Seminar

Abstract

A suite of speciated halogen compounds were measured by chemical ionization mass spectrometry (CIMS) during the recent OASIS campaign (spring 2009) in Barrow, Alaska. Simultaneous measurements of HOBr, BrO, and Br2 were performed during several periods of ozone depletion. The BrO measured by CIMS was found to be in good agreement with that detected by long-path DOAS. Clear diurnal profiles of HOBr (up to 37 pptv) and BrO (up to 40 pptv) were observed with night time levels of both species below the detection limit of the CIMS (~0.5-2 pptv). This is consistent with HOBr and BrO behaving as a short lived chemical species controlled by local photochemistry. The data also indicate that HOBr is cycled very efficiently back to a photolabile form such as Br2. The average mid-day (12:00-17:00 Alaska Standard Time) levels of BrO and HOBr were (6.4± 0.5) and (6.2± 0.5) pptv, respectively, for mid-March to mid-April 2009. BrO and HOBr levels were found to have no clear relationship with ozone but were found to be at the highest levels when ozone was depleted but still above 1 ppbv. Up to 50 pptv of Br2 were detected at night and found to be anti-correlated with ozone indicating that this species is the major nighttime reservoir of active bromine. An analysis of the observed halogen data using a photochemical model to test our understanding of the relationship between BrO and HOBr will be presented.

Cl2, ClNO2, and HCl (for limited periods) were also measured by CIMS during the OASIS campaign. Surprisingly high levels of Cl2 (up to 400 pptv) were observed during the day. However, these levels were found to be consistent with observed HCl and ClNO2, VOC ratios, and independent observations of ClO. The combined data indicate that chlorine chemistry is very active in Barrow and may enhance ozone destruction rates by more than a factor of two. The implications of these results and the potential source(s) of chlorine will be discussed.