-
A three-dimensional (3-D) chemical transport model (CTM), SLIMCAT, has been used to quantify the effect of denitrification on ozone loss for the Arctic winter 2004/2005. The simulated HNO<sub>3</sub> is found to be highly sensitive to the polar stratospheric cloud (PSC) scheme used in the model. Here the standard SLIMCAT full chemistry model, which uses a thermodynamic equilibrium PSC scheme, overpredicts the ozone loss for Arctic winter 2004/2005 due to the overestimation of denitrification and stronger chlorine activation than observed. A model run with a coupled detailed microphysical denitrification scheme, DLAPSE (Denitrification by Lagrangian Particle Sedimentation), is less denitrified than the standard model run and better reproduces the observed HNO<sub>3</sub> as measured by Airborne SUbmillimeter Radiometer (ASUR) and Aura Microwave Limb Sounder (MLS) instruments. Overall, denitrification is responsible for a ~30 % enhancement in O<sub>3</sub> depletion compared with simulations without denitrification for Arctic winter 2004/2005, which is slightly larger than the inferred impact of denitrification on Arctic ozone loss for previous winters from different CTMs simulations. The overestimated denitrification from standard SLIMCAT simulation causes ~5–10 % more ozone loss at ~17 km compared with the simulation using the DLAPSE PSC scheme for Arctic winter 2004/2005. The calculated partial column ozone loss from SLIMCAT using the DLAPSE scheme is about 130 DU by mid-March 2005, which compares well with the inferred column ozone loss from ozonesondes and satellite data (127±21 DU).
-
The heterogeneous oxidation of sulfur dioxide by ozone on CaCO<sub>3</sub> was studied as a function of temperature (230 to 298 K) at ambient pressure. Oxidation reactions were followed in real time using diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) to obtain kinetic and mechanistic data. From the analysis of the spectral features, the formation of sulfate was identified on the surface in the presence of O<sub>3</sub> and SO<sub>2</sub> at different temperatures from 230 to 298 K. The results showed that the heterogeneous oxidation and the rate of sulfate formation were sensitive to temperature. An interesting stage-transition region was observed at temperatures ranging from 230 to 257 K, but it became ambiguous gradually above 257 K. The reactive uptake coefficients at different temperatures from 230 to 298 K were acquired for the first time, which can be used directly in atmospheric chemistry modeling studies to predict the formation of secondary sulfate aerosol in the troposphere. Furthermore, the rate of sulfate formation had a turning point at about 250 K. The sulfate concentration at 250 K was about twice as large as that at 298 K. The rate of sulfate formation increased with decreasing temperature at temperatures above 250 K, while there is a contrary temperature effect at temperatures below 250 K. The activation energy for heterogeneous oxidation at temperatures from 245 K to 230 K was determined to be 14.63 ± 0.20 kJ mol<sup>−1</sup>. A mechanism for the temperature dependence was proposed and the atmospheric implications were discussed.
-
An analysis of chemical composition data of particulate matter samples (TSP, PM<sub>10</sub> and PM<sub>2.5</sub>) collected from 2002 to 2008 in the North Atlantic free troposphere at the Izaña Global Atmospheric Watch (GAW) observatory (Tenerife, Canary Islands) shows that desert dust is very frequently mixed with particulate pollutants in the Saharan Air Layer (SAL). The study of this data set with Median Concentrations At Receptor (MCAR) plots allowed the identification of the potential source regions of the dust and particulate pollutants. Areas located at the south of the southern slope of the Atlas mountains emerge as the most frequent source of the soil desert dust advected to the northern edge of the SAL in summer. Industrial emissions occurring in Northern Algeria, Eastern Algeria, Tunisia and the Atlantic coast of Morocco appear as the most important source of the nitrate, ammonium and a fraction of sulphate (at least 60 % of the sulphate <10 μm transported from some regions) observed in the SAL. These emissions are mostly linked to crude oil refineries, phosphate-based fertilizer industry and power plants. Although desert dust emissions appear as the most frequent source of the phosphorous observed in the SAL, high P concentrations are observed when the SAL is affected by emissions from open mines of phosphate and phosphate based fertilizer industry. The results also show that a significant fraction of the sulphate (up to 90 % of sulphate <10 μm transported from some regions) observed in the SAL may be influenced by soil emissions of evaporite minerals in well defined regions where dry saline lakes (chotts) are present. These interpretations of the MCAR plots are consistent with the results obtained with the Positive Matrix Factorization (PMF2) receptor modelling. The results of this study show that North African industrial pollutants may be mixed with desert dust and exported to the North Atlantic in the Saharan Air Layer.
