Remote sensing of radical precursors in biomass burning plumes

The impact of biomass burning (BB) on air quality and climate change is an increasing problem in the western United States, where fire frequency and burned areas have increased in recent decades. Future changes in our climate and changes in wildfire management will likely further increase the impact of biomass burning on air pollutants such as ozone and particulate matter. Much progress has been made in understanding the emission factors and chemistry of biomass burning plumes. It has become clear that the chemistry in young biomass burning plumes is controlled by the availability of HOx radicals. Two radical precursors, HCHO and HONO, have been identified as the most likely HOx precursors in young plumes. The direct emission of both species from fires fueled by plants common in the western U.S. has been observed in the laboratory and prescribed burns. However, emission factors for larger uncontrolled fires, which are now a regular occurrence in the western U.S., are sparse. Both species also undergo secondary chemistry in the plumes which is thus far not captured well by models. In particular, the chemical formation of HONO in the plume, most likely on the aerosol, remains highly uncertain.

Motivated by these outstanding issues, we propose to investigate the following specific scientific questions:

�?� What are the emissions of HCHO and HONO from large wild fires in the western U.S.?

�?� What is the secondary chemistry of HONO (and HCHO) in BB plumes? Is there a significant volume source of HONO in the plume?

�?� How do the emissions and secondary chemistry of HONO and HCHO impact plume radical chemistry?

To address these questions, we propose airborne remote sensing measurements of HONO, HCHO, NO2, CO, CH4, H2O, O4 (as aerosol proxy), and other gases using a limb-scanning Differential Optical Absorption Spectroscopy instrument on-board the NOAA WP3 during the FIREX 2018 summer deployment. The instrument will perform measurements in limb direction to determine the flight altitude trace gas mixing ratios and to provide semi-quantitative near real-time measurements to identify plumes 10-30 km ahead of the WP3 aircraft. Column averaged measurement of trace gases below the aircraft (HONO, HCHO, NO2, CO) and above the aircraft (only HONO, HCHO, NO2) will be made to assess the integrated trace gas content in plumes. Observations of young plumes and plume development through targeted observations of HONO, HCHO, NO2, CO, and other gases in young plumes below the aircraft will yield emission ratios and, together with in-situ observations from the WP3, also emission factors. Analysis of the observations will be performed with a state-of-the-art 1D chemistry and transport model to study secondary BB plume chemistry, such as the potential formation of HONO on aerosol, and the impact of HCHO and HONO on plume radical chemistry. Our activities will be coordinated with the NOAA WP3 team, continuing previous successful collaborations with NOAA ESRL/CSD.