Interaction between the Mixed-Layer Aerosol Direct Effect and Boundary Layer Depth

The planetary boundary layer (PBL) limits the vertical mixing of aerosol emitted to the lower troposphere. Its depth and the change in its depth over time are as important as emissions to surface air quality and the radiative forcing of tropospheric aerosols. Radiative heating and cooling of the land surface causes a strong diurnal signal in the PBL depth under typical conditions; in other cases, the PBL depth depends strongly on synoptic weather conditions. For both reasons, entrainment to and from the free troposphere, boundary-layer convection, and the vertical distribution of aerosol change on a time scale of minutes to hours. Even intensive radiosonde campaigns rarely take observations more than four times per day. PBL detection in profiles from micropulse lidar (MPL) can fully resolve this cycle. The relationship between aerosol-driven radiative forcing and the PBL depth works in both directions. Not only does the PBL top concentrate the radiative effects of aerosol close to the surface, but the aerosol direct effect heats the mixed layer at the expense of the surface. This change in the vertical distribution of heating stabilizes the lower troposphere, which in turn suppresses the surface convection that drives PBL development. Models have simulated such an effect. To confirm it by PBL depth observations, the study uses long-running MPL deployments, an improved PBL detection algorithm capable of analyzing the resulting time series, and a method for eliminating other variables from the analysis. With additional information from sunphotometer measurements, the MPL profiles can become measurements of aerosol properties with altitude. Comparing these measurements to one another across multiple sites and types of data, and even model output, isolates aerosol radiative forcing as a variable. It also provides the observations to support model simulations, which can assess the sensitivity of PBL depth and development to the aerosol direct effect. Figure: Mean diurnal cycle of PBL depth found in Lamont, OK, USA (UTC-6, left) and Hefei, China (UTC+8, right) for multi-year MPL time series. Time is in hours UTC.