Rainer Vogt

A Mechanism for Halogen Release from Sea-Salt Aerosol in the Remote Marine Boundary Layer

Recent measurements of inorganic chlorine gases [1] and hydrocarbons [2] indicate the presence of reactive chlorine in the remote marine boundary layer; reactions involving chlorine and bromine can affect the concentrations of ozone, hydrocarbons and cloud condensation nuclei.

Iodine chemistry and its role in halogen activation and ozone loss in the marine boundary layer: A model study

A detailed set of reactions treating the gas and aqueous phase chemistry of the most important iodine species in the marine boundary layer (MBL) has been added to a box model which describes Br and Cl chemistry in the MBL. While Br and Cl originate from seasalt, the I compounds are largely derived photochemically from several biogenic alkyl iodides, in particular CH2I2, CH2ClI, C2H5I, C3H7I, or CH3I which are released from the sea. Their photodissociation produces some inorganic iodine gases which can rapidly react in the gas and aqueous phase with other halogen compounds. Scavenging of the iodine species HI, HOI, INO2, and IONO2 by aerosol particles is not a permanent sink as assumed in previous modeling studies. Aqueous-phase chemical reactions can produce the compounds IBr, ICl, and I2, which will be released back into the gas phase due to their low solubility. Our study, although highly theoretical, suggests that almost all particulate iodine is in the chemical form of IO-3. Other aqueous-phase species are only temporary reservoirs and can be re-activated to yield gas phase iodine. Assuming release rates of the organic iodine compounds which yield atmospheric concentrations similar to some measurements, we calculate significant concentrations of reactive halogen gases. The addition of iodine chemistry to our reaction scheme has the effect of accelerating photochemical Br and Cl release from the seasalt. This causes an enhancement in ozone destruction rates in the MBL over that arising from the well established reactions O(1D) + H2O → 2OH, HO2 + O3 → OH + 2O2, and OH + O3 → HO2 + O2. The given reaction scheme accounts for the formation of particulate iodine which is preferably accumulated in the smaller sulfate aerosol particles.

Aerosol pH in the marine boundary layer: A review and model evaluation

Abstract Impacts of sea-salt-aerosol pH on oxidation processes, sulfur cycling, and surface-ocean fertilization are uncertain; estimates vary from pH 9 and the pH-dependence of some transformations is poorly characterized. We modeled these processes under clean and polluted conditions. At pH 8, S (IV) +O 3 in sea salt is the principal S-oxidation pathway. At pH 5.5, S (IV) oxidation by HOCl dominates. Decreased SO 2 solubility at pH 3 slows S (VI) production. The relative contribution of H 2 SO 4(g) scavenging to S (VI) in sea salt increases with decreasing pH. Significant sea-salt dehalogenation is limited to acidified aerosol. Volatilization rates of BrCl and Br 2 do not vary significantly between pH 5.5 and 3, whereas HCl production via acid displacement increases by a factor of 20. At pH 5.5 and 8, virtually all HNO 3 is scavenged by sea salt. Modeled HNO 3 increases at pH 3 but remains substantially lower than particulate NO - 3 . Discrepancies between measurements and modeled results are assessed based on measurement artifacts, uncertainties in rate and equilibrium constants, organic reactants and surface films, and dynamics.

Vehicle exhaust particle size distributions: A comparison of tailpipe and dilution tunnel measurements

