Oxidation capacity of the city air of Santiago, Chile

R. Kurtenbach,Y. F. Elshorbany,G. Villena,M. Rubio,A. R. Rickard,E. Gramsch,P. Wiesen,E. Lissi,J. Kleffmann,M. J. Pilling

doi:10.5194/acp-9-2257-2009

R. Kurtenbach, Y. F. Elshorbany + Show 8 more

Open Access

https://doi.org/10.5194/acp-9-2257-2009

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Abstract

Abstract. The oxidation capacity of the highly polluted urban area of Santiago, Chile has been evaluated during a summer measurement campaign carried out from 8–20 March 2005. The hydroxyl (OH) radical budget was evaluated employing a simple quasi-photostationary-state model (PSS) constrained with simultaneous measurements of HONO, HCHO, O3, NO, NO2, j(O1D), j(NO2), 13 alkenes and meteorological parameters. In addition, a zero dimensional photochemical box model based on the Master Chemical Mechanism (MCMv3.1) has been used to estimate production rates and total free radical budgets, including OH, HO2 and RO2. Besides the above parameters, the MCM model has been constrained by the measured CO and volatile organic compounds (VOCs) including alkanes and aromatics. Both models simulate the same OH concentration during daytime indicating that the primary OH sources and sinks included in the simple PSS model predominate. Mixing ratios of the main OH radical precursors were found to be in the range 0.8–7 ppbv (HONO), 0.9–11 ppbv (HCHO) and 0–125 ppbv (O3). The alkenes average mixing ratio was ~58 ppbC accounting for ~12% of the total identified non-methane hydrocarbons (NMHCs). During the daytime (08:00 h–19:00 h), HONO photolysis was shown to be the most important primary OH radical source comprising alone ~55% of the total initial production rate, followed by alkene ozonolysis (~24%) and photolysis of HCHO (~16%) and O3 (~5%). The calculated average and maximum daytime OH production rates from HONO photolysis was 1.7 ppbv h−1 and 3.1 ppbv h−1, respectively. Based on the experimental results a strong photochemical daytime source of HONO is proposed. A detailed analysis of the sources of OH radical precursors has also been carried out.

Highlights

The physical and chemical properties of the atmosphere are influenced by the presence of trace gases like nitrogen oxides (NOx) and volatile organic compounds (VOCs)
A zero dimensional photochemical box model containing the detailed gas phase mechanism MCMv3.1 was constrained with a suite of ancillary measurements including HONO, HCHO, O3, NOx, PAN, VOCs, j (O1D), j (NO2) and meteorological parameters
The MCM and simple photostationary-state model (PSS) models predict the same OH concentrations showing that the main radical precursors included in the PSS model are dominant and that the OH→RO2 sinks are balanced by the RO2→OH sources

Summary

Introduction

The physical and chemical properties of the atmosphere are influenced by the presence of trace gases like nitrogen oxides (NOx) and volatile organic compounds (VOCs). The oxidising capacity of the atmosphere determines the rate of their removal (Prinn, 2003), and controls the abundance of these trace gases. Since the life time of the trace gases is controlled by the oxidant concentration and by its second-order rate constant (kYi), the method of Geyer et al (2001) is most suitable to calculate the relative importance of each oxidant in the current study. A comparable method based on the reciprocal of the life time (1/τ ) of the oxidized species (Yi) was used to express the total oxidation capacity (Cheng et al, 2008)

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