| 1. |
- Ahlm, Lars, 1976-, et al.
(författare)
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A comparison of dry and wet season aerosol number fluxes over the Amazon rain forest
- 2010
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Ingår i: Atmospheric Chemistry And Physics. - 1680-7316. ; 10:6, s. 3063-3079
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Tidskriftsartikel (refereegranskat)abstract
- Vertical number fluxes of aerosol particles and vertical fluxes of CO2 were measured with the eddy covariance method at the top of a 53m high tower in the Amazon rain forest as part of the LBA (The Large Scale Biosphere Atmosphere Experiment in Amazonia) experiment. The observed aerosol number fluxes included particles with sizes down to 10 nm in diameter. The measurements were carried out during the wet and dry season in 2008. In this study focus is on the dry season aerosol fluxes, with significant influence from biomass burning, and these are compared with aerosol fluxes measured during the wet season. Net particle deposition fluxes dominated in daytime in both seasons and the deposition flux was considerably larger in the dry season due to the much higher dry season particle concentration. The particle transfer velocity increased linearly with increasing friction velocity in both seasons. The difference in transfer velocity between the two seasons was small, indicating that the seasonal change in aerosol number size distribution is not enough for causing any significant change in deposition velocity. In general, particle transfer velocities in this study are low compared to studies over boreal forests. The reasons are probably the high percentage of accumulation mode particles and the low percentage of nucleation mode particles in the Amazon boundary layer, both in the dry and wet season, and low wind speeds in the tropics compared to the midlatitudes. In the dry season, nocturnal particle fluxes behaved very similar to the nocturnal CO2 fluxes. Throughout the night, the measured particle flux at the top of the tower was close to zero, but early in the morning there was an upward particle flux peak that is not likely a result of entrainment or local pollution. It is possible that these morning upward particle fluxes are associated with emission of primary biogenic particles from the rain forest. Emitted particles may be stored within the canopy during stable conditions at nighttime, similarly to CO2, and being released from the canopy when conditions become more turbulent in the morning.
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| 2. |
- Ahlm, Lars, et al.
(författare)
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Emission and dry deposition of accumulation mode particles in the Amazon Basin
- 2010
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Ingår i: Atmospheric Chemistry And Physics. - 1680-7316. ; 10:21, s. 10237-10253
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Tidskriftsartikel (refereegranskat)abstract
- Size-resolved vertical aerosol number fluxes of particles in the diameter range 0.25–2.5 μm were measured with the eddy covariance method from a 53 m high tower over the Amazon rain forest, 60 km NNW of Manaus, Brazil. This study focuses on data measured during the relatively clean wet season, but a shorter measurement period from the more polluted dry season is used as a comparison. Size-resolved net particle fluxes of the five lowest size bins, representing 0.25–0.45 μm in diameter, pointed downward in more or less all wind sectors in the wet season. This is an indication that the source of primary biogenic aerosol particles may be small in this particle size range. In the diameter range 0.5–2.5 μm, vertical particle fluxes were highly dependent on wind direction. In wind sectors where anthropogenic influence was low, net emission fluxes dominated. However, in wind sectors associated with higher anthropogenic influence, net deposition fluxes dominated. The net emission fluxes were interpreted as primary biogenic aerosol emission, but deposition of anthropogenic particles seems to have masked this emission in wind sectors with higher anthropogenic influence. The emission fluxes were at maximum in the afternoon when the mixed layer is well developed, and these emissions were best correlated with horizontal wind speed by the equation log10F=0.47·U+2.26 where F is the emission number flux of 0.5–2.5 μm particles [m−2s−1] and U is the horizontal wind speed [ms−1] at the top of the tower.
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| 3. |
- Eriksson, Patrick, 1964-, et al.
(författare)
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Diurnal variations of humidity and ice water content in the tropical upper troposphere
- 2010
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Ingår i: Atmospheric Chemistry and Physics. - 1680-7316. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- Observational results of diurnal variations of humidity from Odin-SMR and AURA-MLS, and cloud ice mass from Odin-SMR and CloudSat are presented for the first time. Comparisons show that the retrievals of humidity and cloud ice from these two satellite combinations are in good agreement. The retrieved data are combined from four almost evenly distributed times of the day allowing mean values, amplitudes and phases of the diurnal variations around 200 hPa to be estimated. This analysis is applied to six climatologically distinct regions, five located in the tropics and one over the subtropical northern Pacific Ocean. The strongest diurnal cycles are found over tropical land regions, where the amplitude is similar to 7% RHi for humidity and similar to 50% for ice mass. The greatest ice mass for these regions is found during the afternoon, and the humidity maximum is observed to lag this peak by similar to 6 h. Over tropical ocean regions the variations are smaller and the maxima in both ice mass and humidity are found during the early morning. Observed results are compared with output from three climate models (ECHAM, EC-EARTH and CAM3). Direct measurement-model comparisons were not possible because the measured and modelled cloud ice masses represent different quantities. To make a meaningful comparison, the amount of snow had to be estimated from diagnostic parameters of the models. There is a high probability that the models underestimate the average ice mass (outside the 1-sigma uncertainty). The models also show clear deficiencies when it comes to amplitude and phase of the regional variations, but to varying degrees.
