Pieter G. van Zyl

Rapid changes in biomass burning aerosols by atmospheric oxidation

Partially funded by the Academy of Finland (132640, Finnish Centre of Excellence 141135), the Saastamoinen Foundation, and the North-West University (South Africa)

Reevaluating the contribution of sulfuric acid and the origin of organic compounds in atmospheric nanoparticle growth

Aerosol particles formed in the atmosphere are important to the Earths climate system due to their ability to affect cloud properties. At present, little is known about the atmospheric chemistry responsible for the growth of newly formed aerosol particles to climate-relevant sizes. Here combining detailed aerosol measurements with a theoretical framework we found that depending on the gaseous precursors and size of the newly formed particles, the growth was dominated by either sulfuric acid accompanied by ammonium or organic compounds originating in either biogenic emissions or savannah fires. The contribution of sulfuric acid was larger during the early phases of the growth, but in clean conditions organic compounds dominated the growth from 1.5 nm up to climatically relevant sizes. Furthermore, our analysis indicates that in polluted environments the contribution of sulfuric acid to the growth may have been underestimated by up to a factor of 10.

Chapter 20 - Ozone Concentrations and Their Potential Impacts on Vegetation in Southern Africa

Abstract The tropospheric ozone concentrations over southern Africa are relatively high due to large precursor emissions from biomass burning, vegetation and anthropogenic sources and the intensity of solar radiation. In this study, we summarize ozone concentration data from both literature and new measurements. Further, we present data on the accumulated vegetation exposure to ozone in terms of the AOT40 index, which is calculated for the sites with hourly ozone and radiation data, with an aim to assess the potential for adverse effects on vegetation. As the seasonal variation of ozone concentrations is large, we discuss the effect of growing season onset on the stomatal gas exchange and the related potential for vegetation impacts. Finally, future research needs are discussed.

Submicrometer aerosols and excess CO as tracers for biomass burning air mass transport over southern Africa

This paper links surface measurements of biomass burning aerosols and trace gases with trajectory analysis to determine transport pathways for air masses with high and low concentrations. We interpret the long-term atmospheric monitoring record from a remote monitoring station in central southern Africa (North West Province, South Africa). Trace gas analyzers and a Differential Mobility Particle Sizer were used to measure ground level trace gas and submicron aerosol concentrations. Fire signatures were identified based on excess CO above average tropospheric levels, and episodes of enhanced particulate matter concentrations in the 10 to 840 nm size range. Thirty-six biomass burning plumes were reported; 7 had strong signals of excess CO, with ratios between 0.41 and 0.64, while 29 had weak signals ranging between 0.07 and 0.32. Pathways identified for the long-range transport of biomass burning aerosols were as follows: easterly (39% frequency), southwesterly (31%), recirculation (22%), and northerly (8%) flow patterns. CO and Aitken-mode aerosol number strengths were larger for fire emissions arriving in the easterly and southwesterly air masses than for recirculation and northerly air masses. Easterly and southwesterly flows were dominated by Aitken-mode aerosol, whereas accumulation-mode particles dominated in the recirculation and northerly flows. Findings identify biomass burning as a major source of Aitken-mode aerosols. Enhanced CO concentrations, combined with Aitken- and accumulation-mode particle number size distributions, are shown to provide a useful signature of plumes originating over regional biomass combustion events.

Major secondary aerosol formation in southern African open biomass burning plumes

Open biomass burning contributes significantly to air quality degradation and associated human health impacts over large areas. It is one of the largest sources of reactive trace gases and fine particles to Earth’s atmosphere and consequently a major source of cloud condensation nuclei on a global scale. However, there is a large uncertainty in the climate effect of open biomass burning aerosols due to the complexity of their constituents. Here, we present an exceptionally large dataset on southern African savannah and grassland fire plumes and their atmospheric evolution, based on 5.5 years of continuous measurements from 2010 to 2015. We find that the mass of submicrometre aerosols more than doubles on average, in only three hours of daytime ageing. We also evaluate biomass burning aerosol particle size distributions and find a large discrepancy between the observations and current model parameterizations, especially in the 30–100 nm range. We conclude that accounting for near-source secondary organic aerosol formation and using measurement-based size distribution parameterizations in smoke plumes is essential to better constrain the climate and air quality effects of savannah and grassland fires.A substantial amount of secondary aerosols form within hours of biomass burning in southern African savannah and grassland fires, according to analyses of 5.5 years of continuous field measurements.