## 2 Samples and Analytical Procedure
### Aerodyne AMS Measurements  
Aerodyne quadrupole aerosol mass spectrometer (Q-AMS; Aerodyne Research, Inc., Boston, MA, USA) was deployed for the springtime campaign at Cape Hedo, Okinawa, with a time resolution of 10 min [48; 51]. A quadruple mass spectrometer was utilized to analyze the positive ions for unit mass-to-charge (m/z) ratios with a mass resolution of 200. Setup conditions for ambient measurement were the same as for the previous observation at Fukue [52]. A detailed description of the Q-AMS can be found elsewhere [48; 51; 53; 54; 55]. Briefly, aerosols are separated from gaseous species by an aerodynamic lens and vaporized at 600 \({}^{\circ}\)C on a vaporizer. The size cut of the aerodynamic lens is approximately PM\({}_{1.0}\)[51]. The vaporized molecules were then ionized by electron impact at 70 eV, and positive ions were measured by a quadrupole mass spectrometer, which gives the mass spectra of aerosol components [56]. AMS m/z 44 (CO\({}_{2}\)) signals mainly originate from the decarboxylation of organic acids that are vaporized at 600 \({}^{\circ}\)C prior to ionization [40; 41; 46; 57; 58]. AMS data  
Figure 1: Map of Okinawa Island, Japan, with sampling location (Cape Hedo) for aerosols (PM\({}_{1.0}\)) and on-line AMS measurements. were averaged for the integration time of filter samples, i.e., 24 h. Quantified data by AMS were converted to organic aerosol masses using the following equations [51]. \[M_{\mathrm{m/z}}=\frac{1}{CE_{org}}\ \frac{1}{RIE_{org}}\ \frac{MW_{\mathrm{NO3}}}{IE_{ \mathrm{NO3}}}\ \frac{10^{12}}{QN_{A}}\ S_{\mathrm{m/z}} \tag{1}\]  
\[\mathrm{OA}=\sum_{\mathrm{m/z}}M_{\mathrm{m/z}}(\mathrm{m/z}=1-300) \tag{2}\]  
where \(\overline{CE}_{org}\) and \(\overline{RIE}_{org}\) denote the average particle collection efficiency and relative ionization efficiency (RIE) for organics, respectively. Similarly, \(MW_{\mathrm{NO3}}\) (62 g mol\({}^{-1}\)) indicates the molecular weight of nitrate, whereas \(IE_{\mathrm{NO3}}\) indicates ionization efficiency of ammonium nitrate. Q denotes a sample flow rate in cm\({}^{3}\)S\({}^{-1}\), and N\({}_{A}\) denotes Avogadro's number. \(S_{\mathrm{m/z}}\) (Hz) is the signal of count rate at the m/z originating from organic compounds and obtained by the subtraction in the signals from the ambient gas molecules, inorganic species, and instrumental artifacts. On the other hand, \(IE_{\mathrm{NO3}}\) represents the determined monodisperse ammonium nitrate particles from the calibration unit. We assume that the \(\overline{RIE}_{org}\) is 1.4 based on the work of [PERSON] et al. [59], and the CE value is assumed to be 0.5 for organics following the study of [PERSON] et al. [40], who used authentic diacid standards to estimate the CE value. [PERSON] et al. [40] reported that the size distribution of organics in Tokyo is bimodal. Similar case is also reported for AMS measurements in urban areas, e.g., [51; 54; 55; 60]. The small mode (d\({}_{\mathrm{va}}<\) 200 nm) is dominated by aliphatic or aromatic hydrocarbons mainly emitted primarily from combustion sources. The accumulation mode (d\({}_{\mathrm{va}}>\) 200 nm) is dominated by oxygenated organic compounds. The oxygenated organic compounds are the main constituents of secondary organic aerosols. The small-mode organics are externally mixed with inorganic species, and the accumulation-mode organics are internally mixed with inorganic species, e.g., [55; 60]. Therefore, the AMS particle collection efficiency of 0.5 is reasonable for organics in the study site. CE is actually defined for particles and not species and in principle could be a function of particle size for the same composition and physical shape [59].