Aerosol-CO relationship and aerosol effect on ice cloud particle size: Analyses from Aura Microwave Limb Sounder and Aqua Moderate Resolution Imaging Spectroradiometer observations

We examine the relation between Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical thickness (AOT) and Aura Microwave Limb Sounder upper tropospheric (UT) carbon monoxide (CO) to address when CO can be used as a proxy for aerosols. Ice cloud effective radii (re), also from Aqua MODIS, are also analyzed to investigate possible aerosol effects on ice clouds. Our analysis focuses on five regions where ice clouds are collocated with high UT CO loadings: South America (SAM), southern Africa (SAF), northern Africa (NAF), South Asia (SAS), and East Asia (EAS). We find three levels of AOT to CO sensitivity. High AOT sensitivity to CO is characterized by a rapid increase of AOT when CO increases. SAM in August–November and SAF in June–September fall into this category. Moderate AOT sensitivity to CO is characterized by moderate increase of AOT with CO. It includes SAF in October–May, NAF in January–March, SAS in all months, and EAS in August–September. The other months for each region fall into the low sensitivity category. The variations of sensitivity in different regions and seasons result from different emission sources coupled with dynamic influence. CO can be used as an aerosol proxy for the “high” and “moderate” sensitivity cases. During those times, re for polluted clouds is smaller than that for clean clouds, suggesting an indirect effect of aerosol on ice clouds. CO is not a good aerosol index in the low sensitivity cases, in which polluted clouds defined by CO loadings do not show significant differences from clean clouds in re.