Due to the potential ecological effects on terrestrial and aquatic ecosystems from atmospheric deposition in the Athabasca Oil Sands Region (AOSR), Alberta, Canada, this study was implemented to estimate atmospheric nitrogen (N) and sulfur (S) inputs. Passive samplers were used to measure ambient concentrations of ammonia (NH3), nitrogen dioxide (NO2), nitric acid/nitrous acid (HNO3/HONO), and sulfur dioxide (SO2) in the AOSR. Concentrations of NO2 and SO2 in winter were higher than those in summer, while seasonal differences of NH3 and HNO3/HONO showed an opposite trend, with higher values in summer. Concentrations of NH3, NO2 and SO2 were high close to the emission sources (oil sands operations and urban areas). NH3 concentrations were also elevated in the southern portion of the domain indicating possible agricultural and urban emission sources to the southwest. HNO3, an oxidation endpoint, showed wider ranges of concentrations and a larger spatial extent. Concentrations of NH3, NO2, HNO3/HONO and SO2 from passive measurements and their monthly deposition velocities calculated by a multi-layer inference model (MLM) were used to calculate dry deposition of N and S. NH3 contributed the largest fraction of deposited N across the network, ranging between 0.70-1.25kgNha(-1)yr(-1), HNO3/HONO deposition ranged between 0.30-0.90kgNha(-1)yr(-1), and NO2 deposition between 0.03-0.70kgNha(-1)yr(-1). During the modeled period, average dry deposition of the inorganic gaseous N species ranged between 1.03 and 2.85kgNha(-1)yr(-1) and SO4-S deposition ranged between 0.26 and 2.04kgha(-1)yr(-1). Comparisons with co-measured ion exchange resin throughfall data (8.51kgSha(-1)yr(-1)) indicate that modeled dry deposition combined with measured wet deposition (1.37kgSha(-1)yr(-1)) underestimated S deposition. Gas phase NH3 (71%) and HNO3 plus NO2 (79%) dry deposition fluxes dominated the total deposition of NH4-N and NO3-N, respectively.
Hexavalent chromium (Cr(6+)) emitted from welding poses serious health risks to workers exposed to welding fumes. In this study, tetramethylsilane (TMS) was added to shielding gas to control hazardous air pollutants produced during stainless steel welding. The silica precursor acted as an oxidation inhibitor when it decomposed in the high-temperature welding arc, limiting Cr(6+) formation. Additionally, a film of amorphous SiO(2) was deposited on fume particles to insulate them from oxidation. Experiments were conducted following the American Welding Society (AWS) method for fume generation and sampling in an AWS fume hood. The results showed that total shielding gas flow rate impacted the effectiveness of the TMS process. Increasing shielding gas flow rate led to increased reductions in Cr(6+) concentration when TMS was used. When 4.2% of a 30-lpm shielding gas flow was used as TMS carrier gas, Cr(6+) concentration in gas metal arc welding (GMAW) fumes was reduced to below the 2006 Occupational Safety and Health Administration standard (5 μg m(-3)) and the efficiency was >90%. The process also increased fume particle size from a mode size of 20 nm under baseline conditions to 180-300 nm when TMS was added in all shielding gas flow rates tested. SiO(2) particles formed in the process scavenged nanosized fume particles through intercoagulation. Transmission electron microscopy imagery provided visual evidence of an amorphous film of SiO(2) on some fume particles along with the presence of amorphous SiO(2) agglomerates. These results demonstrate the ability of vapor phase silica precursors to increase welding fume particle size and minimize chromium oxidation, thereby preventing the formation of hexavalent chromium.
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