Brown carbon is a distinct type of carbonaceous aerosol in Earth’s atmosphere, recognized for its unique light-absorbing properties. Unlike other carbon forms, brown carbon absorbs light across a wide range of wavelengths, including both visible and ultraviolet light. This characteristic sets it apart and has led to its growing recognition in atmospheric science as a significant component influencing both air quality and climate.
What is Brown Carbon
Brown carbon (BrC) is a complex mixture of organic compounds released during the incomplete combustion of organic matter, often appearing as a brownish smoke. Its distinguishing optical property is its strong absorption of light, particularly in the ultraviolet and short-visible wavelengths, which gives it a characteristic brown or yellowish color. This absorption is due to its intricate chemical structure, containing various light-absorbing molecules known as chromophores, such as polycyclic aromatic hydrocarbons (PAHs) and nitrophenols.
Brown carbon differs significantly from black carbon (BC), commonly known as soot. Black carbon consists primarily of pure carbon in linked forms, is formed during high-temperature combustion, and absorbs light across the entire visible spectrum, appearing black. In contrast, brown carbon is mainly emitted from lower-temperature combustion processes and has a more complex, less-defined chemical structure. While black carbon’s warming effect has been more extensively studied, brown carbon’s unique absorption spectrum means it contributes to atmospheric warming in different ways and at different wavelengths, adding a layer of complexity to climate models.
Sources of Brown Carbon
Biomass burning is the predominant source of brown carbon in the atmosphere, originating from both natural and human-controlled activities. Wildfires and prescribed forest fires release substantial amounts of brown carbon, particularly during smoldering or pyrolysis processes which occur at relatively lower temperatures. Agricultural burning, where crop residues are burned on a large scale in regions like South and Southeast Asia, also contributes significantly to brown carbon emissions.
Residential wood burning for heating and cooking, especially in areas relying on solid fuels, is another major anthropogenic source of brown carbon. While biomass burning is the primary origin, some industrial processes and certain types of vehicular emissions can also release brown carbon components. Globally, brown carbon emissions from both natural and human sources are substantial, with anthropogenic sources accounting for a significant portion.
Environmental Impacts
Brown carbon significantly influences the environment, particularly through its role in climate forcing. As an aerosol, it absorbs solar radiation, leading to a warming effect on the atmosphere. This absorption is especially pronounced in the ultraviolet and short-visible regions of the spectrum. The warming potential of brown carbon is considerable; some studies suggest it can contribute approximately one-fourth as much warming as black carbon, despite often being overlooked in climate simulations.
Its presence reduces atmospheric visibility, contributing to haze and air pollution. Brown carbon is a component of fine particulate matter (PM2.5), which can penetrate deep into the respiratory system. Some of the organic compounds within brown carbon, such as polycyclic aromatic hydrocarbons (PAHs), are known to be toxic and carcinogenic, leading to adverse human health effects. Its role as a light-absorbing component of particulate matter makes it a concern for both regional and global environmental health.
Measuring and Studying Brown Carbon
Scientists employ various methods to identify and quantify brown carbon in the atmosphere. Optical measurements are commonly used, with techniques like spectrophotometry and aethalometers helping to determine its light-absorbing attributes across different wavelengths. These instruments measure indicators of brown carbon’s absorptivity, such as the wavelength-dependent mass absorption cross-section (MAC) or the imaginary component of the refractive index.
Chemical analysis techniques, such as high-resolution mass spectrometry and high-performance liquid chromatography with UV-Vis detectors, provide detailed insights into brown carbon’s molecular composition. These methods help identify specific light-absorbing compounds, or chromophores, within the complex mixture. Additionally, satellite data and aircraft flights collecting aerosol filters are used to assess brown carbon’s global distribution and transport patterns, allowing researchers to estimate its direct radiative effects on a larger scale.