April 2011: AEROFORM project final results
The overall aims of this project were to quantify the importance of aerosol formation for the global atmospheric aerosol, in particular particles with sizes that are relevant to the climate. Specifically to:
1. Use extensive observations from several worldwide observing sites to optimise the representation of particle formation in a global aerosol microphysics model.
We created the largest ever dataset of particle concentrations from 36 sites around the world and analysed them with the GLOMAP aerosol model (Spracklen et al., 2010, ACPD). Several formation mechanisms were tested and we were able to show that boundary layer particle formation is needed to explain the global measurements. We also worked with colleagues at PSI Switzerland to analyse new laboratory measurements and develop a new formation mechanism taking into account organic compounds (Metzger et al., 2010, PNAS). This mechanism captures much better the vertical distribution of aerosol concentrations.
2. Use the model and observations to quantify the relative contribution of particle formation and primary emissions to particle concentrations at different sizes in different regions.
The impact of nucleation on global cloud condensation nuclei (CCN) was first reported in GRL (Spracklen et al., 2008) showing that boundary layer nucleation can account for between 5 and 30% of global CCN. This paper was selected as an Editor’s Highlight. In ACP (Merikanto et al., 2009) we went on to quantify the effect of both free tropospheric and boundary layer nucleation versus primary emissions. We calculated an approximate 40% contribution of all nucleation to surface-level CCN concentrations. This paper demonstrates the enormous importance of nucleation for climate-relevant particles which motivated the project in the first place.
3. Quantify the possible long-term changes in particle formation in response to changes in global emissions and the effects on climate-relevant particle concentrations
This work was published in Merikanto et al. (2010, ACP). We showed that boundary layer nucleation can substantially alter the long-term change in the aerosol indirect forcing. In some important regions, including nucleation in the model led to a 50% change in the indirect effect. In addition, we studied the effect of nucleation on how regional emissions impact CCN (Manktelow et al., 2008, GRL), finding a strong impact on the CCN produced per unit SO2 emission. We also showed that nucleation affects the formation of CCN from forest emissions (Spracklen et al., 2008, Proc Roy Soc). Terpene emissions contribute >50% to regional CCN but the effect is much stronger if nucleation is included in the model. This work was reported in the Guardian on 31 October 2008.
4. If particle formation is demonstrated to be an important process, we propose to develop simplified schemes for the UK Climate Model (the Unified Model) as part of our ongoing UKCA project.
Nucleation was found to be important, and it has been incorporated in the GLOMAP-mode model with input from Dr Graham Mann. The results are very similar to our other model results with the GLOMAP-bin model. GLOMAP-mode is identical to the aerosol code used in UKCA. We will soon test the effect of nucleation in a full climate simulation in UKCA.
Our results have large implications for aerosol and climate modelling. Primarily we have shown that almost half of climate-relevant CCN particles derive from nucleation. These particles will respond to climate change in very different ways to primary particles. We showed this in some of our publications, but there are further implications for long-term climate change that we are now working on.
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