Optical Properties of Boundary Layer Aerosols From High Spectral Resolution Lidar Measurements in a Polluted Urban Environment (Seoul, Korea)

Continuous measurements of aerosol extinction coefficient (sigma(ext)) from the High Spectral Resolution Lidar (HSRL) in Seoul from 2016 to 2018 were used to investigate the diurnal variation of the aerosol optical depth (AOD). Nighttime AOD displayed a larger mean and standard deviation (0.45 +/- 0.47) than daytime (0.40 +/- 0.29). Hygroscopic growth of aerosols under humid conditions was a key factor in the relative enhancement of nighttime AOD. Taking advantage of the HSRL's vertically resolved measurements, the contribution of aerosols within the boundary layer (BL) and free troposphere (FT) to AOD and its temporal variation were investigated. Unlike the diurnal AOD variation, AOD within the BL (AOD(BL)) showed similar diurnal variations with the mixing layer height (MLH), displaying lower nighttime values with a peak around 14-15 local standard time. However, the low correlation between MLH and AOD(BL) (R-2 = 0.06) implied that MLH was not the sole deterministic factor of AOD(BL). A larger AOD of FT aerosols (AOD(FT)) was observed during spring due to frequent elevated dust layers. Using mean sigma(ext) within the BL and surface PM10 concentrations, the mass extinction efficiency (MEE) of aerosols in Seoul was estimated. The PM10 MEE showed a mean of 5.40 g m(-2) and displayed significant variability by PM2.5 to PM10 ratio, season, and ambient relative humidity. The uncertainty of estimated surface PM10 concentrations was minimized when the seasonality and humidity factor in MEE were taken into account (the normalized mean bias decreased from 10.6%, using a single MEE value, to 6.2%). Plain Language Summary The vertical distribution of aerosols and its evolution over time is important for accurately simulating near-surface air quality, since the chemical and transport processes of aerosols vary by altitude. Understanding the optical properties of aerosols by altitude is also important when accounting for aerosol radiative and climate effects. We investigated the temporal variation of aerosol optical properties in Seoul, Korea, using 2-year (2016-2018) High Spectral Resolution Lidar measurements. We found significant hygroscopic growth of aerosols during nighttime that led to enhanced aerosol optical depth (AOD). The diurnal variation of the AOD within the boundary layer (AOD(BL)) resembled that of the mixing layer height (MLH), displaying maximas at 14-15 local standard time and lower values at night. However, due to daytime aerosol dilution and enhancement of aerosol extinction coefficients by hygroscopic growth during nighttime, AOD(BL) and MLH were not necessarily correlated. Significant increases in AOD within the free troposphere (AOD(FT)) were found during spring when long-range transport of dust particles was frequent. Ambient relative humidity, seasons, and the vertical distribution of aerosols (i.e., the contribution of aerosols within the BL to AOD) should be considered when estimating surface particulate matter concentration from remote sensing measurements of AOD.