Abstract
Abstract. We demonstrate a new instrument for in situ detection of atmospheric iodine atoms and molecules based on atomic and molecular resonance and off-resonance ultraviolet fluorescence excited by lamp emission. The instrument combines the robustness, light weight, low power consumption and efficient excitation of radio-frequency discharge light sources with the high sensitivity of the photon counting technique. Calibration of I2 fluorescence is achieved via quantitative detection of the molecule by Incoherent Broad Band Cavity-enhanced Absorption Spectroscopy. Atomic iodine fluorescence signal is calibrated by controlled broad band photolysis of known I2 concentrations in the visible spectral range at atmospheric pressure. The instrument has been optimised in laboratory experiments to reach detection limits of 1.2 pptv for I atoms and 13 pptv for I2, for S/N = 1 and 10 min of integration time. The ROFLEX system has been deployed in a field campaign in northern Spain, representing the first concurrent observation of ambient mixing ratios of iodine atoms and molecules in the 1–350 pptv range.
Highlights
Studies of atmospheric iodine chemistry have been mainly motivated by its impact on the oxidizing capacity of the marine boundary layer (MBL) by catalyzing O3 destruction (Davis et al, 1996; Allan et al, 2000; Saiz-Lopez et al, 2007; Read et al, 2008)
For continuous instrument data logging the photon counting modules (PCM) are interfaced to a PC via an external USB-RS232 multiport interface. 5V DC power for the PCMs and 24V DC for pressure transducers, relays (used to open and close solenoid valves and shutters) and mass flow controllers (MFC)
In order to study the influence of water vapour on the fluorescence signal, fractions of the carrier gas flow were passed through a bubbler containing de-ionised water, resulting in a range of relative humidities (RH) between 50% and 100%
Summary
Studies of atmospheric iodine chemistry have been mainly motivated by its impact on the oxidizing capacity of the marine boundary layer (MBL) by catalyzing O3 destruction (Davis et al, 1996; Allan et al, 2000; Saiz-Lopez et al, 2007; Read et al, 2008). The most important development with respect to Bale et al (2008) is the inclusion of a second detector for collection of molecular iodine off-resonance fluorescence (ORF) This enables: (i) using a much simpler method for calibration of the fluorescence signal by I2 absorption and photolysis, and (ii) measuring concurrently and directly a major iodine source (I2) and active iodine atoms (I). Oscillator strength measurements (see Table 2) indicate that the 178.276 nm transition is ∼20 times stronger than the 183.083 nm line (Clyne and Townsend, 1974; Spietz et al, 2001) Such factor is not observed in emission spectra from iodine lamps (Hikida et al, 1983; Loewenstein and Anderson, 1985; Aleksandrov et al, 1985) as a result of the relative population of the two states of the 2[2]J multiplet. Where the bracketed part is the absorption factor (convolution of the absorption cross section σ (λ) and the lamp photon flux F (λ) over a resonance line, see e.g. Ingle and Crouch, 1988), Y (Pcell) is the fluorescence yield from the 5p4(3P2)6s(2[2]J ) state, is a detection efficiency factor, encompassing geometrical and optical factors and detector quantum efficiency (assuming an uniform spectral sensitivity), τ is the attenuation due to absorption of excitation and fluorescence radiation by other species present in the sample, γ is the sampling efficiency
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