Basic characteristics of atmospheric particles, trace gases and meteorology in a relatively clean Southern African Savannah environment

Laakso, Lauri; Laakso, H.; Aalto, Pasi; Keronen, Petri; Petäj̈ä, Tuukka; Nieminen, T.; Pohja, Toivo; Siivola, Erkki; Kulmala, Markku; Kgabi, Nnenesi A.; Molefe, M.; Mabaso, D.; Phalatse, D.; Pienaar, K.; Kerminen, V.-M.

doi:10.5194/acp-8-4823-2008

Cited by 91 publications

(141 citation statements)

References 42 publications

(51 reference statements)

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“…One of the largest regional pollution sources in North-West Province, Rustenburg mining region, is located approximately 150 km east of Botsalano. Vegetation of the measurement location is typical for mixed bushveld (Laakso et al, 2008).…”

Section: Measurementsmentioning

confidence: 99%

“…The comprehensive measurement description is given by Laakso et al (2008) and Vakkari et al (2011) and here we list only briefly the measurements used in this study.…”

Section: Measurementsmentioning

confidence: 99%

“…One such source, not considered by Ross et al (2003), is the atmospheric newparticle formation (Laakso et al, 2008;Vakkari et al, 2011), which has been observed globally in different environments (e.g. Kulmala et al, 2004).…”

Section: Introductionmentioning

confidence: 99%

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Boundary layer nucleation as a source of new CCN in savannah environment

et al. 2013

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The South African savannah region is a complex environment of air pollution and natural emissions influenced by a strong seasonal cycle in biomass burning and strong precipitation. However, the scarcity of long-term observations means that the knowledge of controlling aerosol processes in this environment is limited. Here we use a recent dataset of 18 months of aerosol size distribution observations trying to understand the annual cycle of cloud condensation nuclei (CCN). Our observations show that the concentration of CCN-sized particles remains, in line with previous studies, high throughout the year with the highest concentrations during the dry winter and the lowest during the wet summer. During the wet season with reduced anthropogenic and biomass burning primary emissions, this pool of CCN is partly filled by boundary layer nucleation with subsequent growth. The enhanced importance of formation and growth during the wet season is addressed to increased biogenic activity together with enhanced free tropospheric removal decreasing the concentration of pre-existing CCN. During the dry season, while frequent new particle formation takes place, particle growth is reduced due to reduced condensing vapour concentrations. Thus in the dry season particles are not able to grow to sizes where they may act as CCN nearly as efficiently as during the wet season. The observations are compared to simulations by a global aerosol model GLOMAP. To our surprise, the global aerosol model utilized to explain the observations was not capable of re-producing the characteristics of particle formation and the annual CCN cycle, despite earlier good performance in predicting the particle concentrations in a number of diverse environments, including the South African savannah region. While the average yearly CCN concentrations of modelled CCN is close to observed concentrations, the characteristics of nucleation bursts and subsequent growth are not captured satisfactory by the model. Our sensitivity tests using different nucleation parameterizations and condensing organic vapour production rates show that neither of these is likely to explain the differences between observed and modelled nucleation and growth rates. A sensitivity study varying 28 modelling parameters indicates that the main uncertainties in the result are due to uncertainties in biomass burning emissions during the dry season, and anthropogenic sulphur emissions during the wet season, both in terms or emitted mass and particle sizes. The uncertainties appear to be mostly related to uncertainties in primary particle emissions, including the emissions variability not captured by monthly emission inventories. The results of this paper also highlights the fact that deficiencies in emissions estimates may result in deficiencies in particle production fluxes, while the end product such as modelled CCN concentration may be in line with observations

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Section: Measurementsmentioning

confidence: 99%

“…The comprehensive measurement description is given by Laakso et al (2008) and Vakkari et al (2011) and here we list only briefly the measurements used in this study.…”

Section: Measurementsmentioning

confidence: 99%

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Boundary layer nucleation as a source of new CCN in savannah environment

et al. 2013

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“…Laakso et al, 2008Laakso et al, , 2012Vakkari et al, 2011Vakkari et al, , 2013Venter et al, 2012;Jaars et al, 2014;Beukes et al, 2013). So far, regional modeling studies for southern Africa have mainly focused on the meteorology (e.g.…”

