D. R. Hastie

Atmospheric concentrations and temporal variations of C1C3 carbonyl compounds at two rural sites in central Ontario

Abstract Measurements of formaldehyde, acetaldehyde, acetone and propionaldehyde concentrations were made at two rural sites in central Ontario. One site (at Egbert, Ont.) is located ≈60 km northwest of Toronto, while the other site (at Dorset, Ont.) is ≈150 km northeast of the Egbert site. Measurements were made using a modified version of a derivatization technique in which sample air is pumped through Teflon tubes packed with silica gel that is coated with 2,4-dinitrophenylhydrazine (DNPH). The product hydrazones were separated and quantified using HPLC. Quantitative determinations of formaldehyde, acetaldehyde and acetone were made for 49 and 47 samples at the Dorset and Egbert sites, respectively, between 25 July and 30 August 1988. The average concentrations determined at the Dorset site for formaldehyde, acetaldehyde, and acetone were 1.6, 0.46 and 1.8 ppb, respectively, and for the Egbert site the corresponding averages were 1.8, 0.57 and 1.6 ppb. A set of 10 samples from the Egbert site were analysed for propionaldehyde yielding an average concentration of 0.03 ppb. The formaldehyde measurements were compared with measurements made at the same time using Tunable Diode Laser Absorption Spectroscopy. The observed concentrations reported here are compared with previously reported measurements of these species and interpreted in terms of atmospheric variables (e.g. meteorology, concentrations of precursor hydrocarbons) influencing their concentrations.

Measurements of photolyzable chlorine and bromine during the Polar Sunrise Experiment 1995

We report measurements of rapidly photolyzable chlorine (Clp; e.g., Cl2 And HOCl) and bromine (Brp; e.g., Br2 and HOBr) in the high Arctic using a newly developed photoactive halogen detector (PHD). Ground level ambient air was sampled daily from mid-February through mid-April in the Canadian Arctic at Alert, Northwest Territories (82.5°N, 62.3°W), as part of the Polar Sunrise Experiment (PSE) 1995. Concentrations of “total photolyzable chlorine” varied from <9 to 100 pptv as Cl2 and that of “total photolyzable bromine” from <4 to 38 pptv as Br2. High concentration episodes of chlorine were observed only prior to sunrise (March 21), while high concentration episodes of bromine were measured throughout the study. The high concentrations of photolyzable chlorine and bromine prior to sunrise suggest a “dark” production mechanism that we assume yields Cl2 and Br2. An inverse correlation of bromine with ozone is clearly present in one major ozone depletion episode at the end of March. A trajectory analysis, taken with the differences in measured levels of photolyzable chlorine and bromine after sunrise, imply different production mechanisms for these two types of species. A steady state analysis of the data for one ozone depletion episode suggests a [Br]/[Cl] ratio in the range 100–300. The high concentrations of photolyzable bromine after sunrise imply the existence of a precursor other than aerosol bromide.

The production of atmospheric NOx and N2O from a fertilized agricultural soil

Abstract The source strength of atmospheric trace gases from rural or remote locations must be quantified in order to assess the effect of such inputs on the background tropospheric chemistry. To assess the importance of biological production of NO x and N 2 O from fertilized agricultural soil, enclosure techniques have been used to determine the emission fluxes of NO x and N 2 O at a site in Southern Ontario, Canada. NO x fluxes on the unfertilized soil range from 1.5 to 41.6 μg(NO) m −2 h −1 . The corresponding N 2 O fluxes are 0–61.8 μg(N 2 O)m −2 h −1 . For the most fertilized soil NO x fluxes range from 3.1 to 583 μg(NO) m −2 h −1 and the N 2 O fluxes from 0 to 446 μg(N 2 O) m −2 h −1 . The fluxes increase linearly with fertilizer application, with 11% of the nitrogen in the fertilizer converted to NO x and 5% to N 2 O. The emission rates were studied as functions of the soil parameters temperature, moisture, ammonium, nitrate and pH, to attempt to understand better the production mechanisms, although a model for the process could not be developed. In rural areas away from transportation corridors the increased NO x emission from fertilized soil may dominate local oxidant production but is not significant on the Province-wide scale.

Determination of the relative ozone and PAN deposition velocities at night

A series of measurements of PAN and ozone was conducted during summer at three rural sites in Canada: Egbert and Dorset, Ontario, and Kejimkujik, Nova Scotia. For nights when a stable surface inversion layer forms, ozone and PAN at the surface are found to undergo first order decay, assumed to be due only to dry deposition. Analysis of the measurement data leads to determination of the relative dry deposition velocities. For all three sites, we find that Vd(O3)/Vd(PAN) = 0.42±0.19, at night. This ratio is roughly a factor of 5–6 times smaller than previously assumed. This smaller relative deposition velocity ratio can have a significant impact on model estimations of PAN concentrations near the surface. We estimate that for these sites, the PAN deposition velocity is at least 0.5 cm/s, and may be greater during daytime. This can have a significant impact on the tropospheric lifetime of PAN.

