Robert Vet

On ozone dry deposition—with emphasis on non-stomatal uptake and wet canopies

Abstract Measurements of O 3 fluxes and concentrations over five different sites were used to study O 3 dry deposition. It was found that high humidity, dew and rain increase O 3 uptake by canopy cuticles. However, the increase by cuticle uptake maybe offset by a decrease in stomatal uptake due to weak solar radiation or stomatal blocking under wet conditions. Thus, during nighttime the overall canopy resistances ( R c ) for O 3 uptake under wet conditions was usually smaller than under dry conditions, while in the daytime, R c for wet canopies could be either larger or smaller compared to dry canopies. This will depend on the relative contributions of the decrease in cuticle resistance and the increase in stomatal resistance. The non-stomatal uptake of O 3 was found to be affected by friction velocity, relative humidity, canopy wetness, leaf area index, etc. Parameterizations for non-stomatal resistance for dry and wet canopies were developed based on the five site O 3 flux data. These equations were found to provide reasonable predictions of non-stomatal canopy resistance based upon comparisons with the nighttime and daytime measurements.

The concentration and deposition of acidity, major ions and trace metals in the snowpack of the Eastern Canadian shield during the winter of 1980–1981

A chemical survey of the snowpack in Eastern Canada, conducted in late January 1981, yielded the concentration and total deposition of major ions and several trace metals. The hydrogen ion concentration ranged from 12.6 (pH 4.90) to 85.0 (pH 4.07) μmole l−1. On an ion-equivalents basis, there was, on average, 60% more nitrate than sulphate in the snowpack. Over a period of average duration 42 days, the mean deposition rates of SO2−4, NO3− and Pb were 12, 38 and 0.016 μmole m−2d−1, respectively. The concentrations of the potentially toxic trace elements Pb, Ni, Cu, V, Cr, Mn and Ba were found to be in the same range as those observed in precipitation in rural areas. All but Mn and Ba were significantly enriched in the snowpack relative to crustal rock composition. They are listed above in order of decreasing enrichment. Results of a study of within-site and between-site variations of ionic and metallic concentrations are also reported.

An Intensive Study of the Size and Composition of Submicron Atmospheric Aerosols at a Rural Site in Ontario, Canada

Atmospheric sampling was conducted at a rural site near Egbert, about 70 km north of Toronto, Ontario, Canada from March 27 to May 8, 2003 to characterize the physical and chemical properties of the ambient aerosol in near real-time. The instrumentation included a tapered element oscillating microbalance (TEOM), an ultrafine condensation particle counter (UCPC), a scanning mobility particle sizer (SMPS), an aerodynamic particle sizer (APS), an aerosol mass spectrometer (AMS), and a particulate nitrate monitor (R&P 8400N) for aerosol measurements. Gas-phase non-methane hydrocarbon compounds (NMHCs) were measured by gas chromatograph-flame ionization detection (GC-FID). Filter samples were also collected for analysis of inorganic ions by ion chromatography (IC). Aerosol properties varied considerably depending upon meteorological conditions and airmass histories. For example, urban and industrial emissions advected from the south strongly influenced the site occasionally, resulting in higher particulate mass with the higher fractions of nitrate and organics. Cleaner northwesterly winds carried aerosols with relatively higher fractions of organics and sulfate. The AMS derived mass size distributions showed that the inorganic species in the particles with vacuum aerodynamic diameters between about 60 nm and 600 nm had mass modal vacuum aerodynamic diameters around 400–500 nm. The particulate organics often exhibited two modes at about 100 nm and 425 nm, more noticeable during fresh pollution events. The small organic mode was well correlated with gas-phase nonmethane hydrocarbons such as ethylbenzene, toluene, and propene, suggesting that the likely sources of small organic particles were combustion related emissions. The particulate nitrate exhibited a diurnal variation with higher concentrations during dark hours and minima in the afternoon. Particulate sulfate and organics showed evidence of photochemical processing with higher levels of sulfate and oxygenated organics in the afternoon. Reasonable agreement among all of the co-located measurements is found, provided the upper size limit of the AMS is considered.

