Improved model of isoprene emissions in Africa using Ozone Monitoring Instrument (OMI) satellite observations of formaldehyde: implications for oxidants and particulate matter

E A Marais,C E Reeves,J G Murphy,K Chance,A Guenther,T P Kurosu,H O T Pye,D J Jacob

doi:10.5194/acp-14-7693-2014

Abstract

Abstract. We use a 2005–2009 record of isoprene emissions over Africa derived from Ozone Monitoring Instrument (OMI) satellite observations of formaldehyde (HCHO) to better understand the factors controlling isoprene emission in the continent and evaluate the impact on atmospheric composition. OMI-derived isoprene emissions show large seasonality over savannas driven by temperature and leaf area index (LAI), and much weaker seasonality over equatorial forests driven by temperature. The commonly used MEGAN (Model of Emissions of Gases and Aerosols from Nature, version 2.1) global isoprene emission model reproduces this seasonality but is biased high, particularly for equatorial forests, when compared to OMI and relaxed-eddy accumulation measurements. Isoprene emissions in MEGAN are computed as the product of an emission factor Eo, LAI, and activity factors dependent on environmental variables. We use the OMI-derived emissions to provide improved estimates of Eo that are in good agreement with direct leaf measurements from field campaigns (r = 0.55, bias = −19%). The largest downward corrections to MEGAN Eo values are for equatorial forests and semi-arid environments, and this is consistent with latitudinal transects of isoprene over western Africa from the African Monsoon Multidisciplinary Analysis (AMMA) aircraft campaign. Total emission of isoprene in Africa is estimated to be 77 Tg C a−1, compared to 104 Tg C a−1 in MEGAN. Simulations with the GEOS-Chem oxidant–aerosol model suggest that isoprene emissions increase mean surface ozone in western Africa by up to 8 ppbv, and particulate matter by up to 1.5 μg m−3, due to coupling with anthropogenic influences.

Highlights

Isoprene is the dominant biogenic non-methane volatile organic compound (NMVOC) emitted by vegetation, accounting for about 50 % of global NMVOC emissions in current inventories (Olivier et al,. 1996; Guenther et al, 2006)
We used a 2005–2009 data set of monthly isoprene emissions in Africa derived from Ozone Monitoring Instrument (OMI) satellite observations of HCHO to study the factors controlling these emissions in different areas of the continent
Our goal was to achieve a better representation of isoprene emission in chemical transport model (CTM), in part through evaluation and improvement of the commonly used MEGAN emission inventory, and to examine the implications for oxidants and aerosols over the continent

Summary

Introduction

Isoprene is the dominant biogenic non-methane volatile organic compound (NMVOC) emitted by vegetation, accounting for about 50 % of global NMVOC emissions in current inventories (Olivier et al,. 1996; Guenther et al, 2006). Isoprene affects the oxidative capacity of the atmosphere through reaction with OH (Ren et al, 2008; Lelieveld et al, 2008) and as a precursor of O3 (Trainer et al, 1987). It is an important precursor for secondary organic aerosols (SOAs) (Claeys et al, 2004) and a temporary reservoir for nitrogen oxide radicals (NOx ≡ NO + NO2) by formation. The widely used global Model of Emissions of Gases and Aerosols from Nature (MEGAN; Guenther et al, 2006, 2012) indicates that 80 % of global isoprene emission takes place in the tropics and 25 % in Africa, but there are large uncertainties in these estimates due to lack of data. We use our OMI-derived isoprene emissions evaluated with local data to better understand the factors controlling isoprene emissions in Africa, improve emission estimates for different African plant functional types (PFTs), and assess the implications for atmospheric oxidants and aerosols

Objectives

Results

Conclusion