Direct measurement of the total reaction rate of OH in the atmosphere

Abstract

In tropospheric chemistry, OH plays a key role. For instance it is essential in converting many trace species from water-insoluble to water-soluble forms, thus assisting in their deposition rate. Due to OH high reactivity, its concentration in the planetary boundary layer (PBL) is very low, at most of the order of 10 6 radicals cm -3. Thus routine measurement of its steady state concentration for comparison with model calculations is non trivial [1,2]. Furthermore, even in the more complete models there still remains some uncertainty as to some of the sources and sinks of OH. Among others it is still somewhat unclear where all the HONO comes from that acts as an early daytime source of OH. Furthermore it is not likely that as yet all the multitude of hydrocarbons that act as non-negligible sinks have been identified.In the present paper we propose a method for investigating the fast kinetics of OH in situ in the atmosphere using a perturbation technique in which high concentration of OH are created by the laser flash photolysis of atmospheric O 3 followed by the reaction of O( 1 D) with H 2 O. Typically we have locally enhanced the hydroxyl radical concentration by 5 orders of magnitude. Thus the OH is easier to detect and its fast disappearance can be followed over a sufficient time period to try to quantify the reaction kinetics. As we shall discuss below, this fast OH kinetics can be reduced to contributions from a small number of reactions, essentially with CO, O 3 , various volatile organic compounds (VOC), NO x and the reaction with OH itself. Consequently if we simultaneously measure the concentrations of CO, O 3 and NO x with the overall OH kinetics, we shall be able to deduce a total hydrocarbon reactivity Σk i [OH] [VOC i ] from the difference between the measured overall OH decay rate and the sum of the reaction rates with CO, O 3 , NO x and OH itself. In a controlled atmosphere with known Σk i [OH] [VOC i ], one can check for possible unknown fast OH reactions. Hence we shall be able to deduce if the fast kinetics of OH in the atmosphere is complete and well understood under condition that the measurements are sufficiently accurate. A final reason for undertaking these measurements of the fast kinetics of OH was that it seemed like a reasonable first choice to test the pump and probe technique for obtaining new information on atmospheric chemistry. Below we describe the experimental method in some detail, and we compare the measured kinetics with those obtained in a model calculation. Experimental data are presented both from controlled atmospheres with known concentrations of chemical constituents, and from the natural PBL. We also mention the possibility of extending these perturbation kinetics measurements from the in situ case to the range resolved remote sensing case, using a DIfferential Absorption Lidar (DIAL) technique. Finally it will be briefly discussed that measurement of Σk i [OH] [VOC i ] could possibly lead to information on local NO x versus VOC limiting of the production of tropos...

A flow-tube based laser-induced fluorescence instrument to measure OH reactivity in the troposphere

“…The lower flow rates (relative to the flow tube technique) through the cell minimise the consumption of ultra clean air for laboratory calibrations, and, since only a small central portion of the cell is photolysed by the laser, radical wall losses are minimised. A related remote sensing method, using a two laser pump-probe lidar, was proposed to measure the OH reactivity, but has not been used for routine measurements (Calpini et al, 1999).…”

Intercomparison of two comparative reactivity method instruments inf the Mediterranean basin during summer 2013

“…Currently, three methods exist to perform direct measurements of total OH reactivity, such as the total OH loss rate measurement (TOLRM) (Kovacs and Brune, 2001;Mao et al, 2009;Ingham et al, 2009;; Pump and probe technique (Calpini et al, 1999;Sadanaga et al, 2004;Yoshino et al, 2006;Lou et al, 2010) and the comparative reactivity method (CRM) (Sinha et al, 2008;Nölscher et al, 2012a;Dolgorouky et al, 2012;Kim et al, 2011;Kumar and Sinha, 2014). A detailed comparison of the three methods can be found in Nölscher et al (2012a) and Hansen et al (2015).…”

“…The pump and probe technique was first pioneered by Calpini et al (1999) and Jeanneret et al (2001) and then adapted by other groups (Sadanaga et al, 2004, Yoshino et al, 2006and Lou et al, 2010. The instrument consists of three main parts: a flow tube to sample ambient air, a pulsed laser to generate OH in the sampling reactor, and a FAGE apparatus to quantify OH.…”

Total OH reactivity measurements using a new fast Gas Chromatographic Photo-Ionization Detector (GC-PID)

“…The first direct measurements of atmospheric total OH reactivity were carried out by Calpini et al (1999) and Jeanneret et al (2001). Later the "pump-and-probe" technique was improved by Sadanaga et al (2004).…”