This paper explores the extent to which standard dilution tunnel measurements of motor vehicle exhaust particulate matter modify particle number and size. Steady state size distributions made directly at the tailpipe, using an ejector pump, are compared to dilution tunnel measurements for three configurations of transfer hose used to transport exhaust from the vehicle tailpipe to the dilution tunnel. For gasoline vehicles run at a steady 50 70 mph, ejector pump and dilution tunnel measurements give consistent results of particle size and number when using an uninsulated stainless steel transfer hose. Both methods show particles in the 10 100 nm range at tailpipe concentrations of the order of 104 particles/cm3. When an insulated hose, or one containing a silicone rubber coupler, is used to test small 4 cylinder gasoline vehicles, a very intense nanoparticle / ultrafine mode at < ~30 nm develops in the dilution tunnel particle size distribution as the vehicle speed is increased to 60 and 70 mph. This nanoparticle mode coincides with a rise of the transfer line temperature to about 180 250 °C. It is much less evident for the full size gasoline sedan, which has cooler exhaust. Both tailpipe and dilution tunnel measurements of diesel vehicle exhaust reveal an accumulation mode peak of ~108 particles/cm3, centered at 80 -100 nm. In this case, even with the uninsulated transfer hose an intense ultrafine peak appears in the dilution tunnel size distributions. This mode is attributed to desorption and/or pyrolysis of organic material, either hydrocarbon deposits on the walls of the steel transfer hose or the silicone rubber, by hot exhaust gases, and their subsequent nucleation in the dilution tunnel. This substantially limits the ability to make accurate particle number and size measurements using dilution tunnel systems. INTRODUCTION Prompted by potential health concerns, the past few years have witnessed a growing interest in particulate matter (PM) measurements, both ambient and from a wide variety of emissions sources. These measurements are conventionally performed by recording PM mass. The ambient standards are written in terms of mass concentrations, and emission regulations are based on mass rates. However, in order to understand better the nature of the mobile source contribution to ambient PM, many research groups are currently extending their investigations to include measurements of the numbers and sizes of particles in motor vehicle exhaust. Because the standard procedure for tailpipe PM measurements utilizes a dilution tunnel to cool the exhaust and to prevent water condensation, concerns have emerged that the test cell measurements of motor vehicle PM do not reflect the “real world” emissions. The root of the concern is that vehicle exhaust, a hot, complex, mixture of gaseous emissions and particles, is transformed differently when diluted in a tunnel as compared to the “real world”. Particles are not immutable; they readily undergo transformations, such as coagulation, condensation, and adsorption, and new particles can be created by nucleation of gaseous particle precursors in the diluted and cooled exhaust. Under “real world” conditions, motor vehicle exhaust is diluted rapidly, in less than one second, and by a large amount, a dilution ratio of greater than 100, into air of variable temperature and humidity. In the test cell, the exhaust is conducted to the dilution tunnel, typically by a 10 cm diameter by 5 m long tube, where it is then diluted by a factor of between 5 and 50, depending on test conditions, using dry air at room temperature. This discrepancy between dilution times, extents, temperature, and humidity can potentially lead to significant differences in the nature of the particulate emissions.

Modelling the chemistry of ozone, halogen compounds, and hydrocarbons in the arctic troposphere during spring

The box model Moccalce has been developed to study the chemistry of the arctic boundary layer. It treats chemical reactions in the gas phase and in the aerosol, as well as exchange between the 2 phases. Photolysis rates vary according to the solar declination during polar sunrise. Apart from the standard tropospheric chemistry of ozone, hydrocarbons, and nitrogen species, the reaction mechanism includes sulfur and the halogens Cl, Br, and I. Modeling an ozone depletion event, we found that iodine species contribute to the chemical destruction of ozone significantly if 10 mixing ratios are about 1 pmol/mol. The reactions of BrO with Bra and 10 are the main pathways of the ozone destruction cycle. Hydrocarbon concentrations decrease during ozone depletion events due to reaction with halogen atoms. The rate of ozone destruction depends on whether the addition of Br to C2H4 and C2 H2 yields inert products or intermediates from which Br can be regenerated. Bromine and HCHO are positively correlated. The model produces HCHO during ozone depletion events, though not as much as reported from field observations. After the destruction of ozone has been competed, the halogen species are converted to halides and subsequently scavenged by aerosol particles.

The kinetics and mechanism of SO2 oxidation by O3 on mineral dust

The oxidation of SO2 by O3 on mineral dust was studied using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Formation of sulfate was observed on the surface. A two-step mechanism that involves physisorbed SO2 followed by oxidation is presented. The formation rate was determined to be first order with respect to SO2 and zero order in O3. The reactive uptake coefficient, γ, was determined from the infrared absorbance, that was calibrated by ion chromatography, and from the geometric or the BET surface area. γSO2 was independent of the SO2 concentration and was determined to be in the order of 10−3 using the geometric surface area, or 10−7 using the BET surface area for [SO2] = 2.2 × 1012 to 2.0 × 1013 and [O3] = 5.6 × 1012 (in units of molecule cm−3). γO3 depended linearly on the O3 concentration and varied from 10−2 to 10−4 using the geometric area or 10−6 to 10−8 using the BET area for [O3] = 1.9 × 1012 to 5.5 × 1013 and [SO2] = 5.4 × 1012 (in units of molecule cm−3). In all experiments surface saturation was observed with an amount of 2 × 1019 sulfate ions g−1 on the mineral dust sample. In the presence of water vapor regeneration of active sites was observed. After several exposures to water vapor corresponding to 80% relative humidity and successive SO2 and O3 treatments the amount of formed sulfate covering the surface was increased by 47% compared to the dry experiments.

