The dilution effect caused by boundary-layer evolution over land has strong influences on air quality. Accurate and continuous measurements of the boundary-layer height over urban areas are therefore needed for complete air-quality assessments. Commercial ceilometers, in combination with reliable and simple methodologies, can be used to retrieve the mixed-layer height, and represent a means of obtaining information on vertical mixing and atmospheric structure above cities. Here, we evaluate various retrieval algorithms based on the gradient method against high-temporal-resolution radiosonde observations. Based on the results, we propose a simple algorithm by using the gradient method, the correction of background noise and the moving averages, with the minimum number of parameters that need to be adjusted to the local properties and the instrument itself. The algorithm is adjusted for Seoul, Korea, and improves the retrieval performance by reducing high-frequency noise. The algorithm is used to investigate the relationship between the evolution of the daytime mixed-layer height and air pollution under a two-layer mixing model where changes in concentration depend only on the urban boundary-layer growth and air entrainment from the free atmosphere. Using 2 months of ceilometer retrievals of mixed-layer height and air-quality data from across the city, we find strong negative correlations for primary emitted pollutants such as NO2, CO, SO2, and particulate matter smaller than 10 µm, and a modest positive correlation for O3. The results provide insight into the significant influence of urban boundary-layer evolution on Seoul’s air quality.
Bibliographical noteFunding Information:
This study was supported by the Korea Polar Research Institute (KOPRI, PN19081) and the National Research Foundation of Korea grant funded by the Korean government (MSIT) (No. NRF-2018R1A5A1024958). J.-W. Hong was supported by the Global Ph.D. Fellowship Program (NRF-2015H1A2A1030932). The assistance provided by T. Knepp, J. Szykman, R. Long and L. Valin for the radiosonde measurements is much appreciated. The authors acknowledge the constructive comments of three anonymous reviewers. The Z ML and air pollution data used in this study are available at 10.22647/EAPL-LEE2017A.
All Science Journal Classification (ASJC) codes
- Atmospheric Science