Comment on acp-2022-840

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> The aerosol mixing state is a crucial physical-chemical property that affects their optical properties and cloud condensation nuclei (CCN) activity. Multiple techniques are commonly employed to determine aerosol mixing states for various applications, and comparisons between these techniques provide insights of the variations in aerosol chemical and physical properties. These techniques include size-resolved CCN activity measurements using a system with CCN counter (CCNC) coupled with a differential mobility analyzer (DMA), a Humidified/Volatility Tandem differential mobility analyzer (H/V-TDMA) which measures aerosol hygroscopicity/volatility distributions, and a single particle soot photometer (SP2) which directly quantifies black carbon (BC) mixing states. This study provides a first time intercomparisons of aerosol mixing state parameters obtained through simultaneous measurements of a DMA-CCNC, a H/VTDMA and a DMA-SP2. The impact of primary aerosols emissions and secondary aerosol formations on the aerosol mixing states and intercomparison results were analyzed. The results showed that differences in mixing state parameters measured by different techniques varied greatly under different conditions. The V-TDMA and DMA-SP2 measurements showed that the non-volatile population identified by the V-TDMA was mainly contributed by BC-containing aerosols. The HTDMA and DMA-SP2 measurements indicated that a substantial proportion of nearly hydrophobic aerosols were not contributed from BC-containing aerosols, but likely originated from fossil fuel combustion and biomass burning emissions. Synthesized comparison results between DMA-CCNC, HTDMA and DMA-SP2 measurements revealed that some of the nearly hydrophobic BC-free particles were CCN-inactive under supersaturated conditions, likely from fossil combustion emissions, while others were CCN-active under supersaturated conditions linked to biomass burning emissions. Fossil fuel combustion-emitted BC-containing aerosols tended to be more externally mixed with other aerosol compositions compared to those emitted from biomass burning activities. These results highlight significant disparities in the mixing states as well as physiochemical properties between aerosol originated from fossil fuel combustion and biomass burning. The formation of secondary nitrate and organic aerosols exerted significant impacts on variations in aerosol mixing states, generally enhancing aerosol hygroscopicity and volatility, while reducing differences in mixing state parameters derived from different techniques, resulting in a reduction in aerosol heterogeneity. The variations in BC-free particle number fractions showed that secondary aerosols tended to form more quickly on BC-free particles than on BC-containing particles. Further comparison of mixing state parameters revealed that the two resolved SOA factors in this study exhibited remarkably different physical properties, indicating that they were likely formed through different pathways. These findings suggest that intercomparisons among aerosol mixing states derived from different techniques can provide deeper insight into aerosol physical properties and how they are impacted by secondary aerosol formation, aiding the investigation of secondary aerosol formation pathways.