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| 4. |
- Fors, Erik, et al.
(författare)
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Hygroscopic properties of Amazonian biomass burning and European background HULIS and investigation of their effects on surface tension with two models linking H-TDMA to CCNC data
- 2010
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Ingår i: Atmospheric Chemistry and Physics. - Copernicus. - 1680-7316. ; 10:12, s. 5625-5639
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Tidskriftsartikel (refereegranskat)abstract
- HUmic-LIke Substances (HULIS) have been identified as major contributors to the organic carbon in atmospheric aerosol. The term "HULIS" is used to describe the organic material found in aerosol particles that resembles the humic organic material in rivers and sea water and in soils. In this study, two sets of filter samples from atmospheric aerosols were collected at different sites. One set of samples was collected at the K-puszta rural site in Hungary, about 80 km SE of Budapest, and a second was collected at a site in Rondonia, Amazonia, Brazil, during the Large-Scale Biosphere-Atmosphere Experiment in Amazonia - Smoke Aerosols, Clouds, Rainfall and Climate (LBA-SMOCC) biomass burning season experiment. HULIS were extracted from the samples and their hygroscopic properties were studied using a Hygroscopicity Tandem Differential Mobility Analyzer (H-TDMA) at relative humidity (RH) < 100%, and a cloud condensation nucleus counter (CCNC) at RH > 100%. The H-TDMA measurements were carried out at a dry diameter of 100 nm and for RH ranging from 30 to 98%. At 90% RH the HULIS samples showed diameter growth factors between 1.04 and 1.07, reaching values of 1.4 at 98% RH. The cloud nucleating properties of the two sets of aerosol samples were analysed using two types of thermal static cloud condensation nucleus counters. Two different parameterization models were applied to investigate the potential effect of HULIS surface activity, both yielding similar results. For the K-puszta winter HULIS sample, the surface tension at the point of activation was estimated to be lowered by between 34% (47.7 mN/m) and 31% (50.3 mN/m) for dry sizes between 50 and 120 nm in comparison to pure water. A moderate lowering was also observed for the entire water soluble aerosol sample, including both organic and inorganic compounds, where the surface tension was decreased by between 2% (71.2 mN/m) and 13% (63.3 mN/m).
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| 5. |
- Gagne, S., et al.
(författare)
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Factors influencing the contribution of ion-induced nucleation in a boreal forest, Finland
- 2010
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Ingår i: Atmospheric Chemistry And Physics. - 1680-7316. ; 10:8, s. 3743-3757
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Tidskriftsartikel (refereegranskat)abstract
- We present the longest series of measurements so far (2 years and 7 months) made with an Ion-DMPS at the SMEAR II measurement station in Hyytiala, Southern Finland. We show that the classification into overcharged (implying some participation of ion-induced nucleation) and undercharged (implying no or very little participation of ion-induced nucleation) days, based on Ion-DMPS measurements, agrees with the fraction of ion-induced nucleation based on NAIS measurements. Those classes are based on the ratio of ambient charged particle to steady-state charged particle concentration, known as the charging state. We analyzed the influence of different parameters on the contribution of ion-induced nucleation to the total particle formation rate. We found that the fraction of ion-induced nucleation is typically higher on warmer, drier and sunnier days compared to colder days with less solar radiation and a higher relative humidity. Also, we observed that bigger concentrations of new particles were produced on days with a smaller fraction of ion-induced nucleation. Moreover, sulfuric acid saturation ratios were smaller for days with a bigger fraction of ion-induced nucleation. Finally, we propose explanations on how these different parameters could influence neutral and ion-induced nucleation, and show that the different mechanisms seem to take place at the same time during an event. For example, we propose that these observed differences could be due to high temperature and low vapors' saturation ratios (water and sulfuric acid) increasing the height of the energy barrier a particle has to reach before it can grow and thus limiting neutral nucleation.
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| 6. |
- Heue, K. -P., et al.
