Long-term in situ observations of biomass burning aerosol at a high altitude station in Venezuela – sources, impacts and interannual variability

T Hamburger,M Matisāns,T Schmeissner,J Ström,P Hoffmann,S Calderon,R Krejci,G Hochschild,A Wiedensohler,P Tunved,J Gross

doi:10.5194/acp-13-9837-2013

Abstract

Abstract. First long-term observations of South American biomass burning aerosol within the tropical lower free troposphere are presented. The observations were conducted between 2007 and 2009 at a high altitude station (4765 m a.s.l.) on the Pico Espejo, Venezuela. Sub-micron particle volume, number concentrations of primary particles and particle absorption were observed. Orographic lifting and shallow convection leads to a distinct diurnal cycle at the station. It enables measurements within the lower free troposphere during night-time and observations of boundary layer air masses during daytime and at their transitional regions. The seasonal cycle is defined by a wet rainy season and a dry biomass burning season. The particle load of biomass burning aerosol is dominated by fires in the Venezuelan savannah. Increases of aerosol concentrations could not be linked to long-range transport of biomass burning plumes from the Amazon basin or Africa due to effective wet scavenging of particles. Highest particle concentrations were observed within boundary layer air masses during the dry season. Ambient sub-micron particle volume reached 1.4±1.3 μm3 cm−3, refractory particle number concentrations (at 300 °C) 510±420 cm−3 and the absorption coefficient 0.91±1.2 Mm−1. The respective concentrations were lowest within the lower free troposphere during the wet season and averaged at 0.19±0.25 μm3 cm−3, 150±94 cm−3 and 0.15±0.26 Mm−1. A decrease of particle concentrations during the dry seasons from 2007–2009 could be connected to a decrease in fire activity in the wider region of Venezuela using MODIS satellite observations. The variability of biomass burning is most likely linked to the El Niño–Southern Oscillation (ENSO). Low biomass burning activity in the Venezuelan savannah was observed to follow La Niña conditions, high biomass burning activity followed El Niño conditions.

Highlights

Atmospheric aerosol particles directly scatter and absorb sunlight (e.g. Haywood and Boucher, 2000) or indirectly perturb the solar radiation by changing the formation and life cycle of clouds (e.g. Lohmann and Feichter, 2005)
The meteorological conditions at the Pico Espejo and their seasonal variations are controlled by the north–south shift of the Intertropical Convergence Zone (ITCZ)
The influence of the ITCZ gets more prominent during the northern hemispheric summer when it migrates towards the north

Summary

Introduction

Atmospheric aerosol particles directly scatter and absorb sunlight (e.g. Haywood and Boucher, 2000) or indirectly perturb the solar radiation by changing the formation and life cycle of clouds (e.g. Lohmann and Feichter, 2005). Lohmann and Feichter, 2005). Atmospheric aerosol particles directly scatter and absorb sunlight Knowledge on their physical and chemical properties and temporal and spatial variability is of major importance when investigating the earth-climate system. Particles in the atmosphere can be considered as a rather short-lived radiatively active species. Their properties feature a high variability in space and time. This makes the quantitative estimate of their climatic effects. Recent and ongoing work helps to reduce these uncertainties (e.g. Forster et al, 2007; Myhre, 2009; Shindell et al, 2009, and references therein)

Methods

Results

Conclusion