Abstract

Abstract. Aerosol optical properties were measured by the DOE/ARM (US Department of Energy Atmospheric Radiation Measurements) Program Mobile Facility during the Two-Column Aerosol Project (TCAP) campaign deployed at Cape Cod, Massachusetts, for a 1-year period (from summer 2012 to summer 2013). Measured optical properties included aerosol light-absorption coefficient (σap) at low relative humidity (RH) and aerosol light-scattering coefficient (σsp) at low and at RH values varying from 30 to 85%, approximately. Calculated variables included the single scattering albedo (SSA), the scattering Ångström exponent (SAE) and the scattering enhancement factor (f(RH)). Over the period of measurement, f(RH = 80%) had a mean value of 1.9 ± 0.3 and 1.8 ± 0.4 in the PM10 and PM1 fractions, respectively. Higher f(RH = 80%) values were observed for wind directions from 0 to 180° (marine sector) together with high SSA and low SAE values. The wind sector from 225 to 315° was identified as an anthropogenically influenced sector, and it was characterized by smaller, darker and less hygroscopic aerosols. For the marine sector, f(RH = 80%) was 2.2 compared with a value of 1.8 obtained for the anthropogenically influenced sector. The air-mass backward trajectory analysis agreed well with the wind sector analysis. It shows low cluster to cluster variability except for air masses coming from the Atlantic Ocean that showed higher hygroscopicity. Knowledge of the effect of RH on aerosol optical properties is of great importance for climate forcing calculations and for comparison of in situ measurements with satellite and remote sensing retrievals. In this sense, predictive capability of f(RH) for use in climate models would be enhanced if other aerosol parameters could be used as proxies to estimate hygroscopic growth. Toward this goal, we propose an exponential equation that successfully estimates aerosol hygroscopicity as a function of SSA at Cape Cod. Further work is needed to determine if the equation obtained is valid in other environments.

Highlights

  • Earth’s atmosphere plays an important role in the planetary energy budget through different processes that shape Earth’s climate

  • An important factor that can modify the role of aerosols in the global energy budget is relative humidity (RH)

  • The aerosol light-absorption coefficient was very low during the measurement period, especially compared with the scattering coefficient; the single scattering albedo (SSA)(550) had a mean value of 0.94 ± 0.04

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Summary

Introduction

Earth’s atmosphere plays an important role in the planetary energy budget through different processes that shape Earth’s climate. An important factor that can modify the role of aerosols in the global energy budget is relative humidity (RH). Continuous measurements of aerosol properties are typically performed under dry conditions (RH < 40 %) as recommended by international networks such as ACTRIS (Aerosols, Clouds, and Trace gases Research InfraStructure Network) or GAW (Global Atmosphere Watch; WMO/GAW, 2003). These measurements at low RH can differ from what would be observed at ambient

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