Short-term variation in atmospheric constituents associated with local front passage observed by a 3-D coherent Doppler lidar and in-situ aerosol/gas measurements

Local fronts, such as sea-breeze and land-breeze fronts, play an important role not only from a meteorological aspect but also for pollutant transport and mixing, in urban areas in particular. Due to the difficulty of measuring three-dimensional (3-D) wind fields, there are few studies focusing on short-term fine-scale spatial and temporal variations in aerosol and gas compositions associated with local front passage. At the end of November 2016, a 3-D coherent Doppler lidar was installed in Fukuoka city (33.55°N, 130.37°E), an urban area in Japan, and 3D-wind fields have been continuously measured since then. Two local fronts with typical density current structure were observed on 3 May and 29 June 2017. On 3 May 2017, there was a stepwise increase in concentrations of aerosol (particularly smaller size aerosol), black carbon (BC) and carbon monoxide (CO), supposedly of surface origin, at the passage of the local front. However, O3 content, which can be higher at higher altitudes, decreased. In another case on 29 June 2017, there was light rain in the early morning and all gas and aerosol contents were low before the passage of the front. At the front passage, there was a stepwise increase in aerosol, BC and CO contents as on 3 May, although O3 content gradually increased after sunrise and no clear change was observed in O3. Vertical aerosol observations by the Doppler lidar indicate that the depth of the front system on 29 June was much thinner (up to 300–400 m) than that on 3 May (up to ∼700 m), and the temperature decrease was larger than on 3 May. These facts suggest that the mixing of air masses within the system with those above or ahead of the system on 29 June was no stronger than on 3 May, and thus the aerosol and gas contents at the ground may be strongly affected by the vertical mixing/dilution of the system as well as by local emission.