Abstract
Abstract. Sulfuric acid clusters stabilized by base molecules are likely to have a significant role in atmospheric new-particle formation. Recent advances in mass spectrometry techniques have permitted the detection of electrically charged clusters. However, direct measurement of electrically neutral clusters is not possible. Mass spectrometry instruments can be combined with a charger, but the possible effect of charging on the composition of neutral clusters must be addressed in order to interpret and understand the measured data. In the present work we have used formation free energies from quantum chemical methods to calculate the evaporation rates of electrically charged (both positive and negative) sulfuric acid–ammonia/dimethylamine clusters. To understand how charging will affect the composition of electrically neutral clusters, we have compared the evaporation rates of the most stable neutral clusters with those of the corresponding charged clusters. Based on the evaporation rates of different molecules from the charged clusters, we determined the most likely resulting cluster composition when a stable neutral cluster is charged and the molecules with the highest evaporation rates are lost from it. We found that all of the most stable neutral clusters will be altered by both positive and negative charging. In the case of charging clusters negatively, base molecules will completely evaporate from clusters with 1 to 3 sulfuric acid molecules in the case of ammonia, and from clusters with 1 or 2 sulfuric acid molecules in the case of dimethylamine. Larger clusters will maintain some base molecules, but the H2SO4 : base ratio will increase. In the case of positive charging, some of the acid molecules will evaporate, decreasing the H2SO4 : base ratio.
Highlights
Atmospheric aerosol particles affect climate both directly and indirectly (IPCC, 2007; Merikanto et al, 2009)
Recent results from the state-of-the-art Cosmics Leaving Outdoor Droplets (CLOUD) experiment at CERN (Kirkby et al, 2011) have shown that, the presence of ammonia can enhance the observed particle formation rate, the rates measured in the experiment are still far from those observed in the atmosphere
Charged DMA-containing clusters must contain at least three sulfuric acid molecules to have a low enough evaporation rate to be abundant under atmospheric conditions, while in the case of ammoniacontaining clusters, four sulfuric acid molecules are needed
Summary
Atmospheric aerosol particles affect climate both directly and indirectly (IPCC, 2007; Merikanto et al, 2009). It has long been known that sulfuric acid clusters need to be stabilized to be able to form aerosol particles. The identity of these stabilizing compounds has been the subject of several studies during the recent years. Different quantum chemical studies have shown that amines stabilize sulfuric acid clusters more efficiently than ammonia (Kurtén et al, 2008, Loukonen et al, 2010; Nadykto et al, 2011).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
