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  • Bartels-Rausch, T., et al. (författare)
  • A review of air-ice chemical and physical interactions (AICI): Liquids, quasi-liquids, and solids in snow
  • 2014
  • Ingår i: Atmospheric Chemistry And Physics. - 1680-7316 .- 1680-7324. ; 14:3, s. 1587-1633
  • Tidskriftsartikel (refereegranskat)abstract
    • Snow in the environment acts as a host to rich chemistry and provides a matrix for physical exchange of contaminants within the ecosystem. The goal of this review is to summarise the current state of knowledge of physical processes and chemical reactivity in surface snow with relevance to polar regions. It focuses on a description of impurities in distinct compartments present in surface snow, such as snow crystals, grain boundaries, crystal surfaces, and liquid parts. It emphasises the microscopic description of the ice surface and its link with the environment. Distinct differences between the disordered air-ice interface, often termed quasi-liquid layer, and a liquid phase are highlighted. The reactivity in these different compartments of surface snow is discussed using many experimental studies, simulations, and selected snow models from the molecular to the macro-scale. Although new experimental techniques have extended our knowledge of the surface properties of ice and their impact on some single reactions and processes, others occurring on, at or within snow grains remain unquantified. The presence of liquid or liquid-like compartments either due to the formation of brine or disorder at surfaces of snow crystals below the freezing point may strongly modify reaction rates. Therefore, future experiments should include a detailed characterisation of the surface properties of the ice matrices. A further point that remains largely unresolved is the distribution of impurities between the different domains of the condensed phase inside the snowpack, i.e. in the bulk solid, in liquid at the surface or trapped in confined pockets within or between grains, or at the surface. While surface-sensitive laboratory techniques may in the future help to resolve this point for equilibrium conditions, additional uncertainty for the environmental snowpack may be caused by the highly dynamic nature of the snowpack due to the fast metamorphism occurring under certain environmental conditions. Due to these gaps in knowledge the first snow chemistry models have attempted to reproduce certain processes like the long-term incorporation of volatile compounds in snow and firn or the release of reactive species from the snowpack. Although so far none of the models offers a coupled approach of physical and chemical processes or a detailed representation of the different compartments, they have successfully been used to reproduce some field experiments. A fully coupled snow chemistry and physics model remains to be developed. © Author(s) 2014.
  • Darbieu, C., et al. (författare)
  • Turbulence vertical structure of the boundary layer during the afternoon transition
  • 2015
  • Ingår i: Atmospheric Chemistry And Physics. - 1680-7316 .- 1680-7324. ; 15:17, s. 10071-10086
  • Tidskriftsartikel (refereegranskat)abstract
    • We investigate the decay of planetary boundary layer (PBL) turbulence in the afternoon, from the time the surface buoyancy flux starts to decrease until sunset. Dense observations of mean and turbulent parameters were acquired during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field experiment by several meteorological surface stations, sounding balloons, radars, lidars and two aircraft during the afternoon transition. We analysed a case study based on some of these observations and large-eddy simulation (LES) data focusing on the turbulent vertical structure throughout the afternoon transition. The decay of turbulence is quantified through the temporal and vertical evolution of (1) the turbulence kinetic energy (TKE), (2) the characteristic length scales of turbulence and (3) the shape of the turbulence spectra. A spectral analysis of LES data, airborne and surface measurements is performed in order to characterize the variation in the turbulent decay with height and study the distribution of turbulence over eddy size. This study highlights the LES ability to reproduce the turbulence evolution throughout the afternoon. LESs and observations agree that the afternoon transition can be divided in two phases: (1) a first phase during which the TKE decays at a low rate, with no significant change in turbulence characteristics, and (2) a second phase characterized by a larger TKE decay rate and a change in spectral shape, implying an evolution of eddy size distribution and energy cascade from low to high wave number. The changes observed either in TKE decay (during the first phase) or in the vertical wind spectra shape (during the second phase of the afternoon transition) occur first in the upper region of the PBL. The higher within the PBL, the stronger the spectra shape changes.
