Long-term high frequency measurements of ethane, benzene and methyl chloride at Ragged Point, Barbados: Identification of long-range transport events

A.T Archibald,C.S Witham,B.R Greally,D Young,D.E Shallcross,A.J Manning,S O’Doherty,M.J Ashfold,Maria Val Martin,Detlev Helmig

doi:10.12952/journal.elementa.000068

Abstract

Abstract Here we present high frequency long-term observations of ethane, benzene and methyl chloride from the AGAGE Ragged Point, Barbados, monitoring station made using a custom built GC-MS system. Our analysis focuses on the first three years of data (2005–2007) and on the interpretation of periodic episodes of high concentrations of these compounds. We focus specifically on an exemplar episode during September 2007 to assess if these measurements are impacted by long-range transport of biomass burning and biogenic emissions. We use the Lagrangian Particle Dispersion model, NAME, run forwards and backwards in time to identify transport of air masses from the North East of Brazil during these events. To assess whether biomass burning was the cause we used hot spots detected using the MODIS instrument to act as point sources for simulating the release of biomass burning plumes. Excellent agreement for the arrival time of the simulated biomass burning plumes and the observations of enhancements in the trace gases indicates that biomass burning strongly influenced these measurements. These modelling data were then used to determine the emissions required to match the observations and compared with bottom up estimates based on burnt area and literature emission factors. Good agreement was found between the two techniques highlight the important role of biomass burning. The modelling constrained by in situ observations suggests that the emission factors were representative of their known upper limits, with the in situ data suggesting slightly greater emissions of ethane than the literature emission factors account for. Further analysis was performed concluding only a small role for biogenic emissions of methyl chloride from South America impacting measurements at Ragged Point. These results highlight the importance of long-term high frequency measurements of NMHC and ODS and highlight how these data can be used to determine sources of emissions 1000’s km away.

Highlights

Long-term trace gas measurement programs have provided invaluable data to further our understanding of the composition and chemistry of the atmosphere; for example the oxidizing capacity of the troposphere (Montzka et al, 2011a; Prinn et al, 2005)
In this study we have used a Lagrangian particle dispersion model (NAME) to investigate the causes of observed enhancements in methyl chloride, ethane and benzene made at the Advanced Global Atmospheric Gases Experiment (AGAGE) monitoring site at Ragged Point, Barbados, during an exemplar event occurring throughout September 2007, and to estimate emissions for the compounds investigated
NAME was run backwards in time from Ragged Point to produce air history maps for each measurement made. Analysis of these air history maps revealed that the majority of air mass history during September 2007 was reflective of background/clean air from the North Atlantic, and that during the enhancement events, air masses originated from the Southern Hemisphere – heavily influenced by North East Brazil (Figure 2)

Summary

Introduction

Long-term trace gas measurement programs have provided invaluable data to further our understanding of the composition and chemistry of the atmosphere; for example the oxidizing capacity of the troposphere (Montzka et al, 2011a; Prinn et al, 2005). Long-range transport of biomass burning and biogenic emissions in Barbados research stations spread throughout the globe making high frequency (∼hourly) atmospheric measurements of a range of Ozone Depleting Substances (ODS), controlled under the Montreal protocol, and Greenhouse Gases (GHG), controlled under the Kyoto protocol. These measurements have, for example, provided hard evidence for the phase out of CFCs and the rise of their replacements (Derwent et al, 2007). In section six we discuss the results of emission estimate calculations, whilst section seven and eight are dedicated to the discussion of the results and conclusions of the study

Objectives

Findings

Discussion

Conclusion