Patricia A. Mulawa

Atmospheric concentrations and the deposition velocity to snow of nitric acid, sulfur dioxide and various particulate species

Abstract A study of deposition velocities to snow was conducted during the 1982–1983 and 1983–1984 winters at the University of Michigan Biological Station in northern Michigan. Weekly measurements were made of dry deposition rates to snow and the atmospheric concentrations of the depositing species. SO 2 , with an average concentration of 2.2 ppb, was the dominant atmospheric sulfur containing species. NO 2 , with an average concentration of 1.8 ppb, was the dominant atmospheric nitrogenous species. NO − 3 deposition was due primarily to HNO 3 , which averaged 0.2 ppb. The HNO 3 deposition velocity averaged 1.4cm s −1 . The SO 2 deposition velocity varied with temperature, averaging 0.15 cm s −1 for samples with appreciable exposure time above − 3°C, and 0.06 cm s −1 for samples which remained below an ambient temperature of −3°C. Deposition velocities of Ca 2+ , Mg 2+ , Na + , K + and NH + 4 were 2.1, 1.5, 0.44, 0.51 and 0.10cm s −1 , respectively. The mass median diameters of these species were 4.4, 2.7, 1.8, 0.9 and 0.46 μm, respectively.

Problems in the sampling and analysis of carbon particulate

Abstract Several thermal and wet chemical methods of separating organic from elemental carbon in particulate samples were examined. It is concluded that none of them represents an ideal separation procedure and that only a method-dependent operational definition of organic and elemental carbon is possible at this time. The best separation method appears to be a thermal procedure using 350°C air oxidation followed by pyrolysis in He at 950°C. There are also difficulties in sampling since dual filter techniques show that adsorption of organic compounds on various filter media accounted for at least 15 per cent of the total organic carbon collected during ambient sampling in Warren, MI. This adsorption further confuses the results and needs to be studied at other sampling sites.

Low molecular weight aliphatic amines in exhaust from catalyst-equipped cars

Emissions from automobiles equipped with three-way catalysts were studied. Aliphatic amine emissions from the cars tested were less than 2.2 mg/mi. Emissions of monomethylamine and dimethylamine were no more than 0.3 mg/mi and 0.1 mg/mi respectively. It is unlikely that any standard gasoline-propelled cars emit significant quantities of amines. (1 diagram, 1 graph, 20 references, 6 tables)

Urban dew: Its composition and influence on dry deposition rates

Abstract The composition of dew collected from a Teflon surface was compared to summer rainwater concentrations at a site in Warren, MI. This comparison showed that natural dew is similar to rainwater with the exception that dew has much higher concentrations of Ca 2+ and Cl − and much lower acidity. Dry deposition rates of several species were measured to artificially-generated dew and a dry surface. It was found that deposition rates were 2–20 times greater to the artificial dew than to the dry surface, indicating that the presence of dew enhances both the retention of dry deposited particles and the absorption of water soluble gases. Measurement of the atmospheric concentrations of the depositing gases permitted the calculation of deposition velocities for particulate Cl − , NO − 3 , SO 2− 4 , Ca 2+ , Mg 2+ , Na + , K + and NH + 4 . Deposition velocities for gaseous HNO 3 , HCl, SO 2 and NH 3 were also determined after correction for particle deposition. These results indicate that acid dew is not a problem at this site. However, the ability of dew to increase the deposition rates of acids and acid precursors to some surfaces suggests that dew may be more acidic at sites with lower deposition rates of basic particles.

An Experimental Evaluation of Remote Sensing-Based Hydrocarbon Measurements: A Comparison to FID Measurements

The purpose of this study was to intercompare hydrocarbon (HC) measurements performed by a number of different instruments: a gas chromatograph (GC), a flame ionization detector (FID), a fourier transform infrared spectrometer (FTIR), a commercially produced non-dispersive infrared analyzer (NDIR), and two remote sensors. These instruments were used to measure total HC concentrations in a variety of samples, including (1) ten different individual HC species, (2) 12 different vehicle exhaust samples, and (3) three different volatilized fuel samples. The 12 exhaust samples were generated by operating two different vehicles on a dynamometer. Each vehicle was operated at different times with three different fuels. The vehicles were operated fuel rich, i.e., with low air/fuel ratios to encourage elevated exhaust HC levels. Some of the exhaust samples were obtained while operating each vehicle at a stoichiometric air/fuel ratio with one spark plug wire disconnected. To quantify the degree to which the various instruments agreed with the FID, a parameter called the response factor was used, where the response factor was defined as the HC/CO2 ratio measured by each instrument divided by the HC/CO2 ratio measured by the dynamometer bench. Of the various instruments, only the GC yielded response factors that were consistently at or close to one. The other instruments typically had values at or below one. For the ten individual HC species studied, the NDIR and remote sensors obtained response factors between 0.05 and 1.0, with the highest response factors being obtained for the alkanes and the lowest response factors obtained for toluene and ethylene. For the exhaust samples, the NDIR and remote sensors obtained response factors between 0.23 and 0.68. For raw fuel samples, the response factors were between 0.44 and 0.68. NDIR and remote sensor measurements correlated very poorly with total HC in exhaust.

