Reply to: "Comment on 'Stratospheric Aerosol Composition Observed by the Atmospheric Chemistry Experiment Following the 2019 Raikoke Eruption' by Boone et al." by Ansmann et al.

The question of stratospheric aerosol type following the Raikoke eruption is revisited. Raman lidar measurements suggest the aerosols are predominately smoke, while Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE‐FTS) results indicate the aerosols are predominately sulfate aerosols. The suggested mechanism of smoke particles self‐lofting into the stratosphere is inconsistent with observations in 2020, when more severe Siberian fires failed to invoke a response even vaguely similar to 2019. A side‐by side comparison of the Sarychev and Raikoke eruptions invalidates model calculations that suggest sulfate aerosols should be at levels too low to explain the observed aerosol loading. Structure in infrared absorption spectra provides conclusive evidence of composition, a unique fingerprint for identifying aerosol type. Such information cannot be misinterpreted so long as there is sufficient resolution and spectral coverage. ACE‐FTS infrared aerosol spectra often have an order of magnitude stronger absorption than that of background sulfate aerosols. These spectra can be accurately reproduced by laboratory measured sulfate aerosol spectroscopic information, providing unambiguous identification of the aerosols as sulfate. Visual inspection of thousands of infrared aerosol spectra from the period following the Raikoke eruption indicates the aerosols in the lower stratosphere are predominately sulfate, with no indication of smoke. The lidar study's identification of the aerosols as smoke was based primarily on observed lidar ratios that were more consistent with a material that absorbed significantly at the lidar wavelengths, inconsistent with expectations for sulfate aerosols. However, this could indicate the presence of a substance dissolved in the sulfate aerosols absorbing at those wavelengths rather than smoke particles. Plain Language Summary: There is a disagreement regarding the type of aerosol observed at high northern latitudes during winter 2019/2020. One group, measuring the atmosphere from the ground with lidars, identifies the aerosols as smoke particles. Another group, measuring atmospheric absorption of sunlight from a satellite, identifies the same aerosols as sulfate (a mixture of water and sulfuric acid). The argument for smoke is inconsistent with the fact that significantly more severe fires the following year did not produce an effect that was even remotely similar. A comparison of measurements from two different volcanic eruptions shows that model calculations of sulfate aerosols for the given conditions appear to be inaccurate. Spectroscopy is used to definitively determine the aerosol type as sulfate because the shape of the spectrum is unique to that aerosol type. There is no flexibility to interpret the measured spectrum as resulting from smoke, because the shape of the observed spectrum would be wrong. An extensive inspection of the measurements, covering many different conditions, revealed no evidence of smoke near altitude 11 km at high latitudes during winter 2019/2020. A substance dissolved in the liquid droplet sulfate aerosol that absorbed at the lidar wavelengths could account for the lidar observations. Key Points: From the infrared aerosol spectra, there is no flexibility to interpret composition as anything but sulfateThere was no significant enhancement in stratospheric aerosols in 2020, when Siberian fires produced 35% greater emissions than 2019Aerosols from the Sarychev and Raikoke eruptions followed a very similar pattern [ABSTRACT FROM AUTHOR]

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