Nitrogen dioxide (NO₂) not only directly affects air quality and participates in secondary chemical reactions but also influences human health. It primarily originates from industrial and vehicular emissions, as well as regional transport at higher altitudes. Therefore, surface in situ measurements alone cannot fully comprehend the high-altitude transport, vertical evolution, and atmospheric chemical processes of NO₂. In this study, we aim to investigate the vertical distribution characteristics and temporal variations of NO₂ in Beijing, given its status as a primary atmospheric pollutant originating from industrial activities and vehicular emissions. With Beijing’s dense population and high vehicle density, vehicular emissions constitute a major source of NO₂. Despite improvements in air quality due to environmental policies, regional transport, especially along the southwest?northeast corridor, remains a significant contributor to NO₂ levels in Beijing. Traditional monitoring methods have limitations in capturing NO₂ transport dynamics, necessitating advanced techniques such as multi-axis differential optical absorption spectroscopy (MAX-DOAS). This research, based on two years of MAX-DOAS observations, is designed to understand NO₂’s vertical distribution and its response to policy interventions and holiday effects. By providing detailed insights into NO₂ behavior under various conditions, we support effective air quality management and policy formulation in Beijing.

Our study examines the spatiotemporal distribution of NO₂ in Beijing using a MAX-DOAS observation station located at the Chinese Academy of Meteorological Sciences from June 1, 2020, to May 31, 2022. Positioned at an elevation of 130 m, the instrument is situated 40 m above ground level with a viewing azimuth of 130°. The station is strategically placed near major NO₂ emission sources from busy traffic areas within a 5 km radius, despite the absence of industrial emissions. The MAX-DOAS instrument comprises modules for collecting sunlight scattered light and signal processing. Sunlight scattered by a right-angle prism is directed onto a single lens fiber, which transmits the light to ultraviolet and visible spectrometers covering wavelengths of 296?408 nm and 420?565 nm, respectively. Observations are automatically taken when the solar zenith angle (SZA) is below 92° across elevation angles ranging from 1° to 90°, with exposure times dynamically adjusted to maintain optimal signal quality. Data processed using QDOAS software undergoes least-squares inversion to derive tropospheric slant column densities (SCDs) of NO₂ within the 338?370 nm wavelength range. This process includes specific parameter settings and aerosol prior profiles, alongside VLIDORT radiative transfer modeling for accurate vertical profile retrievals. Rigorous quality control criteria are applied to ensure a comprehensive analysis of NO₂’s spatiotemporal variations in Beijing, providing critical data support for enhancing air quality management strategies and informing policy development.

The bottom NO₂ volume fraction extracted from NO₂ vertical profiles is well correlated with the NO₂ mass concentration measured at a CNEMC station, Guanyuan (R=0.7723, Fig. 2). The study shows that the highest NO₂ volume fraction near the ground in Beijing occurs in January (17.40×10^-9) and the lowest in April (5.51×10^-9, Fig. 3). We find that NO₂ volume fraction in Beijing varies in the order of winter (15.71×10^-9)>autumn (15.39×10^-9)>spring (8.52×10^-9)>summer (8.06×10^-9) for seasonal variation (Fig. 4). NO₂ profiles all show an exponential shape in different seasons. The averaged diurnal variation of NO₂ in spring and summer exhibits a single peak pattern appearing before 10:00, and it shows a bi-peak pattern in autumn and winter with peaks appearing before 10:00 and after 15:00 (Fig. 5). Moreover, there is not an obvious weekend effect for NO₂ in Beijing from the perspective of concentration variations. However, it shows an obvious weekend effect from the perspective of NO₂ diurnal variations, which mainly manifests in deferred NO₂ peaking (16.16×10^-9) on Saturday morning being larger than that on weekdays and Sunday and the afternoon peak on Sunday being larger than that on weekdays and Saturday. This may be related to the travel cross-cities on Saturday and return on Sunday (Fig. 7). In addition, we also reveal that the reduction of NO₂ in Beijing during major events is significantly greater than that during holidays. The average volume fraction of surface NO₂ during major events and holidays decreases by 29.0% and 18.5%, respectively, compared with the whole observation period (Fig. 8).

During the period from June 1, 2020, to May 31, 2022, we conduct continuous MAX-DOAS NO₂ remote sensing observations in Beijing. Using a least-squares algorithm, we derive NO₂ slant column densities (SCDs) from various elevation angles, enabling us to construct NO₂ vertical profiles for the entire observation period using an optimal estimation method. The correlation coefficient between the derived near-surface NO₂ volume fractions and mass concentrations from CNEMC stations reaches 0.7723, which indicates strong agreement. Key findings include monthly variations in near-surface NO₂ volume fractions, with peak levels observed during winter (17.40×10^-9) and lowest levels in April (5.51×10^-9). NO₂ vertical distribution exhibits a seasonal pattern, with higher volume fractions observed in winter and autumn compared to spring and summer. Daily variations in NO₂ volume fraction show distinct patterns depending on the season: single peaks before 10:00 in spring and summer, and double peaks before 10:00 and after 15:00 in autumn and winter. Notably, NO₂ volume fractions decline more rapidly after 10:00 in spring and summer due to increased boundary layer heights and solar radiation intensity. Weekend effects are also observed, with NO₂ volume fractions decreasing by 1.5% on Saturdays and 7.0% on Sundays compared to weekdays. Weekday, Saturday, and Sunday variations show double-peak patterns, with peaks occurring before 08:00 and after 16:00. Saturdays exhibit the highest peak volume fraction and a delayed peak compared to weekdays and Sundays. Analysis during holidays and major events reveals decreased NO₂ volume fractions of 29.0% and 18.5%, respectively, compared to the overall period. Both periods show double-peak patterns, with peaks before 09:00 and after 16:00, although volume fractions are lower during major events compared to holidays. These findings emphasize the importance of long-term MAX-DOAS NO₂ vertical observations in understanding its influence on the atmospheric environment and supporting NO₂ prevention and control efforts in Beijing.