showed that biogenic soil nitrite can be an important HONO source. (32,33)īesides chemical reactions, Su et al. (29−31) However, multiple scattering effects of light on aerosol sample filters used in those experiments may lead to an overestimation of the observed reaction rates. (2,28) Moreover, laboratory studies found that heterogeneous HNO 3 photolysis on aerosols exhibited a high HONO production rate and has been accounted as an important HONO source. (1,24−27) Additional HONO formation mechanism such as the photosensitized reduction of NO 2 on humic acid surfaces has been proposed. Under atmospherically relevant conditions, the uptake coefficient of NO 2 (γ) on aerosols such as mineral dust, (22,23) soot, (20) and organic particulates (21) is at magnitudes of 10 –4 to 10 –5 is required to explain the observed HONO formation rates of 0.2–2.0 ppb h –1. (20,21) However, the significance of such a source involving NO 2 uptake on aerosols remains controversial. (15,19) In the presence of light, the reaction has been observed to be significantly enhanced and has been considered to be a missing daytime HONO source. (14−17) A heterogeneous reaction of NO 2 on aerosol surfaces (14,18) has been suggested to explain the high HONO concentrations. Emission from combustion processes (10,11) and gas-phase production of HONO (via the reaction of NO with OH (12,13)) are not sufficient to explain the observed high atmospheric HONO concentrations in field studies. The main source of atmospheric HONO has been a mystery for decades. This work advances the understanding of the soil–atmosphere exchange of HONO and the evaluation of its impact on the atmospheric oxidizing capacity. We illustrate the robustness of using * for quantifying soil fluxes of HONO, whereas the laboratory-determined chamber HONO fluxes can largely deviate from those in the real world for the same soil sample. We demonstrate that * is a soil characteristic, which is independent of HONO concentrations in the chamber but varies with different soil water contents. We show a bi-directional soil–atmosphere exchange of HONO and confirm the existence of * over soil: when * is higher than the atmospheric HONO concentration, HONO will be released from soil otherwise, HONO will be deposited. Here, we present a method to retrieve * by conducting controlled dynamic chamber experiments with soil samples applied with different HONO concentrations at the chamber inlet. However, * has not yet been well-validated and quantified. *, the equilibrium gas-phase concentration over the aqueous solution of nitrous acid in the soil, has been suggested as a key parameter for quantifying soil fluxes of HONO. Nitrous acid (HONO) is an important component of the global nitrogen cycle and can regulate the atmospheric oxidative capacity.