Optical Characterization of Continental and Biomass Burning Aerosols over Bozeman Montana: A Case Study of the Aerosol Direct Effect

Abstract

Atmospheric aerosol optical properties were observed from 21 to 27 September 2009 over Bozeman, Montana, during a transitional period in which background polluted rural continental aerosols and well‐aged biomass‐burning aerosols were the dominant aerosol types of extremely fresh biomass‐burning aerosols resulting from forest fires burning in the northwestern United States and Canada. Aerosol optical properties and relative humidity profiles were retrieved using an eye‐safe micropulse water vapor differential absorption lidar (DIAL) (MP‐DIAL), a single‐channel backscatter lidar, a CIMEL solar radiometer as part of the Aerosol Robotic Network (AERONET), a ground‐based integrating nephelometer, and aerosol products from Moderate Resolution Imaging Spectroradiometer (MODIS) Terra and Aqua. Aerosol optical depths (AODs) measured during the case study ranged between 0.03 and 0.17 (0.015 and 0.075) at 532 nm (830 nm) as episodic combinations of fresh and aged biomass‐burning aerosols dominated the optical depth of the pristinely clean background air. Here, a pristinely clean background refers to very low AOD conditions, not that the aerosol scattering and absorption properties are necessarily representative of a clean aerosol type. Diurnal variability in the aerosol extinction to backscatter ratio (Sa) of the background atmosphere derived from the two lidars, which ranged between 55 and 95 sr (50 and 90 sr) at 532 nm (830 nm), showed good agreement with retrievals from AERONET sun and sky measurements over the same time period but were consistently higher than some aerosol models had predicted. Sa measured during the episodic smoke events ranged on average from 60 to 80 sr (50 to 70 sr) at 532 nm (830 nm) while the very fresh biomass‐burning aerosols were shown to exhibit significantly lower Sa ranging between 20 and 40 sr. The shortwave direct radiative forcing that was due to the intrusion of biomass‐burning aerosols was calculated to be on average −10 W/m2 and was shown to compare favorably with regional‐scale forcing calculations using MODIS‐Terra and AERONET data in an effort to assess the accuracy of estimating the regional‐scale aerosol direct radiative forcing effect using aerosol optical properties measured from a single rural site such as Bozeman, Montana.