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Aerosol size distributions have been measured at the Swedish background station Aspvreten (58.8° N, 17.4° E). Different states of the aerosol were determined using a novel application of cluster analysis. The analysis resulted in eight different clusters capturing different stages of the aerosol lifecycle. The atmospheric aerosol size distributions were interpreted as belonging to fresh, intermediate and aged types of size distribution. With aid of back trajectory analysis we present statistics concerning the relation of source area and different meteorological parameters using a non-Lagrangian approach. Source area is argued to be important although not sufficient to describe the observed aerosol properties. Especially processing by clouds and precipitation is shown to be crucial for the evolution of the aerosol size distribution. As much as 60% of the observed size distributions present features that are likely to be related to cloud processes or wet deposition. The lifetime properties of different sized aerosols are discussed by means of measured variability of the aerosol size distribution. Processing by clouds and precipitation is shown to be especially crucial in the size range 100 nm and larger. This indicates an approximate limit for activation in clouds to 100 nm in this type of environment. The aerosol lifecycle is discussed. Size distributions indicating signs of recent new particle formation (~30% of the observed size distributions) represent the first stage in the lifecycle. Aging of the aerosol size distribution may follow two branches: either growth by condensation and coagulation or processing by non-precipitating clouds. In both cases mass is accumulated. Wet removal is the main process capable of removing aerosol mass. Wet deposition is argued to be an important mechanism in reaching a state where nucleation may occur (i.e. sufficiently low aerosol surface area) in environments similar to the one studied.
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This paper shows that most of the so far identified constituents of the tropospheric organic particulate matter belong to a semivolatile fraction for which gas phase diffusion in the lower troposphere is sufficiently fast to establish thermodynamic equilibrium between aerosol particles. For the first time analytical expressions for this process are derived. Inspection of vapor pressure data of a series of organic substances allows a rough estimate for which substances this mixing process must be considered. As general benchmarks we conclude that for typical aerosol radii between 0.1 and 1 µm this mixing process is efficient at 25°C for polar species with molecular weights up to 200 and for non-polar species up to 320. At −10°C, these values are shifted to 150 for polar and to 270 for non-polar substances. The extent of mixing of this semivolatile fraction is governed by equilibrium thermodynamics, leading to a selectively, though not completely, internally mixed aerosol. The internal mixing leads to a systematic depression of melting and deliquescence points of organic and mixed organic/inorganic aerosols, thus leading to an aerosol population in the lower troposphere which is predominantly liquid.
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Polar mesosphere summer echoes (PMSE) are very strong radar echoes primarily studied in the VHF wavelength range from altitudes close to the polar summer mesopause. Radar waves are scattered at irregularities in the radar refractive index which at mesopause altitudes is effectively determined by the electron number density. For efficient scatter, the electron number density must reveal structures at the radar half wavelength (Bragg condition for monostatic radars; ~3 m for typical VHF radars). The question how such small scale electron number density structures are created in the mesopause region has been a longstanding open scientific question for almost 30 years. This paper reviews experimental and theoretical milestones on the way to an advanced understanding of PMSE. Based on new experimental results from in situ observations with sounding rockets, ground based observations with radars and lidars, numerical simulations with microphysical models of the life cycle of mesospheric aerosol particles, and theoretical considerations regarding the diffusivity of electrons in the ice loaded complex plasma of the mesopause region, a consistent explanation for the generation of these radar echoes has been developed. The main idea is that mesospheric neutral air turbulence in combination with a significantly reduced electron diffusivity due to the presence of heavy charged ice aerosol particles (radii ~5–50 nm) leads to the creation of structures at spatial scales significantly smaller than the inner scale of the neutral gas turbulent velocity field itself. Importantly, owing to their very low diffusivity, the plasma structures acquire a very long lifetime, i.e., 10 min to hours in the presence of particles with radii between 10 and 50 nm. This leads to a temporal decoupling of active neutral air turbulence and the existence of small scale plasma structures and PMSE and thus readily explains observations proving the absence of neutral air turbulence at PMSE altitudes. With this explanation at hand, it becomes clear that PMSE are a suitable tool to permanently monitor the thermal and dynamical structure of the mesopause region allowing insights into important atmospheric key parameters like neutral temperatures, winds, gravity wave parameters, turbulence, solar cycle effects, and long term changes.
