Direct Constraints on Secondary HONO Production in Aged Wildfire Smoke From Airborne Measurements Over the Western US

Nitrous acid (HONO) mixing ratios measured in aged wildfire smoke plumes were higher than expected from known homogeneous chemical reactions. In a representative smoke plume, intercepted hours to days downwind of the source, the missing HONO source was highly correlated to particulate nitrate photolysis and NO₂ reactive uptake to particles. Using a multilinear regression involving these two sources, we could explain the missing HONO production in this plume (R² = 0.77). The resulting fit parameters from this plume had good explanatory power (R² = 0.64) for missing HONO production in other fire plumes. The mean enhancement factor for particulate nitrate photolysis relative to gas-phase nitric acid photolysis was 63 and the mean NO₂ reactive uptake coefficient to submicron aerosol surface area forming HONO was 4.9 × 10⁻⁴. Given the likelihood of other neglected secondary HONO sources, these values are upper-limits, suggesting a need to revisit HONO formation mechanisms in aged wildfire smoke.