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Atmospheric hydrogen variations and traffic emissions at an urban site in Finland

Atmospheric hydrogen (H2) mixing ratios were observed over a one year period from summer 2007 to 2008 in Helsinki, Finland. Relatively stable background values of hydrogen were occasionally observed at the site, with minimum in October and maximum between March and May. High hydrogen mixing ratios occurred simultaneously with high carbon monoxide (CO) values and coincided with high traffic flow periods. Carbon monoxide and radon (222Rn) were continuously monitored at the same site and they were used in estimation of the hydrogen emissions from traffic. The morning rush hour slope of ΔH2/ΔCO was in average 0.43±0.03 ppb (H2)/ppb (CO). After correction due to soil deposition of H2 the slope was 0.49±0.07 ppb (H2)/ppb (CO). Using this slope and CO emission statistics, a road traffic emission of about 260 t (H2)/year was estimated for Helsinki in 2007.
Corrigendum to "The role of ammonia in sulfuric acid ion induced nucleation" published in Atmos. Chem. Phys., 8, 2859–2867, 2008
Sulfuric acid and OH concentrations in a boreal forest site

As demonstrated in a number of investigations, gaseous sulfuric acid plays a central role in atmospheric aerosol formation. Using chemical ionization mass spectrometer the gas-phase sulfuric acid and OH concentration were measured in Hyytiälä, SMEAR II station, Southern Finland during 24 March to 28 June 2007. Clear diurnal cycles were observed as well as differences between new particle formation event days and non-event days. Typically, the daily maximum concentrations of gas phase sulfuric acid varied from 3×105 to 2×106 molec cm−3 between non-event and event days. Noon-time OH concentrations varied from 3-6×105 molec cm−3 and not a clear difference between event and non-events was detected. The measured time series were also used as a foundation to develop reasonable proxies for sulfuric acid concentration. The proxies utilized source and sink terms, and the simplest proxy is radiation times sulfur dioxide divided by condensation sink. Since it is still challenging to measure sulfuric acid in ambient concentrations, and due to its significant role in atmospheric particle formation, reasonable proxies are needed. We use all together three different proxies and one chemical box model and compared their results to the measured data. The proxies for the sulfuric acid concentration worked reasonably well, and will be used to describe sulfuric acid concentrations in SMEAR II station, when no measured sulfuric acid data is available. With caution the proxies could be applied to other environments as well.
Analytical treatment of ice sublimation and test of sublimation parameterisations in two–moment ice microphysics models

We derive an analytic solution to the spectral growth/sublimation equation for ice crystals and apply it to idealised cases. The results are used to test parameterisations of the ice sublimation process in two–moment bulk microphysics models. Although it turns out that the relation between number loss fraction and mass loss fraction is not a function since it is not unique, it seems that a functional parameterisation is the best that one can do in a bulk model. Testing a more realistic case with humidity oscillations shows that artificial crystal loss can occur in simulations of mature cirrus clouds with relative humidity fluctuating about ice saturation.
Detailed heterogeneous chemistry in an urban plume box model: reversible co-adsorption of O3, NO2, and H2O on soot coated with benzo[a]pyrene

This study assesses in detail the effects of heterogeneous chemistry on the particle surface and gas-phase composition by modeling the reversible co-adsorption of O3, NO2, and H2O on soot coated with benzo[a]pyrene (BaP) for an urban plume scenario over a period of five days. By coupling the Pöschl-Rudich-Ammann (PRA) kinetic framework for aerosols (Pöschl et al., 2007) to a box model version of the gas phase mechanism RADM2, we are able to track individual concentrations of gas-phase and surface species over the course of several days. The flux-based PRA formulation takes into account changes in the uptake kinetics due to changes in the chemical gas-phase and particle surface compositions. This dynamic uptake coefficient approach is employed for the first time in a broader atmospheric context of an urban plume scenario. Our model scenarios include one to three adsorbents and three to five coupled surface reactions. The results show a variation of the O3 and NO2 uptake coefficients of more than five orders of magnitude over the course of the simulation time and a decrease in the uptake coefficients in the various scenarios by more than three orders of magnitude within the first six hours. Thereafter, periodic peaks of the uptake coefficients follow the diurnal cycle of gas-phase O3-NOx reactions. Physisorption of water vapor reduces the half-life of the coating substance BaP by up to a factor of seven by permanently occupying ~75% of the soot surface. Soot emissions modeled by replenishing reactive surface sites lead to maximum gas-phase O3 depletions of 41 ppbv and 7.8 ppbv for an hourly and six-hourly replenishment cycle, respectively. This conceptual study highlights the interdependence of co-adsorbing species and their non-linear gas-phase feedback. It yields further insight into the atmospheric importance of the chemical oxidation of particles and emphasizes the necessity to implement detailed heterogeneous kinetics in future modeling studies.
Global carbon tetrachloride distributions obtained from the Atmospheric Chemistry Experiment (ACE)

