SI 1 Instrument description Measuring real time gas-phase aerosol precursors The Atmospheric Pressure interface Time Of Flight (APi-TOF) mass spectrometer (TOFWERK A.G.) enables the analysis of the chemical composition of gas-phase atmospheric ions (Junninen et al., 2010) and neutral molecules that can be charged or clustered with nitrate, NO3 -, ions (Jokinen et al., 2012) . The latter when complied to a chemical ionization source. The APi-TOF was run most of the time with an Eisele-type

more »

... al ionization inlet (Aerodyne / University of Helsinki -Eisele and Tanner (1993)) using nitric acid, HNO3, as reagent for chemical ionization (CI, Jokinen et al., 2012). The instrument constantly sampled outside air via a one-meter-long stainless-steel inlet tube, with a diameter of ¾ inch, actively heated (> 5 ℃), insulated and with a U-shaped bend at its extremity, avoiding freezing or sampling of dust or snowflakes. The sample flow was kept constant at 10 L min -1 with a total vacuum flow of ~ 30 L min -1 and a pressurized sheath air flow (pumped from filtered outside air) of ~ 20 L min -1 . The sheath air flow contained the saturated nitric acid vapor (~ 3 mL min -1 ) later ionized to nitrate ions and their associated multimers (i.e., NO3 -, HNO3NO3 -, (HNO3)2NO3 -) after X-ray exposure. After entering the instrument via a 300 μm critical orifice, the sampled molecules -as atmospheric molecules clustered with nitrates ions -were guided into the TOF analyzer (~ 3.5 × 10 -6 mbar) through a succession of chambers. The instrument chambers were at different pressure and monitored at all times: two segmented quadrupole chambers (~ 1.5 mbar and ~ 10 -2 mbar, respectively), and an ion lens assembly (~ 10 -5 mbar). Inside the TOF analyzer, ions are separated according to their mass to charge ratio (m/z). The exact elemental composition of ambient molecules is based on the mass defect of individual recorded signal peaks and can be calculated by combining the individual exact masses of the possible atoms present in the ion cluster. Additionally, an estimated ion concentration can be calculated by normalizing the signal of the detected peaks of interest by the sum of reagent ion signals and by applying a calibration factor. In this work, we estimate a calibration factor of 5 × 10 9 molecules cm -3 , which is based on Sulfuric acid calibration that was performed ahead of the campaign. More information about the CI-APi-TOF instrumentation and the derived qualitative and quantitative products can be found in Jokinen et al. (2012) . To ensure an efficient ionization via proton transfer reaction between the sampled molecules and nitrate ion, we monitored of the total ion counts (TIC) and the respective signal of all nitrate ions (i.e., NO3 -, HNO3NO3and (HNO3)2NO3 -). The time series of the reagent ion signals can be found in Figure S1 . In some instance, spikes of signal loss due to occasional power disturbances or exceptional failure of the pumping system were seen. Therefore, data filtering of measurements having a variation of more than 30 % of the overall median of the TIC signal was applied.