Modelling new particle formation events in the South African savannah

Abstract

Africa is one of the less studied continents with respect to atmospheric aerosols. Savannahs are complex dynamic systems sensitive to climate and land-use changes, but the interaction of these systems with the atmosphere is not well understood. Atmospheric particles, called aerosols, affect the climate on regional and global scales, and are an important factor in air quality. In this study, measurements from a relatively clean savannah environment in South Africa were used to model new particle formation and growth. There already are some combined long-term measurements of trace gas concentrations together with aerosol and meteorological variables available, but to our knowledge this is the first detailed simulation that includes all the main processes relevant to particle formation. The results show that both of the particle formation mechanisms investigated overestimated the dependency of the formation rates on sulphuric acid. From the two particle formation mechanisms tested in this work, the approach that included low volatile organic compounds to the particle formation process was more accurate in describing the nucleation events than the approach that did not. To obtain a reliable estimate of aerosol concentration in simulations for larger scales, nucleation mechanisms would need to include organic compounds, at least in southern Africa. This work is the first step in developing a more comprehensive new particle formation model applicable to the unique environment in southern Africa. Such a model will assist in better understanding and predicting new particle formation-knowledge which could ultimately be used to mitigate impacts of climate change and air quality. Africa is one of the less studied continents with regard to atmospheric aerosols both from a measurement and a modelling point of view. Biomass burning is a significant source of aerosols in Africa, but previous studies have shown that there are also several other sources. 10 A few modelling studies using regional and global models have investigated South Africa, 11,12 but none have used detailed chemistry and aerosol dynamics. In this case study, we used measurements conducted at a semi-clean savannah site in South Africa to simulate new particle formation and growth. The measurements were carried out with a transportable measurement trailer. 13 The observations were used as input to the model to constrain the result. The model used in this work was MALTE (Model to predict new Aerosol formation in the Lower TropospherE). 14,15 Our aim was to investigate the processes creating secondary organic aerosols and explore what factors are important for the growth of these aerosols, especially in southern Africa. Furthermore, we wanted to test the model in a savannah environment, which is very different from the boreal forest ecosystem that the model was developed for. In addition to being a method to enhance our understanding of the processes studied, modelling also allows one to extrapolate to places without measurements. The reliability of such extrapol...

Simulating ozone dry deposition at a boreal forest with a multi-layer canopy deposition model

“…The air temperature (T ), RH and O 3 concentration were linearly interpolated using the observations collected at a height of 16.8 and 33.6 m to arrive at the estimated parameter values at 23 m to allow a direct comparison of the model results with the measurements or being used as input for the model. The missing observed data points of T , RH and O 3 were gap-filled with the method described in Gierens et al (2014).…”

Seasonal and diurnal variations in biogenic volatile organic compounds in highland and lowland ecosystems in southern Kenya

“…A6 to A9). Higher mixing ratios of MTs during the night have been observed earlier in savannas in South Africa (Gierens et al, 2014), and a needleleaf forest in California, USA, and Finland (Bouvier-Brown et al, 2009;Hakola et al, 2012). Even though the MT emissions are expected to be highest during daytime, the mixing ratio of MTs is lower since the mixing, and therefore dilution, is highest during daytime and lowest during the night (Mogensen et al, 2011;Hellén et al, 2018).…”

Statistical exploration of gaseous elemental mercury (GEM) measured at Cape Point from 2007 to 2011

“…As discussed in Lan et al (2012) and the references therein, GEM may be oxidised by q OH, nitrate (NO 3 q ) or halogen (X q ) radicals. Gierens et al (2014) recently indicated that q OH concentrations reach a peak around midday in the interior of SA, and since q OH has a lifetime of ∼ 1 s, its diurnal variation will therefore follow the diurnal variation of the UV radiation with wavelength capable to photolyse O 3 to O 1 D, and therefore achieving peak GEM oxidation during midday. Additionally, the negative constant associated with O 3 in Eq.…”