Comment on “Isotopic evidence for dominant secondary production of HONO in near-ground wildfire plumes” by Chai et al. (2021)

James M Roberts

doi:10.5194/acp-21-16793-2021

Abstract

Abstract. Chai et al. (2021) recently published measurements of wildfire-derived (WF) oxides of nitrogen (NOx) and nitrous acid (HONO) and their isotopic composition. The method used to sample NOx, collection in alkaline solution, has a known 1:1 interference from another reactive nitrogen compound, acetyl peroxynitrate (PAN). Although PAN is thermally unstable, subsequent reactions with nitrogen dioxide (NO2) in effect extend the lifetime of PAN many times longer than the initial decomposition reaction would indicate. This, coupled with the rapid and efficient formation of PAN in WF plumes, means the NOx measurements reported by Chai et al. were severely impacted by PAN. In addition, the model reactions in the original paper included neither the reactions of NO2 with hydroxyl radical (OH) to form nitric acid nor the efficient reaction of larger organic radicals with nitric oxide to form organic nitrates (RONO2).

Highlights

Chai et al present ground-based measurements of nitrous acid (HONO) and the oxides of nitrogen (NOx) and their 15N and 18O isotopic abundances in air masses in the immediate vicinity of wildfire (WF) (Chai et al, 2021)
The main problem with the methodology, sample collection of NOx in alkaline solution, is a 1 : 1 interference from acetyl peroxynitrate (CH3C(O)OONO2, PAN), a ubiquitous product of VOC-NOx photochemistry. We have known this for decades, as alkaline collection and hydrolysis has been used in a number of studies as a means to calibrate PAN sources
The basis of PAN interference with NOx collection is really not in dispute, since PAN is at least 2 orders of magnitude more soluble in water than NO and NO2 (Sander, 2015), and its alkaline hydrolysis is extremely rapid and forms nitrite

Summary

Introduction

Chai et al present ground-based measurements of nitrous acid (HONO) and the oxides of nitrogen (NOx) and their 15N and 18O isotopic abundances in air masses in the immediate vicinity of wildfire (WF) (Chai et al, 2021). The main problem with the methodology, sample collection of NOx in alkaline solution, is a 1 : 1 interference (as N) from acetyl peroxynitrate (CH3C(O)OONO2, PAN), a ubiquitous product of VOC-NOx photochemistry (volatile organic compound).

Results

Conclusion