Laboratory Investigation of Renoxification from the Photolysis of Inorganic Particulate Nitrate

Abstract

Nitrogen oxides (NO x ) play a key role in regulating the oxidizing capacity of the atmosphere through controlling the abundance of O3, OH, and other important gas and particle species. Some recent studies have suggested that particulate nitrate, which is conventionally considered as the ultimate oxidation product of NO x , can undergo “renoxification” via photolysis, recycling NO x and HONO back to the gas phase. However, there are large discrepancies in estimates of the importance of this channel, with reported renoxification rate constants spanning three orders of magnitude. In addition, previous laboratory studies derived the rate constant using bulk particle samples collected on substrates instead of suspended particles. In this work, we study renoxification of suspended submicron particulate sodium and ammonium nitrate through controlled laboratory photolysis experiments using an environmental chamber. We find that, under atmospherically relevant wavelengths and relative humidities, particulate inorganic nitrate releases NO x and HONO less than 10 times as rapidly as gaseous nitric acid, putting our measurements on the low end of recently reported renoxification rate constants. To the extent that our laboratory conditions are representative of the real atmosphere, renoxification from the photolysis of inorganic particulate nitrate appears to play a limited role in contributing to the NO x and OH budgets in remote environments. These results are based on simplified model systems; future studies should investigate renoxification of more complex aerosol mixtures that represent a broader spectrum of aerosol properties to better constrain the photolysis of ambient aerosols.

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

“…Hence, it is expected that the real EF should be much lower than the inferred ones (EFinferred) from P(HONO)nitrate = Pun. Therefore, 606 EF values were inferred, in the range of 15.6 to 1072, with a mean of 173 ± 98, which is much higher than those (around 1) determined in recent flow tube or smog chamber studies (Shi et al, 2021;Wang et al, 2021). The minimum (EFinferred_mini = 15.6) is at a similar level to field studies of Romer et al (2018) and Zhou et al (2003), and the lower values in the laboratory studies (Bao et al, 2018;Ye et al, 2016Ye et al, , 2017Zhou et al, 2011).…”

“…Another flow tube study (Laufs and Kleffmann, 2016) also reported a slow HONO formation from secondary heterogeneous reactions of NO2 produced during HNO3 photolysis. Besides, a very recent chamber study (Shi et al, 2021) found that the EF values of airborne nitrate were lower than 10 (generally around 1), which also indicates an insignificant contribution of nitrate photolysis to HONO formation. Furthermore, when considering the large variation of EF values (from digits to thousands) in the model, model performance on HONO simulations could be improved but accompanied by large uncertainties (Fu et al, 2019;Liu et al, 2019b).…”

Is the ocean surface a source of nitrous acid (HONO) in the marine boundary layer?

“…15-fold during the day, based on a typical particle surface area for the marine boundary layer of 150 µm 2 cm -3 (VandenBoer et al, 2013) and a daytime boundary layer height of 1000 m; this ratio is even larger at night). Aerosols can be a source of HONO in the marine boundary layer via the rapid photolysis of nitrate-containing particles; however there is significant uncertainty to the importance of this route, with reported rates spanning three orders of magnitude (Kasibhatla et al, 2018;Reed et al, 2017;Shi et al, 2021;Ye et al, 2016). Zhang et al (2016) included a parameterisation for an enhanced NO2 to HONO conversion rate over the sea surface based upon the results of Zha et al, (2014) and calculated that this marine source accounted for 9% of the observed HONO mixing ratio in Hong Kong (implying a significant influence over OH production).…”

“…Using a 0-D box model, Reed et al (2017) were only able to reproduce the observed diurnal trends in HONO and NOx at CVAO by including the rapid photolysis of particle nitrate. Although significant uncertainty remains over the importance of nitrate photolysis (Shi et al, 2021), the modelling result of Reed et al suggests that particle nitrate photolysis may represent the dominant daytime source of HONO at CVAO, as opposed to photochemical conversion of NO2 to HONO on the ocean surface.…”

A Comprehensive Observational Based Multiphase Chemical Model Analysis of the Sulfur Dioxide Oxidations in both Summer and Winter

“…As proposed in a previous study, the particulate nitrate photolysis can explain the missing source of sulfate in Beijing haze (Zheng et al, 2020). However, according to the recent laboratory report (Shi et al, 2021), the nitrate photolysis enhancement factor is no larger than 2 at all RH ranges. We also included the calculation of nitrate photolysis in this study due to the high loading of particle nitrate and found that the contribution was rather small (~0.008 μg m -3 h -1 in winter haze periods); thus, we did not include this pathway in the figures.…”