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Turbulent dispersion in cloud-topped boundary layers

Compared to dry boundary layers, dispersion in cloud-topped boundary layers has received less attention. In this LES based numerical study we investigate the dispersion of a passive tracer in the form of Lagrangian particles for four kinds of atmospheric boundary layers: 1) a dry convective boundary layer (for reference), 2) a "smoke" cloud boundary layer in which the turbulence is driven by radiative cooling, 3) a stratocumulus topped boundary layer and 4) a shallow cumulus topped boundary layer. We show that the dispersion characteristics of the smoke cloud boundary layer as well as the stratocumulus situation can be well understood by borrowing concepts from previous studies of dispersion in the dry convective boundary layer. A general result is that the presence of clouds enhances mixing and dispersion – a notion that is not always reflected well in traditional parameterization models, in which clouds usually suppress dispersion by diminishing solar irradiance. The dispersion characteristics of a cumulus cloud layer turn out to be markedly different from the other three cases and the results can not be explained by only considering the well-known top-hat velocity distribution. To understand the surprising characteristics in the shallow cumulus layer, this case has been examined in more detail by 1) determining the velocity distribution conditioned on the distance to the nearest cloud and 2) accounting for the wavelike behaviour associated with the stratified dry environment.
Modelled radiative forcing of the direct aerosol effect with multi-observation evaluation

A high-resolution global aerosol model (Oslo CTM2) driven by meteorological data and allowing a comparison with a variety of aerosol observations is used to simulate radiative forcing (RF) of the direct aerosol effect. The model simulates all main aerosol components, including several secondary components such as nitrate and secondary organic carbon. The model reproduces the main chemical composition and size features observed during large aerosol campaigns. Although the chemical composition compares best with ground-based measurement over land for modelled sulphate, no systematic differences are found for other compounds. The modelled aerosol optical depth (AOD) is compared to remote sensed data from AERONET ground and MODIS and MISR satellite retrievals. To gain confidence in the aerosol modelling, we have tested its ability to reproduce daily variability in the aerosol content, and this is performing well in many regions; however, we also identified some locations where model improvements are needed. The annual mean regional pattern of AOD from the aerosol model is broadly similar to the AERONET and the satellite retrievals (mostly within 10–20%). We notice a significant improvement from MODIS Collection 4 to Collection 5 compared to AERONET data. Satellite derived estimates of aerosol radiative effect over ocean for clear sky conditions differs significantly on regional scales (almost up to a factor two), but also in the global mean. The Oslo CTM2 has an aerosol radiative effect close to the mean of the satellite derived estimates. We derive a radiative forcing (RF) of the direct aerosol effect of −0.35 Wm−2 in our base case. Implementation of a simple approach to consider internal black carbon (BC) mixture results in a total RF of −0.28 Wm−2. Our results highlight the importance of carbonaceous particles, producing stronger individual RF than considered in the recent IPCC estimate; however, net RF is less different. A significant RF from secondary organic aerosols (SOA) is estimated (close to −0.1 Wm−2). The SOA also contributes to a strong domination of secondary aerosol species for the aerosol composition over land. A combination of sensitivity simulations and model evaluation show that the RF is rather robust and unlikely to be much stronger than in our best estimate.
Relating observations of contrail persistence to numerical weather analysis output

The potential for using high-resolution meteorological data from two operational numerical weather analyses (NWA) to diagnose and predict persistent contrail formation is evaluated using two independent contrail observation databases. Contrail occurrence statistics derived from surface and satellite observations between April 2004 and June 2005 are matched to the humidity, vertical velocity, wind shear and atmospheric stability derived from analyses from the Rapid Update Cycle (RUC) and the Advanced Regional Prediction System (ARPS) models. The relationships between contrail occurrence and the NWA-derived statistics are analyzed to determine under which atmospheric conditions persistent contrail formation is favored within NWAs. Humidity is the most important factor determining whether contrails are short-lived or persistent, and persistent contrails are more likely to appear when vertical velocities are positive. The model-derived atmospheric stability and wind shear do not appear to have a significant effect on contrail occurrence.
Uncertainty in global CCN concentrations from uncertain aerosol nucleation and primary emission rates