Atmospheric Deposition to the Turkey Lakes Watershed: Temporal Variations and Characteristics

We investigated the atmospheric concentrations and deposition fluxes of major ions to the Turkey Lakes Watershed (TLW) between 1980 and 1996. During that time, daily SO42− concentrations in precipitation decreased markedly, while NO3−, NH4+, and H+ concentrations remained roughly constant. It appears that precipitation acidity did not decrease in spite of declining SO42− concentrations due to a concurrent and counterbalancing decrease in the concentrations of Ca2+, Mg2+, and K+ in precipitation. The reasons for the decline in base cations are unknown, but this decline is probably related to decreasing emissions of soil-derived particles from agricultural, industrial, and road sources. A similar situation was seen during the same period in other parts of Canada, the eastern United States, and Europe. Wet, dry, and total (wet + dry) deposition fluxes of sulphur (S) and nitrogen (N) were estimated annually for the years 1980–96. The 17-year mean annual total (wet + dry) deposition of S to the watershed was estimated at 38.5 mmol m−2 y−1 (range 24.3–50.3). Total S deposition decreased by 35% from the early 1980s (1982–84) to the mid-1990s (1994–96), a decline consistent with the 23% decline in annual SO2 emissions in eastern North America during the same period. In contrast, the annual total (wet + dry) deposition of oxidized N ranged from 39.8 to 60.4 mmol m−2 y−1, with a 15-year mean of 50.1 mmol m−2 y−1 and a net increase of 10% between the early 1980s (1983–85) and the mid-1990s (1994–96). This is in keeping with a 10% increase in NOx emissions in eastern North America during the same period. For both S and N (oxidized), wet deposition dominated over dry deposition as the major mechanism for atmospheric input to the watershed. Annually, wet deposition accounted for approximately two-thirds of the total atmospheric deposition of both S and N. Dry S deposition was due more to gaseous SO2 deposition (two-thirds of dry S deposition) than to particulate SO42− deposition (one-third of dry S deposition). Dry deposition of oxidized N, however, was dominated (95%) by gaseous HNO3 deposition, with minimal input from particulate NO3− deposition. Compared to several selected watershed/forest sites in Canada, the United States, and Europe, the estimated total deposition of S and N at the TLW was relatively high during the measurement period.

Eulerian model evaluation field study (EMEFS): A summary of surface network measurements and data quality

From June 1988 to May 1990, a Joint Canada-United States measurement program known as EMEFS made daily air and precipitation measurements across eastern North America. Surface data were collected by five participating networks at approximately 130 sites for use in assessing regional air pollutant and deposition model performance. This paper describes the measurement program and focuses on its associated quality assurance program. The concentration measurements were found to have overall uncertainties of ± 2.3% for SO42− and NO3− for precipitation and, for the air concentrations, ± 6.4% for SO2, ± 5.4% for SO42−, and ± 10.3% for NO3−. These uncertainty levels generally satisfied the program data quality objectives for use in model evaluation studies and represent the quality of contemporary air precipitation chemistry networks.

A Comparison of the Precipitation Chemistry Measurements Obtained by the CAPMoN and NADP/NTN Networks

Precipitation chemistry measurementsobtained by the Canadian Air and PrecipitationMonitoring Network (CAPMoN) and the U.S. NationalAtmospheric Deposition Program/National Trends Network(NADP/NTN) have been examined using more than 7 yrof collocated data from two sites, namely, Sutton,Quebec, Canada and State College, Pennsylvania, U.S.A.In the case of the CAPMoN data, weeklyprecipitation-weighted mean concentrations, totalsample depths and total standard gauge depths werecomputed from daily data and compared to thecorresponding weekly sampling data of the NADP/NTNnetwork. Seasonal and annual precipitation-weightedmean concentrations and deposition values were alsocomputed for both networks and compared. Statisticallysignificant between-network biases were found to existin the weekly results for most of the measuredvariables, particularly standard gauge depth, sampledepth, pH, H+, NO3-,NH4+,Na+; the NADP/NTN values were consistently lowerthan those of CAPMoN with the exception of pH andNa+. The magnitude of the biases was less than35% of the median CAPMoN weekly value for the 7 yr. For most of the measured parameters, thevariability of the between-network differencesrepresented less than 20% of the median CAPMoN weeklyvalue. Both the between-network biases andvariabilities were functions of several physicalparameters, the most dominant being the sample depthand the ionic concentration. For seasonal and annualdeposition values, statistically significantbetween-network biases were found for H+,SO42-, NO3-,Ca2+,NH4+ for both periods; for Mg2+ andK+ for seasonal data; and Cl- for yearlydata, with the NADP/NTN deposition values being lowerthan those of CAPMoN. The relative biases ranged from7 to 37%. Part of the between-network bias in thedeposition estimates was directly attributable to astrong bias in the standard gauge depths of the two networks.

Evaluation of a non-stomatal resistance parameterization for SO2 dry deposition

Abstract Measurements of SO2 fluxes and concentrations at 5 different sites were used to study SO2 dry deposition under different canopy wetness conditions and to evaluate a non-stomatal resistance parameterization. It was found that high humidity, dew and rain increase SO2 uptake by canopy cuticles and the magnitude of the enhanced SO2 uptake by dew and rain is different. A non-stomatal resistance parameterization originally developed for O3 was applied to SO2 with adjustments to some input parameters. The modified parameterization was found to perform well compared to measurements. The non-stomatal resistance parameterization represents an improvement over other existing parameterizations due to its inclusion of meteorological variations allowing it to predict very reasonable diurnal variations of SO2 dry deposition over wet canopies.