Performance Evaluation of a Novel Sampling and Measurement System for Exhaust Particle Characterization

This paper presents a novel partial flow sampling system for the characterization of airborne exhaust particle emissions. The sampled aerosol is first conditioned in a porous dilutor and then subsequent ejector dilutors are used to decrease its concentration to the range of the instrumentation used. First we examine the sensitivity of aerosol properties to boundary sampling conditions. This information is then used to select suitable sampling parameters to distinguish both the nucleation and the accumulation mode. Selecting appropriate sampling parameters, it is demonstrated that a distinct nucleation mode can be formed and measured with different instruments. Using these parameters we examine the performance of the system over transient vehicle operation. Additionally, we performed calculations of particle losses in the various components of the system which are then used to correct signals from the instruments. Several quality characteristics are then discussed, such as the repeatability and reproducibility of the measurements and the potential to derive total emission rate with a partial flow sampling system. Comparisons in different laboratories show that repeatability (intra-laboratory variability) is in the order of 10% for accumulation mode particles and 50% for nucleation mode ones. Reproducibility (inter-laboratory variability) values are in the range of ±20-30%. Finally, we compared laboratory size distributions with ambient samples obtained chasing a vehicle. This demonstrated that the sampling system accurately reproduced the accumulation mode particles as well as the potential for nucleation mode formation. This sampling system has been used in the framework of a European project for measurement of emissions of a number of light duty vehicles and heavy duty engines.

Identification of diesel exhaust particles at an Autobahn, urban and rural location using single-particle mass spectrometry

Abstract A single-particle mass spectrometer (LAMPAS-2) was operated at an Autobahn (high-speed highway with significant heavy-duty diesel traffic), an urban and a rural site in the vicinity of Aachen (Germany). The single-particle mass spectra could be classified into eight classes, representing different types of mineral particles, inorganic salt particles, and carbonaceous aerosol particles. At all three sites characteristic patterns of diesel exhaust particles with and without secondary compounds (ammonium sulfate/nitrate) were observed. The relative contribution of diesel soot to the number of 0.5 μm particles was 23% or 35% at the rural and the Autobahn site, respectively. The absolute number of diesel exhaust particles was three times larger at the Autobahn site. At the urban site the diesel exhaust particle contribution ranged from 10% to 35%, depending on the local operation of heavy-duty construction vehicles. Elemental carbon and carbonaceous particles made up the majority number of the 0.5 μm particles, and showed a decreasing percentage towards 2 μm particle size. As expected mineral-soil-derived particles showed the reverse size distribution. The data sets were also analyzed using a reference pattern obtained from exhaust particles of a light-duty diesel vehicle as a fingerprint. A similar trend of the contribution to the diesel-exhaust-like particle class was found, although the absolute numbers were somewhat different. On-line single-particle mass spectrometry proved to be a promising tool to identify individual particles if characteristic reference spectra were available.

Single particle MS, SNMS, SIMS, XPS, and FTIR spectroscopic analysis of soot particles during the AIDA campaign

Abstract Within the framework of the AIDA soot aerosol campaign diesel soot particles, spark generated soot particles, and aerosol mixtures were characterized with respect to their chemical state using different surface sensitive analysis methods. A comparison between diesel soot and graphite spark generated soot revealed a significant difference in the chemical composition of the particle surfaces. No distinct change from external to internal mixing could be detected by single particle mass spectrometry for mixtures of diesel soot and (NH4)2SO4 aerosol since the spectra of diesel soot and (NH4)2SO4 aerosol were surprisingly similar due to sulfate on the surface of diesel soot particles and traces of carbon impurities on ammonium sulfate particles. In addition to the expected formation of new particles a considerable change of the soot particle surface was detected while exposing diesel soot or spark generated soot to α-pinene and ozone, indicating a surface layer formed by oxidation products of α-pinene. However, the oxygen-containing hydrocarbon fragments detected by single particle mass spectrometry were distinctly different for the two soot types, which can be explained by either the different product adsorption or ionization behavior. Depositions of α-pinene reaction products on the surface could be confirmed by QMS-SIMS and XPS for particles of both types of soot. Due to the high mass resolution of TOF-SIMS acidic derivatives were identified as reaction products of α-pinene and ozone. The analytical methods applied in this work elucidated the different properties of spark generated soot compared to diesel soot. Therefore, spark generated soot should only be used with care as a general diesel soot surrogate.

Composition of Semi-volatile Particles from Diesel Exhaust

Vehicle exhaust particles from diesel passenger vehicles were studied in terms of volatility and chemical composition. Condensation of semi-volatile compounds leads to particle growth during exhaust dilution and cooling. The particle growth was observed to be particle surface related. At higher vehicle speed and load some of the semi-volatile material forms nucleation particles that are dominating the particle number concentration. The nucleation mode is completely volatile at 180°C and consists mainly of sulfate. The amount of organic material is smaller. The organics/sulfate ratio is larger for the soot mode indicating an earlier condensation process of organics before they are incorporated in the nucleation process. Under typical atmospheric dilution conditions most of the semi-volatile material is present in the soot mode. The semi-volatile material evaporates at temperature between 130°C and 180°C. Thermal treatment using a thermodenuder enables complete evaporation of the nucleation particles, however not all material from the soot particles is removed.