[show abstract][hide abstract] ABSTRACT: In situ measurements of ozone, photochemically active bromine compounds, and other trace gases over the Arctic Ocean in April 2008 are used to examine the chemistry and geographical extent of ozone depletion in the arctic marine boundary layer (MBL). Data were obtained from the NOAA WP-3D aircraft during the Aerosol, Radiation, and Cloud Processes affecting Arctic Climate (ARCPAC) study and the NASA DC-8 aircraft during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) study. Fast (1 s) and sensitive (detection limits at the low pptv level) measurements of BrCl and BrO were obtained from three different chemical ionization mass spectrometer (CIMS) instruments, and soluble bromide was measured with a mist chamber. The CIMS instruments also detected Br2. Subsequent laboratory studies showed that HOBr rapidly converts to Br2 on the Teflon instrument inlets. This detected Br2 is identified as active bromine and represents a lower limit of the sum HOBr+Br2. The measured active bromine is shown to likely be HOBr during daytime flights in the arctic. In the MBL over the Arctic Ocean, soluble bromide and active bromine were consistently elevated and ozone was depleted. Ozone depletion and active bromine enhancement were confined to the MBL that was capped by a temperature inversion at 200–500 m altitude. In ozone-depleted air, BrO rarely exceeded 10 pptv and was always substantially lower than soluble bromide that was as high as 40 pptv. BrCl was rarely enhanced above the 2-pptv detection limit, either in the MBL, over Alaska, or in the arctic free troposphere.
ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2010; · 5.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: In situ measurements of ozone and photochemically active bromine compounds over the Arctic Ocean near Alaska in April 2008 are used to examine the causes and geographical extent of arctic ozone depletion. Data were obtained from the NOAA WP-3D aircraft during the Aerosol, Radiation, and Cloud Processes affecting Arctic Climate (ARCPAC) study and the NASA DC-8 aircraft during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) study. Fast (1 s) and sensitive (detection limits at the low pptv level) measurements of BrCl and BrO were obtained from three different chemical ionization mass spectrometer (CIMS) instruments, and soluble bromide was measured by a mist chamber. The CIMS instruments also detected Br2 that was likely formed from atmospheric HOBr that rapidly converts to Br2 on Teflon inlets. This detected Br2 is identified as active bromine and represents a minimum of the sum HOBr + Br2. In the marine boundary layer over the Arctic Ocean, active bromine was consistently elevated and ozone was depleted (Figure 1). Ozone depletion was confined to the marine boundary layer that was tightly capped by a temperature inversion at 200-500 m altitude (Figure 2). Figure 1. Map of Alaska showing the flight tracks of the NOAA WP-3D aircraft on 5 days in April, 2008 during the ARCPAC study. Ozone mixing ratios less than 20 ppbv are shown as symbols. Figure 2. 1 s measurements of ozone, CO, and potential temperature, and 10 s averages of bromine, measured during a descent of the NOAA WP-3D aircraft over the Arctic Ocean within 30 km of the Alaskan coast on 15 April 2008.
[show abstract][hide abstract] ABSTRACT: This paper describes a modelling study of several HOx and NOx species (OH, HO2, organic peroxy radicals, NO3 and N2O5) in the marine boundary layer. A model based upon the Master Chemical Mechanism (MCM) was constrained to observations of chemical and physical parameters made onboard the NOAA ship R/V Brown as part of the New England Air Quality Study (NEAQS) in the summer of 2004. The model was used to calculate [OH] and to determine the composition of the peroxy radical pool. Modelled [NO3] and [N2O5] were compared to in-situ measurements by Cavity Ring-Down Spectroscopy. The comparison showed that the model generally overestimated the measurements by 30–50%, on average. The model results were analyzed with respect to several chemical and physical parameters, including uptake of NO3 and N2O5 on fog droplets and on aerosol, dry deposition of NO3 and N2O5, gas-phase hydrolysis of N2O5 and reactions of NO3 with NMHCs and peroxy radicals. The results suggest that fog, when present, is an important sink for N2O5 via rapid heterogeneous uptake. The comparison between the model and the measurements were consistent with values of the heterogeneous uptake coefficient of N2O5 (γN2O5)>1×10−2, independent of aerosol composition in this marine environment. The analysis of the different loss processes of the nitrate radical showed the important role of the organic peroxy radicals, which accounted for a significant fraction (median: 15%) of NO3 gas-phase removal, particularly in the presence of high concentrations of dimethyl sulphide (DMS).
ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2008; · 5.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: . Cavity ring-down spectroscopy is a relatively new and quite sensitive technique for the measurement of gas-phase optical extinction.
It holds the potential for simple, direct and sensitive measurement of the concentrations of a variety of trace gases in the
atmosphere. For example, detection of the nitrate radical, NO3, and its companion, dinitrogen pentoxide, N2O5, has been demonstrated with a sensitivity of 0.25 pptv (1σ). This paper considers several of the requirements for the application
of cavity ring-down spectroscopy to concentration measurements of trace gases in ambient air. These include detection sensitivity,
measurement of an accurate zero in the presence of competing absorbers, cavity stability and mirror cleanliness, laser line-width
effects, saturation effects, Rayleigh scattering, the influence of atmospheric aerosols and sampling issues for reactive species.
Examples drawn from our work on NO3 and N2O5 detection in the field illustrate these considerations.
Applied Physics B 01/2002; 75(2):173-182. · 1.78 Impact Factor
[show abstract][hide abstract] ABSTRACT: Direct, in situ observations of NO3, N2O5 and HNO3 in the marine boundary layer during NEAQS have provided new insights into the nocturnal budget for NOx. Comparison between the behavior of these compounds in the NEAQS data set and from a continental site in Boulder, CO shows that N2O5 can act as both a reservoir for NOx and an efficient sink that converts it to nitric acid. The total removal of NOx pollution will depend strongly on the efficiency of these processes. Diurnal averages of NO3, N2O5 and HNO3 during NEAQS show that the nitric acid production during the night is comparable to that during the day, and that N2O5 hydrolysis reactions produce a significant quantity of gas phase nitric acid.