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Black carbon-containing aerosol particles play an important role in the direct and indirect radiative forcing of climate. However, the magnitude and sign of the net radiative effect is strongly dependent on the physical properties of the black carbon (BC) component of the particles, such as mass concentration, number size distribution and mixing state. Here we use a global aerosol model combined with aircraft measurements of BC particle number and size from the Single Particle Soot Photometer (SP2) to assess the realism with which these physical properties are predicted by global models. The comparison reveals a substantial mismatch between the measured and modelled BC size distribution over the size range of the SP2 instrument (90–400 nm BC diameter). The model predicts BC particle number concentrations a factor ~3.5–5.7 higher than measured and a mode diameter that is ~40–65 nm smaller than observed. More than ~90% of the model particles with dry diameters ≳260 nm contain BC, while the observations suggest only 14% on average. These model–observation biases in the BC properties are considerably greater than for the overall particle distribution, suggesting that the discrepancy is associated with model assumptions about the size and mixing state of the emitted carbonaceous particles. We expect the discrepancy in BC size distribution to be common among most global aerosol models, with implications for model estimates of absorption optical depth and direct radiative forcing., Atmospheric Chemistry and Physics, 13 (9), ISSN:1680-7375, ISSN:1680-7367

It is important to understand the relative contribution of primary and secondary particles to regional and global aerosol so that models can attribute aerosol radiative forcing to different sources. In large-scale models, there is considerable uncertainty associated with treatments of particle formation (nucleation) in the boundary layer (BL) and in the size distribution of emitted primary particles, leading to uncertainties in predicted cloud condensation nuclei (CCN) concentrations. Here we quantify how primary particle emissions and secondary particle formation influence size-resolved particle number concentrations in the BL using a global aerosol microphysics model and aircraft and ground site observations made during the May 2008 campaign of the European Integrated Project on Aerosol Cloud Climate Air Quality Interactions (EUCAARI). We tested four different parameterisations for BL nucleation and two assumptions for the emission size distribution of anthropogenic and wildfire carbonaceous particles. When we emit carbonaceous particles at small sizes (as recommended by the Aerosol Intercomparison project, AEROCOM), the spatial distributions of campaign-mean number concentrations of particles with diameter >50 nm (N50) and >100 nm (N100) were well captured by the model (R2≥0.8) and the normalised mean bias (NMB) was also small (−18% for N50 and −1% for N100). Emission of carbonaceous particles at larger sizes, which we consider to be more realistic for low spatial resolution global models, results in equally good correlation but larger bias (R2≥0.8, NMB = −52% and −29%), which could be partly but not entirely compensated by BL nucleation. Within the uncertainty of the observations and accounting for the uncertainty in the size of emitted primary particles, BL nucleation makes a statistically significant contribution to CCN-sized particles at less than a quarter of the ground sites. Our results show that a major source of uncertainty in CCN-sized particles in polluted European air is the emitted size of primary carbonaceous particles. New information is required not just from direct observations, but also to determine the "effective emission size" and composition of primary particles appropriate for different resolution models., Atmospheric Chemistry and Physics, 11 (23), ISSN:1680-7375, ISSN:1680-7367