Isotopically Tracking Nitrogen Oxides, Nitrous Acid, Nitric Acid, and Particulate Nitrate from Wildfire during FIREX-AQ

Abstract:

Wildfires are an important source of reactive nitrogen species including nitrogen oxides (NO_x = NO + NO₂) and nitrous acid (HONO), as well as products such as nitric acid (HNO₃) and particulate nitrate (p-NO₃^-). These species strongly influence air quality and climate. In summer of 2019 (July 25 —August 21), as part of the FIREX-AQ ground mission, we drove ~3000 miles and sampled smoke of different ages in both daytime and nighttime from 5 different wildfires in the western US. Using recently developed methods that have been validated in both laboratory and field settings, we conducted high time resolution collection of NO_x, NO₂, HONO, HNO₃, and p-NO₃^- for off-line characterization of isotopic composition including δ¹⁵N, δ¹⁸O and Δ¹⁷O (=δ¹⁷O−0.52×δ¹⁸O). δ¹⁵N serves as a potential tracker of sources and reactive nitrogen cycling pathways; δ¹⁸O and Δ¹⁷O provide key information about oxidation pathways (e.g., O₃ versus RO₂). δ¹⁵N for NO_x and HONO results range from -4.9‰ to +5.2‰ and -8‰ to +5.2‰ respectively, with the majority overlapping with our prior lab controlled burning results, suggesting the capability of δ¹⁵N to distinguish biomass burning emissions from other sources (i.e. vehicular emissions, soil emissions). In addition, our measurements show δ¹⁸O and Δ¹⁷O for NO₂ and HONO vary significantly in day vs night, as well as in young smoke vs aged smoke, reflecting different oxidizing mechanisms under different smoke conditions. Preliminary results of isotopic signatures of all the above reactive nitrogen species will be presented, and their relationships will be explored to identify wildfire derived reactive nitrogen source signatures and to constrain atmospheric processing pathways (e.g. secondary formation of HONO) for different fires under various smoke conditions depending on fuel types, fire conditions and meteorological conditions.