Modification, calibration, and performance of the Ultra-High Sensitivity Aerosol Spectrometer for particle size distribution and volatility measurements during the Atmospheric Tomography Mission (ATom) airborne campaign

Abstract

Atmospheric aerosol is a key component of the chemistry and climate of the Earth's atmosphere. Accurate measurement of the concentration of atmospheric particles as a function of their size is fundamental to investigations of particle microphysics, optical characteristics, and chemical processes. We describe the modification, calibration, and performance of two commercially available, Ultra-High Sensitivity Aerosol Spectrometers (UHSASs) as used on the NASA DC-8 aircraft during the Atmospheric Tomography Mission (ATom). To avoid sample flow issues related to pressure variations during aircraft altitude changes, we installed a laminar flow meter on each instrument to measure sample flow directly at the inlet as well as flow controllers to maintain constant volumetric sheath flows. In addition, we added a compact thermodenuder operating at 300°C to the inlet line of one of the instruments. With these modifications, the instruments are capable of making accurate (ranging from 7% for Dp<0.07μm to 1% for Dp>0.13μm), precise ( 1000 to 225hPa, while simultaneously providing information on particle volatility.We assessed the effect of uncertainty in the refractive index (n) of ambient particles that are sized by the UHSAS assuming the refractive index of ammonium sulfate (n Combining double low line 1.52). For calibration particles with n between 1.44 and 1.58, the UHSAS diameter varies by +4/-10% relative to ammonium sulfate. This diameter uncertainty associated with the range of refractive indices (i.e., particle composition) translates to aerosol surface area and volume uncertainties of +8.4/-17.8 and +12.4/-27.5%, respectively. In addition to sizing uncertainty, low counting statistics can lead to uncertainties of <20% for aerosol surface area and <30% for volume with 10s time resolution. The UHSAS reduction in counting efficiency was corrected for concentrations >1000cm-3.Examples of thermodenuded and non-thermodenuded aerosol number and volume size distributions as well as propagated uncertainties are shown for several cases encountered during the ATom project. Uncertainties in particle number concentration were limited by counting statistics, especially in the tropical upper troposphere where accumulation-mode concentrations were sometimes <20cm-3 (counting rates ∼ 5Hz) at standard temperature and pressure.