(författare)
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Observations of the 2008 Kasatochi volcanic SO2 plume by CARIBIC aircraft DOAS and the GOME-2 satellite
- 2010
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Ingår i: Atmospheric Chemistry And Physics. - Copernicus. - 1680-7316. ; 10:10, s. 4699-4713
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Tidskriftsartikel (refereegranskat)abstract
- The 2008 Kasatochi volcanic eruption emitted 1.5-2.5 Tg SO2 into the upper troposphere and lower stratosphere. Parts of the main volcanic plume (gases and particles) reached central Europe a week after the eruption and were detected there by the CARIBIC (Civil Aircraft for Regular investigation of the Atmosphere based on an Instrument Container) flying observatory. The plume was also observed by the GOME-2 satellite instrument, only a few hours after the CARIBIC aircraft had crossed the plume, thus giving a unique opportunity to compare results. Trajectories and local wind speeds are investigated in detail using the GOME-2 and CARIBIC observations for better comparison of the results from these two observational systems. A comparison of the satellite spatial pattern with the local observations of the wind speed and the trajectory model TRAJKS showed a slight discrepancy, which has to be considered for satellite validation. Hence, it appears that detailed analyses of wind speeds are required. Emitted and secondary particles, partly measured and sampled by the CARIBIC in situ instruments, affected the DOAS SO2 measurements, of both CARIBIC and GOME-2. Overall GOME-2 and the CARIBIC SO2 measurements agree very well. The major uncertainties remain the actual wind speed needed to properly correct for the advection of the plume between the different overpass times and effects of aerosols on DOAS retrievals. The good agreement can be seen as validation for both GOME-2 and CARIBIC DOAS observations.
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| 7. |
- Holst, Thomas, et al.
(författare)
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BVOC ecosystem flux measurements at a high latitude wetland site
- 2010
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Ingår i: Atmospheric Chemistry and Physics. - 1680-7316. ; 10:4, s. 1617-1634
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we present summertime concentrations and fluxes of biogenic volatile organic compounds (BVOCs) measured at a sub-arctic wetland in northern Sweden using a disjunct eddy-covariance (DEC) technique based on a proton transfer reaction mass spectrometer (PTR-MS). The vegetation at the site was dominated by Sphagnum, Carex and Eriophorum spp. The measurements reported here cover a period of 50 days (1 August to 19 September 2006), approximately one half of the growing season at the site, and allowed to investigate the effect of day-to-day variation in weather as well as of vegetation senescence on daily BVOC fluxes, and on their temperature and light responses. The sensitivity drift of the DEC system was assessed by comparing H3O+-ion cluster formed with water molecules (H3O+(H2O) at m37) with water vapour concentration measurements made using an adjacent humidity sensor, and the applicability of the DEC method was analysed by a comparison of sensible heat fluxes for high frequency and DEC data obtained from the sonic anemometer. These analyses showed no significant PTR-MS sensor drift over a period of several weeks and only a small flux-loss due to high-frequency spectrum omissions. This loss was within the range expected from other studies and the theoretical considerations. Standardised (20 degrees C and 1000 mu mol m(-2) s(-1) PAR) summer isoprene emission rates found in this study of 329 mu g Cm-2 (ground area) h(-1) were comparable with findings from more southern boreal forests, and fen-like ecosystems. On a diel scale, measured fluxes indicated a stronger temperature dependence than emissions from temperate or (sub) tropical ecosystems. For the first time, to our knowledge, we report ecosystem methanol fluxes from a sub-arctic ecosystem. Maximum daytime emission fluxes were around 270 mu g m(-2) h(-1) (ca. 100 mu g Cm-2 h(-1)), and during most nights small negative fluxes directed from the atmosphere to the surface were observed.
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| 8. |
- Khosrawi, Farahnaz, et al.
(författare)
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Particle formation in the Arctic free troposphere during the ASTAR 2004 campaign: A case study on the influence of vertical motion on the binary homogeneous nucleation of H2SO4/H2O
- 2010
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Ingår i: Atmospheric Chemistry And Physics. - 1680-7316. ; 10, s. 1105-1120
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Tidskriftsartikel (refereegranskat)abstract
- During the ASTAR (Arctic Study of Tropospheric Aerosol and Radiation) campaign nucleation mode particles (4 to 13 nm) were quite frequently observed at altitudes below 4000 m. However, in the upper free troposphere, nucleation mode particles were only observed once, namely during the flight on 24 May 2004 (7000 m). To investigate if vertical motion are the reason for this difference that on one particular day nucleation mode particles were observed but not on the other days we employ a microphysical box model. The box model simulations were performed along air parcel trajectories calculated 6-d backwards based on European Center for Medium-Range Weather Forecasts (ECMWF) meteorological analyses using state parameters such as pressure and temperature in combination with additional parameters such as vertical stability. Box model simulations were performed for the 24 May where nucleation mode particles were observed (nucleation event) as well as for the day with measurements before and after (22 and 26 May) which are representative for no nucleation (none nucleation event). A nucleation burst was simulated along all trajectories, however, in the majority of the simulations the nucleation rate was either too low or too high so that no nucleation mode particles were left at the time were the measurements were performed. Further, the simulation results could be divided into three cases. Thereby, we found that for case 1 the temperature was the only driving mechanism while for case 2 and 3 vertical motion have influenced the formation of new particles. The reason why nucleation mode particles were observed on 24 May, but not on the other day, can be explained by the conditions under which particle formation occurred. On 24 May the particle formation was caused by a slow updraft, while on the other two days the particle formation was caused by a fast updraft.
