Abstract

We conducted long-term (2017–2022) continuous observations of water vapor concentration in the lower atmosphere (0–1 km) over Tsukuba and Chiba, Japan, using the multi-axis differential optical absorption spectroscopy (MAX-DOAS) technique within the framework of the international Air Quality and Sky Research Remote Sensing Network (A-SKY). The accuracy of MAX-DOAS-derived lower-atmospheric water vapor concentration was validated against radiosonde measurements in Tsukuba (sample size = 1203), yielding a strong correlation (R = 0.971), thereby confirming the reliability of this method. Additionally, we demonstrated the capability of a four-azimuth-viewing MAX-DOAS system, comprising four A-SKY/MAX-DOAS instruments, to capture horizontal inhomogeneities in lower-atmospheric water vapor over Chiba. Analysis of the four directional data sets revealed a correlation between these inhomogeneities and atmospheric instability (increase in inhomogeneities during atmospheric instability). Under stable atmospheric conditions, the correlation coefficient between any two azimuths exceeded 0.95, whereas in unstable conditions, it decreased below 0.95 in all directions, occasionally dropping below 0.90. To further investigate this relationship, we identified 15 cases of significant horizontal inhomogeneity under unstable conditions, 10 of which coincided with the presence of a stationary front north of Chiba. Local analysis (LA) by the Japan Meteorological Agency indicated an inflow of warm, moist air toward the front, likely contributing to both the observed inhomogeneity and atmospheric instability. Additionally, while these inhomogeneities were captured by A-SKY/MAX-DOAS, they were not adequately detected in the lower-atmospheric LA, despite being within the error range of A-SKY/MAX-DOAS. This highlights the critical role of A-SKY/MAX-DOAS in monitoring lower-atmospheric inhomogeneities that conventional LA has underestimated.