# Evaluation of the SOA Formation in the Reaction of Furfural with Atmospheric Oxidants
## 3 Results and Discussion
### Gas-Phase Products
Two analogous mechanisms have been proposed by considering the reaction with NOx. The same products were detected as in the absence of NOx. Besides, the presence of NOx leads to the formation of nitrate and peroxyacylnitrate products. Figure 1: (**a**) FTIR spectra of furfural. (**b**) Furfural with chlorine atoms after 30 min of reaction. (**c**) furfural with chlorine atoms in the presence of NO after 20 min of reaction. (**d**) 5-chloro-2-(5\(H\))-furanone. The formation of 5-chloro-2(5\(H\))-furanone is evidenced in Figure 1c along with sets of IR bands at 792, 1262 and 1744 cm\({}^{-1}\) and 890, 1163 and 1700 cm\({}^{-1}\), which are absorption patterns attributed to -OONO\({}_{2}\) and -ONO\({}_{2}\) groups, respectively. Moreover, the band at 1798 cm\({}^{-1}\) may be due to the presence of a substituted lactone and the band at 1843 cm\({}^{-1}\) may be due to the presence of a carbonyl group of a nitrate compound. For GC-TOFMS experiments, four product peaks were observed at 10.03, 12.02, 13.23 and 23.73 min in the gas chromatogram (see Figure 2). Peaks at 10.03 and 13.23 min were assigned to maleic anhydride and 5-chloro-2-(5\(H\))-furanone (P1), respectively. The peak at 12.02 min was identified as being due to 2-nitrofuran. The assignment was confirmed by comparing the retention times and mass spectra with those of commercial samples: 2-nitrofuran and maleic anhydride. For the identification of 5-chloro-2-(5\(H\))-furanone, the IR and MS spectra obtained for the synthesized products in our previous work were employed [17]. Matches were not found in the database for the peak observed at 23.73 min, but according to the FTIR data, one would expect the formation of a nitrate compound containing a carbonyl group. Therefore, taking in account the FTIR data, the _m/z_ value (141) and pattern observed (Figure S2), this peak is tentatively assigned to 4-nitrooxidanyl-\(\gamma\)-crotonlactone (P4, Scheme 2). However, the presence of this product was not confirmed because it is not commercially available. The formation of 2-furylnitroketone and 2-nitrofuran (P5) can be explained by the reaction between the intermediate radicals generated and NO\({}_{2}\), which is present in the NOx mixture (red route in Scheme 2). These radicals react slowly with O\({}_{2}\) since it is a slightly exothermic and highly reversible reaction, but, alternatively, they would react with other species such as O\({}_{3}\), NO\({}_{2}\) and RO\({}_{2}\)[10]. Analogous mechanisms were proposed for the reaction of furfural with OH radicals. According to the literature, 2-oxo-3-pentene-1,5-dialdehydehyde, 5-hydroxy-2-(5\(H\))-furanone, 4-oxo-2-butenoic acid and maleic anhydride are the main products expected for the OH oxidation of furfural [10]. From this list of proposed products, only the formation of maleic anhydride was observed in our experiments by GC-TOFMS. The FTIR spectra are in this case more difficult to analyze due to the overlap of the OH precursors employed (CH\({}_{3}\)ONO + NO) and the decomposition products (formaldehyde, CO and NO\({}_{2}\)). However, the IR bands observed in the product spectra after 30 min of photolysis (see Figure S3, Supplementary Materials) have been associated with the presence of -ONO\({}_{2}\) and -OONO\({}_{2}\) groups, as described previously. These bands may correspond to the products P4 and P5, which may be formed when certain levels of NOx are present in the gas mixture. Figure 2: General hydrogen abstraction mechanism in the reaction of furfural with atmospheric oxidants X (Cl atoms, OH or NO\({}_{3}\) radicals). The proposed route for the reaction in the presence of NOx is shown in red. In the case of the reaction of furfural with NO\({}_{3}\), the precursor employed (N\({}_{2}\)O\({}_{5}\)) decomposes into NO\({}_{2}\) and nitric acid (HNO\({}_{3}\)). Assuming a reaction mechanism similar to that proposed for the reaction with chlorine atoms and OH radicals, one would expect the formation of the addition product 5-nitrate-2-(5\(H\))-furanone (P1), maleic anhydride (P2) and other nitrate products.