# Evaluation of the SOA Formation in the Reaction of Furfural with Atmospheric Oxidants
## 3 Results and Discussion
### Gas-Phase Products
For the reactions of furfural with chlorine atoms in the absence and presence of NOx, the molar yields obtained for 5-chloro-2-(5\(H\))-furanone were similar for both reactions, although the levels of HCl and CO quantified were lower for the reaction in the presence of NOx. Yields of HCl and CO are overestimated due to wall secondary reactions and traces of humidity. For the reaction of furfural with the OH radical, only the levels of CO (using FTIR) and maleic anhydride (using GC-MS/TOF) were confirmed. Both products lead to primary concentration-time profiles, as in the case of chlorine reactions. In this case, the yield obtained for the anhydride was slightly higher than that obtained for the reaction with chlorine atoms. For the reaction with NO\({}_{3}\) radicals, only the levels of NO\({}_{2}\), CO and HNO\({}_{3}\) were confirmed by FTIR analysis. A progressive increase in the concentration of NO\({}_{2}\) during the reaction can be seen (Figure S7 Supplementary Materials), but this began to decay over time as a result of secondary reactions. The high concentrations detected initially are attributable to the thermal decomposition of the precursor (N\({}_{2}\)O\({}_{5}\)). The yield was estimated to be (78.3 \(\pm\) 6.6)%. In this case, the high levels of NO\({}_{2}\) detected could be of great importance since it can react with the reaction intermediates generated. The concentration profiles for CO and HNO\({}_{3}\) are more or less constant, with a slight decay observed for the latter throughout the reaction. This trend can be attributed to the deposition of nitric acid on the reactor walls. However, HNO\({}_{3}\) levels are difficult to evaluate since, as mentioned before, they are strongly influenced by the existence of secondary reactions. Molar yields of (58.6 \(\pm\) 8.6)% and (20 \(\pm\) 3)% for 2-nitrofuran and maleic anhydride, respectively, were quantified by GC-TOFMS. As can be seen from the results in Table 1, this yield is considerably higher than in the case of the other oxidants, which suggests in this case an extra route for the generation of maleic anhydride (Scheme 1). As mentioned above, the curve contours observed for maleic anhydride after 2 h of reaction suggest loss due to secondary reactions. The molar yield for the 2-furylnitroketone could not be obtained since it is not commercially available. The studied oxidants led to the formation of the common product maleic anhydride. This compound may be generated by hydrogen abstraction of the -CHO group (via I), since the formation of its coproduct (5-hydroxy-2-(5\(H\))-furanone) via II has not been confirmed. For reactions with chlorine atoms and the OH radical, the yield for maleic anhydride obtained is between 6 and 9%. However, this yield is underestimated because it is suspected that this compound undergoes a photolysis process [20]. However, in the case of the nitrate radical, as mentioned above, the anhydride has an extra contribution by the additional pathway. Its yield (\(\sim\)20%) is approximately 2-3 times greater than for other reactions. The results obtained in this study confirm that for reactions with chlorine atoms and the OH and NO\({}_{3}\) radicals, furfural reacts by both mechanisms, namely C5 addition and aldehyde hydrogen abstraction. According to the estimated yield for 2-nitrofuran, it is believed that the abstraction process occurs to a greater extent for nitrate radicals. The reaction with NO\({}_{3}\) radicals led to the formation of various nitrated products such as 2-nitrofuran (P5) and unconfirmed products