Aerosol extinction profiles at 550 nm were retrieved by applying multi-axis differential optical absorption spectroscopy (MAX-DOAS) and lookup table. Then the tropospheric NO₂ and HCHO vertical column densities were retrieved using a two-step method from 28 July to 5 August of 2015 in Shanghai. The retrieved results were compared with the satellite products, and then their diurnal variation was observed. A consistency check was performed before the inversion to obtain a correction factor. Based on the sensitivity of geometric angles to oxygen dimer air mass factor (O₄ AMF, AMF is the ratio of the slanted column density to the vertical column density), the parameterization scheme of geometric angles in the lookup table is optimized. The results show that the aerosol increased significantly in the afternoon. The diurnal variation of tropospheric NO₂ and HCHO vertical column densities (VCDs) are bimodal and unimodal patterns respectively, and their values are higher than those of GOME-2 and OMI satellite products. A process of aerosol reduction and recovery are related to ground particulates and meteorological elements. The chemical sensitivity of local ozone production also has a clear diurnal variation.

This study adopts MAX-DOAS technology and optimal estimation methods to analyze the vertical profiles of aerosols, HONO, and O₃ in Kunming during winter 2024, thereby clarifying their vertical distribution characteristics and diurnal variation rules. Aerosols are predominantly concentrated in the near-surface layer, with concentrations decreasing gradually with increasing altitude; their concentrations rise in the morning, reach a peak at 10 a.m., and then decline, with the maximum extinction coefficient reaching 1.3 km⁻¹. HONO concentrations decrease exponentially with height, being the highest around sunrise, the lowest at noon, and then gradually accumulating, with the maximum concentration exceeding 3.14 × 10¹⁰ molec·cm⁻³. O₃ achieves its maximum concentration in the 2.6–4 km altitude range in the afternoon (the highest concentration > 1.16 × 10¹² molec·cm⁻³), while near-surface O₃ concentrations increase steadily from morning onward. Model analysis demonstrates that without considering HONO, OH radicals shift from net production to net loss in the morning, leading to reduced O₃ production rates and a decrease of approximately 8 ppb in the maximum O₃ concentration. These findings highlight the significant vertical and diurnal variations in the three pollutants, emphasize HONO’s crucial role in OH radical generation and O₃ formation, provide valuable scientific support for air pollution control, and confirm HONO’s important impact on regional atmospheric chemistry and the distribution of key pollutants.

The tropospheric vertical column concentration (VCDtrop) of NO₂, SO₂, and HCHO was retrieved, respectively, by employing the geometric method (Geomtry), simplified model method (Model), and look-up table method (Table) with the observation spectra of the multi-axis differential absorption spectroscopy instrument (MAX-DOAS). The correlation and relative differences of the inversion results obtained by these three algorithms, as well as the changes in quantiles, were explored. The comparative analysis reveals that the more concentrated the vertical distribution height of gas components is in the near-surface layer, the better the conformity of the VCDtrop retrieved by different algorithms. However, the increase in relative differences is also related to the diurnal variation of gas components. The influence of aerosols on the inversion of the VCDtrop is greater than the change in the vertical distribution height of the gas component itself. The near-surface concentration and distribution height of gas components are the internal factors that give rise to relative differences in the inversion of the VCDtrop by different algorithms, while aerosols are one of the extremely important external reasons. The VCDtrop inverted by Geomtry without considering the influence of aerosols is generally larger except for NO₂. Model sets up aerosols in accordance with the height and meteorological conditions of the atmospheric environment. Table can invert the aerosol profile in real time. Compared with Model, it shows a significant improvement in the refined setting of aerosols. Moreover, while obtaining the vertical distribution of aerosols, it can invert the diurnal variation of the VCDtrop. The VCDtrop inverted by Table is the smallest, and the relative difference with Model is on average about 10% smaller. The relative difference of the VCDtrop for the same height (aerosol optical thickness) quantile is 7–15% (about 25% lower on average). When comparing the inversion results of Table with the Ozone Monitoring Instrument (OMI) satellite product, the MAX-DOAS inversion results of NO₂, SO₂, and HCHO are all larger than the OMI product. This is related to the different observation methods of the MAX-DOAS and OMI and the configuration between the aerosol layer and the distribution height of gas components.