-
Within the atmospheric research community, there is a strong interest in integrated datasets, combining data from several instrumentations. This integration is complicated by the different characteristics of the datasets, inherent to the measurement techniques. Here we have compared two carbon monoxide time series (1997 till 2007) acquired at the high-Alpine research station Jungfraujoch (3580 m above sea level), with two well-established measurement techniques, namely in situ surface concentration measurements using Non-Dispersive Infrared Absorption technology (NDIR), and ground-based remote sensing measurements using solar absorption Fourier Transform Infrared spectrometry (FTIR). The profile information available in the FTIR signal allowed us to extract an independent layer with a top height of 7.18 km above sea level, appropriate for comparison with our in situ measurements. We show that, even if both techniques are able to measure free troposphere CO concentrations, the datasets exhibit marked differences in their overall trends (−3.21 ± 0.03 ppb year<sup>−1</sup> for NDIR vs. −0.8 ± 0.4 ppb year<sup>−1</sup> for FTIR). Removing measurements that are polluted by uprising boundary layer air has a strong impact on the NDIR trend (now −2.62 ± 0.03 ppb year<sup>−1</sup>), but its difference with FTIR remains significant. Using the LAGRANTO trajectory model, we show that both measurement techniques are influenced by different source regions and therefore are likely subject to exhibit significant differences in their overall trend behaviour. However the observation that the NDIR-FTIR trend difference is as significant before as after 2001 is at odds with available emission databases which claim a significant Asian CO increase after 2001 only.
-
The first in situ point observations of iodine monoxide (IO) at a clean marine site were made using a laser-induced fluorescence instrument deployed at Mace Head, Ireland in August 2007. IO mixing ratios of up to 49.8 pptv (equivalent to pmol mol<sup>−1</sup>; 1 s average) were observed at day-time low tide, well in excess of previous observed spatially-averaged maxima. A strong anti-correlation of IO mixing ratios with tide height was evident and the high time resolution of the observations showed IO peaked in the hour after low tide. The temporal delay in peak IO compared to low tide has not been observed previously but coincides with the time of peak aerosol number previously observed at Mace Head. <br><br> A long path-differential optical absorption spectroscopy instrument (with a 2 × 6.8 km folded path across Roundstone Bay) was also based at the site for 3 days during the point measurement observation period. Both instruments show similar temporal trends but the point measurements of IO are a factor of ~6–10 times greater than the spatially averaged IO mixing ratios, providing direct empirical evidence of the presence of inhomogeneities in the IO mixing ratio near the intertidal region.
-
The Weather Research and Forecasting model coupled with chemistry (WRF-Chem) is modified to include a volatility basis set (VBS) treatment of secondary organic aerosol formation. The VBS approach, coupled with SAPRC-99 gas-phase chemistry mechanism, is used to model gas-particle partitioning and multiple generations of gas-phase oxidation of organic vapors. In addition to the detailed 9-species VBS, a simplified mechanism using 2 volatility species (2-species VBS) is developed and tested for similarity to the 9-species VBS in terms of both mass and oxygen-to-carbon ratios of organic aerosols in the atmosphere. WRF-Chem results are evaluated against field measurements of organic aerosols collected during the MILAGRO 2006 campaign in the vicinity of Mexico City. The simplified 2-species mechanism reduces the computational cost by a factor of 2 as compared to 9-species VBS. Both ground site and aircraft measurements suggest that the 9-species and 2-species VBS predictions of total organic aerosol mass as well as individual organic aerosol components including primary, secondary, and biomass burning are comparable in magnitude. In addition, oxygen-to-carbon ratio predictions from both approaches agree within 25 %, providing evidence that the 2-species VBS is well suited to represent the complex evolution of organic aerosols. Model sensitivity to amount of anthropogenic semi-volatile and intermediate volatility (S/IVOC) precursor emissions is also examined by doubling the default emissions. Both the emission cases significantly under-predict primary organic aerosols in the city center and along aircraft flight transects. Secondary organic aerosols are predicted reasonably well along flight tracks surrounding the city, but are consistently over-predicted downwind of the city. Also, oxygen-to-carbon ratio predictions are significantly improved compared to prior studies by adding 15 % oxygen mass per generation of oxidation; however, all modeling cases still under-predict these ratios downwind as compared to measurements, suggesting a need to further improve chemistry parameterizations of secondary organic aerosol formation.
-
The global tropospheric distribution of molecular hydrogen (H<sub>2</sub>) and its uptake by the soil are simulated using a model called CHemical AGCM (atmospheric general circulation model) for the Study of the Environment and Radiative forcing (CHASER), which incorporates a two-layered soil diffusion/uptake process component. The simulated distribution of deposition velocity over land is influenced by regional climate, and has a global average of 3.3×10<sup>−2</sup> cm s<sup>−1</sup>. In the region north of 30° N, the amount of soil uptake shows a large seasonal variation corresponding to change in biological activity due to soil temperature and change in diffusion suppression by snow cover. In the temperate and humid regions in the mid- to low- latitudes, the uptake is mostly influenced by the soil air ratio, which controls the gas diffusivity in the soil. In the semi-arid regions, water stress and high temperatures contribute to the reduction of biological activity, as well as to the seasonal variation in the deposition velocity. A comparison with the observations shows that the model reproduces both the distribution and seasonal variation of H<sub>2</sub> relatively well. The global burden and tropospheric lifetime of H<sub>2</sub> are 150 Tg and 2.0 yr, respectively. The seasonal variation in H<sub>2</sub> mixing ratios at the northern high latitudes is mainly controlled by a large seasonal change in the soil uptake. In the Southern Hemisphere, seasonal change in net chemical production and inter-hemispheric transport are the dominant causes of the seasonal cycle, while large biomass burning contributes significantly to the seasonal variation in the tropics and subtropics. Both observations and the model show large inter-annual variations, especially for the period 1997–1998, associated with large biomass burning in the tropics and at Northern Hemisphere high latitudes. The soil uptake shows relatively small inter-annual variability compared with the biomass burning signal. Given that the thickness of biologically inactive layer plays an important role in the soil uptake of H<sub>2</sub>, its value in the model is chosen to achieve agreement with the observed H<sub>2</sub> trends. Uncertainty of the estimated soil uptake flux in the semi-arid region is still large, reflecting the discrepancy in the observed and modeled seasonal variations.