  • Gaita, Samuel Mwaniki, et al. (författare)
  • Source apportionment and seasonal variation of PM2.5 in a Sub-Saharan African city: Nairobi, Kenya
  • 2014
  • Ingår i: Atmospheric Chemistry And Physics. - 1680-7316 .- 1680-7324. ; 14, s. 9977-9991
  • Tidskriftsartikel (refereegranskat)abstract
    • Sources of airborne particulate matter and their seasonal variation in urban areas in Sub-Saharan Africa are poorly understood due to lack of long-term measurement data. In view of this, filter samples of airborne particulate matter (particle diameter ?2.5 ?m, PM2.5) were collected between May 2008 and April 2010 at two sites (urban background site and suburban site) within the Nairobi metropolitan area. A total of 780 samples were collected and analyzed for particulate mass, black carbon (BC) and 13 trace elements. The average PM2.5 concentration at the urban background site was 21±9.5 ?g m?3, whereas the concentration at the suburban site was 13±7.3 ?g m?3. The daily PM2.5 concentrations exceeded 25 ?g m?3 (the World Health Organization 24 h guideline value) on 29% of the days at the urban background site and 7% of the days at the suburban site. At both sites, BC, Fe, S and Cl accounted for approximately 80% of all detected elements. Positive matrix factorization analysis identified five source factors that contribute to PM2.5 in Nairobi, namely traffic, mineral dust, industry, combustion and a mixed factor (composed of biomass burning, secondary aerosol and aged sea salt). Mineral dust and traffic factors were related to approximately 74% of PM2.5. The identified source factors exhibited seasonal variation, apart from the traffic factor, which was prominently consistent throughout the sampling period. Weekly variations were observed in all factors, with weekdays having higher concentrations than weekends. The results provide information that can be exploited for policy formulation and mitigation strategies to control air pollution in Sub-Saharan African cities.
  • Hallquist, Åsa M., et al. (författare)
  • Particle and gaseous emissions from individual diesel and CNG buses
  • 2013
  • Ingår i: Atmospheric Chemistry And Physics. - 1680-7316 .- 1680-7324. ; 13:10, s. 5337-5350
  • Tidskriftsartikel (refereegranskat)abstract
    • In this study size-resolved particle and gaseous emissions from 28 individual diesel-fuelled and 7 compressed natural gas (CNG)-fuelled buses, selected from an in-use bus fleet, were characterised for real-world dilution scenarios. The method used was based on using CO2 as a tracer of exhaust gas dilution. The particles were sampled by using an extractive sampling method and analysed with high time resolution instrumentation EEPS (10 Hz) and CO2 with a non-dispersive infrared gas analyser (LI-840, LI-COR Inc. 1 Hz). The gaseous constituents (CO, HC and NO) were measured by using a remote sensing device (AccuScan RSD 3000, Environmental System Products Inc.). Nitrogen oxides, NOx, were estimated from NO by using default NO2/NOx ratios from the road vehicle emission model HBEFA3.1. The buses studied were diesel-fuelled Euro III–V and CNG-fuelled Enhanced Environmentally Friendly Vehicles (EEVs) with different after-treatment, including selective catalytic reduction (SCR), exhaust gas recirculation (EGR) and with and without diesel particulate filter (DPF). The primary driving mode applied in this study was accelerating mode. However, regarding the particle emissions also a constant speed mode was analysed. The investigated CNG buses emitted on average a higher number of particles but less mass compared to the diesel-fuelled buses. Emission factors for number of particles (EFPN) were EFPN, DPF = 4.4 ± 3.5 × 1014, EFPN, no DPF = 2.1 ± 1.0 × 1015 and EFPN, CNG = 7.8 ± 5.7 ×1015 kg fuel−1. In the accelerating mode, size-resolved emission factors (EFs) showed unimodal number size distributions with peak diameters of 70–90 nm and 10 nm for diesel and CNG buses, respectively. For the constant speed mode, bimodal average number size distributions were obtained for the diesel buses with peak modes of ~10 nm and ~60 nm. Emission factors for NOx expressed as NO2 equivalents for the diesel buses were on average 27 ± 7 g (kg fuel)−1 and for the CNG buses 41 ± 26 g (kg fuel)−1. An anti-relationship between EFNOx and EFPM was observed especially for buses with no DPF, and there was a positive relationship between EFPM and EFCO.
