AbstractNitrous 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.