Black carbon absorption at the global scale is affected by particle-scale diversity in composition.

Laura Fierce,Tami C Bond,Francisco Mena,Susanne E Bauer,Nicole Riemer

doi:10.1038/ncomms12361

Abstract

Atmospheric black carbon (BC) exerts a strong, but uncertain, warming effect on the climate. BC that is coated with non-absorbing material absorbs more strongly than the same amount of BC in an uncoated particle, but the magnitude of this absorption enhancement (Eabs) is not well constrained. Modelling studies and laboratory measurements have found stronger absorption enhancement than has been observed in the atmosphere. Here, using a particle-resolved aerosol model to simulate diverse BC populations, we show that absorption is overestimated by as much as a factor of two if diversity is neglected and population-averaged composition is assumed across all BC-containing particles. If, instead, composition diversity is resolved, we find Eabs=1−1.5 at low relative humidity, consistent with ambient observations. This study offers not only an explanation for the discrepancy between modelled and observed absorption enhancement, but also demonstrates how particle-scale simulations can be used to develop relationships for global-scale models.

Highlights

Atmospheric black carbon (BC) exerts a strong, but uncertain, warming effect on the climate
This study investigates errors in modelled light absorption caused by ignoring diversity in particle composition, and shows that absorption is strongly affected by the distribution of components among individual particles
Several particle types were included in the simulations, this study focuses on the evolution of the BC-containing particles only, which originate from combustion sources

Summary

Introduction

Atmospheric black carbon (BC) exerts a strong, but uncertain, warming effect on the climate. To identify factors influencing light absorption by BC, we performed a nonparametric regression on a series of simulations to identify the key independent variables that most affect Eabs Through this nonparametric analysis, we derived a relationship for Eabs as a function of the populationaveraged mass fraction of aerosol components and the environmental relative humidity (RH), such that the relationship for Eabs accounts for particle-level variation in composition but depends only on variables that many global models already track. We derived a relationship for Eabs as a function of the populationaveraged mass fraction of aerosol components and the environmental relative humidity (RH), such that the relationship for Eabs accounts for particle-level variation in composition but depends only on variables that many global models already track By applying this nonparametric relationship to output from a global model, we demonstrate that light absorption by BC at the global scale depends strongly on particle-scale diversity in composition

Methods

Results

Conclusion