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The global chemical transport model Oslo CTM2 has been extended to includethe formation, transport and deposition of secondary organic aerosol (SOA).Precursor hydrocarbons which are oxidised to form condensible species includeboth biogenic species such as terpenes and isoprene, as well as speciesemitted predominantly by anthropogenic activities (toluene, m-xylene,methylbenzene and other aromatics). A model simulation for 2004 gives anannual global SOA production of approximately 55 Tg. Of this total, 2.5 Tgis found to consist of the oxidation products of anthropogenically emittedhydrocarbons, and about 15 Tg is formed by the oxidation products ofisoprene. The global production of SOA is increased to about69 Tg yr−1 by allowing semi-volatile species to partition to ammoniumsulphate aerosol. This brings modelled organic aerosol values closer to thoseobserved, however observations in Europe remain significantly underestimated.Allowing SOA to partition into ammonium sulphate aerosol increases thecontribution of anthropogenic SOA from about 4.5% to 9.4% of the totalproduction. Total modelled organic aerosol (OA) values are found to representa lower fraction of the measured values in winter (when primary organicaerosol (POA) is the dominant OA component) than in summer, which may be anindication that estimates of POA emissions are too low. Additionally, formeasurement stations where the summer OA values are higher than in winter,the model generally underestimates the increase in summertime OA. In order tocorrectly model the observed increase in OA in summer, additional SOA sourcesor formation mechanisms may be necessary. The importance of NO3 as anoxidant of SOA precursors is found to vary regionally, causing up to50%–60% of the total amount of SOA near the surface in polluted regionsand less than 25% in more remote areas, if the yield of condensibleoxidation products for β-pinene is used for NO3 oxidation of allterpenes. Reducing the yield for α-pinene and limonene oxidation inline with recent measurements reduces the global fraction of SOA formed fromNO3 oxidation products from 27% to about 21%. This study underscores theneed for SOA to be represented in a more realistic way in global aerosolmodels in order to better reproduce observations of organic aerosol burdensin industrialised and biomass burning regions

Secondary organic aerosol in the global aerosol – chemical transport model Oslo CTM2