Measurements of Photolyzable Halogen Compounds and Bromine Radicals During the Polar Sunrise Experiment 1997

As part of the Polar Sunrise Experiment (PSE) 1997, concentrations of halogen species thought to be involved in ground level Arctic ozone depletion were made at Alert, NWT, Canada (82.5°N, 62.3°W) during the months of March and April, 1997. Measurements were made of photolyzable chlorine (Cl2 and HOCl) and bromine (Br2 and HOBr) using the Photoactive Halogen Detector (PHD), and bromine radicals (BrOx) using a modified radical amplifier. During the sampling period between Julian Day 86 (March 27) and Day 102 (April 12), two ozone depletion episodes occurred, the most notable being on days 96-99, when ozone levels were below detectable limits (≈1 ppbv). Concentrations of BrOx above the 4 pptv detection limit were found for a significant part of the study, both during and outside of depletion events. The highest BrOx concentrations were observed at the end of the depletion event, when the concentration reached 15 pptv. We found substantial amounts of Br2 in the absence of O3, indicating that O3 is not a necessary requirement for production of Br2. There is also Br2 present when winds are from the south, implying local scale (e.g. from the snowpack) production. During the principal O3 depletion event, the HOBr concentration rose to ≈260 pptv, coincident with the BrOx maximum. This implies a steady state HO2 concentration of 6 pptv. During a partial O3 depletion event, we estimate that the flux of Br2 from the surface is about 10 times greater than that for Cl2.

The sensitivity of the radical amplifier to ambient water vapour

The radical amplifier is an instrument used to measure radical concentrations in the troposphere. The critical parameter in determining the sensitivity is the chain length, which is shown to decrease with increasing water vapour in the reactor. When compared to measurements in dry air, this decrease is a factor 2 at a relative humidity of 40%. This suggests that field measurements using the radical amplifier may be underestimating the ambient radical concentration by a similar factor. One source of this deterioration in performance is an increase in the loss of radicals to the walls of the reactor, although there also appears to be a contribution from a water dependence on the gas phase chemistry.

Measurements of alkyl and multifunctional organic nitrates at a rural site in Ontario

Measurements of alkyl and multifunctional organic nitrates were conducted at a rural site in Ontario during periods with varying levels of photochemical activity. On August 6 and August 21-23, 1992, a total of 17 organic nitrates (12 C3-C6 alkyl nitrates, 4 C2-C4 hydroxynitrates and 1,2-dinitrooxybutane) were quantitatively determined in atmospheric samples. The sum of the organic nitrate concentrations was found to be correlated with ozone and ranged from 12 to 140 parts per trillion (volume). The total concentration of organic nitrates measured contributed from 0.5 to 3.0% to the total odd nitrogen species. On average, the alkyl nitrates represented 82%, the hydroxynitrates 16%, and the dinitrate 2% of the total measured organic nitrates. Unidentified organic nitrate peaks determined from an organic nitrate selective detector were found to contribme an additional 0.25% to the total odd nitrogen budget. The distribution of alkyl and hydroxy nitrates measured was found to be reasonably consistent with computed relative production rates, for photochemically active air masses. Although the inclusion of the multifunctional organic nitrates does not significantly change the measured contribution of these species to NO.,, it is shown that the isoprene nitrates may s , x contribute as much as the combined contribution of all the measured organic nitrates.

Observational evidence for the impact of the lake breeze circulation on ozone concentrations in southern Ontario

Abstract Very rapid increases in the concentrations of ozone and ozone precursors, in the late afternoon, have been observed at a rural and an urban site in southern Ontario. Ozone concentration increases of 30 ppbv in a few minutes have been observed. These increases occur simultaneously with the arrival of a Lake Ontario lake breeze front as identified from meteorological measurements and visible satellite imagery. This indicates that polluted air masses from over Lake Ontario are being transported inland by the lake breeze. Aircraft measurements of ozone, NOx, and hydrocarbons show such an air mass moving inland. Chemical measurements at the sites show that the polluted air masses are not of local origin, but are of similar age to those regularly encountered in rural areas.

The influence of the nocturnal boundary layer on secondary trace species in the atmosphere at Dorset, Ontario

The impact of the nocturnal boundary layer (NBL) on the concentrations of O3, H2O2, PAN and CH2O has been studied for two 4-day periods, one in the summer of 1989 and the other in the spring of 1990. Where the presence of the nocturnal layer is clear, O3, PAN and CH2O concentrations decrease rapidly and these species appear to be deposited to the surface. Break-up of the inversion in the morning returns concentrations to levels typical of the previous day. H2O2 is removed much faster than the other species and it is replenished much more slowly. This appears to be due to the H2O2 dissolving in water droplets resulting from the rapid cooling of the air. There is evidence for a morning maximum in PAN, likely due to the combination of high concentrations being brought downwards during the break-up of the NBL followed by thermal decomposition.

Exchange of nitrous oxide within the Hudson Bay lowland

The source strength of atmospheric trace gases from natural ecosystems must be quantified in order to assess the effect of such inputs on the background tropospheric chemistry. A static chamber technique and a gas exchange technique were used to determine the emissions of nitrous oxide from five sites within the Hudson Bay Lowland, as part of the Northern Wetland Study. Two mechanisms, one diffusive and the other episodic, were found likely to be responsible for the emissions of nitrous oxide. The annual diffusive flux ranged from -3.8 mg(N20)/m 2 in a treed bog to 7.9 mg(N20)/m 2 in an open fen. The addition of the episodic flux, increased this range to -2.1 mg(N20)/m 2 and 18.5 mg(N20)/m 2 respectively. These episodic emissions occurred in from 2.5 % to 16.7 % of the samples during the late summer peak emission period. Since the gas exchange rate could not detect the episodic emissions, it was found to be a poor method for water emission rate determination within the wetland. LANDSAT-Thermatic Mapper (TM) imagery was used to scale the emissions, from the chamber level to an integrated average over the entire Hudson Bay Lowland. The total emission rate of N20 from the Hudson Bay Lowland, was determined to be 1.2 Gg(N20)/year, of which 80% was attributed to episodic emissions.