Extended Data Figure 3: Elemental ratios measured by the AMS during ammonium sulphate seed addition.

The period corresponds to that shown in Fig. 2b. At 09:00 ammonium sulphate was added, and there is a slight decrease in the O/C ratio, consistent with more SVOCs being able to condense at higher SOA loading. Before the seed addition, the data are noisier owing to the relatively low SOA loading, but during this period we expect only ELVOCs to be able to condense onto the particles. The calculated O/C and H/C ratios for the gas phase ELVOCs are shown in shaded pink and grey, respectively. Again, at higher loadings the values start to diverge, consistent with an increased contribution from SVOCs to SOA mass. The O/C and H/C were calculated according to an improved elemental analysis methodology (M.C. et al., manuscript in preparation). The light orange dots depict the SOA O/C calculated using the old methodology commonly used until now, yielding values that are lower by roughly 0.15–0.2. These values are consistent with previous α-pinene ozonolysis experiments¹⁶ as well as average O/C values measured in Hyytiälä⁷⁶. It should, however, be noted that the ability of the AMS to provide accurate O/C is dependent on the specific compounds, and together with possible particle-phase evolution, the gas and particle phase O/C is not expected to match perfectly even if completely error-free measurements were possible.