Investigating cloud absorption effects: Global absorption properties of black carbon, tar balls, and soil dust in clouds and aerosols

Abstract

This study examines modeled properties of black carbon (BC), tar ball (TB), and soil dust (SD) absorption within clouds and aerosols to understand better Cloud Absorption Effects I and II, which are defined as the effects on cloud heating of absorbing inclusions in hydrometeor particles and of absorbing aerosol particles interstitially between hydrometeor particles at their actual relative humidity (RH), respectively. The globally and annually averaged modeled 550 nm aerosol mass absorption coefficient (AMAC) of externally mixed BC was 6.72 (6.3–7.3) m 2/g, within the laboratory range (6.3–8.7 m 2/g). The global AMAC of internally mixed (IM) BC was 16.2 (13.9–18.2) m 2/g, less than the measured maximum at 100% RH (23 m 2/g). The resulting AMAC amplification factor due to internal mixing was 2.41 (2–2.9), with highest values in high RH regions. The global 650 nm hydrometeor mass absorption coefficient (HMAC) due to BC inclusions was 17.7 (10.6–19) m 2/g, ∼9.3% higher than that of the IM‐AMAC. The 650 nm HMACs of TBs and SD were half and 1/190th, respectively, that of BC. Modeled aerosol absorption optical depths were consistent with data. In column tests, BC inclusions in low and mid clouds (CAE I) gave column‐integrated BC heating rates ∼200% and 235%, respectively, those of interstitial BC at the actual cloud RH (CAE II), which itself gave heating rates ∼120% and ∼130%, respectively, those of interstitial BC at the clear‐sky RH. Globally, cloud optical depth increased then decreased with increasing aerosol optical depth, consistent with boomerang curves from satellite studies. Thus, CAEs, which are largely ignored, heat clouds significantly.