## 3 Results and Discussions
### Temporal Variations and Comparison of m/z 44 Signal with COO of Total Carboxylic Acids
During the period from 29 March to 5 April, m/z 44 signals decreased consistently, while the COO concentrations obtained by off\(-\)line GC measurements increased and then decreased. Interestingly, ambient temperature and RH were both lower during this period. Higher temperature and relative humidity (RH) enhanced the oxidation of organics in the atmosphere [82]. Thus, there are other factors that control the COO concentrations of diacids. Some unidentified carboxylic acids may contribute to the m/z 44 signals, which include hydroxy dicarboxylic acids [83; 84], mid-chain hydroxy and/or keto carboxylic acids [85; 86; 87], and semi-volatile monoacids, including formic and acetic acids in particle phase [27; 80; 88]. The amounts of COO estimated from diacids and monoacids, including fatty acids, measured by GC show a strong correlation (R = 0.93) with m/z 44 (COO\({}^{+}\)) signals obtained by AMS (Figure 6). Calculated loadings of m/z 44 (COO\({}^{+}\)) by AMS and COO (carboxylic acids) by GC agree well, suggesting that carboxylic acids are detected by AMS. This study confirms that m/z 44 signals can be used as reliable markers for oxygenated organic aerosols because major diacids such as oxalic acid are good tracers of secondary organic aerosols (SOA) via photochemical processing in the atmosphere [7; 89]. Oxalic acid is not only produced by photochemical processing of organics in daytime, but also produced at night by the oxidation of VOCs with ozone or other oxidants. However, [PERSON] et al. (2010) reported that the daytime formation of oxalic acid always overwhelms the nighttime formation. [PERSON] et al. [29] reported a strong correlation between WSOC- and AMS-derived m/z 44 (COO\({}^{+}\)) signals and suggested that WSOC is enriched with carboxylic acids, although carboxylic acids were not measured. Figure 5: Temporal variations in the calculated COO of total diacids, total oxoacids, benzoic acid, and total fatty acids obtained by GC, and m/z 44 (COO\({}^{+}\)) signals obtained by AMS. Diacids-Derived COO vs. OM\({}_{\rm AMS}\) Ratios: Implication for Photochemical Processing during Long-Range Transport to Okinawa  
Oxalic acid is an end product in the oxidation of longer-chain diacids that are mainly formed in the atmosphere by the photochemical processes. Oxalic acid is more abundant in submicron aerosols (0.65-1.1 \(\upmu\)m) in Okinawa, suggesting an enhanced photochemical processing of diacids and other precursors in fine mode [82]. Photochemical aging is important in the western North Pacific Rim [75]. These studies demonstrate that the secondary formation of diacids is crucial in Okinawa. AMS\(-\)derived m/z 44 signals are dominated by COO\({}^{+}\), which mainly originates from the thermal decarboxylation of organic acids [59; 64]. Hence, the m/z 44/OM\({}_{\rm AMS}\) ratio can be used as diagnostic tracer for photochemical processing of organic aerosols [40; 59; 90]. We found that oxalic acid accounts for 16 \(\pm\) 3% of m/z 44 and 3.7 \(\pm\) 0.9% of OM\({}_{\rm AMS}\) in the Okinawa aerosols. This is the first report on the contribution of oxalic acid to O\({}_{\rm AMS}\) to discuss the photochemical processing. Here, we calculated the oxalic\(-\)CO\(-\)to\(-\)OM ratios to better understand the photochemically aged organic aerosols from Okinawa; Okinawa aerosols are seriously aged [48; 76]. The oxalic\(-\)COO to OM\({}_{\rm AMS}\) ratios ranged from 0.048-0.11 (av. 0.076 \(\pm\) 0.018) for aged aerosols from Okinawa. Hence, this value should be used for the highly processed aerosols. The average ratios of malonic\(-\)COO/OM\({}_{\rm AMS}\) and succinic\(-\)COO/OM\({}_{\rm AMS}\) are limited to 0.014. For longer-chain diacids, the ratios are still lower than those of malonic and succinic acids. This study shows the COO of specific diacids to OM\({}_{\rm AMS}\) ratios for the first time in aged aerosols. These ratios are higher when chemical species are more processed.