The role of Highly Oxygenated Molecules ( HOMs ) and 1 ambient ions characterized in the boreal forest . 2 3

In order to investigate the role of the ambient ions in the boreal forest we have performed measurements 17 to chemically characterize the composition of negatively charged clusters containing highly oxygenated molecules 18 (HOMs). Additionally, we compared this information with the chemical composition of the neutral gas phase 19 molecules detected in the ambient air during the same period. The chemical composition of the ions was retrieved 20 using an Atmospheric Pressure interface Time-Of-Flight mass spectrometer (APi-TOF) while the gas phase 21 neutral molecules (mainly sulphuric acid and HOMs) were characterized using the same mass spectrometer 22 coupled to a nitrate-based chemical ionization unit (CI-APi-TOF). Overall, we divided the identified HOMs in 23 two classes: HOMs containing only carbon, hydrogen and oxygen and nitrogen-containing HOMs or 24 organonitrates (ONs). During the day, among the ions, in addition to the well-known pure sulphuric acid clusters, 25 we found a large number of HOMs clustered with the two most common deprotonated inorganic acids, nitrate 26 (NO3) or bisulphate (HSO4), with the first one being more abundant. During the night, the distribution of ions, 27 mainly composed by HOM clustered with NO3, was very similar to the neutral compounds that are detected in 28 the CI-APi-TOF as adducts with the primary ion (NO3). 29 For the first time, we identified several clusters containing up to 40 carbon atoms clustered with NO3. At this 30 regard, we think that these ions are formed by up to 4 oxidized α-pinene units. However, while we know that the 31 dimer is probably formed by a covalent bond between two α-pinene oxidised units, it is still not clear what is the 32 bonding that formed these big clusters. 33 Finally, diurnal profiles of the negative ions were consistent with the neutral compounds revealing that ONs peak 34 during the day while HOMs are more abundant at night-time. However, during the day, a fraction of the negative 35 charge is taken up by the pure sulphuric acid clusters causing differences between ambient ions and neutral 36 compounds (i.e. less available charge for HOM and ON). 37 38