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The age of stratospheric air is computed with a trajectory model, using ECMWF ERA-40 3D-Var and operational 4D-Var winds. Analysis as well as forecast data are used. In the latter case successive forecast segments are put together to get a time series of the wind fields. This is done for different forecast segment lengths. The sensitivity of the computed age to the forecast segment length and assimilation method are studied, and the results are compared with observations and with results from a chemistry transport model that uses the same data sets. A large number of backward trajectories are started in the stratosphere, and from the fraction of these trajectories that has passed the tropopause the age of air is computed. First, for ten different data sets 50-day backward trajectories starting in the tropical lower stratosphere are computed. The results show that in this region the computed cross-tropopause transport decreases with increasing forecast segment length. Next, for three selected data sets (3D-Var 24-h and 4D-Var 72-h forecast segments, and 4D-Var analyses) 5-year backward trajectories are computed that start all over the globe at an altitude of 20km. For all data sets the computed ages of air in the extratropics are smaller than the observation-based age. For 4D-Var forecast series they are closest to the observation-based values, but still 0.5-1.5 year too small. Compared to the difference in age between the results for the different data sets, the difference in age between the trajectory and the chemistry transport model results is small.
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In this study we present a complex case study of a Stratosphere-to-Troposphere Transport (STT) event down to the surface of a low topography region in Northern Greece, during the second fortnight of March 2000. During this event our surface station at Livadi (23°15 E/40°32 N, 850 m a.s.l.), was influenced by very different synoptic systems developing over Eastern Europe, N. America and the N. Atlantic, the last one evolving to a cut-off low over France/Spain. This is the first study, to our knowledge, that presents a down to the surface STT event in the eastern Mediterranean. The intrusion is primarily captured with the use of the cosmogenic radionuclide <sup>7</sup>Be, which increased to 9.07 mBq m<sup>-3</sup> and 9.37 mBq m<sup>-3</sup> on 30 and 31 March 2000, respectively. A <sup>7</sup>Be concentration of around 8 mBq m<sup>-3</sup> recorded during parallel measurements at Thessaloniki (20 m a.s.l.) gives strong evidence that air of stratospheric origins has even gone down to sea level. A rapid increase of 10–15 ppb is also observed in the surface ozone concentration on 31 March 2000. The relative increase of both tracers is consistent with a volume fraction of stratospheric air at the surface of about 5%, but the substantial increase in <sup>7</sup>Be flags more clearly the event. Trajectory analyses, in conjunction with the evolution of the synoptic situation described by potential vorticity maps, are used for the exact identification of the different intrusions and the attribution of each intruding parcel of stratospheric air to a certain filament of high PV. Finally, the persistency of the stratospheric layers in the troposphere is another interesting point of this case study. The vast majority of the trajectories spent 7–10 days in the troposphere before reaching the surface at Livadi station.
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In the past, differential optical absorption spectroscopy (DOAS) has mostly been employed for atmospheric trace gas retrieval in the UV/Vis spectral region. New spectrometers such as SCIAMACHY onboard ENVISAT also provide near infrared channels and thus allow for the detection of greenhouse gases like CH<sub>4</sub>, CO<sub>2</sub>, or N<sub>2</sub>O. However, modifications of the classical DOAS algorithm are necessary to account for the idiosyncrasies of this spectral region, i.e. the temperature and pressure dependence of the high resolution absorption lines. Furthermore, understanding the sensitivity of the measurement of these high resolution, strong absorption lines by means of a non-ideal device, i.e. having finite spectral resolution, is of special importance. This applies not only in the NIR, but can also prove to be an issue for the UV/Vis spectral region. <P style="line-height: 20px;"> This paper presents a modified iterative maximum a posteriori-DOAS (IMAP-DOAS) algorithm based on optimal estimation theory introduced to the remote sensing community by rodgers76. This method directly iterates the vertical column densities of the absorbers of interest until the modeled total optical density fits the measurement. Although the discussion in this paper lays emphasis on satellite retrieval, the basic principles of the algorithm also hold for arbitrary measurement geometries. <P style="line-height: 20px;"> This new approach is applied to modeled spectra based on a comprehensive set of atmospheric temperature and pressure profiles. This analysis reveals that the sensitivity of measurement strongly depends on the prevailing pressure-height. The IMAP-DOAS algorithm properly accounts for the sensitivity of measurement on pressure due to pressure broadening of the absorption lines. Thus, biases in the retrieved vertical columns that would arise in classical algorithms, are obviated. Here, we analyse and quantify these systematic biases as well as errors due to variations in the temperature and pressure profiles, which is indispensable for the understanding of measurement precision and accuracy in the near infrared as well as for future intercomparisons of retrieval algorithms.
