Formation of nitrogen-containing gas phase products from the heterogeneous (photo)reaction of NO2 with gallic acid

Heterogeneous reaction of gas phase NO2 with atmospheric humic-like substances (HULIS) is potentially an important source of volatile organic compounds (VOCs) including nitrogen (N)-containing compounds, a class of brown carbon of emerging importance. However, the role of ubiquitous water-soluble aerosol components in this multiphase chemistry, namely nitrate and iron ions, remains largely unexplored. Here, we used secondary electrospray ionization ultrahigh-resolution mass spectrometry for real-time measurements of VOCs formed during the heterogeneous reaction of gas phase NO2 with a solution containing gallic acid (GA) as a proxy of HULIS at pH 5 relevant for moderately acidic aerosol particles. Results showed that the number of detected N-containing organic compounds largely increased from 4 during the NO2 reaction with GA in the absence of nitrate and iron ions to 55 in the presence of nitrate and iron ions. The N-containing compounds have reduced nitrogen functional groups, namely amines, imines and imides. These results suggest that the number of N-containing compounds is significantly higher in deliquescent aerosol particles due to the influence of relatively higher ionic strength from nitrate ions and complexation/redox reactivity of iron cations compared to that in the dilute aqueous phase representative of cloud, fog, and rain water.


Figure S2 :
Figure S2: Van Krevelen plot showing the correlation between H/C ratio with O/C ratio obtained for the gas phase compounds detected in heterogeneous reaction of NO2 with GA/Fe(III)/NO3 -([NO3 -] = 0.05 mol L -1 ) in dark.

Figure S3 :
Figure S3: N-containing organic compounds are divided into subgroups related to the O/N ratio in their chemical composition.The y-axis represents the contribution of each subgroup to the total intensity of CHON compounds in ESI -ion mode detected during the heterogeneous reaction of NO2 with GA (A) in dark, and (B) under irradiation (C) GA/NO3 -([NO3 -] = 0.05 mol L -1 ) in dark and (D) under irradiation, (E) GA/NO3 -([NO3 -] = 0.5 mol L -1 ) in dark, and (F) under irradiation.

Figure S4 :
Figure S4: N-containing organic compounds are divided into subgroups related to the O/N ratio in their chemical composition.The y-axis represents the contribution of each subgroup to the total intensity of CHON compounds in ESI -ion mode detected in heterogeneous reaction of NO2 with GA/Fe(III) (A) in dark, and (B) under irradiation, (C) GA/NO3 -/Fe(III) ([NO3 -] = 0.5 mol L -1 ) in dark, and (D) under irradiation.

Figure S5 :
Figure S5: N-containing organic compounds are divided into subgroups related to the O/N ratio in their chemical composition.The y-axis represents the contribution of each subgroup to the total intensity of CHON compounds in ESI -ion mode detected in heterogeneous reaction of NO2 with GA/NO3 -/Fe(III) ([NO3 -] = 0.05 mol L -1 ) under irradiation.

Figure S6 :
Figure S6: Typical signal of HONO formed during the reaction of NO2 (50 ppb) with GA (1×10 -4 mol L −1 ) in the presence of NO3 -([NO3 -] = 0.05 mol L −1 ) in dark and under irradiation.The error bar on HONO signal (red) corresponds to 10% uncertainties in the measurements of HONO by LOPAP.

Figure S7 :
Figure S7: Typical signal of HONO (green) formed during the reaction of NO2 (50 ppb) with GA (1×10 -4 mol L −1 ) with GA in the presence of Fe(III) (3×10 -5 mol L −1 ), in dark and under irradiation.The error bar on HONO signal (red) corresponds to 10% uncertainties in the measurements of HONO by LOPAP.

Figure S9 :
Figure S9: Simplified illustration of the experimental set up used in this study.

Figure S10 :
Figure S10: The absorption spectra of GA (1 × 10 -4 mol L -1 ) in the presence of NaNO3 (Ieff = 0.05 M, 0.5 M)/Fe(III) (1 × 10 -6 mol L -1 ), the photon flux of Xe lamp (black dashed line).The left axis corresponds to the absorption spectra of GA in presence of NaNO3 and Fe(III), and the right axis corresponds to the spectral irradiance of xenon lamp.

Figure S11 :
Figure S11: The tentative pathway of HONO formation in dark.

Figure S13 :
Figure S13: A tentative reaction mechanism describing the formation of putrescine (A11) in the presence of Fe(III) and NO3 -in dark.

Table S3 :
Observed compounds upon heterogeneous NO2 processing of GA in dark.Formulae are shown in their neutral forms.

Table S4 :
Observed compounds upon heterogeneous NO2 processing of GA under simulated sunlight irradiation.Formulae are shown in their neutral forms.

Table S5 :
Observed compounds upon heterogeneous NO2 processing of GA/Fe(III) in dark.Formulae are shown in their neutral forms.

Table S6 :
Observed compounds upon heterogeneous NO2 processing of GA/Fe(III) under simulated sunlight irradiation.Formulae are shown in their neutral forms.