Resolving the photochemical complexity of trace gases in transitional environments using multi-directional MAX-DOAS

Introduction: Nitrogen dioxide (NO₂), nitrous acid (HONO), and formaldehyde (HCHO) are key reactive trace gases for assessing regional air quality and atmospheric chemistry. While NO₂ is primarily associated with anthropogenic emissions, HONO and HCHO also originate from natural sources, making human-driven pollution less discernable, particularly in heterogeneous environments. These species are also involved in atmospheric photochemical processes, causing complex spatial and temporal patterns. Multi-axis differential optical absorption spectroscopy (MAX-DOAS) enables the retrieval of near-surface concentrations (NSCs) and vertical column densities (VCDs) providing temporal consistency and resolving spatial complexity.

Methods: Multi-directional MAX-DOAS measurements were performed from the CNR-IIA atmospheric observatory, located 40 km northeast of Rome. The surrounding area includes natural ecosystems, urban settlements, and agricultural areas within the southern Tiber River basin. NO₂, HONO, and HCHO NSCs and VCDs were retrieved along different azimuth viewing directions through the application of inversion to differential slant column densities. Quality-controlled retrievals were analyzed to assess directional variability of the trace gases and of the HONO/NO₂ and HCHO/NO₂ ratios, used as indicators of atmospheric oxidation processes and photochemical regimes.

Results: Significant azimuthal differences were observed in both NSCs and VCDs, indicating heterogeneous distributions of reactive trace gases around the observatory complex environment. Elevated NO₂ levels were associated with directions influenced by urban emissions, while higher HONO and HCHO values were found in predominantly rural directions, demonstrating dependency from secondary production processes and biogenic sources. The HONO/NO₂ ratio showed pronounced directional and temporal variability, suggesting differences in HONO formation pathways, while variations in the HCHO/NO₂ ratio highlighted changes in VOC-related oxidation activity related to vegetation presence and seasonality.

Conclusions: The results demonstrate the capability of multi-directional MAX-DOAS measurements to characterize trace gases variability, improve the understanding of air-quality dynamics and atmospheric oxidative capacity in heterogeneous environments affected by natural and anthropogenic influences.