Chemical apportionment of southern African aerosol mass and optical depth

B I Magi

doi:10.5194/acp-9-7643-2009

Abstract

Abstract. This study characterizes the aerosol over extratropical and tropical southern Africa during the biomass burning season by presenting an aerosol mass apportionment and aerosol optical properties. Carbonaceous aerosol species account for 54% and 83% of the extratropical and tropical aerosol mass, respectively, which is consistent with the fact that the major source of particulate matter in southern Africa is biomass burning. This mass apportionment implies that carbonaceous species in the form of organic carbon (OC) and black carbon (BC) play a critical role in the aerosol optical properties. By combining the in situ measurements of aerosol mass concentrations with concurrent measurements of aerosol optical properties at a wavelength of 550 nm, it is shown that 80–90% of the aerosol scattering is due to carbonaceous aerosol, and the derived mass scattering cross sections (MSC) for OC and BC are 3.9±0.6 m2/g and 1.6±0.2 m2/g, respectively. Derived values of mass absorption cross sections (MAC) for OC and BC are 0.7±0.6 m2/g and 8.2±1.1 m2/g, respectively. The values of MAC imply that ~26% of the aerosol absorption in southern Africa is due to OC, with the remainder due to BC. The results in this study provide important constraints for aerosol properties in a region dominated by biomass burning and should be integrated into climate models to improve aerosol simulations.

Highlights

Biomass burning is a major source of particulate matter in the atmosphere over Africa during the dry season
After applying the proxy calibration correction, the mass concentrations derived from teflon and quartz filter samples can be analyzed together with those reported by E2003
The speciated mass apportionment reveals that the primary driving force for the increase in PM2.5 is an increase in organic carbon (OC) and black carbon (BC) mass

Summary

Introduction

This study synthesizes in situ measurements of aerosol mass concentrations obtained by aircraft in the southern African regional haze and originally published in SAFARI2000 studies by Eatough et al (2003), Gao et al (2003), and Kirchstetter et al (2003). Without this solution, aerosol mass concentrations presented by Eatough et al (2003), Gao et al (2003), and Kirchstetter et al (2003) could only be analyzed independently. Magi: Chemical apportionment of southern African aerosol mass are discussed, results and analysis are presented in Sect.

Background

Miscalibrated flow meters

Proxy calibration

Results

Aerosol optical depth apportionment

Mass scattering and mass absorption cross sections