The Precision of Precipitation Chemistry Measurements in the Canadian Air and Precipitation Monitoring Network (CAPMoN)

Precision estimates are presented for precipitation chemistry and depth measurements made by the Canadian Air and Precipitation Monitoring Network (CAPMoN). The estimates were made for daily measurements of ion concentration and precipitation depth as well as for weekly, 28-day, seasonal and annual precipitation-weighted mean concentrations and depths. The data on which the estimates are based were collected from collocated samplers at five CAPMoN sites during the period 1985 to 1993. The data pairs from the collocated samplers were used to calculate the between-instrument error defined as 1/√2 times the difference between the paired sample concentrations (or depths). For all of the ion concentrations and depths, the between-sampler errors were found not to be normally distributed, but the normality of the distributions improved with the length of the (volume-weighting) time period considered. A set of quantitative measures of overall network precision were derived in absolute (mg L-1) and relative (%) units. These included the Modified Median Absolute Deviation (M.MAD), the P90% probability values and the Coefficient of Variation (CoV). The latter, defined as the percent ratio of the M.MAD to the median concentration (or depth), represents the relative precision at the center of the error and concentration (and depth) distributions. Based on the CoV values, the relative precision of the CAPMoN measurements was very high (better than 4%) for SO42-, NO3-, pH, H+, NH4+, sample depth and standard gauge depth, and not as high (between 10 and <35%) for Cl-, Na+, Ca2+, Mg2+, and K+. The ions with the lowest median concentrations had the poorest relative precision since so many of the concentrations were at or near the analytical detection limit. Except for the sample and standard gauge depths, both the absolute and relative precision improved with the length of the precipitation-weighting period. Detailed statistical testing established that the precision of the daily measurements is dependent on a number of factors, the most dominant being sample depth and concentration, i.e., the absolute precision improves with increasing sample depth and decreasing concentration. The strength of these relationships diminished with the length of the precipitation-weighting period being considered. Laboratory-related sources of imprecision were found to account for less than 4% of the overall daily measurement imprecision for most species, while field-related sources of imprecision accounted for the balance. Specialized plots are shown which allow data users to estimate the absolute and relative precision at any concentration and depth value.

Evaluation of elemental and black carbon measurements from the GAViM and IMPROVE networks

The GAViM program provides fine particulate and visibility data for several remote locations in Canada. Two long-term intercomparison studies between the GAViM and a major U.S. aerosol monitoring network, IMPROVE, were used to evaluate the uncertainty in the analytical data produced by proton induced x-ray emission (PIXE), proton elastic scattering analysis (PESA), and gravimetric analysis. GAViM and IMPROVE agreed well for elements from Fe to Zn where PIXE is the most sensitive; the relative difference between the 2 networks for Fe and Zn was <2%. Some lighter elements, e.g., sodium or sulphur, revealed a difference of 10-20%. Furthermore, an empiric conversion scheme for the GAViM absorption data produced by the laser integrated plate method (LIPM) was derived from the comparison to the IMPROVE thermal/optical reflectance (TOR) data. This conversion depends on the aerosol composition and is therefore site specific. It allows estimation of the elemental carbon concentrations from the historic raw light absorption values obtained by LIPM. If the mass attenuation coefficient of the fine aerosol collected at the 2 remote GAViM sites is assumed to be equal to 10 m 2 /g, then the results imply that the light absorption coefficient measured by LIPM is generally higher than the true value by up to a factor of 1.3 or 1.8, respectively. In both cases, LIPM overestimated the black carbon content, mostly for the lightly loaded samples.

Evaluation and Improvement of a Dry Deposition Model using SO2 and O3 Measurements over a Mixed Forest

A dry deposition model (RDM) for operational application has beenevaluated and modified in the present study. Field measurements of friction velocity and dry deposition velocity of SO2 andO3 over a mixed forest have been used to evaluate RDM. It was found that RDM predicts friction velocities very close to measurements and thus it can predict reasonable aerodynamic resistance. RDM overestimated O3 deposition during dry nighttime conditions and underestimated both O3 andSO2deposition for early morning hours. It could not predict the mean diurnal variation in deposition velocity for either O3 or SO2 deposition under wet surface conditions. Modifications have been made for O3 and SO2 dry deposition based on the comparison of results and based upon additional published data. Compared to an earlier version of RDM, the modified versionpredicts better results for O3 and SO2 dry deposition,especially under rain and dew conditions.