The first study of the global atmospheric distribution of carbon tetrachloride (CCl4), as a function of altitude and latitude, was performed using solar occultation measurements obtained by the Atmospheric Chemistry Experiment (ACE) mission using Fourier transform spectroscopy. A total of 8703 profile measurements were taken in the upper troposphere and lower stratosphere between February 2004 and August 2007. The zonal distribution of carbon tetrachloride displays a slight hemispheric asymmetry and decreasing concentration with increasing altitude at all latitudes. Maximum carbon tetrachloride concentrations are situated below 10 km in altitude with VMR (Volume Mixing Ratio) values of 100–130 ppt (parts per trillion). The highest concentrations are located about the Equator and at mid-latitudes, particularly for latitudes in heavily industrialised regions (20–45° N), with values declining towards the poles. Global distributions obtained from ACE were compared with predictions from three chemistry transport models showing good agreement in terms of the vertical gradient despite an overall offset. The ACE dataset gives unique global and temporal coverage of carbon tetrachloride and its transport through the atmosphere. An estimated lifetime for carbon tetrachloride of 34±5 years was determined through correlation with CFC-11.
Vehicular emission of volatile organic compounds (VOCs) from a tunnel study in Hong Kong

Vehicle emissions of volatile organic compounds (VOCs) were determined at the Shing Mun Tunnel, Hong Kong in summer and winter of 2003. One hundred and ten VOCs were quantified in this study. The average concentration of the total measured VOCs at the inlet and outlet of the tunnel were 81 250 pptv and 117 850 pptv, respectively. Among the 110 compounds, ethene, ethyne and toluene were the most abundant species in the tunnel. The total measured VOC emission factors ranged from 67 mg veh−1 km−1 to 148 mg veh−1 km−1, with an average of 115 mg veh−1 km−1. The five most abundant VOCs observed in the tunnel were, in decreasing order, ethene, toluene, n-butane, propane and i-pentane. These five most abundant species contributed over 38% of the total measured VOCs emitted. The high propane and n-butane emissions were found to be associated with liquefied petroleum gas (LPG)-fueled taxis. Fair correlations were observed between marker species (ethene, i-pentane, n-nonane, and benzene, toluene, ethylbenzene and xylenes – BTEX) with fractions of gasoline-fueled or diesel-fueled vehicles. Moreover, ethene, ethyne, and propene are the key species that were abundant in the tunnel but not in gasoline vapors or LPG. The ozone formation potential from the VOCs in Hong Kong was evaluated by the maximum increment reactivity (MIR). It was found to be 568 mg of ozone per vehicle per kilometer traveled. Among them, ethene, propene and toluene contribute most to the ozone-formation reactivity.
Investigation of ship-plume chemistry using a newly-developed photochemical/dynamic ship-plume model

A photochemical/dynamic ship-plume model, which can consider the ship-plume dynamics and ship-plume chemistry, simultaneously, was developed to gain a better understanding of atmospheric impact of ship emissions. The model performance was then evaluated by a comparison with the observation data measured on a NOAA WP-3D flight during the Intercontinental Transport and Chemical Transformation 2002 (ITCT 2K2) airborne field campaign. The simulation conditions and parameters, such as meteorological conditions, emission rates, and background gas and particulate species concentrations, were obtained directly and/or inferred indirectly from the ITCT 2K2 observation data. The model-predicted concentrations showed good agreement with the observed concentrations of five ambient species (NOx, NOy, ozone, HNO3, and H2SO4) at the eight plume transects by the WP-3D flight with strong correlations around the 1:1 line (0.64≤R≤0.85). In addition, a set of tests were carried out to approximate the magnitude of the reaction probability of HNO3 onto sea-salt particles in the model-observation comparison framework. These results suggest that the reaction probability of HNO3 onto sea-salt particles may be in the order of 0.05–0.1. The equivalent NOx lifetime throughout the "entire plume" was also estimated from photochemical/dynamic ship-plume modeling. The NOx lifetimes estimated throughout the entire ship plume ranged from 2.64 h to 3.76 h under stable to neutral stability conditions. The short NOx lifetime over the entire ship plume clearly shows that the ship-plume chemistry shortens the NOx lifetime considerably. Therefore, the ship-plume chemistry model should be used to model the changes in ship-plume chemical compositions and better evaluate the atmospheric impact of ocean-going ship emissions.
Cloud condensation nuclei in pristine tropical rainforest air of Amazonia: size-resolved measurements and modeling of atmospheric aerosol composition and CCN activity

