In this contribution a new analytical technique is presented for the direct mass spectrometric (MS) detection of gas-phase trace species at atmospheric pressure. Employing resonance-enhanced multiphoton ionization (REMPI) close to the inlet nozzle orifice, i.e., at high molecule densities, the sensitivity of the instrument has been increased by up to 3 orders of magnitude as compared to conventional REMPI-MS with ionization in a differentially coupled ion source. Furthermore, adiabatic cooling, resonant ionization, and mass-selective detection establish a highly selective analytical technique. Several atmospherically relevant compounds were investigated. The current detection limit for NO is 0.9 pptv, for acetaldehyde 1.7 pptv, for CO 15 pptv, and for 2,5-dichlorotoluene 12 pptv. We discuss APLI with regard to applications in medical and environmental research.

We report on the development of a quasi-simultaneous highly selective method for NO and NO2 detection at the ultratrace level. Atmospheric pressure laser ionization (APLI), recently introduced by our group, is used to detect both compounds at low parts per trillion by volume (pptv) mixing ratios. APLI is based on resonance-enhanced multiphoton ionization mass spectrometry. Two-color pump-probe experiments employing a single excimer pumped dye laser combination allow for the ultrasensitive measurement of NO and NO2 within a narrow range of maximum pumping efficiency of the laser dye Coumarin 120. NO is detected via excitation of the long-lived A 2sigma+ (nu' = 1) level at 215.36 nm and subsequently ionized with 308-nm radiation provided by the excimer pump laser. NO2 is ionized after double resonant excitation of the A2B1 and 3psigma manifolds in a (1 + 1' + 1(')) process using 431.65 + 308 nm. The selectivity of the NO measurement exceeds 2,000 with respect to NO2 and N2O5. For NO2, a selectivity of>3,000 with respect to N2O5 and organic nitrates is observed. The current APLI detection limit of NO and NO2 is 0.5 and 5 pptv, respectively, with a 20-s integration time.

Experiments presented in this contribution demonstrate a heterogeneous source of several chlorine oxides, in particular OClO and ClClO 2 , from ClO radicals passed over water-ice surfaces at low pressures. ClO radicals were generated in a flow system and reaction products were monitored by UV-vis absorption spectroscopy and time-of-flight mass spectrometry using electron-impact and resonance-enhanced multiphoton ionization, respectively. In all experiments an efficient release of OClO into the gas phase was observed upon ice eVaporation. No such gas-phase products directly formed in the initial interaction of ClO with the ice surface were detected. To explain these findings, it is proposed that a ClO‚H 2 O complex is formed in a rapidly established equilibrium between ClO monomers and gas-phase H 2 O. The existence of this complex as well as a surface-enhanced ClO-recombination process is assumed to be responsible for the observed efficient reactive uptake of ClO radicals onto the ice surface. Several possible reaction pathways resulting in the formation of the experimentally observed products are presented. As an alternative pathway, H 2 O-facilitated gas phase disproportionation of ClO yielding hypochlorous and chlorous acid and subsequent deposition on the surface is considered. The observation of OClO evolving from an ice surface previously exposed to ClO radicals, as well as the lack of any symmetric ClOOCl dimer formation in the presence of high water mixing ratios, carries some possible atmospheric implications: First, there is currently a missing source of OClO in the chemically perturbed polar vortex. The ClO + BrO reaction is presently believed to be the only source of OClO in the stratosphere, although several studies show this reaction system to severely underestimate OClO production in this atmospheric subsystem. It is suggested that this experimentally observed heterogeneous source of OClO could carry implications for the total atmospheric OClO budget. Second, the ClO‚H 2 O complex could directly or indirectly affect the ClOOCl formation rate and thus strongly impact homogeneous chlorine chemistry.

In this paper we demonstrate the feasibility of ultra-trace resonance enhanced multiphoton ionization (REMPI) detection employing a small broad-bandwidth solid state laser system. The results reported here are compared with measurements carried out with a conventional excimer pumped dye laser combination. Mass selected broad-bandwidth REMPI spectra for the environmentally relevant nitrogen oxides NO and NO2 are presented. Tunable broad-bandwidth laser radiation with a spectral resolution of > 10 cm(-1) in the wavelength range 560-400 nm was employed for the detection of NO2. For NO detection, the range 230-224 nm was covered. Laser radiation was generated using an optical parametric oscillator pumped by an unseeded Nd:YAG laser. A mobile time-of-flight mass spectrometer equipped with an atmospheric pressure laser ionization source allowed for mass selective parent ion detection at m/z 30 for NO and m/z 46 for NO2. The limit of detection was 10 pptV for NO and 20 pptV for NO2. A selectivity of > 2000 for both compounds with respect to N2O5, organic nitrates and NO2 in the case of NO is reported. An improved laser system currently under construction is expected to provide detection limits below pptv mixing ratios for both nitrogen oxides in a 20 s integration interval.

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