Abstract
Abstract. The Norwegian Earth System Model (NorESM) is evaluated against atmospheric observations of aerosol number concentrations. The model is extended to include an explicit mechanism for new particle formation and secondary organic aerosol (SOA) formation from biogenic precursors. Three nucleation mechanisms are included in NorESM: binary sulfuric acid, activation type, and organic nucleation. Ten model experiments are conducted to study the sensitivity of the simulated aerosol number concentrations to nucleation, SOA formation, black carbon size distribution and model meteorology. Simulated vertical profiles are evaluated against 12 flight campaigns. Comparison of monthly averaged aerosol number concentrations against 60 measurement sites reveals that the model with explicit nucleation and SOA scheme performs well in terms of correlation coefficient, R2 = 0.41 and a bias of −6%. NorESM generally overestimates the amplitude of the seasonal cycle, possibly due to underestimated sinks or exaggerated sensitivity to biogenic precursors.
Highlights
Atmospheric aerosol particles affect climate by scattering and absorbing radiation, but by acting as cloud condensation nuclei (CCN) and affecting the dynamical and radiative properties of clouds
We have developed NorESM1-M to include an explicit mechanism for nucleation, nuclei growth and secondary organic aerosol (SOA) formation
If the background sink is lowered, the relative importance of nucleation increases. This can be shown by comparing simulations ActNuc_BC24 and ActNuc_BC48 with their respective _NoNuc counterparts: with a lower sink, nucleation increases continental aerosol number concentration by 90 % (ActNuc_BC48 vs. NoNuc_BC48), while a higher background sink shows a lower sensitivity of 50 % to nucleation (ActNuc_BC24 vs. NoNuc_BC24)
Summary
Atmospheric aerosol particles affect climate by scattering and absorbing radiation, but by acting as cloud condensation nuclei (CCN) and affecting the dynamical and radiative properties of clouds. Information on the aerosol size distribution and number concentration has been included in global aerosol models for more than a decade (Von Salzen et al, 2000; Ghan et al, 2001; Adams and Seinfeld, 2002). Development of global aerosol models has both improved simulated aerosol number concentrations and revealed crucial information on the relative importance of aerosol processes in the atmosphere. Aerosols form either by emission of primary particles or by formation of secondary aerosol material in the atmosphere. While anthropogenic emissions contribute only a small amount to the total aerosol mass, the addition to aerosol number concentration can be 35 % (Makkonen et al, 2012b). The comparison of the sensitivity simulations gives an indication of the relative importance of secondary aerosol formation and primary aerosol emission to aerosol number concentrations
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