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

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 D p < 0.07 µm to 1 % for D p > 0.13 µm), precise (< ±1.2 %), and continuous (1 Hz) measurements of size-resolved particle number concentration over the diameter range of 0.063-1.0 µm at ambient pressures of > 1000 to 225 hPa, 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 = 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 10 s time resolution. The UHSAS reduction in counting efficiency was corrected for concentrations > 1000 cm −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 < 20 cm −3 (counting rates ∼ 5 Hz) at standard temperature and pressure.

Characterization of a catalyst-based conversion technique to measure total particulate nitrogen and organic carbon and comparison to a particle mass measurement instrument

“…Previous laboratory studies show UHSAS and CPC number concentration comparisons in excellent agreement (Cai et al, 2008;Kupc et al, 2018); however, occasionally only a ∼ 90 % counting efficiency for the UHSAS was observed when compared to the CPC. These differences are attributed to particle coincidence at high concentrations (> 1000 cm −3 ) and to inefficient particle mixing before reaching the instruments.…”

“…The UHSAS manufacturer recommended calibration uses PSL microspheres, which are well characterized and have a known refractive index (n = 1.58) and shape. Because the UHSAS sizing is sensitive to particle refractive index, a new sizing calibration curve was produced for each studied particle type (i.e., refractive index) using a DMA to select particles for a range of known sizes (Kupc et al, 2018). These calibration curves were used to retrieve accurate particle size distributions that properly accounted for the multiply charged particles.…”

“…These differences are attributed to particle coincidence at high concentrations (> 1000 cm −3 ) and to inefficient particle mixing before reaching the instruments. Corrections for particle coincidence were applied (Kupc et al, 2018) though we expect differences due to particle mixing add an additional 10 % uncertainty to the measurements. The UHSAS-and CPC-measured particle number concentrations were generally within 10 % of each other; however, the CPC values did not require coincidence corrections and had a better signal-to-noise ratio.…”

Size-resolved mixing state of black carbon in the Canadian high Arctic and implications for simulated direct radiative effect

“…The UHSAS was calibrated during operations using polystyrene latex spheres (PSLs), which have a refractive index of n = 1.59. Kupc et al (2018) investigated the size dependence of UHSAS measurements on the assumed refractive index of the particles being measured. For particles with diameters smaller than 600 nm and real refractive indices of n = 1.44-1.58, the diameter measured by the UHSAS varied by +4 to −10 % (relative to the refractive index of ammonium sulfate; n = 1.52).…”

“…One potential issue with using the UHSAS aboard an aircraft is that as the pressure changes during ascent and descent, the sample flow at the inlet of the chamber can deviate from the measured and regulated flow at the outlet of the chamber (Brock et al, 2011;Kupc et al, 2018), which results in inaccurate particle concentration measurements. Brock et al (2011) saw particle number deviations from a reference counter of ∼ 10-15 % on ascent and descent.…”