Radicals in the marine boundary layer during NEAQS 2004: a model study of day-time and night-time sources and sinks

• Main Authors: R. Sommariva, H. D. Osthoff, S. S. Brown, T. S. Bates, T. Baynard, D. Coffman, J. A. de Gouw, P. D. Goldan, W. C. Kuster, B. M. Lerner, H. Stark, C. Warneke, E. J. Williams, F. C. Fehsenfeld, A. R. Ravishankara, M. Trainer
• Format: Article
• Language: English
• Published: Copernicus Publications 2009-05-01
• Series: Atmospheric Chemistry and Physics
• Online Access: http://www.atmos-chem-phys.net/9/3075/2009/acp-9-3075-2009.pdf

This paper describes a modelling study of several HO<sub>x</sub> and NO<sub>x</sub> species (OH, HO<sub>2</sub>, organic peroxy radicals, NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub>) in the marine boundary layer. A model based upon the Master Chemical Mechanism (MCM) was constrained to observations of chemical and physical parameters made onboard the NOAA ship R/V <i>Brown</i> as part of the New England Air Quality Study (NEAQS) in the summer of 2004. The model was used to calculate [OH] and to determine the composition of the peroxy radical pool. Modelled [NO<sub>3</sub>] and [N<sub>2</sub>O<sub>5</sub>] were compared to in-situ measurements by Cavity Ring-Down Spectroscopy. The comparison showed that the model generally overestimated the measurements by 30–50%, on average. <br><br> The model results were analyzed with respect to several chemical and physical parameters, including uptake of NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> on fog droplets and on aerosol, dry deposition of NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub>, gas-phase hydrolysis of N<sub>2</sub>O<sub>5</sub> and reactions of NO<sub>3</sub> with NMHCs and peroxy radicals. The results suggest that fog, when present, is an important sink for N<sub>2</sub>O<sub>5</sub> via rapid heterogeneous uptake. The comparison between the model and the measurements were consistent with values of the heterogeneous uptake coefficient of N<sub>2</sub>O<sub>5</sub> (γ<sub>N<sub>2</sub>O<sub>5</sub></sub>)>1×10<sup>−2</sup>, independent of aerosol composition in this marine environment. The analysis of the different loss processes of the nitrate radical showed the important role of the organic peroxy radicals, which accounted for a significant fraction (median: 15%) of NO<sub>3</sub> gas-phase removal, particularly in the presence of high concentrations of dimethyl sulphide (DMS).