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On the transition between heterogeneous and homogeneous freezing

Box model simulations of an uplifting and adiabatically cooling cloud of aerosol have been performed in order to study the transition between cirrus formation dominated by homogeneous nucleation of ice to that dominated by heterogeneous nucleation. The aerosol was assumed to consist of an internal mixture of sulfuric acid solution droplets with inclusions of soot. The parametrisation of De Mott et al. (1997) was used to simulate the heterogeneous nucleation of ice in such droplets with soot inclusions. The simulations show that the transition from heterogeneous to homogeneous nucleation occurs over a narrow range of soot concentration. Thus it seems to be possible to fix critical concentrations of heterogeneous ice nuclei which must be exceeded if heterogeneous freezing dominates cirrus formation. A formula has been derived that allows to compute the critical concentrations of heterogeneous ice nuclei as a function of temperature, updraft speed, ambient pressure, and supersaturation at which heterogeneous freezing occurs. Generally, homogeneous nucleation dominates in regions with updrafts stronger than 20 cm s -1, with the exception of heavily polluted areas which could be common in the northern hemisphere due to air traffic, where updrafts of the order 1 m s-1 may be necessary to render heterogeneous nucleation unimportant. According to the present results it cannot be excluded that heterogeneous nucleation plays a more important role for cirrus formation in the northern midlatitudes than anywhere else. A possible consequence of these results is that air pollution may lead to a higher coverage of cirrus clouds, but then these clouds will be optically thinner than clouds formed by homogeneous freezing, with the exception of regions where condensation trails are frequent.
Chemical characteristics assigned to trajectory clusters during the MINOS campaign

During the Mediterranean Intensive Oxidant Study (MINOS) in August 2001 a total of 14 measurement flights were performed with the DLR Falcon jet aircraft from Heraklion, Crete. One objective of this campaign was to investigate the role of long-range transport of pollutants into the Mediterranean area. An analysis of 5-day back trajectories indicates that in the lower troposphere (0-4 km) air masses originated from eastern and western Europe, in the mid-troposphere (4-8 km) from the North Atlantic Ocean region and in the upper troposphere (8-14 km) from North Atlantic Ocean/North America (NANA) as well as South Asia. We allocated all back trajectories to clusters based on their ending height and source region. The mixing ratios of ozone, nitrogen oxide, total reactive oxidized nitrogen (NOy), formaldehyde, methanol, acetonitrile, acetone, peroxyacetyl nitrate (PAN), carbon dioxide, carbon monoxide and methane measured along the flight tracks are examined in relation to the different cluster trajectories. In the lower troposphere the mean trace gas mixing ratios of the eastern Europe cluster trajectories were significantly higher than those from western Europe. In the upper troposphere air from the NANA region seems to be influenced by the stratosphere, in addition, air masses were transported from South Asia, being influenced by strong convection in the Indian monsoon.
Heterogeneous conversion of NO2 on secondary organic aerosol surfaces: A possible source of nitrous acid (HONO) in the atmosphere?

The heterogeneous conversion of NO2 on different secondary organic aerosols (SOA) was investigated with the focus on a possible formation of nitrous acid (HONO). In one set of experiments different organic aerosols were produced in the reactions of O3 with alpha-pinene, limonene or catechol and OH radicals with toluene or limonene, respectively. The aerosols were sampled on filters and exposed to humidified NO2  mixtures under atmospheric conditions. The estimated upper limits for the uptake coefficients of NO2  and the reactive uptake coefficients NO2  -> HONO are in the range of 10-6 and 10-7, respectively. The integrated HONO formation for 1 h reaction time was <1013 cm-2 geometrical surface and <1017 g-1 particle mass. In a second set of experiments the conversion of NO2 into HONO in the presence of organic particles was carried out in an aerosol flow tube under atmospheric conditions. In this case the aerosols were produced in the reaction of O3 with beta-pinene, limonene or catechol, respectively. The upper limits for the reactive uptake coefficients NO2 -> HONO were in the range of 7 x 10-7 - 9 x 10-6. The results from the present study show that heterogeneous formation of nitrous acid on secondary organic aerosols (SOA) is unimportant for the atmosphere.
Organic acids as cloud condensation nuclei: Laboratory studies of highly soluble and insoluble species

