Abstract. Hydroxyl (OH) radical reactivity (k OH ) has been measured for 18 years with different measurement techniques. In order to compare the performances of instruments deployed in the field, two campaigns were conducted performing experiments in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich in October 2015 and April 2016. Chemical conditions were chosen either to be representative of the atmosphere or to test potential limitations of instruments. All types of instruments that are currently used for atmospheric measurements were used in one of the two campaigns. The results of these campaigns demonstrate that OH reactivity can be accurately measured for a wide range of atmospherically relevant chemical conditions (e.g. water vapour, nitrogen oxides, various organic compounds) by all instruments. The precision of the measurements (limit of detection < 1 s −1 at a time resolution of 30 s to a few minutes) is higher for instruments directly detecting hydroxyl radicals, whereas the indirect comparative reactivPublished by Copernicus Publications on behalf of the European Geosciences Union. H. Fuchs et al.: OH reactivity comparison in SAPHIRity method (CRM) has a higher limit of detection of 2 s −1 at a time resolution of 10 to 15 min. The performances of the instruments were systematically tested by stepwise increasing, for example, the concentrations of carbon monoxide (CO), water vapour or nitric oxide (NO). In further experiments, mixtures of organic reactants were injected into the chamber to simulate urban and forested environments. Overall, the results show that the instruments are capable of measuring OH reactivity in the presence of CO, alkanes, alkenes and aromatic compounds. The transmission efficiency in Teflon inlet lines could have introduced systematic errors in measurements for low-volatile organic compounds in some instruments. CRM instruments exhibited a larger scatter in the data compared to the other instruments. The largest differences to reference measurements or to calculated reactivity were observed by CRM instruments in the presence of terpenes and oxygenated organic compounds (mixing ratio of OH reactants were up to 10 ppbv). In some of these experiments, only a small fraction of the reactivity is detected. The accuracy of CRM measurements is most likely limited by the corrections that need to be applied to account for known effects of, for example, deviations from pseudo first-order conditions, nitrogen oxides or water vapour on the measurement. Methods used to derive these corrections vary among the different CRM instruments. Measurements taken with a flowtube instrument combined with the direct detection of OH by chemical ionisation mass spectrometry (CIMS) show limitations in cases of high reactivity and high NO concentrations but were accurate for low reactivity (< 15 s −1 ) and low NO (< 5 ppbv) conditions.

Abstract. The calibration of field instruments used to measure concentrations of OH and HO2 worldwide has traditionally relied on a single method utilising the photolysis of water vapour in air in a flow tube at atmospheric pressure. Here the calibration of two FAGE (fluorescence assay by gaseous expansion) apparatuses designed for HOx (OH and HO2) measurements have been investigated as a function of external pressure using two different laser systems. The conventional method of generating known concentrations of HOx from H2O vapour photolysis in a turbulent flow tube impinging just outside the FAGE sample inlet has been used to study instrument sensitivity as a function of internal fluorescence cell pressure (1.8–3.8 mbar). An increase in the calibration constants CHO and CHO2 with pressure was observed, and an empirical linear regression of the data was used to describe the trends, with ΔCHO = (17 ± 11) % and ΔCHO2 = (31.6 ± 4.4)% increase per millibar air (uncertainties quoted to 2σ). Presented here are the first direct measurements of the FAGE calibration constants as a function of external pressure (440–1000 mbar) in a controlled environment using the University of Leeds HIRAC chamber (Highly Instrumented Reactor for Atmospheric Chemistry). Two methods were used: the temporal decay of hydrocarbons for calibration of OH, and the kinetics of the second-order recombination of HO2 for HO2 calibrations. Over comparable conditions for the FAGE cell, the two alternative methods are in good agreement with the conventional method, with the average ratio of calibration factors (conventional : alternative) across the entire pressure range, COH(conv)/COH(alt) = 1.19 ± 0.26 and CHO2(conv)/CHO2(alt) = 0.96 ± 0.18 (2σ). These alternative calibration methods currently have comparable systematic uncertainties to the conventional method: ~ 28% and ~ 41% for the alternative OH and HO2 calibration methods respectively compared to 35% for the H2O vapour photolysis method; ways in which these can be reduced in the future are discussed. The good agreement between the very different methods of calibration leads to increased confidence in HOx field measurements and particularly in aircraft-based HOx measurements, where there are substantial variations in external pressure, and assumptions are made regarding loss rates on inlets as a function of pressure.

