Abstract. We have analyzed one year (July 2006-July 2007 of measurement data from a relatively clean background site located in dry savannah in South Africa. The annualmedian trace gas concentrations were equal to 0.7 ppb for SO 2 , 1.4 ppb for NO x , 36 ppb for O 3 and 105 ppb for CO. The corresponding PM 1 , PM 2.5 and PM 10 concentrations were 9.0, 10.5 and 18.8 µg m −3 , and the annual median total particle number concentration in the size range 10-840 nm was 2340 cm −3 . During Easterly winds, influence of industrial sources approximately 150 km away from the measurement site was clearly visible, especially in SO 2 and NO x concentrations. Of gases, NO x and CO had a clear annual, and SO 2 , NO x and O 3 clear diurnal cycle.Atmospheric new-particle formation was observed to take place in more than 90% of the analyzed days. The days with no new particle formation were cloudy or rainy days. The formation rate of 10 nm particles varied in the range of 0.1-28 cm −3 s −1 (median 1.9 cm −3 s −1 ) and nucleation mode particle growth rates were in the range 3-21 nm h −1 (median 8.5 nm h −1 ). Due to high formation and growth rates, observed new particle formation gives a significant contribute to the number of cloud condensation nuclei budget, having a potential to affect the regional climate forcing patterns.
Abstract. This study is based on 18 months (20 July 2006-5 February 2008) of continuous measurements of aerosol particle size distributions, air ion size distributions, trace gas concentrations and basic meteorology in a semi-clean savannah environment in Republic of South Africa. New particle formation and growth was observed on 69% of the days and bursts of non-growing ions/sub-10 nm particles on additional 14% of the days. This new particle formation frequency is the highest reported from boundary layer so far. Also the new particle formation and growth rates were among the highest reported in the literature for continental boundary layer locations; median 10 nm formation rate was 2.2 cm −3 s −1 and median 10-30 nm growth rate 8.9 nm h −1 . The median 2 nm ion formation rate was 0.5 cm −3 s −1 and the median ion growth rates were 6.2, 8.0 and 8.1 nm h −1 for size ranges 1.5-3 nm, 3-7 nm and 7-20 nm, respectively. The growth rates had a clear seasonal dependency with minimum during winter and maxima in spring and late summer. The relative contribution of estimated sulphuric acid to the growth rate was decreasing with increasing particle size and could explain more than 20% of the observed growth rate only for the 1.5-3 nm size range. Also the air mass history analysis indicated the highest formation and growth rates to be associated with the area of highest VOC (Volatile Organic Compounds) emissions following from biological activity rather than the highest estimated sulphuric acid concentrations. The frequency of new particle formation, however, increased nearly monotonously with the estimated sulphuric acid reaching 100% at H 2 SO 4 concentration of 6 · 10 7 cm −3 , which suggests the formation and growth to be independent of each other.
This study is based on 18 months (20 July 2006–5 February 2008) of continuous measurements of aerosol particle size distributions, air ion size distributions, trace gas concentrations and basic meteorology in a semi-clean savannah environment in Republic of South Africa. New particle formation and growth was observed on 69% of the days and bursts of non-growing ions/sub-10 nm particles on additional 14% of the days. The new particle formation and growth rates were among the highest reported in the literature for continental boundary layer locations; median 10 nm formation rate was 2.2 cm<sup>−3</sup>s<sup>−1</sup> and median 10–30 nm growth rate 8.9 nm h<sup>−1</sup>. The median 2 nm ion formation rate was 0.5 cm<sup>−3</sup>s<sup>−1</sup> and the median ion growth rates were 6.2, 8.0 and 8.1 nm h<sup>−1</sup> for size ranges 1.5–3 nm, 3–7 nm and 7–20 nm, respectively. Three different approaches were used to study the origin of the formation and growth rates: seasonal variation, air mass history analysis and estimated sulphuric acid contribution to the growth. The growth rates had a clear seasonal dependency with minimum during winter and maxima in spring and late summer and the air mass history analysis indicated the highest formation and growth rates to be associated with the area of highest VOC (Volatile Organic Compounds) emissions rather than the highest estimated sulphuric acid concentrations. The relative contribution of estimated sulphuric acid to the growth rate was decreasing with increasing particle size and could explain more than 20% of the observed growth rate only for the 1.5–3 nm size range. The implication is that the sulphuric acid alone is not enough to explain the growth, but the highest growth rates seem to originate in VOC emissions following from biological activity. The frequency of new particle formation, however, increased nearly monotonously with the estimated sulphuric acid reaching 100% at H<sub>2</sub>SO<sub>4</sub> concentration of 4×10<sup>7</sup>cm<sup>−3</sup>, which suggests the formation and growth to be independent of each other
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). <br><br> 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. <br><br> 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. <br><br> 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
The activity concentrations of 238 U, 232 Th and 40 K were determined using gamma spectrometry (HPGe detector) and their radiological hazards are presented in this study for shore sediment samples from the North Dune beach of Henties Bay, Namibia. The activity concentrations were found to range from 25.32 ± 1.24 Bq.kg À1 to 232.33 ± 0.62 Bq.kg À1 with an average value of 175.59 ± 0.92 Bq.kg À1 for 238 U, BDL to 77.99 ± 45 Bq.kg À1 with an average value of 40.17 ± 27 Bq.kg À1 for 232 Th and 222.39 ± 8 Bq.kg À1 to 482.16 ± 10 Bq.kg-1 with an average value of 349.66 ± 8 Bq.kg À1 for 40 K. In order to assess the radiological hazard of the shore sediment samples, the radiological hazard indices such as absorbed dose rates (ADR), radium equivalent (R eq) annual effective dose equivalent (AEDE), the hazard indices (H ex and H in), and the excess lifetime cancer risk (ELCR) were calculated. The values obtained in the present study were compared with world acceptable limits and it was found that the average values of radium equivalent (Req) and external hazard index (Hex) were below the world allowable limits. However, the average values of absorbed dose rates (ADR), annual effective dose equivalent (AEDE) (outdoor and indoor), internal hazard index (H in) and excess lifetime cancer risk (ELCR) exceeded the world acceptable limits.
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