The ability of sub-micron-sized organic acid particles to act as cloud condensation nuclei (CCN) has been examined at room temperature using a newly constructed continuous-flow, thermal-gradient diffusion chamber (TGDC). The organic acids studied were: oxalic, malonic, glutaric, oleic and stearic. The CCN properties of the highly soluble acids - oxalic, malonic and glutaric - match very closely Köhler theory predictions which assume full dissolution of the dry particle and a surface tension of the growing droplet equal to that of water. In particular, for supersaturations between 0.3 and 0.6, agreement between the dry particle diameter which gives 50% activation and that calculated from Köhler theory is to within 3nm on average. In the course of the experiments, considerable instability of glutaric acid particles was observed as a function of time and there is evidence that they fragment to some degree to smaller particles. Stearic acid and oleic acid, which are both highly insoluble in water, did not activate at supersaturations of 0.6% with dry diameters up to 140nm. Finally, to validate the performance of the TGDC, we present results for the activation of ammonium sulfate particles that demonstrate good agreement with Köhler theory if solution non-ideality is considered. Our findings support earlier studies in the literature that showed highly soluble organics to be CCN active but insoluble species to be largely inactive.
Inverse transport for the verification of the Comprehensive Nuclear Test Ban Treaty

An international monitoring system is being built as a verification tool for the Comprehensive Test Ban Treaty. Forty stations will measure on a worldwide daily basis the concentration of radioactive noble gases. The paper introduces, by handling preliminary real data, a new approach of backtracking for the identification of sources of passive tracers after positive measurements. When several measurements are available the ambiguity about possible sources is reduced significantly. The approach is validated against ETEX data. A distinction is made between adjoint and inverse transport shown to be, indeed, different though equivalent ideas. As an interesting side result it is shown that, in the passive tracer dispersion equation, the diffusion stemming from a time symmetric turbulence is necessarily a self-adjoint operator, a result easily verified for the usual gradient closure, but more general.
Atmospheric impact of the 1783–1784 Laki eruption: Part I Chemistry modelling

Results from the first chemistry-transport model study of the impact of the 1783–1784 Laki fissure eruption (Iceland: 64°N, 17°W) upon atmospheric composition are presented. The eruption released an estimated 61 Tg(S) as SO2 into the troposphere and lower stratosphere. The model has a high resolution tropopause region, and detailed sulphur chemistry. The simulated SO2 plume spreads over much of the Northern Hemisphere, polewards of ~40°N. About 70% of the SO2 gas is directly deposited to the surface before it can be oxidised to sulphuric acid aerosol. The main SO2 oxidants, OH and H2O2, are depleted by up to 40% zonally, and the lifetime of SO2 consequently increases. Zonally averaged tropospheric SO2 concentrations over the first three months of the eruption exceed 20 ppbv, and sulphuric acid aerosol reaches ~2 ppbv. These compare to modelled pre-industrial/present-day values of 0.1/0.5 ppbv SO2 and 0.1/1.0 ppbv sulphate. A total sulphuric acid aerosol yield of 17–22 Tg(S) is produced. The mean aerosol lifetime is 6–10 days, and the peak aerosol loading of the atmosphere is 1.4–1.7 Tg(S) (equivalent to 5.9–7.1 Tg of hydrated sulphuric acid aerosol). These compare to modelled pre-industrial/present-day sulphate burdens of 0.28/0.81 Tg(S), and lifetimes of 6/5 days, respectively. Due to the relatively short atmospheric residence times of both SO2 and sulphate, the aerosol loading approximately mirrors the temporal evolution of emissions associated with the eruption. The model produces a reason-able simulation of the acid deposition found in Greenland ice cores. These results appear to be relatively insensitive to the vertical profile of emissions assumed, although if more of the emissions reached higher levels (>12 km), this would give longer lifetimes and larger aerosol yields. Introducing the emissions in episodes generates similar results to using monthly mean emissions, because the atmospheric lifetimes are similar to the repose periods between episodes. Most previous estimates of the global aerosol loading associated with Laki did not use atmospheric models; this study suggests that these earlier estimates have been generally too large in magnitude, and too long-lived. Environmental effects following the Laki eruption may have been dominated by the widespread deposition of SO2 gas rather than sulphuric acid aerosol.
Spatial variation of aerosol properties over Europe derived from satellite observations and comparison with model calculations

