| 1. |
- Engvall, Ann-Christine, et al.
(författare)
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Changes in aerosol properties during spring-summer period in the Arctic troposphere
- 2008
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Ingår i: Atmospheric Chemistry And Physics. - 1680-7316 .- 1680-7324. ; 8:3, s. 445-462
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Tidskriftsartikel (refereegranskat)abstract
- The change in aerosol properties during the transition from the more polluted spring to the clean summer in the Arctic troposphere was studied. A six-year data set of observations from Ny-Ålesund on Svalbard, covering the months April through June, serve as the basis for the characterisation of this time period. In addition four-day-back trajectories were used to describe air mass histories. The observed transition in aerosol properties from an accumulation-mode dominated distribution to an Aitken-mode dominated distribution is discussed with respect to long-range transport and influences from natural and anthropogenic sources of aerosols and pertinent trace gases. Our study shows that the air-mass transport is an important factor modulating the physical and chemical properties observed. However, the air-mass transport cannot alone explain the annually repeated systematic and rather rapid change in aerosol properties, occurring within a limited time window of approximately 10 days. With a simplified phenomenological model, which delivers the nucleation potential for new-particle formation, we suggest that the rapid shift in aerosol microphysical properties between the Arctic spring and summer is mainly driven by the incoming solar radiation in concert with transport of precursor gases and changes in condensational sink.
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| 2. |
- Engvall, Ann-Christine, et al.
(författare)
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In-situ airborne observations of the microphysical properties of the Arctic tropospheric aerosol during late spring and summer
- 2008
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Ingår i: Tellus. Series B, Chemical and physical meteorology. - : Stockholm University Press. - 0280-6509 .- 1600-0889. ; 60:3, s. 392-404
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Tidskriftsartikel (refereegranskat)abstract
- In-situ aerosol data collected in the Arctic troposphere during a three-week period in 2004 were analysed. The measurements took place during late spring, i.e., at the time of the year when the characteristics of the aerosol distribution change from being accumulation-mode dominated to being primarily of the Aitken-mode type, a process that previously has been observed in the boundary layer. To address the question whether this transition is also detectable in the free troposphere of an aircraft-measured data from the ASTAR 2004 campaign were analysed. In this study, we present vertically as well as temporally results from both ground-based and airborne measurements of the total number concentrations of particles larger than 10 and 260 nm. Aircraft-measured size distributions of the aerosol ranging from 20 to 2200 nm have been evaluated with regard to conditions in the boundary layer as well as in the free troposphere. Furthermore an analysis of the volatile fraction of the aerosol population has been performed both for the integrated and size-distributed results. From these investigations we find that the transition takes place in the entire troposphere.
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| 3. |
- Engvall, Ann-Christine, 1976-
(författare)
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Properties and Origin of Arctic Aerosols
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The present thesis deals with the origin and physics of aerosols in the Arctic atmosphere. These show a large annual variability due to changes of the photochemical and cloud processes as well as of the synoptic-scale atmospheric pressure patterns. High concentrations of anthropogenic trace gases and particles are found in the atmosphere during winter and spring, whereas the summer period is least affected as regards human impact. The thesis is based on a synthesis of aerosol observations from ground stations as well as research aircraft. A major goal was to study the shift that the Arctic aerosol-size distribution undergoes from spring to summer, a transition that takes place during a rather short period of around 10 days. Six years of aerosol, chemical, and transport data are investigated for the April-June period. This analysis indicates that the rapid transition is governed by a delicate balance between insolation and the source and sink processes affecting the aerosol. In-situ observations show that exchange processes between the boundary layer and the free troposphere may be a key component governing the temporal evolution of the aerosol during summer. It has been concluded that air-borne measurements are essential for establishing the vertical distribution of the aerosol (knowledge of which may be essential when analysing long-term and point measurements). As emphasized in the thesis, insights concerning this vertical structure are especially valuable when layers aloft show concentrations of soot or light-absorbing aerosol and, in addition, the environment is highly reflecting, as is the case in the Arctic. Such plumes, transported from lower latitudes and difficult to detect from the surface, are suggested to have contributed to the high-altitude Arctic warming trend observed during the last two decades. The results in this thesis underline that merging long-term observations with aircraft measurements is highly useful when studying aerosol and its effects.
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| 4. |
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| 5. |
- Engvall, Ann-Christine, et al.