-
Forests are the dominant source of volatile organic compounds into the atmosphere, with isoprene being the most significant species. The oxidation chemistry of these compounds is a significant driver of local, regional and global atmospheric composition. Observations made over Borneo during the OP3 project in 2008, together with an observationally constrained box model are used to assess our understanding of this oxidation chemistry. In line with previous work in tropical forests, we find that the standard model based on MCM chemistry significantly underestimates the observed OH concentrations. Geometric mean observed to modelled ratios of OH and HO<sub>2</sub> in airmasses impacted with isoprene are 5.32<sub>−4.43</sub><sup>+3.68</sup> and 1.18<sub>−0.30</sub><sup>+0.30</sup> respectively, with 68 % of the observations being within the specified variation. We implement a variety of mechanistic changes into the model, including epoxide formation and unimolecular decomposition of isoprene peroxy radicals, and assess their impact on the model success. We conclude that none of the current suggestions can simultaneously remove the bias from both OH and HO<sub>2</sub> simulations and believe that detailed laboratory studies are now needed to resolve this issue.
-
Atmospheric loadings of secondary organic aerosols (SOA) are significantly under-predicted by climate models. In these models, SOA particles are assumed liquid-like droplets at equilibrium with the gas-phase. In sharp contrast, our recent laboratory and field measurements show that SOA particles are non-rigid, highly viscous, spherical, quasi-solids, and do not behave like liquid droplets. They evaporate at rates much lower than predicted by models, and are consequently not at equilibrium with the gas phase. In addition, our data show that SOA particles trap hydrophobic organics, whose presence further reduces evaporation rates, and that aging these particles nearly stops evaporation. Measurements of the evaporation kinetics of ambient SOA particles under vapor-free conditions at room temperature showed that less than 20 % of particle mass evaporates in 4 h. <br><br> In this study, we examine, for the first time, these groundbreaking observations to present a new, experimentally based picture of the phase and evaporation behavior of SOA particles. We conclude that to first order SOA can be reasonably approximated to be non-evaporating. We use a simplified approach to investigate the implications of this near-irreversible gas-particle partitioning behavior in a box model and a 3-D chemical transport model, both of which, for the first time, include multi-generational gas-phase chemistry with functionalization and fragmentation reactions, and compare them to traditional reversible partitioning models. Results indicate that the revised irreversible partitioning approach yields slightly higher SOA loadings than traditional reversible partitioning approach when functionalization reactions, pushing SOA species to lower volatility bins are dominant. However, when fragmentation reactions play a major role, the revised irreversible partitioning approach predicts significantly higher SOA than the traditional approach. In addition to irreversibility, functionalization, and fragmentation, we explore the utility of lower activity coefficient to account for complex molecular interactions within particles and show that this approach predicts considerably higher SOA loadings. Using the 3-D Weather Research and Forecasting (WRF) model, coupled with Chemistry (WRF-Chem) modeling example of the Mexico City region, we demonstrate that when fragmentation is taken into account, irreversible partitioning increases predicted SOA loadings and lifetimes significantly compared to traditional models. When lower activity coefficient is also included, predicted SOA loadings in the Mexico City plateau increase by more than a factor of 3.
-
The release of vast quantities of methane into the atmosphere as a result of clathrate destabilization is a potential mechanism for rapid amplification of global warming. Previous studies have calculated the enhanced warming based mainly on the radiative effect of the methane itself, with smaller contributions from the associated carbon dioxide or ozone increases. Here, we study the effect of strongly elevated methane (CH<sub>4</sub>) levels on oxidant and aerosol particle concentrations using a combination of chemistry-transport and general circulation models. A 10-fold increase in methane concentrations is predicted to significantly decrease hydroxyl radical (OH) concentrations, while moderately increasing ozone (O<sub>3</sub>). These changes lead to a 70 % increase in the atmospheric lifetime of methane, and an 18 % decrease in global mean cloud droplet number concentrations (CDNC). The CDNC change causes a radiative forcing that is comparable in magnitude to the longwave radiative forcing ("enhanced greenhouse effect") of the added methane. Together, the indirect CH<sub>4</sub>-O<sub>3</sub> and CH<sub>4</sub>-OH-aerosol forcings could more than double the warming effect of large methane increases. Our findings may help explain the anomalously large temperature changes associated with historic methane releases.
