Measurements of cloud condensation nuclei (CCN), aerosol size distribution and chemical composition were obtained at the UNH‐AIRMAP Thompson Farms site, during the ICARTT 2004 campaign. This work focuses on the analysis of a week of measurements, during which semiurban and continental air were sampled. Predictions of CCN concentrations were carried out using “simple” Köhler theory; the predictions are subsequently compared with CCN measurements at 0.2%, 0.3%, 0.37%, 0.5% and 0.6% supersaturation. Using size‐averaged chemical composition, CCN are substantially overpredicted (by 35.8 ± 28.5%). Introducing size‐dependent chemical composition substantially improved closure (average error 17.4 ± 27.0%). CCN closure is worse during periods of changing wind direction, suggesting that the introduction of aerosol mixing state into CCN predictions may sometimes be required. Finally, knowledge of the soluble salt fraction is sufficient for description of CCN activity.

[1] In situ observations of aerosol and cloud condensation nuclei (CCN) and the GISS GCM Model II' with an online aerosol simulation and explicit aerosol-cloud interactions are used to quantify the uncertainty in radiative forcing and autoconversion rate from application of Köhler theory. Simulations suggest that application of Köhler theory introduces a 10-20% uncertainty in global average indirect forcing and 2-11% uncertainty in autoconversion. Regionally, the uncertainty in indirect forcing ranges between 10-20%, and 5-50% for autoconversion. These results are insensitive to the range of updraft velocity and water vapor uptake coefficient considered. This study suggests that Köhler theory (as implemented in climate models) is not a significant source of uncertainty for aerosol indirect forcing but can be substantial for assessments of aerosol effects on the hydrological cycle in climatically sensitive regions of the globe. This implies that improvements in the representation of GCM subgrid processes and aerosol size distribution will mostly benefit indirect forcing assessments. Predictions of autoconversion, by nature, will be subject to considerable uncertainty; its reduction may require explicit representation of size-resolved aerosol composition and mixing state.

[1] This study quantitatively assesses the sensitivity of cloud droplet number (CDNC) to errors in cloud condensation nuclei (CCN) predictions that arise from application of Köhler theory. The CDNC uncertainty is assessed by forcing a droplet activation parameterization with a comprehensive dataset of CCN activity and aerosol size and chemical composition obtained during the ICARTT field campaign in August 2004. Our analysis suggests that, for a diverse range of updraft velocity, droplet growth kinetics and airmass origin, the error in predicted CDNC is (at most) half of the CCN prediction error. This means that the typical 20-50% error in ambient CCN closure studies would result in a 10-25% error in CDNC. For the first time, a quantitative link between aerosol and CDNC prediction errors is available, and can be the basis of a robust uncertainty analysis of the first aerosol indirect effect.

The impact of new particle formation on regional air quality and CCN formation is for the first time explored using the UAM-AERO air quality model. New particles are formed by ternary nucleation of sulfuric acid, ammonia and water; subsequent growth of clusters to large sizes is driven by condensation of sulfuric acid and organic vapors, as described by the recently developed nano-Köhler theory. Application of the model in Athens (GAA) and Marseilles (GMA) reveals higher sulfuric acid condensational sink and gaseous sulfuric acid (hence nucleation rate) for the latter. However, limited quantities of organic vapors in the GMA inhibit the growth of the formed clusters; therefore new particle formation is more efficient in the GAA. A sensitivity analysis demonstrates that (1) uncertainty in vaporization enthalpy does not affect organic carbon formed by nucleation, and (2) an accommodation coefficient of unity gives excellent agreement of condensation sink with in-situ observations. Nucleation affects the aerosol size distribution, and can be an important contributor to CCN; locally it can be more important than chemical ageing of pre-existing aerosols.