(författare)
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The radiative effect of an aged, internally mixed Arctic aerosol originating from lower-latitude biomass burning
- 2009
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Ingår i: Tellus. Series B, Chemical and physical meteorology. - : Stockholm University Press. - 0280-6509 .- 1600-0889. ; 61:4, s. 677-684
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Tidskriftsartikel (refereegranskat)abstract
- Arctic-haze layers and their radiative effects have been investigated previously in numerous studies as they are known to have an impact on the regional climate. In this study, we report on an event of an elevated aerosol layer, notably consisting of high-absorbing soot particles, observed in the European Arctic free troposphere the 2007 April 14 during the ASTAR 2007 campaign. The ca. 0.5 km vertically thick aerosol layer located at an altitude of around 3 km had a particle-size distribution mode around 250 nm diameter. In this study, we quantify the radiative effect aerosol layers have on the Arctic atmosphere by using in situ observations. Measurements of particles size segregated temperature stability using thermal denuders, indicate that the aerosol in the optically active size range was chemically internally mixed. In the plume, maximum observed absorption and scattering coefficients were 3 x 10(-6) and 20 x 10(-6) m(-1), respectively. Observed microphysical and optical properties were used to constrain calculations of heating rates of an internally mixed aerosol assuming two different surface albedos that represent snow/ice covered and open ocean. The average profile resulted in a heating rate in the layer of 0.2 K d(-1) for the high-albedo case and 0.15 K d(-1) for the low albedo case. This calculated dependence on albedo based on actual observations corroborates previous numerical simulations. The heating within the plume resulted in a measurable signal shown as an enhancement in the temperature of a few tenths of a degree. Although the origin of the aerosol plume could not unambiguously be determined, the microphysical properties of the aerosol had strong similarities with previously reported biomass burning plumes. With a changing climate, short-lived pollutants such as biomass plumes may become more frequent in the Arctic and have important radiative effects at regional scale.
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| 6. |
- Stjernberg, Ann-Christine Engvall, et al.
(författare)
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Low concentrations of near-surface ozone in Siberia
- 2012
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Ingår i: Tellus. Series B, Chemical and physical meteorology. - : Stockholm University Press. - 0280-6509 .- 1600-0889. ; 64, s. 1-13
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Tidskriftsartikel (refereegranskat)abstract
- Siberia with its large area covered with boreal forests, wetlands and tundra is believed to be an important sink for ozone via dry deposition and reactions with biogenic volatile organic compounds (BVOCs) emitted by the forests. To study the importance of deposition of ozone in Siberia, we analyse measurements of ozone mixing ratios taken along the Trans-Siberian railway by train, air-borne measurements and point measurements at the Zotino station. For all data, we ran the Lagrangian particle dispersion model FLEXPART in backward mode for 20 d, which yields the so-called potential emission sensitivity (PES) fields. These fields give a quantitative measure of where and how strongly the sampled air masses have been in contact with the surface and hence possible influenced by surface fluxes. These fields are further statistically analysed to identify source and sink regions that are influencing the observed ozone. Results show that the source regions for the surface ozone in Siberia are located at lower latitudes: the regions around the Mediterranean Sea, the Middle East, Kazakhstan and China. Low ozone mixing ratios are associated to transport from North West Russia, the Arctic region, and the Pacific Ocean. By calculating PES values for both a passive tracer without consideration of removal processes and for an ozone-like tracer where dry deposition processes are included, we are able to quantify the ozone loss occurring en route to the receptor. Strong correlations between low ozone concentrations and the spatially integrated footprints from FLEXPART, especially during the period summer to autumn, indicate the importance of the Siberian forests as a sink for tropospheric ozone.
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| 7. |
- Ström, Johan, et al.
(författare)
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On small particles in the Arctic summer boundary layer : observations at two different heights near Ny-Ålesund, Svalbard
- 2008
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Ingår i: Tellus. Series B, Chemical and physical meteorology. - : Stockholm University Press. - 0280-6509 .- 1600-0889. ; 61:2, s. 473-482
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Tidskriftsartikel (refereegranskat)abstract
- Concurrent observations of particle number densities and size distributions observed at two different heights (near ocean level and 475 m above sea level) in Ny-Ålesund, Svalbard were studied with respect to the diurnal variation during a summer period in June 2004. The results show that observed variation in particle number density in the Arctic boundary layer may be strongly modulated by vertical mixing and dilution. The particles appeared to be formed in the early morning when solar intensity reached about 30% of the mid-day intensity. Based on differences in the observed number densities at the two heights it appears as if particles are formed in the lower part of the boundary layer. The formation rate of 10 nm diameter particles is estimated to be 0.11 cm−3 s−1 and the growth rate is in a range between 1 and 2 nm h−1.
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