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High tropospheric NO<sub>2</sub> amounts are occasionally detected by space-borne spectrometers above cloudy scenes. For monitoring of near-ground air pollution such data are not directly applicable because clouds shield the highly polluted planetary boundary layer (PBL). We present a method based on trajectories which implicitly estimates the additional sub-cloud NO<sub>2</sub> distribution in order to model concentrations at ground stations. The method is applied to a transboundary pollution transport episode which led to high NO<sub>2</sub> vertical tropospheric column densities (VTCs) over middle Europe observed by the Global Ozone Monitoring Experiment (GOME) instrument above clouds on 17 February 2001. The case study shows that pollution originally residing near the ground in central Germany, the Ruhr area and adjacent parts of the Netherlands and Belgium has been advected to higher tropospheric levels by a passing weather front. Combining the above-cloud NO<sub>2</sub> VTCs with trajectory information covering the GOME columns and including their sub-cloud part yields an estimate of the total NO<sub>2</sub> distribution within the tropospheric columns. The highly polluted air masses are then traced by forward trajectories starting from the GOME columns to move further to the Alpine region and their impact there is assessed. Considering ground-based in-situ measurements in the Alpine region, we conclude that for this episode, at least 50% of the NO<sub>2</sub> concentration recorded at the sites can be attributed to transboundary transport during the frontal passage. This study demonstrates the potential of using NO<sub>2</sub> VTCs from GOME detected above clouds when combined with transport modelling.
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Airborne measurements of acetone, methanol, PAN, acetonitrile (by Proton Transfer Reaction Mass Spectrometry), and CO (by Tunable Diode Laser Absorption Spectroscopy) have been performed during the Mediterranean Intensive Oxidants Study (MINOS August 2001). We have identified ten biomass burning plumes from strongly elevated acetonitrile mixing ratios. The characteristic biomass burning signatures obtained from these plumes reveal secondary production of acetone and methanol, while CO photochemically declines in the plumes. Mean excess mixing ratios - normalized to CO - of 1.8%, 0.20%, 3.8%, and 0.65% for acetone, acetonitrile, methanol, and PAN, respectively, were found. By scaling to an assumed global annual source of 663-807Tg CO, biomass burning emissions of 25-31 and 29-35 Tg/yr for acetone and methanol are estimated, respectively. Our measurements suggest that the present biomass burning contributions of acetone and methanol are significantly underestimated due to the neglect of secondary formation within the plume. Median acetonitrile mixing ratios throughout the troposphere were around 150pmol/mol, in accord with current biomass burning inventories and an atmospheric lifetime of ~6 months.
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Diurnal variation of number size distribution (particle size 3-800nm) and modal parameters (geometric standard deviation, geometric mean diameter and modal aerosol particle concentration) in a highly polluted urban environment was investigated during October and November 2002 in New Delhi, India. Continuous monitoring for more than two weeks with the time resolution of 10min was conducted using a Differential Mobility Particle Sizer (twin DMPS). The results indicated clear increase in Aitken mode (25-100nm) particles during traffic peak hours, but towards the evenings there were more Aitken mode particles compared to the mornings. Also high concentrations of accumulation mode particles (>100nm) were detected in the evenings only. In the evenings, biomass/refuse burning and cooking are possible sources beside the traffic. We have also shown that nucleation events are possible in this kind of atmosphere even though as clear nucleation events as observed in rural sites could not be detected. The formation rate of 3nm particles (J3) of the observed events varied from 3.3 to 13.9cm<sup>-3</sup>s<sup>-1</sup> and the growth rate varied from 11.6 to 18.1nmh<sup>-1</sup> showing rapid growth and high formation rate, which seems to be typical in urban areas.
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Vertical gradients of mixing ratios of volatile organic compounds have been measured in a Ponderosa pine forest in Central California (38.90° N, 120.63° W, 1315m). These measurements reveal large quantities of previously unreported oxidation products of short lived biogenic precursors. The emission of biogenic precursors must be in the range of 13-66µmol m<sup>-2</sup>h<sup>-1</sup> to produce the observed oxidation products. That is 6-30 times the emissions of total monoterpenes observed above the forest canopy on a molar basis. These reactive precursors constitute a large fraction of biogenic emissions at this site, and are not included in current emission inventories. When oxidized by ozone they should efficiently produce secondary aerosol and hydroxyl radicals.
