Browsing by Autor "A. Gannet Hallar"

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Identification of topographic features influencing aerosol observations at high altitude stations
(Copernicus Publications, 2018) Martine Collaud Coen; Elisabeth Andrews; Diego Aliaga; Marcos Andrade; Hristo Angelov; Nicolas Bukowiecki; Marina Ealo; Paulo Fialho; H. Flentje; A. Gannet Hallar
Abstract. High altitude stations are often emphasized as free tropospheric measuring sites but they remain influenced by atmospheric boundary layer (ABL) air masses due to convective transport processes. The local and meso-scale topographical features around the station are involved in the convective boundary layer development and in the formation of thermally induced winds leading to ABL air lifting. The station altitude alone is not a sufficient parameter to characterize the ABL influence. In this study, a topography analysis is performed allowing calculation of a newly defined index called ABL-TopoIndex. The ABL-TopoIndex is constructed in order to correlate with the ABL influence at the high altitude stations and long-term aerosol time series are used to assess its validity. Topography data from the global digital elevation model GTopo30 were used to calculate five parameters for 43 high and 3 middle altitude stations situated on five continents. The geometric mean of these five parameters determines a topography based index called ABL-TopoIndex, which can be used to rank the high altitude stations as a function of the ABL influence. To construct the ABL-TopoIndex, we rely on the criteria that the ABL influence will be low if the station is one of the highest points in the mountainous massif, if there is a large altitude difference between the station and the valleys or high plains, if the slopes around the station are steep, and finally if the inverse drainage basin potentially reflecting the source area for thermally lifted pollutants to reach the site is small. All stations on volcanic islands exhibit a low ABL-TopoIndex, whereas stations in the Himalayas and the Tibetan Plateau have high ABL-TopoIndex values. Spearman's rank correlation between aerosol optical properties and number concentration from 28 stations and the ABL-TopoIndex, the altitude and the latitude are used to validate this topographical approach. Statistically significant (SS) correlations are found between the 5th and 50th percentiles of all aerosol parameters and the ABL-TopoIndex, whereas no SS correlation is found with the station altitude. The diurnal cycles of aerosol parameters seem to be best explained by the station latitude although a SS correlation is found between the amplitude of the diurnal cycles of the absorption coefficient and the ABL-TopoIndex.
Identification of water-soluble organic carbon in non-urban aerosols using ultrahigh-resolution FT-ICR mass spectrometry: organic anions
(CSIRO Publishing, 2012) Lynn Mazzoleni; Parichehr Saranjampour; Megan M. Dalbec; Vera Samburova; A. Gannet Hallar; Barbara Zielińska; Douglas H. Lowenthal; Steve Kohl
Environmental context Aerosol water-soluble organic carbon is a complex mixture of thousands of organic compounds which may have a significant influence on the climate-relevant properties of atmospheric aerosols. Using ultrahigh resolution mass spectrometry, more than 4000 individual molecular formulas were identified in non-urban aerosol water-soluble organic carbon. A significant fraction of the assigned molecular formulas were matched to assigned molecular formulas of laboratory generated secondary organic aerosols. Abstract Water-soluble organic carbon (WSOC) is a complex mixture of thousands of organic compounds which may have significant influence on the climate-relevant properties of atmospheric aerosols. An improved understanding of the molecular composition of WSOC is needed to evaluate the effect of aerosol composition upon aerosol physical properties. In this work, ultrahigh-resolution Fourier transform–ion cyclotron resonance mass spectrometry (FT-ICR MS) was used to characterise aerosol WSOC collected during the summer of 2010 at the Storm Peak Laboratory (3210 m ASL) near Steamboat Springs, CO. Approximately 4000 molecular formulas were assigned in the mass range of 100–800 Da after negative-ion electrospray ionisation and more than 50 % of them contained nitrogen or sulfur. The double bond equivalents (DBEs) of the molecular formulas were inversely proportional to the O : C ratio, despite a relatively constant H : C ratio of ~1.5. Despite the range of DBE values, the elemental ratios and the high number of oxygen atoms per formula indicate that a majority of the compounds are aliphatic to olefinic in nature. These trends indicate significant non-oxidative accretion reaction pathways for the formation of high molecular weight WSOC components. In addition, a significant number of molecular formulas assigned in this work matched those previously identified as secondary organic aerosol components of monoterpene and sesquiterpene ozonolysis.
The topography contribution to the influence of the atmospheric boundary layer at high altitude stations
(2017) Martine Collaud Coen; Elisabeth Andrews; Diego Aliaga; Marcos Andrade; Hristo Angelov; Nicolas Bukowiecki; Marina Ealo; Paulo Fialho; H. Flentje; A. Gannet Hallar
Abstract. High altitude stations are often emphasized as free tropospheric measuring sites but they remain influenced by atmospheric boundary layer (ABL) air masses due to convective transport processes. The local and meso-scale topographical features around the station are involved in the convective boundary layer development and in the formation of thermally induced winds leading to ABL air lifting. The station altitude is not a sufficient parameter to characterize the ABL influence. Topography data from the global digital elevation model GTopo30 were used to calculate 5 parameters for 46 high altitude stations situated in five continents. The geometric mean of these 5 parameters determines a topography based index called ABL-TopoIndex which can be used to rank the high altitude stations as a function of the ABL influence. To construct the ABL-TopoIndex, we rely on the criteria that the ABL influence will be low if the station is one of the highest points in the mountainous massif, if there is a large altitude difference between the station and the valleys or plateaus, if the slopes around the station are steep, and finally if the drainage basin for air convection is small. All stations on volcanic islands exhibit a low ABL-TopoIndex whereas stations in the Himalaya and the Tibetan Plateau have high ABL-TopoIndex values. Spearman's rank correlation between aerosol optical properties and number concentration from 28 stations and the ABL-TopoIndex, the altitude and the latitude are used to validate this topographical approach. Statistically significant (s.s.) correlations are found between the 5 and 50 percentiles of all aerosol parameters and the ABL-TopoIndex whereas no s.s. correlation is found with the station altitude. The diurnal cycles of aerosol parameters seem to be best explained by the station latitude although a s.s. correlation is found between the amplitude of the diurnal cycles of the absorption coefficient and the ABL-TopoIndex. Finally, the main flow paths for air convection were calculated for various ABL heights.