Atmospheric photochemistry of aromatic hydrocarbons: OH budgets during SAPHIR chamber experiments

S Nehr,A Hofzumahaus,F Rohrer,H Fuchs,R Tillmann,X Li,A Wahner,B Bohn,H.-P Dorn,R Häseler

doi:10.5194/acp-14-6941-2014

Abstract

Abstract. Current photochemical models developed to simulate the atmospheric degradation of aromatic hydrocarbons tend to underestimate OH radical concentrations. In order to analyse OH budgets, we performed experiments with benzene, toluene, p-xylene and 1,3,5-trimethylbenzene in the atmosphere simulation chamber SAPHIR. Experiments were conducted under low-NO conditions (typically 0.1–0.2 ppb) and high-NO conditions (typically 7–8 ppb), and starting concentrations of 6–250 ppb of aromatics, dependent on OH rate constants. For the OH budget analysis a steady-state approach was applied in which OH production and destruction rates (POH and DOH) have to be equal. The POH were determined from measurements of HO2, NO, HONO, and O3 concentrations, considering OH formation by photolysis and recycling from HO2. The DOH were calculated from measurements of the OH concentrations and total OH reactivities. The OH budgets were determined from DOH/POH ratios. The accuracy and reproducibility of the approach were assessed in several experiments using CO as a reference compound where an average ratio DOH/POH = 1.13 ± 0.19 was obtained. In experiments with aromatics, these ratios ranged within 1.1–1.6 under low-NO conditions and 0.9–1.2 under high-NO conditions. The results indicate that OH budgets during photo-oxidation experiments with aromatics are balanced within experimental accuracies. Inclusion of a further, recently proposed OH production via HO2 + RO2 reactions led to improvements under low-NO conditions but the differences were small and insignificant within the experimental errors.

Highlights

Large amounts of volatile organic compounds (VOCs) are released into the Earth’s atmosphere from both biogenic and anthropogenic sources with an estimated global emission rate of the order of 1012 kg per year (Piccot et al, 1992; Arneth et al, 2011)
Toluene, xylenes and trimethylbenzenes are among the most abundant organic trace constituents observed in the urban environment (Fortin et al, 2005; Johnson et al, 2010)
The self-cleaning ability of the atmosphere mainly results from the presence of OH radicals that initiate the oxidation processes of most VOCs, including aromatics (Ehhalt, 1999)

Summary

Introduction

Large amounts of volatile organic compounds (VOCs) are released into the Earth’s atmosphere from both biogenic and anthropogenic sources with an estimated global emission rate of the order of 1012 kg per year (Piccot et al, 1992; Arneth et al, 2011). Major emission sources of aromatics are vehicle exhaust, solvent usage and residential wood burning (Hawthorne et al, 1988; Niedojadlo et al, 2007). Once emitted to the atmosphere, organic compounds are oxidized photochemically and are removed by wet or dry deposition. The self-cleaning ability of the atmosphere mainly results from the presence of OH radicals that initiate the oxidation processes of most VOCs, including aromatics (Ehhalt, 1999). The predominant primary atmospheric OH source is the photolysis of O3 (Finlayson-Pitts and Pitts, 2000). At wavelengths below about 330 nm, O3 is photolysed to electronically excited O(1D) that subsequently may react with water vapour to give OH: O3 + hν → O(1D) + O2 λ < 330 nm (R1)

Methods

Findings

Discussion

Conclusion