What Can TCAP Observations Tell Us about Temporal Aerosol Changes and Their Impact on Daily Average Radiative Forcing?

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Description

While numerous studies have revealed strong temporal aerosol variability for many urban/industrial regions around the world, we do not know to what degree temporal aerosol variability can modify the daily average direct aerosol radiative forcing (DARF) and, consequently, the DARF at climatologically relevant time scales (e.g., months or years). To estimate the impact of temporal aerosol changes on the 24-h average DARF, we take advantage of an unique dataset obtained from ground-based observations during the recent the Two-Column Aerosol Project (TCAP) on Cape Cod, which is typically downwind of several large metropolitan areas including Boston and New York City. This unique TCAP dataset integrates data from numerous instruments for sampling aerosol, cloud and radiative properties and includes data from the Multi-Filter Rotating Shadowband Radiometer (MFRSR), a Scanning Mobility Particle Sizer (SMPS), an Aerodynamic Particle Sizer (APS), a three-wavelength nephelometer and a suite of instruments to measure the aerosol chemical composition. We analyze the temporal variability of the column (MFRSR data), near-surface (SMPS, APS, nephelometer, chemical composition) aerosol properties and related back-trajectories derived from WRF-Chem in the context of the climate-relevant 24-h average DARF. In particularly, we found large (up to 20% on average) diurnal changes of the aerosol optical depth. We illustrate how the variability of aerosol properties impacts the DARF at different time scales, suggest a simple way for reducing potentially large (up to 100%) errors associated with sparse temporal sampling, and outline the expected application of our results to the evaluation and improvement of regional and global climate models.