Amir A. Aliabadi

Characterization and Parametrization of Reynolds Stress and Turbulent Heat Flux in the Stably-Stratified Lower Arctic Troposphere Using Aircraft Measurements

Aircraft measurements are used to characterize properties of clear-air turbulence in the lower Arctic troposphere. For typical vertical resolutions in general circulation models, there is evidence for both downgradient and countergradient vertical turbulent transport of momentum and heat in the mostly statically stable conditions within both the boundary layer and the free troposphere. Countergradient transport is enhanced in the free troposphere compared to the boundary layer. Three parametrizations are suggested to formulate the turbulent heat flux and are evaluated using the observations. The parametrization that accounts for the anisotropic nature of turbulence and buoyancy flux predicts both observed downgradient and countergradient transport of heat more accurately than those that do not. The inverse turbulent Prandtl number is found to only weakly decrease with increasing gradient Richardson number in a statistically significant way, but with large scatter in the data. The suggested parametrizations can potentially improve the performance of regional and global atmospheric models.

Evidence for marine biogenic influence on summertime Arctic aerosol

We present vertically-resolved observations of aerosol composition during pristine summertime Arctic background conditions. The methansulfonic acid (MSA)-to-sulfate ratio peaked near the surface (mean 0.10), indicating a contribution from ocean-derived biogenic sulfur. Similarly, the organic aerosol (OA)-to-sulfate ratio increased towards the surface (mean 2.0). Both MSA-to-sulfate and OA-to-sulfate ratios were significantly correlated with FLEXPART-WRF-predicted airmass residence time over open water, indicating marine influenced OA. External mixing of sea salt aerosol from a larger number fraction of organic, sulfate and amine-containing particles, together with low wind speeds (median 4.7 m s−1), suggests a role for secondary organic aerosol formation. Cloud condensation nuclei concentrations were nearly constant (∼120 cm−3) when the OA fraction was <60% and increased to 350 cm−3 when the organic fraction was larger and residence times over open water were longer. Our observations illustrate the importance of marine-influenced OA under Arctic background conditions, which are likely to change as the Arctic transitions to larger areas of open water.

Ventilation strategies and air quality management in passenger aircraft cabins: A review of experimental approaches and numerical simulations

The cabins of passenger aircraft experience one of the most complex indoor environments among all other means of mass public transportation. Given the large number of passengers that use these aircraft each day worldwide, suitable ventilation strategies for air quality management must be employed to control the spread of airborne contaminants while ensuring passenger comfort. In the current review article, the different ventilation strategies (existing and proposed) used aboard commercial aircraft, and the common airborne contaminants encountered in cabins are discussed through a critical survey of key studies performed mainly in the last two decades. The research methodologies adopted by these studies, which vary from experimental measurements to numerical simulations, are also analyzed in a systematic manner. Based on the available literature, best practices for aircraft ventilation and air quality research are identified for each research methodology. Current research gaps are also discussed and future research topics are suggested for various research methodologies.

Flow and temperature dynamics in an urban canyon under a comprehensive set of wind directions, wind speeds, and thermal stability conditions

Atmospheric flow and temperature dynamics in the urban roughness sublayer exhibit numerous complexities that cannot all be investigated using models or scaled-down experiments, thus these complexities necessitate careful field observations. Such dynamics were studied under a comprehensive set of wind directions, wind speeds, and thermal stability conditions in a field campaign held in Guelph, Ontario, Canada, from 13th to 25th of August 2017. The urban site was a quasi-two-dimensional canyon with unit canyon aspect ratio. Beside characterizing thermal stability, inertial effects, urban heat island intensity, and mean properties of the atmosphere, the turbulence statistics were studied carefully as functions of roof-level wind angle, diurnal time, the building Reynolds number, or the bulk Richardson number. Turbulence statistics in the vertical direction were observed to be influenced by local conditions, such as flow properties and nearby surface temperatures. These statistics also indicated presence of small integral lengthscales and short integral timescales. On the other hand, turbulence statistics in the horizontal directions were influenced by non-local conditions, such as horizontal heterogeneity in heating or urban morphology. These statistics indicated presence of large integral lengthscales and long integral timescales. In addition, a rigorous scaling analysis was performed to seek significant correlations between turbulence statistics and other known flow variables both locally or as measured at a nearby non-urban rural station. Variances scaled more successfully to mean quantities than covariances (Reynolds stresses and turbulence kinematic heat fluxes). In addition, the statistics in the vertical direction scaled more successfully compared to horizontal directions. Statistics in the horizontal directions, particularly in the along-canyon axis direction, were poorly scaled, suggesting presence of unorganized and irregular turbulence structures influenced by non-local conditions. Temperature difference between the atmosphere and nearby surfaces as well as measured velocity scales showed to scale vertical heat fluxes successfully.

Aircraft measurements of High Arctic springtime aerosol show evidence for vertically varying sources, transport and composition

The sources, chemical transformations and removal mechanisms of aerosol transported to the Arctic are key factors that control Arctic aerosol-climate interactions. Our understanding of sources and processes is limited by a lack of vertically resolved observations in remote Arctic regions. We present vertically resolved observations of trace gases and aerosol composition in High Arctic springtime, made largely north of 80◦N, during the NETCARE campaign. Trace gas gradients observed on these flights defined the polar dome as north of 66 – 68.5◦N and below potential temperatures of 283.5 – 287.5 K (Bozem et al., 5 2018). In the polar dome, we observe evidence for vertically varying source regions and chemical processing. These vertical changes in sources and chemistry lead to systematic variation in aerosol composition as a function of potential temperature. We show evidence for sources of aerosol with higher organic aerosol (OA), ammonium (NH4) and refractory black carbon (rBC) content in the upper polar dome. Based on FLEXPART-ECMWF calculations, air masses sampled at all levels inside the polar dome (i.e., potential temperature < 280.5 K, altitude < ∼3.5 km) subsided during transport over transport times of at 10 least 10 days. Air masses at the lowest potential temperatures, in the lower polar dome, had spent long times (>10 days) in the Arctic, while air masses in the upper polar dome had entered the Arctic more recently. These differences in transport history were closely related to aerosol composition. In the lower polar dome, the measured sub-micron aerosol mass was dominated by sulphate (mean 74%), with lesser contributions from rBC (1%), NH4 (4%) and OA (20%). At higher altitudes and warmer potential temperatures, OA, NH4 and rBC contributed 42%, 8% and 2% of aerosol mass, respectively. A qualitative indication 15 for the presence of sea salt showed that sodium chloride contributed to sub-micron aerosol in the lower polar dome, but was not detectable in the upper polar dome. Our observations suggest that long-term, surface-based measurements underestimate the contribution of OA, rBC and NH4 to aerosol transported to the Arctic troposphere in spring. 1 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-628 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 24 August 2018 c

Modelling Regional Air Quality in the Canadian Arctic: Simulation of an Arctic Summer Field Campaign

Model simulations of an Arctic summer field campaign were carried out. The model results were compared with observational data from both ground-based monitoring and in situ measurements on-board multiple mobile platforms. The model was able to well capture regional sources and transport affecting the Arctic air quality. It is shown that the study area was impacted by North American (NA) regional biomass burning emissions. The model-observation comparison also corroborates previous findings on possible roles of marine-biogenic sources in aerosol production in the Arctic MBL during summertime.