Aerosol optical depths (AOD) and Angström coefficients over Europe were retrieved using data from the ATSR-2 radiometer on board the ESA satellite ERS-2, for August 1997. Taking advantage of the nadir and forward view of the ATSR-2, the dual view algorithm was used over land to eliminate the influence of the surface reflection. Over sea the AOD was retrieved using only the forward observations. Retrieved aerosol optical properties are in good agreement with those from ground-based sunphotometers. The AOD and Angström coefficients together yield information on the column integrated effective aerosol distribution.  Observed regional variations of the AOD and Angström coefficient are related to anthropogenic emissions of aerosol precursors such as SO2 and NOx in the major European industrial and urban areas, and their subsequent transformation into the aerosol phase. The influence of anthropogenic aerosols such as ammonium sulphate and ammonium nitrate on the total AOD is estimated using a regional chemistry transport model. Sulphate is estimated to contribute from 15% in very clean areas to 70% in polluted areas, the contribution of nitrate is between 5% and 25% over most of Europe. This paper shows the great importance of nitrate in summer over The Netherlands.
An investigation of ozone and planetary boundary layer dynamics over the complex topography of Grenoble combining measurements and modeling

This paper concerns an evaluation of ozone (O3) and planetary boundary layer (PBL) dynamics over the complex topography of the Grenoble region through a combination of measurements and mesoscale model (METPHOMOD) predictions for three days, during July 1999. The measurements of O3 and PBL structure were obtained with a Differential Absorption Lidar (DIAL) system, situated 20 km south of Grenoble at Vif (310 m ASL). The combined lidar observations and model calculations are in good agreement with atmospheric measurements obtained with an instrumented aircraft (METAIR). Ozone fluxes were calculated using lidar measurements of ozone vertical profiles concentrations and the horizontal wind speeds measured with a Radar Doppler wind profiler (DEGREANE. The ozone flux patterns indicate that the diurnal cycle of ozone production is controlled by local thermal winds. The convective PBL maximum height was some 2700 m above the land surface while the nighttime residual ozone layer was generally found between 1200 and 2200 m. Finally we evaluate the magnitude of the ozone processes at different altitudes in order to estimate the photochemical ozone production due to the primary pollutants emissions of Grenoble city and the regional network of automobile traffic.
Differences across the ITCZ in the chemical characteristics of the Indian Ocean MBL aerosol during INDOEX

The water soluble inorganic part of the sub-micrometer aerosol was measured from two research vessels over the Indian Ocean during the winter monsoon season (February and March) as part of the INDOEX project in 1998 and 1999. Additional measurements were made of gas phase SO2 from one of the vessels in 1999. All samples collected north of the Inter Tropical Convergence Zone, ITCZ, were clearly affected by continental, anthropogenic sources. A sharp transition occurred across the ITCZ with concentrations of nss-SO42-, NH4+ and nss-K+ being lower by a factor of 7-15, >20 and >40, respectively, on the southern side of the ITCZ. The contribution from DMS to the sub-micrometer nss-SO42- was estimated to be up to 40% in clean air north of the ITCZ but less than 10% in polluted air originating from India. South of the ITCZ virtually all nss-SO42- was likely to be derived from oxidation of DMS. The concentration of SO2 decreased rapidly with distance from the Indian coast, the molar ratio SO2/nss-SO42- reaching values below 5% after 35 h travel time over the ocean. Surprisingly, MSA, which is derived from DMS, also showed higher concentrations in the sub-micrometer aerosol north of the ITCZ than south of it. This could be explained by the larger sub-micrometer surface area available north of the ITCZ for the condensation of MSA. South of the ITCZ a major part of the MSA was found on the super-micrometer particles. An analysis based on the air trajectories showed that systematic variation in the observed concentrations was associated with variations in the transport from source regions. For example, differences in time since air parcels left the Arabian or Indian coasts was shown to be an important factor for explaining the substantial differences in absolute concentrations.
On the formation of radiation fogs under heavily polluted conditions

We have studied the effect of gaseous pollutants on fog droplet growth in heavily polluted air using a model that describes time-dependent sulfate production in the liquid phase and thermodynamical equilibrium between the droplets and the gas phase. Our research indicates that the oxidation of SO2 to sulfate has a significant effect on fog droplet growth especially when hygroscopic trace gases, for example HNO3 and NH3 are present. The increased sulfate production by dissolution of hygroscopic gases results from increased pH (caused by absorption of ammonia) and from the increased size of the fog/smog droplets. Our results indicate that unactivated fogs may become optically very thick when the droplet concentrations are on the order of several thousand per cubic centimeter of air.