Modelling multi-phase halogen chemistry in the coastal marine boundary layer: investigation of the relative importance of local chemistry vs. long-range transport

D Lowe,J R Dorsey,G Mcfiggans,J Ryder,R Leigh

doi:10.5194/acp-11-979-2011

Abstract

Abstract. Measurements of significant concentrations of IO, I2 and BrO in a semi-polluted coast environment at Roscoff, in North-West France, have been made as part of the Reactive Halogens in the Marine Boundary Layer (RHaMBLe) campaign undertaken in September 2006. We use a one-dimensional column model, with idealised I2 emissions predicted using macroalgael maps and tidal data from the littoral area surrounding Roscoff, to investigate the probable causes for these observations. The coupled microphysical and chemical aerosol model simulates mixed-phase halogen chemistry using two separate particle modes, seasalt and non-seasalt, each comprising of eight size-sections. This work confirms the finding of a previous study that the BrO measurements are most likely caused by unknown, local sources. We find that the remote observations of IO and I2 are best replicated using the I2 recycling mechanism suggested by previous studies, but that such a mechanism is not wholly necessary. However in-situ measurements of I2 can only be explained by invoking an I2 recycling mechanism. We suggest that focussed observations of the changes in NOx and NOy concentrations, as well as changes in the nitrate fraction of the non-seasalt aerosol mode, in the presence of I2 bursts could be used to determine the atmospheric relevance of the predicted I2 recycling mechanism.

Highlights

Research into tropospheric iodine and bromine radical species has been driven by their potential to influence important atmospheric chemical processes. These include changing the oxidative capacity of the atmosphere via destruction of ozone in catalytic cycles (Davis et al, 1996; Read et al, 2008), and by altering the partitioning of NOx and HOx (McFiggans et al, 2000; Platt and Honninger, 2003; Bloss et al, 2005); and the formation and growth of new particles (Hoffmann et al, 2001; O’Dowd et al, 2002; McFiggans et al, 2004)
We have reduced the midday peak photolysis rates for I2O2, I2O3 and I2O4 by 50% to 0.03 to match the reductions in the rest of the photolysis rates
Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) IO and OIO observations are likely the result of localised emissions from macroalgae, which are diluted with cleaner air to get average mixing ratios

Summary

Introduction

Research into tropospheric iodine and bromine radical species has been driven by their potential to influence important atmospheric chemical processes. These include changing the oxidative capacity of the atmosphere via destruction of ozone in catalytic cycles (Davis et al, 1996; Read et al, 2008), and by altering the partitioning of NOx and HOx (McFiggans et al, 2000; Platt and Honninger, 2003; Bloss et al, 2005); and the formation and growth of new particles (Hoffmann et al, 2001; O’Dowd et al, 2002; McFiggans et al, 2004). The Reactive Halogens in the Marine Boundary Layer (RHaMBLe) project at Roscoff, Brittany, was designed to investigate Marine Boundary Layer (MBL) chemistry in semipolluted conditions. See McFiggans et al (2010) for a detailed overview of the campaign

Objectives

Results

Conclusion