Indirect Effects of Secondary Organic Aerosol on Cirrus Clouds

Abstract Secondary organic aerosols (SOA) have been identified as a potential source of depositional ice nucleating particles and thus may have a radiative effect on cirrus clouds. This study develops a global model to examine the radiative effect of SOA on cirrus clouds using different treatments for the size distribution of SOA. The SOA from new particle formation by organics and their subsequent growth has a radiative effect of 0.35 ± 0.06 W m −2 , while the radiative effect of SOA calculated by assuming a fixed size distribution is 0.31 ± 0.08 W m −2 . This positive radiative effect on cirrus clouds opposes the negative effect of anthropogenic soot on cirrus clouds. In addition, the inclusion of SOA as an ice nucleating particle changes the background ice crystal number concentration, which impacts the calculation of radiative forcing from other aerosols. The radiative forcing of aircraft soot is estimated to be −0.11 ± 0.03 W m −2 when including SOA formed from new particle formation by organics and growth. This is less negative than simulations that do not include ice nucleation from SOA. The change in SOA formed from organic nucleation from the preindustrial period to the present day causes a positive forcing of 0.02 ± 0.04 W m −2 . It is important to use a size distribution based on the explicit formation mechanism for SOA to calculate their radiative effects. The simulation using an assumed fixed size distribution incorrectly results in a negative forcing of SOA between the present day and preindustrial atmospheres because it does not correctly calculate the change of SOA in the accumulation mode.