Potentially Important Contribution of Biomass Burning Aerosols to Global Ice Nucleating Particles Concentration

Ice nucleating particles (INP) capable of nucleating ice crystals via immersion freezing at temperatures above approximately -35C have the potential to strongly influence cloud glaciation, and therefore may significantly influence global precipitation and climate feedbacks. Commonly, mineral dust and marine organics have been shown to demonstrate ice nucleating activity in various geographical regions. In addition to dust and marine aerosols, biomass burning aerosols (BBA) have been recently shown to act as immersion-mode INP between -30C and -20C. Laboratory measurements suggested that crystalline minerals formed during the combustion process are responsible for this ice nucleation activity. However, the role of BBA as INP is not well-characterized and poorly understood. BBA ice nucleation activity has been explored on a regional scale in field measurements and modeling studies, but the contribution of BBA to the global INP budget is uncertain. Large uncertainties stem from poor knowledge of the types of fuels that yield INPs and the global coverage of those fuel types. Further uncertainty arises from the unknown size distribution of the particles that contain the crystalline minerals resulting in ice nucleation. However, with some estimates and constraints on these uncertainties, the relative importance of ice nucleation activity of BBA compared to, for example, dust and marine organics can be quantified.

In this work, we investigated the global importance of BBA as INP. For this, we used a global climate model, specifically the UK Met Office Unified Model (UM). Field campaigns over four different regions – Pacific Northwest United States, Continental South America, Southern Ocean, and Northeastern Arctic, mostly conducted as part of the Atmospheric Radiation Measurement program of the Department of Energy, are used to evaluate our model. Preliminary BBA INP concentrations were obtained using parameterizations derived from laboratory and field campaign measurements. In these parameterizations, the number concentration of BBA INPs is represented as a function of the size-resolved surface area concentration of the particles and the freezing temperature. We use sensitivity studies to understand the potential importance of uncertainties in size and fuel type. Our simulations suggest that INP concentrations from BBA may be of comparable importance to mineral dust and sea spray/marine organic aerosols over large regions of the atmosphere in the seasons when biomass burning is frequent.