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Atmospheric aerosol is a key component of the chemistry and climate of the\udEarth's atmosphere. Accurate measurement of the concentration of atmospheric\udparticles as a function of their size is fundamental to investigations of\udparticle microphysics, optical characteristics, and chemical processes. We\uddescribe the modification, calibration, and performance of two commercially\udavailable, Ultra-High Sensitivity Aerosol Spectrometers (UHSASs) as used on\udthe NASA DC-8 aircraft during the Atmospheric Tomography Mission (ATom). To\udavoid sample flow issues related to pressure variations during aircraft\udaltitude changes, we installed a laminar flow meter on each instrument to\udmeasure sample flow directly at the inlet as well as flow controllers to\udmaintain constant volumetric sheath flows. In addition, we added a compact\udthermodenuder operating at 300 °C to the inlet line of one of the\udinstruments. With these modifications, the instruments are capable of making\udaccurate (ranging from 7 % for Dp < 0.07 µm to 1 %\udfor Dp > 0.13 µm), precise\ud(< ±1.2 %), and continuous (1 Hz) measurements of size-resolved particle number\udconcentration over the diameter range of 0.063–1.0 µm at ambient\udpressures of > 1000 to 225 hPa, while simultaneously providing\udinformation on particle volatility. We assessed the effect of uncertainty in the refractive index (n) of ambient\udparticles that are sized by the UHSAS assuming the refractive index of\udammonium sulfate (n =  1.52). For calibration particles with n between 1.44 and\ud1.58, the UHSAS diameter varies by +4/−10 % relative to ammonium\udsulfate. This diameter uncertainty associated with the range of refractive\udindices (i.e., particle composition) translates to aerosol surface area and\udvolume uncertainties of +8.4/−17.8 and +12.4/−27.5 %,\udrespectively. In addition to sizing uncertainty, low counting statistics can\udlead to uncertainties of < 20 % for aerosol surface area and\ud< 30 % for volume with 10 s time resolution. The UHSAS reduction\udin counting efficiency was corrected for concentrations > 1000 cm−3. Examples of thermodenuded and non-thermodenuded aerosol number and volume\udsize distributions as well as propagated uncertainties are shown for several\udcases encountered during the ATom project. Uncertainties in particle number\udconcentration were limited by counting statistics, especially in the\udtropical upper troposphere where accumulation-mode concentrations were\udsometimes < 20 cm−3 (counting rates  ∼  5 Hz) at\udstandard temperature and pressure

atmospheric measurement techniques

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