## 3 Results and Discussions
### Temporal Variations and Comparison of m/z 44 Signal with COO of Total Carboxylic Acids
Oxalic acid is produced by the photochemical oxidation of various precursors and is enriched in fine aerosols as an end product of chain reactions of longer-chain diacids and related compounds in the atmosphere [76; 91; 92; 93; 94; 95]. Several studies have reported higher ratios of oxalic acid to OC when the air mass is transported from urban to downwind remote/rural sites [13; 21; 49; 93; 96]. Hence, the diacid-COO/OC ratio can be used as a proxy for photochemical processing. OA\({}_{\rm AMS}\) showed strong correlations with the COO of oxalic acid (R = 0.84), malonic acid (0.93), and succinic acid (0.89). Further, we found a strong correlation (R = 0.92) of OA\({}_{\rm AMS}\)  
Figure 6: Correlation plot for the m/z 44 (COO\({}^{+}\)) signal measured by AMS and COO calculated for all carboxylic acids (diacids, ox oxacids, benzoic acid, and fatty acids) measured by GC. with SO\({}_{4}\)\({}^{2-}\). Sulfate is mainly formed in the aqueous phase by the atmospheric oxidation of SO\({}_{2}\). We also found a good correlation between diacids\(-\)COO/Org\({}_{\text{AMS}}\) (except for four points) and ambient temperature (R = 0.66). The temperature is always correlated with solar radiation [97]. Hence, diacids\(-\)COO/Org\({}_{\text{AMS}}\) can be used as a tracer for photochemical processing in both gaseous and aqueous phases. In general, an OC/EC ratio exceeding a threshold of 2.0 is used to indicate the photochemical production of secondary organic aerosols [98]. [PERSON] et al. [99] reported higher OC/EC ratios for remote and rural sites. The average OC/EC ratios in this study were 3.9. Hence, the secondary formation of organic aerosols is important in Okinawa due to photochemical aging during long-range transport. As discussed above, m/z 44 /OM\({}_{\text{AMS}}\) ratio can be used as a diagnostic tracer for the photochemical processing of organic aerosols. [PERSON] et al. [40] reported that m/z 44/OM\({}_{\text{AMS}}\) ratio < 0.04 is associated with less processed organics; the ratio of 0.04 to 0.08 is related with moderately processed organics; and the ratio > 0.08 is involved with highly processed organics. In this study, we found that in all the samples, m/z 44/OM\({}_{\text{AMS}}\) ratios (range: 0.09-0.13) were greater than 0.08, supporting the notion that organic aerosols from Okinawa were subjected to serious photochemical processing during the atmospheric transport from East Asia to the western Pacific Rim. Carbonaceous components (OC and EC) have been used to evaluate the secondary formation of organic aerosols, e.g., [100]. Concentrations of OC and EC ranged from 0.41 to 2.49 m\({}^{-3}\) (av. 1.2 \(\mu\)g m\({}^{-3}\)) and 0 to 0.85 (0.36 \(\mu\)g m\({}^{-3}\)), respectively. The OC-to-EC ratio is a useful tool to determine the sources of aerosols ([PERSON] and [PERSON], 2014b and references therein). The OC/EC ratios exceeding 2.0 are derived from the enhanced production of SOA [101]. The averaged OC/EC ratio in the present study is ca. 3, suggesting a significant contribution of SOA. [PERSON] et al. [48] reported that low volatility (LV)-OOA is the dominant organics produced by secondary processes over Okinawa, being consistent with the previous results from Cape Hedo [102, 103]. Based on the enhanced OC/EC, m/z 44 (COO\({}^{+}\))/OM\({}_{\text{AMS}}\) and COO of specific diacid/OM\({}_{\text{AMS}}\) ratios, we consider that the ambient aerosols from Okinawa are photochemically processed during long-range transport from the Asian Continent. In addition, C\({}_{3}\)/C\({}_{4}\) ratios were used as a proxy of photochemical aging of OA; <1 for primary emissions [12, 14, 49, 86] and >1 for aged aerosols [11]. The average C\({}_{3}\)/C\({}_{4}\) ratio of 1.3 in this study again suggests a significant photochemical aging of organic aerosols from Okinawa. We found that the majority of OA is a low-volatility oxygenated organic aerosol (LV\(-\)OOA). Our mass spectra showed strong peaks at m/z = 18, 28, and 44, which are typical for organic acids and LV-OOA [40, 48, 103].