print page

Global nitrous acid emissions by wildfires revealed from space:

A study published in Nature Geoscience by the Royal Belgian Institute for Space Aeronomy.

On Monday September 21, 2020, the prestigious Nature Geosciences journal published an article presenting the first satellite detection of nitrous acid (HONO) using the TROPOMI satellite instrument.

This work is the result of a fruitful collaboration between two BIRA-IASB teams, the UV-Visible observation group and the Tropospheric Chemistry Modelling group, and the group of R. Volkamer at Colorado University.

This publication (Theys et al., 2020) presents the first unambiguous detection of HONO from TROPOMI and reveals the existence of enhanced HONO over wildfires (see example in Figure 1) consistently for major ecosystems.

Figure 1: HONO emissions by fires in Victoria and New South Wales (Australia) as observed by TROPOMI on 4 Jan 2020.

The importance of HONO in atmospheric chemistry stems from its role as precursor of the OH radical. The latter is among the most important oxidizing molecules and controls the degradation of pollutants and greenhouse gases, and contributes to ozone formation and photochemical smog. As such, accurate determination of HONO sources are highly relevant to the modeling of climate and air quality. Because the release of HONO by fires is poorly understood, current models usually neglect HONO from those sources.

In collaboration with the Volkamer group of the University of Colorado Boulder, we validated the TROPOMI measurements with aircraft data obtained during the Biomass Burning Fluxes of Trace Gases and Aerosol (BB-FLUX) field campaign. A qualitative comparison is shown in Figure 2 which corroborates the detection of HONO in fire plumes.

Figure 2: (left) Comparison between TROPOMI and aircraft HONO slant column measurements of the Rabbit Foot Fire (Idaho, USA) on August 12, 2018, (right) picture of the King Air research aircraft used during BB-FLUX campaign (credit: Rainer Volkamer and Kyle Zarzana, University of Colorado, Boulder).

Our study demonstrates that pyrogenic HONO emission assessments have been underestimated by a factor of 2-4 for all ecosystem types (savannas, tropical and extra-tropical forests). Supported by model simulations from the BIRA-IASB Tropospheric Chemistry Modelling group, we estimate that HONO emissions account for about two-thirds of OH production in fresh wildfire plumes and can have a significant impact on atmospheric composition at regional level.

The findings from this study are ground for further research. The full implications of the enhanced HONO sources from fire events on climate and air quality need to be assessed. Furthermore, the exact formation of HONO in the fires is still not understood.



This work has been performed in the frame of the TROPOMI and the BB-FLUX projects. We acknowledge financial support from ESA S5P MPC (4000117151/16/I-LG), Belgium Prodex TRACE-S5P (PEA 4000105598) projects. The BB-FLUX project is supported by the US National Science Foundation award AGS-1754019.

Online paper


N. Theys, UV-Visible research group at BIRA-IASB: theys (at) aeronomie (dot) be.


Share this article


Last update on 01 Oct 2020