical Investigation of Gas Scavenging by Weak Precipitation

A one-dimensional cloud model with size-resolved microphysics and size-resolved aqueous-phase chemistry, driven by prescribed dynamics, has been used to study gas scavenging by weak precipitation developed from low-level, warm stratiform clouds. The dependence of the gas removal rate on the physical and chemical properties of precipitation has been explored under controlled initial conditions. It is found that the removal of four gaseous species (SO₂, NH₃, H₂O₂ and HNO₃) strongly depends on the total droplet surface area, regardless the mean size of droplets. The removal rates also correlate positively with the precipitation rate, especially for precipitation having a mean radius larger than 20 μm. The dependence of the scavenging coefficients on the total droplet surface area is stronger than on the precipitation rate. The removal rates of SO₂, NH₃ and H₂O₂ by precipitation strongly depend on the others' initial concentrations. When NH₃ (or H₂O₂) concentration is much lower than that of SO₂, the removal rate of SO₂ is then controlled by the concentration of H₂O₂ (or NH₃). The removal of NH₃ (or H₂O₂) also directly depends on the concentration of SO₂. NH₃ and H₂O₂ can also indirectly affect each other's removal rate through interaction with SO₂. The scavenging coefficient of SO₂ increases with the concentration ratio of NH₃ to SO₂ if the ratio is larger than 0.5, while the scavenging coefficient of NH₃ increases with the concentration ratio of SO₂ to NH₃ when the ratio is smaller than 1. The scavenging coefficient of H₂O₂ generally increases with the concentration ratio of SO₂ to H₂O₂. Although the Henry's law equilibrium approach seems to be able to simulate gas scavenging by cloud droplets, it causes large errors when used for simulating the scavenging of soluble gas species by droplets of precipitating sizes.     

Hoarfrost and rime chemistry in Poland—An introductory analysis from meteorological perspective

Hoarfrost and rime analysis was based on the collection of samples between 2003 and 2006 from 8 sites, which represent both lowland (northern) and mountainous (southern) parts of Poland. On the other hand 4 of these sites belong to “urban” and 4 to “rural” category. pH, conductivity, SO₄²⁻, NO₃⁻, Cl⁻, H⁺, NH₄⁺, Ca²⁺, Mg²⁺, Na⁺, K⁺ have been determined in order to study the chemistry of hoarfrost and rime. Higher total inorganic ionic content (TIC) in hoarfrost and rime (2.46 meq·l⁻¹ and 1.23 meq·l⁻¹ respectively) was observed when compared with precipitation (0.37 meq·l⁻¹). Large variability of TIC and chemical composition of individual samples were typical at each of the measurement sites depending on emission patterns, atmospheric conditions and local terrain topography. Higher concentrations of both hoarfrost and rime occurred in southern (mountainous) rather than in northern (lowland) part of Poland which can be explained by worse pollutant dispersion conditions in the south. The surprisingly low hoarfrost concentrations in urban coastal stations in the area of the Bay of Gdansk were attributed to the cleaning effect of nocturnal breeze-type circulation, best pronounced in cool part of the year. Due to relatively high pollutant concentration and long duration, hoarfrost and rime are at least significant factors in environmental processes in different ecosystems in Poland.     

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

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 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.     

Sensitivity Analysis of Photochemical Indicators for O<Subscript>3</Subscript> Chemistry Using Automatic Differentiation

Photochemical indicators for determination of O₃–NO_x–ROG sensitivity and their sensitivity to model parameters are studied for a variety of polluted conditions using a comprehensive mixed-phase chemistry box model and the novel automatic differentiation ADIFOR tool. The main chemical reaction pathways in all phases, interfacial mass transfer processes, and ambient physical parameters that affect the indicators are identified and analyzed. Condensed mixed-phase chemical mechanisms are derived from the sensitivity analysis.Our results show that cloud chemistry has a significant impact on the indicators and their sensitivities, particularly on those involving H₂O₂, HNO₃, HCHO, and NO_z. Caution should be taken when applying the established threshold values of indicators in regions with large cloud coverage. Among the commonly used indicators, NO_y and O₃/NO_y are relatively insensitive to most model parameters, whereas indicators involving H₂O₂, HNO₃, HCHO, and NO_z are highly sensitive to changes in initial species concentrations, reaction rate constants, equilibrium constants, temperature, relative humidity, cloud droplet size, and cloud water content.     

The heterogeneous formation of N2O over bulk condensed phases in the presence of SO2 at high humidities

In view of the uncertainty of the origin of the secular increase of N₂O, we studied heterogeneous processes that contribute to formation of N₂O in an environment that comes as close as possible to exhaust conditions containing NO and SO₂, among other constituents. The N₂O formation was followed using electron capture gas chromatography (ECD-GC). The other reactants and intermediates (SO₂, NO, NO₂ and HONO) were monitored using gas phase UV-VIS absorption spectroscopy. Experiments were conducted at 298 and 368 K as well as at dry and high humidity (approaching 100% rh) conditions. There is a significant heterogeneous rate of N ₂ O formation at conditions that mimic an exhaust plume from combustion processes.The simultaneous presence of NO, SO₂, O₂ in the gas phase and condensed phase water, either in the bulk liquid or adsorbed state has been confirmed to be necessary for the production of significant levels of N₂O. The stoichiometry of the overall reaction is: 2 NO+SO₂+H₂O N₂O+H₂SO₄. The maximum rate of N₂O formation occurred at the beginning of the reaction and scales with the surface area of the condensed phase and is independent of its volume. A significant rate of N₂O formation at 368 K at 100% rh was also observed in the absence of a bulk substrate. The diffusion of both gas and liquid phase reactants is not rate limiting as the reaction kenetics is dominated by the rate ofN₂O formation under the experimental conditions used in this work. The simultaneous presence of high humidity (90–100% rh at 368 K) and bulk condensed phase results in the maximum rate and final yield of N₂O approaching 60% and 100% conversion after one hour in the presence of amorphous carbon and fly-ash, respectively.Work performed in partial fulfillment of the requirements of Dr ès Sciences at EPFL.