Spatiotemporal distribution and formation mechanisms of HONO in agricultural areas based on long-term 2D MAX-DOAS observations

Nitrous acid (HONO) is a key precursor of hydroxyl radicals (OH·), exerting an important influence on regional atmospheric oxidation capacity and the formation of ozone (O₃) and secondary aerosols. However, its multiple sources and complex formation pathways lead to substantial uncertainties in source apportionment and process constraints. In agricultural regions in particular, the contributions of soil microbial emissions and heterogeneous conversion on soil/aerosol surfaces remain poorly constrained by long-term observations, resulting in systematic underestimation in models.

To this end, leveraging our self-developed 2D MAX-DOAS remote-sensing observation network spanning typical regions across China, we conducted a two-year continuous campaign (2022–2023) at an agricultural site in Shouxian County, Anhui Province (32.44 °N,116.79 °E). Vertical profiles of HONO, NO₂, and aerosols were retrieved with the PriAM algorithm. Data consistency and instrumental stability were evaluated via dual-instrument intercomparison, enabling an investigation of HONO spatiotemporal variability, formation mechanisms, and estimated emission fluxes in agricultural environments.

The two systems showed excellent agreement for HONO, NO₂, and aerosols, with R² up to 0.90, demonstrating robust long-term stability. HONO exhibited pronounced near-surface accumulation, being mainly confined below 0.5 km and decreasing exponentially with altitude. Diurnal variations displayed a clear morning–evening bimodal pattern in spring, autumn, and winter, typically peaking at 09:00 and 16:00 Beijing time (BJT). In summer, this bimodality weakened due to enhanced photolysis and dilution associated with a deeper boundary layer, leading to a much smaller diurnal amplitude.

Seasonally, HONO emission fluxes showed a pronounced winter maximum and summer minimum. Winter accumulation was promoted by low temperature, high humidity, a shallow boundary layer, and sustained NO₂ supply. Autumn was mainly influenced by residual nitrogen inputs during harvest and straw burning, whereas spring enhancements were closely linked to increased soil emissions following fertilization during wheat regreening. In summer, stronger photolysis and more efficient vertical mixing inhibited accumulation. High-HONO events predominantly occurred under RH>70 % and T<10 °C, indicating that moist reactive interfaces and stable stratification jointly favor HONO formation and accumulation. Within ±2 weeks of fertilization, near-surface HONO, NO₂, and aerosol concentrations increased synchronously, with maximum enhancements of 1500 %, 200 %, and 700 %, respectively. The HONO/NO₂ ratio increased markedly after fertilization and decreased with altitude, suggesting direct HONO release from reactive nitrogen in soils via microbial processes, with additional contributions from heterogeneous NO₂ reactions on soil and aerosol surfaces. Potential Source Contribution Function (PSCF) analysis further indicated that elevated HONO during the spring fertilization period was dominated by local sources, with limited influence from long-range transport.

This study provides key vertically resolved observational evidence to quantitatively constrain the magnitude and spatiotemporal evolution of agriculturally driven HONO sources, thereby supporting improved HONO parameterization in regional chemical models and assessments of its impact on atmospheric oxidation capacity.