Potentially large contribution of biomass-burning aerosol to global ice nucleating particle concentrations and implications for aerosol lifecycle and cloud microphysics

Anthropogenic and natural wildfires are a large and increasing source of aerosols

aerosols

Small particles or droplets, such as wildfire smoke, volcanic gases, or salty sea spray, that float in the air. They are emitted by both natural events and human activities.

Source: NASA Global Climate Change Source

to the atmosphere. Ice nucleating particles

ice nucleating particles

Tiny specks of material suspended in the air that act as starting points for the formation of ice crystals in clouds.

Source: Pacific Northwest National Laboratory Source

substantially affect cloud lifetime, precipitation and radiative effects, and thus also the aerosol lifecycle. Several laboratory and field measurements suggest that biomass-burning aerosols may be significant sources of ice nucleating particles. However, the processes involved and the implications for clouds are poorly understood and not currently represented in atmospheric models. Black carbon

black carbon

The sooty black material emitted from gas and diesel engines, coal-fired power plants, and other sources that burn fossil fuel. It comprises a significant portion of particulate matter or PM, which is an air pollutant.

Source: EPA Source

is the only burning-derived ice nucleating particle to have been included in models so far. However, laboratory and field experiments, including those performed by the principal investigator’s group at Carnegie Mellon University, suggest black carbon in biomass-burning aerosol is a poor ice nucleating particle. The Carnegie Mellon experiments combined with detailed aerosol analysis at the Department of Energy’s Environmental Molecular Sciences Laboratory (EMSL) led us to conclude that previously unknown mineral inclusions produced by the biomass

biomass

Material that comes from living things, including trees, crops, grasses, and animals and animal waste. Some kinds of biomass, such as wood and biofuels, can be burned to produce energy.

Source: EPA Global Climate Change Source

fuel combustion itself are the main source of the ice nucleating particles in biomass-burning aerosol, not black carbon. 

Biomass-burning aerosol from burning tall grasses and shrubs contain elevated amounts of mineral-forming elements. Aerosol from these fuels has higher crystalline mineral content and thus higher ice nucleation activity than aerosol emitted by burning wood. Further experiments discovered that biomass-burning aerosol from tall grass fuel combustion has enhanced ice-nucleation ability caused by atmospheric aging.

atmospheric aging

Refers to the gradual alteration of atmospheric constituents, including gases and aerosols, through chemical, physical, and biological processes over time.

Source: Source

We think this happens when organic carbon aerosol coatings that conceals the ice-active minerals are removed. Together these recent findings present a very different understanding of the sources of ice nucleating particles from biomass burning. We hypothesize that minerals produced during combustion are the dominant source of biomass ice nucleating particles and are globally important compared to other significant ice nucleating particle sources such as mineral dust and marine aerosols.

This project will use laboratory measurements and field measurements from the Atmospheric Radiation Measurement (ARM) data archive, and a global and regional weather and climate model. We will first quantify the potential global contribution of biomass burning to ice nucleating particles in mixed-phase clouds, then evaluate how aging may affect biomass-burning ice nucleating particles in case studies derived primarily from the ARM BBOP field campaign. Additional lab experiments on biomass-burning aerosol and analysis of our existing lab and EMSL data as well as relevant biomass-burning aerosol and ice nucleating particles field data in the ARM archive will be used to generate the needed model parameterizations of aerosol and ice nucleating particle properties. We will use information and key parameters derived from a framework developed at Colorado State University to simulate and understand the evolution of organic aerosol in biomass-burning smoke plumes. These results will allow us to describe the evaporation of organic aerosol, which may further increase the ice activity of biomass-burning aerosol as it travels through and is processed by the atmosphere.

We will carefully test the representation of background ice nucleating particles from dust and marine organic aerosol in this model using ARM datasets. Once we are confident in the ability of the model to represent these other ice nucleating particle types, we will evaluate the relative importance of ice nucleating particles from biomass burning. With an approximate representation of the aging of organic aerosol, we will also include in our model new understanding on how this aging affects the ability of biomass-burning aerosol to act as an ice nucleating particle. We will perform extensive sensitivity studies to understand the importance of vegetation/fuel type, size distribution, and other uncertainties to the global importance of ice nuclei from biomass burning.