# Evaluation of the SOA Formation in the Reaction of Furfural with Atmospheric Oxidants
## 4 Conclusions  
The main products detected were 5-chloro-2-(5\(H\))-furanone and maleic anhydride for the reaction with chlorine atoms. 2-nitrofuran was observed in the presence of NOx. The formation of 2-furylketone and 5-nitrate-2-(5H)-furanone has also been tentatively proposed for the reactions with nitrate radicals and chlorine atoms in the presence of NOx. A general mechanism for the atmospheric oxidation of furfural has been proposed. The attack of the oxidants occurs mainly by a double bond addition at the C5 position. Hydrogen abstraction is also observed for all of the studied oxidants. Furthermore, the high maleic anhydride yield obtained for nitrate reactions suggests a contribution from an extra route. When oxidation of furfural takes place under high NOx conditions, an alternative route with NO/NO\({}_{2}\) has been proposed. This behavior has been observed for nitrate and Cl/NOx reactions. The low yields obtained for the gas-phase products identified (~13-23% for chlorine and ~9% for OH in terms of total carbon) suggest the formation of furfural products in a condensed phase. Figure 5: Size distribution of SOA particles formed in the reaction of furfural with chlorine atoms at different times, \(\overline{\text{\text{\text{\text{\text{\text{\text{\text{\text{ \text{ \text{ \text{ \text{ \text  
This statement was confirmed in our experiments, where secondary organic aerosol was observed for the reaction of furfural with chlorine atoms, OH radicals and ozone. The formation of particulate matter in the reaction of furfural with chlorine atoms, OH radicals and ozone w observed. The aerosol yields obtained for these reactions varied in the 0.3-3% range, with the reactions with chlorine atoms giving the highest yield, Y. In this reaction, the influence of the oxidant/furfural ratio was studied and an average yield of 6% was obtained for most of the reactions. When [Cl2]0/[ROG]0 is \(\leq\)1, lower yields of around 2-3% were obtained. Although the aerosol yields obtained in this work are not particularly high, the generation of an aerosol is expected to have greater relevance in coastal areas, where the concentrations of chlorine atoms are higher and these reactions become more important. The presence of organic aerosols in the atmosphere can influence the terrestrial radiative balance since they absorb and disperse solar and terrestrial radiation and influence the generation of clouds and heterogeneous reactions [11; 12]. In addition, the particulate matter observed is made up of ultrafine particles, in the order of nanometers, and they can easily penetrate the airways and reach the alveolar region. However, more experiments are required to better understand the atmospheric oxidation of furfural. The following are available online at [[http://www.mdpi.com/2073-4433/11/9/927/s1](http://www.mdpi.com/2073-4433/11/9/927/s1)]([http://www.mdpi.com/2073-4433/11/9/927/s1](http://www.mdpi.com/2073-4433/11/9/927/s1)). Scheme S1: Resonance forms generated after the attack of the oxidant X. Figure S1: FTIR spectra of maleic anhydride. Figure S2: Mass spectra obtained for commercial 2-nitrofuran (a) and the products detected at 12.02 (b) and 23.73 min (c). Figure S3: IR product spectra for furfural, with OH radicals at t = 0 and t = 30 min of reaction. IR spectra of the HCHO detected Figure S4: IR residual products spectra after 60 min of reaction of NO\({}_{3}\) and furfural (a) and database spectra for N\({}_{2}\)O\({}_{5}\) (b) and HNO\({}_{3}\) (c). Figure S5: Concentration-time profiles of the products observed by GC-MS/TOF for the reaction of furfural with chlorine atoms in the presence of NOx. Figure S6: Integrated peak area-time profiles of the products observed by GC-MS/TOF for the reaction of furfural with chlorine atoms in the presence of NOx. Figure S7a: Concentration-time profiles of the products observed by FTIR for the reaction of furfural with nitrate radical. Figure S7b: Concentration-time profiles of the products observed by GC-MS/TOF for the reaction of furfural with nitrate radical.