  • Ruuskanen, T. M., et al. (författare)
  • Concentrations and fluxes of aerosol particles during the LAPBIAT measurement campaign at Varrio field station
  • 2007
  • Ingår i: Atmospheric Chemistry And Physics. - 1680-7316 .- 1680-7324. ; 7:14, s. 3683-3700
  • Tidskriftsartikel (refereegranskat)abstract
    • The LAPBIAT measurement campaign took place in the Varrio SMEAR I measurement station located in Eastern Lapland in the spring of 2003 between 26 April and 11 May. In this paper we describe the measurement campaign, concentrations and fluxes of aerosol particles, air ions and trace gases, paying special attention to an aerosol particle formation event broken by a air mass change from a clean Arctic air mass with new particle formation to polluted one approaching from industrial areas of Kola Peninsula, Russia, lacking new particle formation. Aerosol particle number flux measurements show strong downward fluxes during that time. Concentrations of coarse aerosol particles were high for 1-2 days before the nucleation event (i.e. 28-29 April), very low immediately before and during the observed aerosol particle formation event (30 April) and increased moderately from the moment of sudden break of the event. In general particle deposition measurements based on snow samples show the same changes. Measurements of the mobility distribution of air ions showed elevated concentrations of intermediate air ions during the particle formation event. We estimated the growth rates in the nucleation mode size range. For particles <10 nm, the growth rate increases with size on 30 April. Dispersion modelling made with model SILAM support the conclusion that the nucleation event was interrupted by an outbreak of sulphate-rich air mass in the evening of 30 April that originated from the industry at Kola Peninsula, Russia. The results of this campaign highlight the need for detailed research in atmospheric transport of air constituents for understanding the aerosol dynamics.
  • Spolaor, A., et al. (författare)
  • Seasonality of halogen deposition in polar snow and ice
  • 2014
  • Ingår i: Atmospheric Chemistry And Physics. - 1680-7316 .- 1680-7324. ; 14
  • Tidskriftsartikel (refereegranskat)abstract
    • The atmospheric chemistry of iodine and bromine in Polar regions is of interest due to the key role of halogens in many atmospheric processes, particularly tropospheric ozone destruction. Bromine is emitted from the open ocean but is enriched above first-year sea ice during springtime bromine explosion events, whereas iodine emission is at- tributed to biological communities in the open ocean and hosted by sea ice. It has been previously demonstrated that bromine and iodine are present in Antarctic ice over glacial– interglacial cycles. Here we investigate seasonal variability of bromine and iodine in polar snow and ice, to evaluate their emission, transport and deposition in Antarctica and the Arc- tic and better understand potential links to sea ice. We find that bromine and iodine concentrations and Br enrichment (relative to sea salt content) in polar ice do vary seasonally in Arctic snow and Antarctic ice. Although seasonal vari- ability in halogen emission sources is recorded by satellite- based observations of tropospheric halogen concentrations, seasonal patterns observed in snowpack are likely also in- fluenced by photolysis-driven processes. Peaks of bromine concentration and Br enrichment in Arctic snow and Antarc- tic ice occur in spring and summer, when sunlight is present. A secondary bromine peak, observed at the end of summer, is attributed to bromine deposition at the end of the polar day. Iodine concentrations are largest in winter Antarctic ice strata, contrary to contemporary observations of summer maxima in iodine emissions. These findings support previous observations of iodine peaks in winter snow strata attributed to the absence of sunlight-driven photolytic re-mobilisation of iodine from surface snow. Further investigation is required to confirm these proposed mechanisms explaining observa- tions of halogens in polar snow and ice, and to evaluate the extent to which halogens may be applied as sea ice proxies.