The indirect effect of aerosols on climate is highly uncertain and limits our ability to assess anthropogenic climate change. The foundation of this uncertainty is uncertainty in the number of cloud condensation nuclei (CCN), which itself stems from uncertainty in aerosol nucleation, primary emission and growth rates. In this paper, we use a global general circulation model with aerosol microphysics to assess how the uncertainties in aerosol nucleation, emission and growth rates affect our prediction of CCN(0.2%) concentrations. Using several nucleation rate parameterizations that span six orders of magnitude of globally averaged nucleation rates, the tropospheric average CCN(0.2%) concentrations vary by 17% and the boundary layer average vary by 12%. This sensitivity of tropospheric average CCN(0.2%) to the nucleation parameterizations increases to 33% and 20% when the total primary emissions are reduced by a factor of 3 and the SOA condensation rates are increased by a factor of 3.5, respectively. These results show that it is necessary to better understand global nucleation rates when determining CCN concentrations. When primary emissions rates are varied by a factor of 3 while using a binary nucleation parameterization, tropospheric average CCN(0.2%) concentrations also vary by 17%, but boundary layer average vary by 40%. Using the fastest nucleation rate parameterization, these changes drop to 3% and 22%, respectively. These results show the importance of reducing uncertainties in primary emissions, which appear from these results to be somewhat more important for CCN than the much larger uncertainties in nucleation. These results also show that uncertainties in nucleation and primary emissions are more important when sufficient condensable material is available to grow them to CCN sizes. The percent change in CCN(0.2%) concentration between pre-industrial times and present day does not depend greatly on the nucleation rate parameterization used for our base case scenarios; however, because other factors, such as primary emissions and SOA, are uncertain in both time periods, this may be a coincidence.
Influence of non-ideality on condensation to aerosol

Secondary organic aerosol (SOA) is a complex mixture of water and organic molecules. Its composition is determined by the presence of semi-volatile or non-volatile compounds, their saturation vapor pressure and activity coefficient. The activity coefficient is a non-ideality effect and is a complex function of SOA composition. In a previous publication, the detailed chemical mechanism (DCM) for α-pinene oxidation and subsequent aerosol formation BOREAM was presented. In this work, we investigate with this DCM the impact of non-ideality by simulating smog chamber experiments for α-pinene degradation and aerosol formation and taking the activity coefficient into account of all molecules in the aerosol phase. Several versions of the UNIFAC method are tested for this purpose, and missing parameters for e.g. hydroperoxides and nitrates are inferred from fittings to activity coefficient data generated using the SPARC model. Alternative approaches to deal with these missing parameters are also tested, as well as an activity coefficient calculation method based on Hansen solubility parameters (HSP). It turns out that for most experiments, non-ideality has only a limited impact on the interaction between the organic molecules, and therefore on SOA yields and composition, when water uptake is ignored. The reason is that often, the activity coefficient is on average close to 1 and, specifically for high-VOC experiments, partitioning is not very sensitive on the activity coefficient because the equilibrium is shifted strongly towards condensation. Still, for ozonolysis experiments with low amounts of volatile organic carbon (low-VOC), the UNIFAC parameterization of Raatikainen et al. leads to significantly higher SOA yields (by up to a factor 1.6) compared to the ideal case and to other parameterizations. Water uptake is model dependent, in the order: ideal > UNIFAC-Raatikainen > UNIFAC-Peng > UNIFAC-Hansen ≈ UNIFAC-Magnussen ≈ UNIFAC-Ming. In the absence of salt dissolution, phase splitting from pure SOA is unlikely.
Increasing ozone in marine boundary layer inflow at the west coasts of North America and Europe

An effective method is presented for determining the ozone (O3) mixing ratio in the onshore flow of marine air at the North American west coast. By combining the data available from all marine boundary layer (MBL) sites with simultaneous wind data, decadal temporal trends of MBL O3 in all seasons are established with high precision. The average springtime temporal trend over the past two decades is 0.46 ppbv/yr with a 95% confidence limit of 0.13 ppbv/yr, and statistically significant trends are found for all seasons except autumn, which does have a significantly smaller trend than other seasons. The average trend in mean annual ozone is 0.34±0.09 ppbv/yr. These decadal trends at the North American west coast present a striking comparison and contrast with the trends reported for the European west coast at Mace Head, Ireland. The trends in the winter, spring and summer seasons compare well at the two locations, while the Mace Head trend is significantly greater in autumn. Even though the trends are similar, the absolute O3 mixing ratios differ markedly, with the marine air arriving at Europe in all seasons containing 7±2 ppbv higher ozone than marine air arriving at North America. Further, the ozone mixing ratios at the North American west coast show no indication of stabilizing as has been reported for Mace Head. In a larger historical context the background boundary layer O3 mixing ratios over the 130 years covered by available data have increased substantially (by a factor of two to three), and this increase continues at present, at least in the MBL of the Pacific coast region of North America. The reproduction of the increasing trends in MBL O3 over the past two decades, as well as the difference in the O3 mixing ratios between the two coastal regions will present a significant challenge for global chemical transport models. Further, the ability of the models to at least semi-quantitatively reproduce the longer-term, historical trends may an even greater challenge.
Severe ozone air pollution in the Persian Gulf region