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| 9. |
- Kuhn, Thomas, et al.
(författare)
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Characterising aerosol transport into the Canadian high Arctic using aerosol mass spectrometry and Lagrangian modelling
- 2010
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Ingår i: Atmospheric Chemistry and Physics. - Copernicus GmbH. - 1680-7316. ; 10:21
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Tidskriftsartikel (övrigt vetenskapligt)abstract
- We report the analysis of measurements made using an aerosol mass spectrometer (AMS; Aerodyne Research Inc.) that was installed in the Polar Environment Atmospheric Research Laboratory (PEARL) in summer 2006. PEARL is located in the Canadian high Arctic at 610 m above sea level on Ellesmere Island (80° N 86° W). PEARL is unique for its remote location in the Arctic and because most of the time it is situated within the free troposphere. It is, therefore, well suited as a receptor site to study the long-range tropospheric transport of pollutants into the Arctic. Some information about the successful year-round operation of an AMS at a high Arctic site such as PEARL will be reported here, together with design considerations for reliable sampling under harsh low-temperature conditions. Computational fluid dynamics calculations were made to ensure that sample integrity was maintained while sampling air at temperatures that average 40 °C in the winter and can be as low as 55 °C. Selected AMS measurements of aerosol mass concentration, size and chemical composition recorded during the months of August, September and October 2006 will be reported. The air temperature was raised to about 20 deg;C during sampling, but the short residence time in the inlet system (∼25 s) ensured that less than 10% of semivolatiles such as ammonium nitrate were lost. During this period, sulfate was, at most times, the predominant aerosol component with on average 0.115 μg-3 (detection limit 0.003mg-3). The second most abundant component was undifferentiated organic aerosol, with on average 0.11 Î1/4g mg3 (detection limit 0.04 I1/4g mg3). The nitrate component, which averaged 0.007 mg-3, was above its detection limit (0.002 Î1/4g mg3), whereas the ammonium ion had an apparent average concentration of 0.02 g mg-3, which was approximately equal to its detection limit. A few episodes, having increased mass concentrations and lasting from several hours to several days, are apparent in the data. These were investigated further using a statisticalanalysis to determine their common characteristics. High correlations among some of the components arriving during the short-term episodes provide evidence for common sources. Lagrangian methods were also used to identify the source regions for some of the episodes. In all cases, these coincided with the arrival of air that had contacted the surface at latitudes below about 60° N. Most of these lower-latitude footprints were on land, but sulfate emissions from shipping in the Atlantic were also detected. The Lagrangian results demonstrate that there is direct transport of polluted air into the high Arctic (up to 80° N) from latitudes down to 40° N on a time scale of 2-3 weeks. The polluted air originates in a wide variety of industrial, resource extraction and petroleum-related activity as well as from large population centres
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| 10. |
- Lohmann, U., et al.
(författare)
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Total aerosol effect : radiative forcing or radiative flux perturbation?
- 2010
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Ingår i: Atmospheric Chemistry And Physics. - 1680-7316. ; 10:7, s. 3235-3246
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Tidskriftsartikel (refereegranskat)abstract
- Uncertainties in aerosol radiative forcings, especially those associated with clouds, contribute to a large extent to uncertainties in the total anthropogenic forcing. The interaction of aerosols with clouds and radiation introduces feedbacks which can affect the rate of precipitation formation. In former assessments of aerosol radiative forcings, these effects have not been quantified. Also, with global aerosol-climate models simulating interactively aerosols and cloud microphysical properties, a quantification of the aerosol forcings in the traditional way is difficult to define properly. Here we argue that fast feedbacks should be included because they act quickly compared with the time scale of global warming. We show that for different forcing agents (aerosols and greenhouse gases) the radiative forcings as traditionally defined agree rather well with estimates from a method, here referred to as radiative flux perturbations (RFP), that takes these fast feedbacks and interactions into account. Based on our results, we recommend RFP as a valid option to compare different forcing agents, and to compare the effects of particular forcing agents in different models.
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