Centre for Atmospheric Chemistry
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Browsing Centre for Atmospheric Chemistry by Author "Aklilu, Y."
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Item Open Access Aerosol particle size distributions in the lower Fraser Valley: evidence for particle nucleation and growth.(Copernicus Publications, 2004) Mozurkewich, M.; Chan, T.W.; Aklilu, Y.; Verheggen, B.Particle size distributions from 9 to 640nm diameter were measured at Eagle Ridge in the lower Fraser Valley from 13 August to 1 September 2001 as part of the Pacific 2001 Air Quality Study. The site was on top of a ridge, about 300m above the valley floor, in a predominantly agricultural area about 70km ESE of Vancouver. To further characterize the particles, their hygroscopic properties (affinity for water) were measured. The maximum of the number distributions was generally between 40 and 100nm diameter, but the number distribution was sometimes dominated by ultrafine particles with diameters below 40nm. These ultrafine particles, which appeared to some extent on all days, were frequently associated with elevated levels of CO and NOx, as expected for fresh vehicular emissions. The appearance of these fresh emissions was most pronounced when the growing mixed layer reached the altitude of the site. In contrast, pronounced nucleation events occurred on the five cleanest days; these resulted in particle number concentrations as high as 5x104 particles cm-3 and growth rates of 5 to 10nmhr-1. Nucleation appears to have been triggered when the UV flux reached about 25Wm-2. The growth of these newly formed particles was probably driven by the photochemical oxidation of biogenic organic compounds. Dramatic growth events were also observed on the afternoons of the more polluted days; these produced an extremely narrow mode σ<0.3) at a diameter of about 40nm. Rainy days showed low number concentrations with the size distributions shifted to small sizes. On one of these days there was evidence of nucleation not far from the site; this may have been occurring in the vicinity of the clouds.Item Open Access Determination of external and internal mixing of organic and inorganic aerosol components from hygroscopic properties of sub micrometer particles during a field study in the lower Fraser Valley(Taylor & Francis, 2004) Aklilu, Y.; Mozurkewich, M.A tandem differential mobility analyzer (TDMA) system was developed to measure particle growth factors over a range of relative humidities from 50-85%. This system was deployed in the Lower Fraser Valley, and data was collected at Golden Ears Provincial Park and Eagle Ridge Mountain during the Pacific 2001 field study. Following humidification, particles either had a single hygroscopic mode or a bimodal distribution with more hygroscopic and less hygroscopic modes. Growth factors for the less hygroscopic mode were found to be consistent throughout the study. Monomodal distributions were the more common result. Their growth factor ranged between that of the less and the more hygroscopic modes. Using observed and calculated growth factors, relative organic and inorganic fractions were estimated. Particles at these sites were largely organic. Organic fractions near unity were found at Golden Ears forest and during rainy days at Eagle Ridge.Item Open Access Hygroscopicity of particles at two rural, urban influenced sites during Pacific 2001: Comparison with estimates of water uptake from particle composition(Elsevier, 2006) Aklilu, Y.; Mozurkewich, M.; Prenni, A.J.; Kreidenweis, S.M.; Alfarra, M.R.; Allan, J.D.; Anlauf, K.; Brook, J.; Leaitch, W.R.; Sharma, S.; Boudries, H.; Worsnop, D.R.Hygroscopicity of particles was measured at Langley (rural) and Eagle Ridge (semi-rural) as part of the Pacific 2001 field campaign. The measured growth factors at the two sites were comparable. However, differences in particle composition as measured by an Aerosol Mass Spectrometer were evident at these two sites. Sulphate mass concentration was found to be similar at the two sites, while higher nitrate and organic mass were observed at Eagle Ridge. Higher growth factors were observed when the air mass was impacted by SO2 sources, while lower growth factors were observed when the air mass was affected by urban emissions. To examine the hygroscopic role of the different particle components, expected growth factors were calculated from the composition data and compared to measured growth factors. Calculations were done using the Zdanovskii, Stokes and Robinson (ZSR) mixing rule. Sulphate fraction played a dominant role in particle hygroscopicity at both sites. Calculated growth factors were within the uncertainty of the measurements, except when the nitrate fraction was high. The results imply that particulate nitrate takes up much less water than ammonium nitrate, indicating that the ZSR mixing rule fails for nitrate. Small variations of organic growth factors with source regions suggest that secondary organic matter is more hygroscopic than primary organic matter.