Measurements of Photolyzable Chlorine and Bromine During the Polar Sunrise Experiment

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

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

DMS Emissions to the Atmosphere from the Lakes of the Canadian Boreal Region

Dimethyl sulfide (DMS) concentrations were measured over 2 years in 10 lakes situated in northwestern Ontario. Concentrations varied from 0.1 nmol L−1 to 100 nmol L−1 (geometric mean of 1.2 nmol L−1) during the ice-free season (April to November) of 1995 and 1996. Shallow (1–5 m depth and 1–5 ha area) and medium depth (5–10 m depth and 5–15 ha area) lakes exhibited higher surface water DMS concentrations than deeper and larger lakes (32 m depth and 20–56 ha area). During the fall in the medium size lakes (5–12 m depth and <15 ha area), DMS concentrations increased markedly by a factor of as much as 100 because of circulation of deeper, high-DMS water from the hypolimnion (lake turnover). The estimated seasonal mean fluxes of DMS from shallow and medium depth lakes to the atmosphere ranged between 0.058 and 15 μmol S m−2 d−1 respectively. The shallow and medium depth lakes tend to release higher DMS than the large and deeper lakes. Extrapolation of flux estimates indicates that the lakes of the Canadian Boreal Shield emit 1.5 Gg yr−1 biogenic sulfur (DMS) to the atmosphere, and this is approximately 83% of the total annual biogenic sulfur (DMS) emissions from the Canadian Boreal Shield (lakes plus terrestrial). Compared to the annual anthropogenic emissions by five smelters in the boreal region, these emissions are small (0.08%)

Measurement Technique for the Determination of Photolyzable Chlorine and Bromine in the Atmosphere

A technique has been developed to enable measurement of photolyzable chlorine and bromine at trace levels in the troposphere. In this method, ambient air is drawn through a cylindrical flow cell, which is irradiated with a Xe arc lamp. In the reaction vessel of the photoactive halogen detector (PHD), photolytically active molecules Clp (including Cl2, HOCl, ClNO, ClNO2, and ClONO2) and Brp (including Br2, HOBr, BrNO, BrNO2, and BrONO2) are photolyzed, and the halogen atoms produced react with propene to form stable halogenated products. These products are then sampled and subsequently separated and detected by gas chromatography. The system is calibrated using low concentration mixtures of Cl2 and Br2 in air from commercially available permeation sources. We obtained detection limits of 4 pptv and 9 pptv as Br2 and Cl2, respectively, for 36 L samples

Long-term trends of the black carbon concentrations in the Canadian Arctic

International audienceDuring the winter and spring the North American Arctic is impacted by anthropogenic black carbon (BC) in ''Arctic Haze'' pollution from sources mainly located in Europe and Russia. This black carbon, while suspended in the atmosphere and in surface snow, has a significant effect on radiative forcing of the Arctic atmosphere. Routine ground-level observations of aerosol black carbon by optical absorption have been made at a Canadian Arctic location, Alert (82.5°N, 62.5°W), Nunavut since 1989. A 3-year intensive study was conducted to compare BC obtained by the thermal analysis and optical absorption methods, so that the seasonal variations in the ''operational'' absorption cross sections of the aerosol could be determined. A time series analysis indicated that black carbon concentrations undergo a strong seasonal variation superimposed upon a long-term trend. The latter shows a decrease of about 55% in BC concentrations between 1989 and 2002 at Alert. Factors responsible for these trends such as changes in emissions and atmospheric transport support the hypothesis that BC emissions from the former USSR are mostly responsible for the observed decreasing trend. Transport from other sectors such as North America and Europe are not as prevalent at Alert