Recently it was discovered that over the Middle East during summer ozone mixing ratios can reach a pronounced maximum in the middle troposphere. Here we extend the analysis to the surface and show that especially in the Persian Gulf region conditions are highly favorable for ozone air pollution. We apply the EMAC atmospheric chemistry-climate model to investigate long-distance transport and the regional formation of ozone. Further, we make use of available in situ and satellite measurements and compare these with model output. The results indicate that the region is a hot spot of photochemical smog where European Union air quality standards are violated throughout the year. Long-distance transports of air pollution from Europe and the Middle East, natural emissions and stratospheric ozone conspire to bring about relatively high background ozone mixing ratios. This provides a hotbed to strong and growing indigenous air pollution in the dry local climate, and these conditions are likely to get worse in the future.
Intensification of tropical cyclones in the GFS model

Special forecasts from the Global Forecast System (GFS) model were used in this study to evaluate how the intensification process in a tropical cyclone is represented in this model. Several tropical cyclones that developed in 2005 were analyzed in terms of the storm-scale circulation rather than more traditional measures such as maximum wind or minimum central pressure. The primary balance governing the circulation in the planetary boundary layer is between the convergence of environmental vorticity, which tends to spin up the storm, and surface friction, which tends to spin it down. In addition, we employ recently developed ideas about the relationship between precipitation and the saturation fraction of the environment to understand the factors controlling mass, and hence vorticity convergence. The budget of moist entropy is central to this analysis. Two well-known governing factors for cyclone intensification emerge from this study; surface moist entropy fluxes, dependent in the model on sea surface temperature and cyclone-generated surface winds, and ventilation of the system by dry environmental air. Quantitative expressions for the role of these factors in cyclone intensification are presented in this paper.
Isoprene photooxidation: new insights into the production of acids and organic nitrates

We describe a nearly explicit chemical mechanism for isoprene photooxidation guided by chamber studies that include time-resolved observation of an extensive suite of volatile compounds. We provide new constraints on the chemistry of the poorly-understood isoprene δ-hydroxy channels, which account for more than one third of the total isoprene carbon flux and a larger fraction of the nitrate yields. We show that the cis branch dominates the chemistry of the δ-hydroxy channel with less than 5% of the carbon following the trans branch. The modelled yield of isoprene nitrates is 12&plusmn3% with a large difference between the δ and β branches. The oxidation of these nitrates releases about 50% of the NOx. Methacrolein nitrates (modelled yield ~15±3% from methacrolein) and methylvinylketone nitrates (modelled yield ~11±3% yield from methylvinylketone) are also observed. Propanone nitrate, produced with a yield of 1% from isoprene, appears to be the longest-lived nitrate formed in the total oxidation of isoprene. We find a large molar yield of formic acid and suggest a novel mechanism leading to its formation from the organic nitrates. Finally, the most important features of this mechanism are summarized in a condensed scheme appropriate for use in global chemical transport models.
Organic nitrate and secondary organic aerosol yield from NO3 oxidation of β-pinene evaluated using a gas-phase kinetics/aerosol partitioning model

The yields of organic nitrates and of secondary organic aerosol (SOA) particle formation were measured for the reaction NO3+β-pinene under dry and humid conditions in the atmosphere simulation chamber SAPHIR at Research Center Jülich. These experiments were conducted at low concentrations of NO3 (NO3+N2O5<10 ppb) and β-pinene (peak~15 ppb), with no seed aerosol. SOA formation was observed to be prompt and substantial (~50% mass yield under both dry conditions and at 60% RH), and highly correlated with organic nitrate formation. The observed gas/aerosol partitioning of organic nitrates can be simulated using an absorptive partitioning model to derive an estimated vapor pressure of the condensing nitrate species of pvap~5×10−6 Torr (6.67×10−4 Pa), which constrains speculation about the oxidation mechanism and chemical identity of the organic nitrate. Once formed the SOA in this system continues to evolve, resulting in measurable aerosol volume decrease with time. The observations of high aerosol yield from NOx-dependent oxidation of monoterpenes provide an example of a significant anthropogenic source of SOA from biogenic hydrocarbon precursors. Estimates of the NO3+β-pinene SOA source strength for California and the globe indicate that NO3 reactions with monoterpenes are likely an important source (0.5–8% of the global total) of organic aerosol on regional and global scales.