Abstract
Abstract. A global model perturbed parameter ensemble of 60 simulations was used to explore how combinations of six parameters related to secondary organic aerosol (SOA) formation affect particle number concentrations and organic aerosol mass. The parameters represent the formation of organic compounds with different volatilities from biogenic and anthropogenic sources. The most plausible parameter combinations were determined by comparing the simulations against observations of the number concentration of particles larger than 3 nm diameter (N3), the number concentration of particles larger than 50 nm diameter (N50), and the organic aerosol (OA) mass concentration. The simulations expose a high degree of model equifinality in which the skill of widely different parameter combinations cannot be distinguished against observations. We therefore conclude that, based on the observations we have used, a six-parameter SOA scheme is under-determined. Nevertheless, the model skill in simulating N3 and N50 is clearly determined by the low-volatility and extremely low-volatility compounds that affect new particle formation and growth, and the skill in simulating OA mass is determined by the low-volatility and semi-volatile compounds. The biogenic low-volatility class of compounds that grow nucleated clusters and condense on all particles is found to have the strongest effect on the model skill in simulating N3, N50, and OA. The simulations also expose potential structural deficiencies in the model: we find that parameter combinations that are best for N3 and N50 are worst for OA mass, and the ensemble exaggerates the observed seasonal cycle of particle concentrations – a deficiency that we conclude requires an additional anthropogenic source of either primary or secondary particles.
Highlights
About 20 %–50 % of lower-tropospheric fine aerosol mass in continental mid-latitudes and almost 90 % in the tropics is composed of organic material (Kanakidou et al, 2005)
Overall we find that when particle number concentrations are low, the main difference between simulations that produce a high amount of organic aerosol (OA) and those that do not is the relative concentrations of B_LVOC, which grows freshly nucleated clusters before they can be scavenged by coagulation
The ranges for each parameter were chosen to encompass maximum uncertainty associated with organic compounds that affect three different stages of secondary organic aerosol (SOA) formation − nucleation, cluster growth, and particle growth
Summary
About 20 %–50 % of lower-tropospheric fine aerosol mass in continental mid-latitudes and almost 90 % in the tropics is composed of organic material (Kanakidou et al, 2005). The major fraction of organic aerosol has been found to be secondary (Zhang et al, 2007; Jimenez et al, 2009), formed as a result of atmospheric oxidation of volatile organic compounds (VOCs) leading to secondary organic aerosol (SOA). Despite the importance of SOA for climate, a comprehensive characterization of atmospheric VOCs, their reaction pathways, and their SOA formation potential has not yet been possible. Characterization of VOCs is challenging because of the very large number of compounds involved and their diverse sources; tens of thousands of VOCs have been identified in the atmosphere and yet more still remain to be detected (Goldstein and Galbally, 2007). For the SOA precursor gases that have been identified, questions remain about their emission sources, their chemical conversion to SOA, and the effects of atmospheric chemical composition and oxidants on SOA formation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
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
