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

Abstract

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

Atmospheric new particle formation as a source of CCN in the eastern Mediterranean marine boundary layer

“…The observed values of GR on 29, 30 and 31 August were 3.3, 1.8 and 3.6 nm h −1 , respectively, which are lower than the annually averaged (±SD) GR of 5.2 ± 3.4 nm h −1 reported by Pikridas et al (2012) at the same site for the 10-25 nm size range. By following the approach of Laakso et al (2013), we could follow the growth of newly formed particles up to about 50-60 nm in particle diameter until another NPF occurred or the particle growth was interrupted by an air mass or cloud cover change. These features are suggestive of the convolution of nucleation with condensational growth of both new and preexisting particles formed in the previous day (or even earlier) to produce CCN size range particles.…”

“…When no CCN measurements are available, a commonly used proxy for the CCN number concentration is the total number concentration of particles larger than some threshold diameter, D, denoted as N D (Paasonen et al, 2013;Laakso et al, 2013). Just prior to the nucleation event period (28 August-2 September) and after it, our CCN counter was offline the DMA due to technical problems, so that it was recording the total CCN number concentration at 0.2 % supersaturation, CCN 0.2 .…”

Analysis of nucleation events in the European boundary layer using the regional aerosol–climate model REMO-HAM with a solar radiation-driven OH-proxy

“…Many other studies using global aerosol-climate models have demonstrated the importance of atmospheric NPF for regional and global aerosol number concentration and cloud condensation nuclei budgets (Merikanto et al, 2009;Pierce et al, 2007;Pierce and Adams, 2009;Wang and Penner, 2009;Yu and Luo, 2009;Trivitayanurak et al, 2008;Jung et al, 2010;Laakso et al, 2013;Scott et al, 2014), each study assessing which parametrization leads to the best agreement with observations in their model. Global models have a large grid size (usually 200-300 km when aerosols are included), hence the number concentration of newly formed particles and size distribution are prone to large uncertainties.…”

Ambient aromatic hydrocarbon measurements at Welgegund, South Africa

“…Beukes et al (2014), Laakso et al (2013) and Venter et al (2012) indicated that O 3 is currently the most problematic pollutant in South Africa. Tropospheric O 3 impacts air quality, food security (Zunckel et al, 2006) and regional climate change (Fry et al, 2013).…”