The Key Role of Sulfate in the Photochemical Renoxification on Real PM_2.5

Abstract

The active nitrogen species (HONO, NO, and NO2) have important impacts on the atmospheric oxidative capacity and the transformation of many atmospheric species. In this study, a fast photochemical renoxification rate of adsorbed HNO3/NO3 – to active nitrogen species (HONO, NO, and NO2) was detected on real urban PM2.5, and sulfate was found to play a key role in this process. Different from the reported direct photolysis pathway, the photochemical reaction of HNO3/NO3 – on PM2.5 is dominated by a photosensitizing mechanism. Acidic protons are proved to be essential for this pathway. The role of sulfate, because of the nonvolatility of its conjugated acid, is to conserve the necessary acidic protons when interacting with HNO3 and thus maintain its photoreactivity. This work implies that sulfate will have important implications in atmospheric nitrogen cycling by accelerating the release of nitrogen oxides from photochemical renoxification of HNO3/NO3 – adsorbed on ambient particulates and thus can cause major environmental problems.

Atmospheric Measurements at the Foot and the Summit of Mt. Tai – Part I: HONO Formation and Its Role in the Oxidizing Capacity of the Upper Boundary Layer

“…Moreover, significant differences between EF values obtained from field studies and laboratory studies indicate a complex process of pNO3 photolysis that may be influenced by various environmental parameters, e.g., the aerosol pNO3 loading and the aerosol composition (Bao et al, 2018(Bao et al, , 2020Ye et al, 2016Ye et al, , 2017, and experimental laboratory conditions, e.g., collected particles on the filter or generated airborne particles (Shi et al, 2021;Wang et al, 2021). We, therefore, suggest that this process still needs further field or laboratory constraints.…”

“…The photolysis of particulate nitrate (pNO3) was found to be an important HONO source in low NOx areas such as forest canopy and marine boundary layer. High enhancement factors (EF = J(pNO3)/J(HNO3)), within the range of tens to thousands, were proposed in forest area, marine boundary layer, and polluted areas like the NCP (Bao et al, 2020;Ye et al, 2016Ye et al, , 2017Zhou et al, 2007Zhou et al, , 2011. However, model studies with field constraints (Romer et al, 2018;Xue et al, 2020) found that the EF was moderate (7-30) rather than tens to thousands obtained in laboratory studies (Bao et al, 2020;Ye et al, 2016Ye et al, , 2017Zhou et al, 2007).…”

“…High enhancement factors (EF = J(pNO3)/J(HNO3)), within the range of tens to thousands, were proposed in forest area, marine boundary layer, and polluted areas like the NCP (Bao et al, 2020;Ye et al, 2016Ye et al, , 2017Zhou et al, 2007Zhou et al, , 2011. However, model studies with field constraints (Romer et al, 2018;Xue et al, 2020) found that the EF was moderate (7-30) rather than tens to thousands obtained in laboratory studies (Bao et al, 2020;Ye et al, 2016Ye et al, , 2017Zhou et al, 2007). Moreover, a recent laboratory flow tube study (Wang et al, 2021) revealed that the EF was lower than 1 in the aqueous phase.…”

The Positive Effect of Formaldehyde on the Photocatalytic Renoxification of Nitrate on TiO<sub>2</sub> Particles

“…Reed et al, 2017). Some researchers have suggested that deposited NO 3 − and HNO 3 can be recycled back to gas phase NO x under illumination, via the renoxification process (Schuttlefield et al, 2008;Romer et al, 2018;Bao et al, 2020;Shi et al, 2021b). Photolytic renoxification occurs under light with a wavelength of < 350 nm, through the photolysis of NO 3 − /HNO 3 adsorbed on the solid surface to generate NO x .…”

Amplified role of potential HONO sources in O&lt;sub&gt;3&lt;/sub&gt; formation in North China Plain during autumn haze aggravating processes

“…in the atmosphere has received extensive attention over the past several years, and the nitrate photolysis frequency (Jnitrate) is still argued. In the laboratory studies, some researchers (Bao et al, 2018;Ye et al, 2016Ye et al, , 2017 showed that nitrate photolysis was an important HONO source, the measured Jnitrate was 1-3 orders larger than the gaseous nitric acid (HNO3) photolysis frequency (JHNO3) and could reach up to 10 -4 s -1 , and a number of substances including humic acid (Yang et al, 2018), sulfate (Bao et al, 2020) and TiO2 (Xu et al, 2021) might enhance the reaction significantly; while Shi et al (2021) found that the Jnitrate/JHNO3 ratio was <10 when using suspended submicron particulate sodium and ammonium nitrate rather than PM2.5 samples. In the field studies combining with model simulations, Kasibhatla et al (2018) compared NOx observations in Cape Verde Atmospheric Observatory with GEOS-Chem (Goddard Earth Observing System-Chemistry) model simulations and reported a Jnitrate/JHNO3 ratio of 25-50, Romer et al (2018) reported a Jnitrate/JHNO3 ratio of < 30 based on observations of NOx (= NO + NO2) and HNO3 over the Yellow Sea and a box model simulation.…”