Issue 6, 2021

Insights into HONO sources from observations during a solar eclipse


Nitrous acid (HONO) is a major, and often the dominant, precursor to primary OH radical production in the daytime boundary layer, driving the removal of many primary pollutants and formation of secondary species such as ozone and many aerosol components. A number of photochemical HONO production mechanisms have been proposed, alongside homogeneous gas-phase reactions, to account for field observations of daytime HONO. The range of production mechanisms show varying dependencies upon precursor species such as NO2, available surfaces for heterogeneous reactions, and dark/photoenhanced aspects. Here, we exploit measurements of HONO and related species during a near-total solar eclipse as a natural perturbation to the atmospheric photochemistry to assess the characteristics of the production mechanisms occurring at an urban background location. Little variation in HONO abundance was observed in response to changing light levels during the eclipse, pointing to relatively balanced photochemical source and (well-understood) sink terms. We employ a series of simple kinetic simulations to explore the consistency of different potential source mechanisms with the observations, finding evidence for a dominant role for photochemical processing of traffic-derived NO2 upon surfaces producing HONO, alongside indications of a smaller contribution from direct vehicular emissions. Other mechanisms involving dark heterogeneous reactions were not, in isolation, consistent with the observations. The critical role of NO2, ultimately derived overwhelmingly from local road traffic emissions at this location, points to significant future reductions in daytime HONO production with vehicle fleet evolution and reduction of tailpipe emissions.

Graphical abstract: Insights into HONO sources from observations during a solar eclipse

Supplementary files

Article information

Article type
13 Feb 2021
29 Jun 2021
First published
20 Jul 2021
This article is Open Access
Creative Commons BY license

Environ. Sci.: Atmos., 2021,1, 395-405

Insights into HONO sources from observations during a solar eclipse

A. Singh, L. R. Crilley, F. D. Pope and W. J. Bloss, Environ. Sci.: Atmos., 2021, 1, 395 DOI: 10.1039/D1EA00010A

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

Read more about how to correctly acknowledge RSC content.

Social activity