The importance of size ranges in aerosol instrument intercomparisons: a case study for the Atmospheric Tomography Mission

Abstract

Abstract. Aerosol intercomparisons are inherently complex as they convolve\ninstrument-dependent detection efficiencies vs. size (which often change\nwith pressure, temperature, or humidity) and variations in the sampled\naerosol population, in addition to differences in chemical detection\nprinciples (e.g., inorganic-only nitrate vs. inorganic plus\norganic nitrate for two instruments). The NASA Atmospheric Tomography\nMission (ATom) spanned four separate aircraft deployments which sampled the remote marine troposphere from 86 ∘ \u2009S to 82 ∘ \u2009N over\ndifferent seasons with a wide range of aerosol concentrations and\ncompositions. Aerosols were quantified with a set of carefully characterized and calibrated instruments, some based on particle sizing and some on composition measurements. This study aims to provide a critical evaluation of inlet transmissions impacting aerosol intercomparisons, and of aerosol quantification during ATom, with a focus on the aerosol mass spectrometer (AMS). The volume determined from physical sizing instruments (aerosol microphysical properties, AMP, 2.7\u2009nm to 4.8\u2009 µm optical diameter) is compared in detail with that derived from the chemical measurements of the AMS and the single particle soot photometer (SP2). Special attention was paid to characterize the upper end of the AMS size-dependent transmission with in-field calibrations, which we show to be critical for accurate comparisons across instruments with inevitably different size cuts. Observed differences between campaigns emphasize the importance of characterizing AMS transmission for each instrument and field study for meaningful interpretation of instrument comparisons. Good agreement (regression slope\u2009 =0.949 and 1.083 for ATom-1 and ATom-2, respectively; SD\u2009 =0.003 ) was found between the composition-based volume (including AMS-quantified sea salt) and that derived from AMP after applying the AMS inlet transmission. The AMS captured, on average, 95±15 \u2009% of the standard PM 1 volume (referred to as the URG Corp. standard cut 1\u2009 µm cyclone operated at its nominal efficiency). These results support the absence of significant unknown biases and the appropriateness of the accuracy estimates for AMS total mass and volume for the mostly aged air masses encountered in ATom. The particle size ranges (and their altitude dependence) that are sampled by the AMS and complementary composition instruments (such as soluble acidic gases and aerosol, SAGA, and particle analysis by laser mass spectrometry, PALMS) are investigated to inform their use in future studies.