Abstract. OH reactivity (k OH ) is the total pseudo-first-order loss rate coefficient describing the removal of OH radicals to all sinks in the atmosphere, and is the inverse of the chemical lifetime of OH. Measurements of ambient OH reactivity can be used to discover the extent to which measured OH sinks contribute to the total OH loss rate. Thus, OH reactivity measurements enable determination of the comprehensiveness of measurements used in models to predict air quality and ozone production, and, in conjunction with measurements of OH radical concentrations, to assess our understanding of OH production rates. In this work, we describe the design and characterisation of an instrument to measure OH reactivity using laser flash photolysis coupled to laserinduced fluorescence (LFP-LIF) spectroscopy. The LFP-LIF technique produces OH radicals in isolation, and thus minimises potential interferences in OH reactivity measurements owing to the reaction of HO 2 with NO which can occur if HO 2 is co-produced with OH in the instrument. Capabilities of the instrument for ambient OH reactivity measurements are illustrated by data collected during field campaigns in London, UK, and York, UK. The instrumental limit of detection for k OH was determined to be 1.0 s −1 for the campaign in London and 0.4 s −1 for the campaign in York. The precision, determined by laboratory experiment, is typically < 1 s −1 for most ambient measurements of OH reactivity. Total uncertainty in ambient measurements of OH reactivity is ∼ 6 %. We also present the coupling and characterisation of the LFP-LIF instrument to an atmospheric chamber for measurements of OH reactivity during simulated experiments, and provide suggestions for future improvements to OH reactivity LFP-LIF instruments.

Abstract.The reaction CH 3 C(O)O 2 + HO 2 → CH 3 C(O)OOH + O 2 (Reaction R5a), CH 3 C(O)OH + O 3 (Reaction R5b), CH 3 + CO 2 + OH + O 2 (Reaction R5c) was studied in a series of experiments conducted at 1000 mbar and (293 ± 2) K in the HIRAC simulation chamber. For the first time, products, (CH 3 C(O)OOH, CH 3 C(O)OH, O 3 and OH) from all three branching pathways of the reaction have been detected directly and simultaneously. Measurements of radical precursors (CH 3 OH, CH 3 CHO), HO 2 and some secondary products HCHO and HCOOH further constrained the system. Fitting a comprehensive model to the experimental data, obtained over a range of conditions, determined the branching ratios α (R5a) = 0.37 ± 0.10, α (R5b) = 0.12 ± 0.04 and α (R5c) = 0.51 ± 0.12 (errors at 2σ level). Improved measurement/model agreement was achieved using k (R5) = (2.4 ± 0.4) × 10 −11 cm 3 molecule −1 s −1 , which is within the large uncertainty of the current IUPAC and JPL recommended rate coefficients for the title reaction. The rate coefficient and branching ratios are in good agreement with a recent study performed by Groß et al. (2014b); taken together, these two studies show that the rate of OH regeneration through Reaction (R5) is more rapid than previously thought. GEOS-Chem has been used to assess the implications of the revised rate coefficients and branching ratios; the modelling shows an enhancement of up to 5 % in OH concentrations in tropical rainforest areas and increases of up to 10 % at altitudes of 6-8 km above the equator, compared to calculations based on the IUPAC recommended rate coefficient and yield. The enhanced rate of acetylperoxy consumption significantly reduces PAN in remote regions (up to 30 %) with commensurate reductions in background NO x .

Abstract. Nitrous acid (HONO) has been quantitatively measured in situ by differential photolysis at 385 and 395 nm, and subsequent detection as nitric oxide (NO) by the chemiluminescence reaction with ozone (O3). The technique has been evaluated by Fourier transform infrared (FT-IR) spectroscopy to provide a direct HONO measurement in a simulation chamber and compared side by side with a long absorption path optical photometer (LOPAP) in the field. The NO–O3 chemiluminescence technique is robust, well characterized, and capable of sampling at low pressure, whilst solid-state converter technology allows for unattended in situ HONO measurements in combination with fast time resolution and response.