Atmospheric aerosol particles serving as cloud condensation nuclei (CCN) are key elements of the hydrological cycle and climate. We have measured and characterized CCN at water vapor supersaturations in the range of S=0.10–0.82% in pristine tropical rainforest air during the AMAZE-08 campaign in central Amazonia. The effective hygroscopicity parameters describing the influence of chemical composition on the CCN activity of aerosol particles varied in the range of κ≈0.1–0.4 (0.16±0.06 arithmetic mean and standard deviation). The overall median value of κ≈0.15 was by a factor of two lower than the values typically observed for continental aerosols in other regions of the world. Aitken mode particles were less hygroscopic than accumulation mode particles (κ≈0.1 at D≈50 nm; κ≈0.2 at D≈200 nm), which is in agreement with earlier hygroscopicity tandem differential mobility analyzer (H-TDMA) studies. The CCN measurement results are consistent with aerosol mass spectrometry (AMS) data, showing that the organic mass fraction (forg) was on average as high as ~90% in the Aitken mode (D≤100 nm) and decreased with increasing particle diameter in the accumulation mode (~80% at D≈200 nm). The κ values exhibited a negative linear correlation with forg (R2=0.81), and extrapolation yielded the following effective hygroscopicity parameters for organic and inorganic particle components: κorg≈0.1 which can be regarded as the effective hygroscopicity of biogenic secondary organic aerosol (SOA) and κinorg≈0.6 which is characteristic for ammonium sulfate and related salts. Both the size dependence and the temporal variability of effective particle hygroscopicity could be parameterized as a function of AMS-based organic and inorganic mass fractions (κp=κorg×forg +κinorg×finorg). The CCN number concentrations predicted with κp were in fair agreement with the measurement results (~20% average deviation). The median CCN number concentrations at S=0.1–0.82% ranged from NCCN,0.10≈35 cm−3 to NCCN,0.82≈160 cm−3, the median concentration of aerosol particles larger than 30 nm was NCN,30≈200 cm−3, and the corresponding integral CCN efficiencies were in the range of NCCN,0.10/NCN,30≈0.1 to NCCN,0.82/NCN,30≈0.8. Although the number concentrations and hygroscopicity parameters were much lower in pristine rainforest air, the integral CCN efficiencies observed were similar to those in highly polluted megacity air. Moreover, model calculations of NCCN,S assuming an approximate global average value of κ≈0.3 for continental aerosols led to systematic overpredictions, but the average deviations exceeded ~50% only at low water vapor supersaturation (0.1%) and low particle number concentrations (≤100 cm−3). Model calculations assuming a constant aerosol size distribution led to higher average deviations at all investigated levels of supersaturation: ~60% for the campaign average distribution and ~1600% for a generic remote continental size distribution. These findings confirm earlier studies suggesting that aerosol particle number and size are the major predictors for the variability of the CCN concentration in continental boundary layer air, followed by particle composition and hygroscopicity as relatively minor modulators. Depending on the required and applicable level of detail, the information and parameterizations presented in this paper should enable efficient description of the CCN properties of pristine tropical rainforest aerosols of Amazonia in detailed process models as well as in large-scale atmospheric and climate models.
The impact of resolution on ship plume simulations with NOx chemistry

A high resolution chemical transport model of the marine boundary layer is designed in order to investigate the detailed chemical evolution of a ship plume in a tropical location. To estimate systematic errors due to finite model resolution, otherwise identical simulations are run at a range of model resolutions. Notably, to obtain comparable plumes in the different simulations, it is found necessary to use an advection scheme consistent with the Large Eddy Model representation of sub-grid winds for those simulations with degraded resolution. Our simulations show that OH concentration, NOx lifetime and ozone production efficiency of the model change by 8%, 32% and 31% respectively between the highest (200 m×200 m×40 m) and lowest resolution (9600 m×9600 m×1920 m) simulations. Interpolating to the resolution of a typical global composition transport model (CTM, 5°×5°), suggests that a CTM overestimates OH, NOx lifetime and ozone production efficiency by approximately 15%, 55% and 59% respectively. For the first time, by explicitly degrading the model spatial resolution we show that there is a significant reduction in model skill in accurately simulating the aforementioned quantities due to the coarse resolution of these CTMs and the non-linear nature of atmospheric chemistry. These results are significant for the assessment and forecasting of the climate impact of ship NOx and indicate that for realistic representation of ship plume emissions in CTMs, some suitable parametrisation is necessary at current global model resolutions.