A comparison of nitric acid and particulate nitrate measurements by the penetration and denuder difference methods

Measurements of ambient nitric acid and particulate nitrates were made by the dual-filter, denuder difference and penetration methods for 53 days during the summer of 1981 in Warren, Michigan. Nitric acid average concentrations determined by the penetration method (3.71 μg m−3) and the denuder difference method (3.11 μg m−3) were significantly lower than the dual-filter measurements (4.52 μg m−3). Similarly, nonvolatile particulate nitrate averaged 0.95 μg m−3 by penetration and 1.10 μg m−3 by denuder difference compared to 1.53 μg m−3 by the dual-filter method. These comparisons indicated that the penetration and denuder difference methods achieve significant reductions in nitric acid and nonvolatile particulate nitrate interferences. Based upon its operational simplicity, the denuder difference method is recommended.

Atmospheric carbonaceous species measurement methods comparison study: General Motors results

The General Motors Research Laboratories participated in both the field sampling and round-robin portions of the Carbonaceous Species Intercomparison study that was held in Glendora, CA, during the summer of 1986. Five samplers were operated during the field study. The average particulate elemental carbon (EC) concentrations determined from the five samplers agreed to within 12%. Large differences were observed in the concentrations of particulate organic carbon (OC) determined from the five samplers. Some of the differences are attributed to losses of OC from the filters due to volatilization during the collection period. The amount of volatilization varies with the length of the sampling time and the filter face velocity. In addition, the adsorption of gas-phase organic compounds caused a significant positive interference in the determination of OC. Our OC and EC results for the round-robin samples were compared to the values obtained by the other participating laboratories. The average ratio of our res...

A laboratory investigation of the routes of HNO3 dry deposition to coniferous seedlings

White pine, Norway spruce and red spruce seedlings were exposed to nitric acid vapor concentrations of 10 to 120 ppb in constant stirred tank reactors. Nitric acid dry deposition rates were determined from both the change in nitric acid concentration in the reactor flow stream and from the amount of nitrogen recovered from the seedlings. Nitric acid labeled with 15N was used to distinguish dry-deposited nitrogen in the plant from the nitrogen that was already present. It was found that dry deposition occurs via three routes: surface deposition, trans-cuticular deposition, and stomatal deposition. Resistance to surface deposition is very low (< 4.8 m2-s mol(-1)) for a freshly washed surface, but increases as the surface adsorption sites are occupied. Resistance to trans-cuticular uptake averaged 206 m2-s mol(-1). Stomatal resistance can be calculated from the rate of water diffusion out of the plant. Eighty per cent of the nitric acid deposited via the trans-cuticular and stomatal routes was assimilated by the plant. However, none of the nitric acid deposited on the surface was assimilated. In rural areas with coniferous forests, the combination of low ambient nitric acid concentrations and low initial surface resistance means that most nitric acid will be dry deposited on the tree surface, and thus will not be directly assimilated.

Light-duty motor vehicle exhaust particulate matter measurement in the Denver, Colorado, area

A study of particulate matter (PM) emissions from in-use, light-duty vehicles was conducted during the summer of 1996 and the winter of 1997 in the Denver, CO, region. Vehicles were tested as received on chassis dynamometers on the Federal Test Procedure Urban Dynamometer Driving Schedule (UDDS) and the IM240 driving schedule. Both PM10 and regulated emissions were measured for each phase of the UDDS. For the summer portion of the study, 92 gasoline vehicles, 10 diesel vehicles, and 9 gasoline vehicles with visible smoke emissions were tested once. For the winter, 56 gasoline vehicles, 12 diesel vehicles, and 15 gasoline vehicles with visible smoke were tested twice, once indoors at 60 °F and once outdoors at the prevailing temperature. Vehicle model year ranged from 1966 to 1996. Impactor particle size distributions were obtained on a subset of vehicles. Continuous estimates of the particle number emissions were obtained with an electrical aerosol analyzer. This data set is being provided to the Northern Front Range Air Quality Study program and to the State of Colorado and the U.S. Environmental Protection Agency for use in updating emissions inventories.

Ambient concentrations, scavenging ratios, and source regions of acid related compounds and trace metals during winter in northern michigan

Abstract Daily measurements the atmospheric cocnentrations of HNO 3 , NO 3 - , NO 2 , SO 2 , SO 4 2− , NH 4 + , and several trace metals were made at the University of Michigan Biological Station over a 124-day period during the 1984–1985 winter. The composition of the daily precipitation was also determined. The relative contributions of scavenged NO 3 − and HNO 3 to the precipitation was estimated by assuming that the NO 3 − scavenging ratio was the same as that of trace metals with a similar particle size. Similarly, the SO 4 2− and SO 2 contributions were based on the scavenging ratios of NH 4 + and trace metals. On this basis, it was determined that the event median NO 3 − and HNO 3 scavenging ratios were 500 and 3500, respectively. HNO 3 scavenging accounted for 83% of the total scavenged NO 3 − . Scavenging of SO 4 2− accounted for all the snow SO 4 2− in 67% of the events. In the remaining events, some SO 2 was scavenged, with a median scavenging ratio of 219. Overall, 67% of the snowfall acidity appeared to be due to HNO 3 scavenging. Backward air-mass trajectories that were calculated for each event were used to determine the general source regions of the acidic species. Snow associated with air masses from the south and west accounted for 81 and 75% of the deposited NO 3 − and SO 4 2− , respectively.