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confidence: 99%

The anthropogenic contribution to atmospheric black carbon concentrations in southern Africa: a WRF-Chem modeling study

et al. 2015

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Abstract. South Africa has one of the largest industrialized economies in Africa. Emissions of air pollutants are particularly high in the Johannesburg-Pretoria metropolitan area, the Mpumalanga Highveld and the Vaal Triangle, resulting in local air pollution. This study presents and evaluates a setup for conducting modeling experiments over southern Africa with the Weather Research and Forecasting model including chemistry and aerosols (WRF-Chem), and analyzes the contribution of anthropogenic emissions to the total black carbon (BC) concentrations from September to December 2010.The modeled BC concentrations are compared with measurements obtained at the Welgegund station situated ca. 100 km southwest of Johannesburg. An evaluation of WRF-Chem with observational data from ground-based measurement stations, radiosondes, and satellites shows that the meteorology is modeled mostly reasonably well, but precipitation amounts are widely overestimated and the onset of the wet season is modeled approximately 1 month too early in 2010. Modeled daily mean BC concentrations show a temporal correlation of 0.66 with measurements, but the total BC concentration is underestimated in the model by up to 50 %.Sensitivity studies with anthropogenic emissions of BC and co-emitted species turned off show that anthropogenic sources can contribute up to 100 % to BC concentrations in the industrialized and urban areas, and anthropogenic BC and co-emitted species together can contribute up to 60 % to PM 1 levels. Particularly the co-emitted species contribute significantly to the aerosol optical depth (AOD). Furthermore, in areas of large-scale biomass-burning atmospheric heating rates are increased through absorption by BC up to an altitude of about 600 hPa.

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“…A detailed description of the measurement instruments, operation procedures and data analysis, as well as calibration and maintenance procedures has been presented by Beukes et al (2014), , Petäjä et al (2013, Hirsikko et al (2012), Venter et al (2012), Vakkari et al (2011) and Laakso et al (2008).…”

Section: Measurement Methodsmentioning

confidence: 99%

Ambient aromatic hydrocarbon measurements at Welgegund, South Africa

et al. 2014

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Abstract. Aromatic hydrocarbons are associated with direct adverse human health effects and can have negative impacts on ecosystems due to their toxicity, as well as indirect negative effects through the formation of tropospheric ozone and secondary organic aerosol, which affect human health, crop production and regional climate. Measurements of aromatic hydrocarbons were conducted at the Welgegund measurement station (South Africa), which is considered to be a regionally representative background site. However, the site is occasionally impacted by plumes from major anthropogenic source regions in the interior of South Africa, which include the western Bushveld Igneous Complex (e.g. platinum, base metal and ferrochrome smelters), the eastern Bushveld Igneous Complex (platinum and ferrochrome smelters), the Johannesburg-Pretoria metropolitan conurbation (> 10 million people), the Vaal Triangle (e.g. petrochemical and pyrometallurgical industries), the Mpumalanga Highveld (e.g. coal-fired power plants and petrochemical industry) and also a region of anticyclonic recirculation of air mass over the interior of South Africa. The aromatic hydrocarbon measurements were conducted with an automated sampler on Tenax-TA and Carbopack-B adsorbent tubes with heated inlet for 1 year. Samples were collected twice a week for 2 h during daytime and 2 h during night-time. A thermal desorption unit, connected to a gas chromatograph and a mass selective detector was used for sample preparation and analysis. Results indicated that the monthly median (mean) total aromatic hydrocarbon concentrations ranged between 0.01 (0.011) and 3.1 (3.2) ppb. Benzene levels did not exceed the local air quality standard limit, i.e. annual mean of 1.6 ppb. Toluene was the most abundant compound, with an annual median (mean) concentration of 0.63 (0.89) ppb. No statistically significant differences in the concentrations measured during daytime and night-time were found, and no distinct seasonal patterns were observed. Air mass back trajectory analysis indicated that the lack of seasonal cycles could be attributed to patterns determining the origin of the air masses sampled. Aromatic hydrocarbon concentrations were in general significantly higher in air masses that passed over anthropogenically impacted regions. Inter-compound correlations and ratios gave some indications of the possible sources of the different aromatic hydrocarbons in the source regions defined in the paper. The highest contribution of aromatic hydrocarbon concentrations to ozone formation potential was also observed in plumes passing over anthropogenically impacted regions.

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Basic characteristics of atmospheric particles, trace gases and meteorology in a relatively clean Southern African Savannah environment

Cited by 91 publications

References 42 publications

Boundary layer nucleation as a source of new CCN in savannah environment

Boundary layer nucleation as a source of new CCN in savannah environment

The anthropogenic contribution to atmospheric black carbon concentrations in southern Africa: a WRF-Chem modeling study

Ambient aromatic hydrocarbon measurements at Welgegund, South Africa

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