  • Zdanowicz, Christian, 1966- (författare)
  • Historical black carbon deposition in the Canadian High Arctic: A 190-year long ice-core record from Devon Island
  • ????
  • Ingår i: Atmospheric Chemistry And Physics. - Copernicus Publications. - 1680-7316 .- 1680-7324.
  • Tidskriftsartikel (refereegranskat)abstract
    • Black carbon aerosol (BC) emitted from natural and anthropogenic sources (e.g., wildfires, coal burning) can contribute to magnify climate warming at high latitudes by darkening snow- and ice-covered surfaces, thus lowering their albedo. Modeling the atmospheric transport and deposition of BC to the Arctic is therefore important, and historical archives of BC accumulation in polar ice can help to validate such modeling efforts. Here we present a 190-year ice-core record of refractory BC (rBC) deposition on Devon ice cap, Canada, spanning calendar years 1810-1990, the first such record ever developed from the Canadian Arctic. The estimated mean deposition flux of rBC on Devon ice cap for 1963-1990 is 0.2 mg m-2 a-1, which is low compared to most Greenland ice-core sites over the same period. The Devon ice cap rBC record also differs from existing Greenland records in that it shows no evidence of a substantial increase in rBC deposition during the early-mid 20th century, which, for Greenland, has been attributed to mid-latitude coal burning emissions. The deposition of other contaminants such as sulfate and Pb increased on Devon ice cap in the 20th century but without a concomitant rise in rBC. Part of the difference with Greenland may be due to local factors such as wind scouring of winter snow at the coring site on Devon ice cap. Air back-trajectory analyses also suggest that Devon ice cap receives BC from more distant North American and Eurasian sources than Greenland, and aerosol mixing and removal during long-range transport over the Arctic Ocean likely masks some of the specific BC source-receptor relationships. Findings from this study underscore the large variability in BC aerosol deposition across the Arctic region that may arise from different transport patterns. This variability needs to be accounted for when estimating the large-scale albedo lowering effect of BC deposition on Arctic snow/ice.
  • Aalto, Tuula, et al. (författare)
  • Modeling atmospheric CO2 concentration profiles and fluxes above sloping terrain at a boreal site
  • 2006
  • Ingår i: Atmospheric Chemistry and Physics. - Copernicus Gesellschaft Mbh. - 1680-7316. ; 6:2, s. 303-314
  • Tidskriftsartikel (refereegranskat)abstract
    • CO2 fluxes and concentrations were simulated in the planetary boundary layer above subarctic hilly terrain using a three dimensional model. The model solves the transport equations in the local scale and includes a vegetation sub-model. A WMO/GAW background concentration measurement site and an ecosystem flux measurement site are located inside the modeled region at a hilltop and above a mixed boreal forest, respectively. According to model results, the concentration measurement at the hill site was representative for continental background. However, this was not the case for the whole model domain. Concentration at few meters above active vegetation represented mainly local variation. Local variation became inseparable from the regional signal at about 60-100 m above ground. Flow over hills changed profiles of environmental variables and height of inversion layer, however CO2 profiles were more affected by upwind land use than topography. The hill site was above boundary layer during night and inside boundary layer during daytime. The CO2 input from model lateral boundaries dominated in both cases. Daily variation in the CO2 assimilation rate was clearly seen in the CO2 profiles. Concentration difference between the hill site and the forest site was about 5 ppm during afternoon according to both model and measurements. The average modeled flux to the whole model region was about 40% of measured and modeled local flux at the forest site.
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