Contribution from biogenic organic compounds to particle growth during the 2010 BEACHON-ROCS campaign in a Colorado temperate needleleaf forest

New particle formation (NPF) is an important at- mospheric phenomenon. During an NPF event, particles first form by nucleation and then grow further in size. The growth step is crucial because it controls the number of particles that can become cloud condensation nuclei. Among various phys- ical and chemical processes contributing to particle growth, condensation by organic vapors has been suggested as im- portant. In order to better understand the influence of bio- genic emissions on particle growth, we carried out modeling studies of NPF events during the BEACHON-ROCS (Bio- hydro-atmosphere interactions of Energy, Aerosol, Carbon, H2O, Organics & Nitrogen - Rocky Mountain Organic Car- bon Study) campaign at Manitou Experimental Forest Ob- servatory in Colorado, USA. The site is representative of the semi-arid western USA. With the latest Criegee intermedi- ate reaction rates implemented in the chemistry scheme, the model underestimates sulfuric acid concentration by 50 %, suggesting either missing sources of atmospheric sulfuric acid or an overestimated sink term. The results emphasize the contribution from biogenic volatile organic compound emis- sions to particle growth by demonstrating the effects of the oxidation products of monoterpenes and 2-Methyl-3-buten- 2-ol (MBO). Monoterpene oxidation products are shown to influence the nighttime particle loadings significantly, while their concentrations are insufficient to grow the particles dur- ing the day. The growth of ultrafine particles in the daytime appears to be closely related to the OH oxidation products of MBO.