Chemistry of Brown Carbon
Organic aerosol particles are generally white in color, and therefore they scatter visible solar radiation back to space. This scattering prevents visible solar radiation from reaching the Earth’s surface thus cooling the climate. Light-absorbing “brown carbon” aerosols counteract this effect due to their ability to absorb solar radiation thus warming the climate. The term “brown carbon” refers to aerosol particles that contain organic compounds, which are capable of absorbing near UV and visible light . Brown carbon decreases atmospheric visibility, deteriorates air quality in urban areas, and affects regional climate. The goal of our research is to understand the chemical composition and chemical aging of various types of brown carbon aerosols.
Primary Brown Carbon
The largest source of brown carbon in the atmosphere is the burning of biomass, such as wood and grass. The smoke produced during inefficient combustion of biomass is highly complex and has brownish color (as shown in the photograph of particulate matter collected on a filter during a Black Spruce controlled burn). We have been investigating the chemical composition of brown carbon with help of a high performance liquid chromatography (HPLC) coupled to photo diode array (PDA) and high resolution mass spectrometry (HRMS) detectors in collaboration with Dr. Julia Laskin and Dr. Alexander Laskin from Purdue University (previously from EMSL, PNNL). Examples of our recent publications on biomass burning smoke relying on the HPLC-PDA-HRMS include Refs. [114, 123, 127].
Secondary Brown Carbon
Secondary brown carbon refers to compounds produced directly in the atmosphere by multi-phase chemistry . For example, photooxidation of many aromatic compounds leads to light-absorbing SOA material. We have investigated the UV-Vis and fluorescence properties of brown carbon produced by the photooxidation of benzene, toluene, p-xylene, and naphthalene [93, 133]. We also found that photooxidation of indole, a compound emitted by plants and livestock, forms brown carbon containing known indole-derived dies . Finally, we measured mass absorption coefficients for a number of other types of secondary SOA .
A while back , we observed that SOA produced by oxidation of limonene changes from colorless to orange-brown when exposed to ammonia, a common atmospheric pollutant. This type of browning chemistry is not limited to SOA produced from limonene; we have been able to quantify the mass absorption coefficients of brown carbon material produced by this mechanism for a number of SOA types . The following images show this transformation for an aerosol filter sample
The highly colored compounds produced by aging reactions are generated when ammonia reacts with carbonyl compounds present in the SOA. In collaboration with Dr. Alexander Laskin and Dr. Julia Laskin, we have been able to detect imine compounds that are likely responsible for the light absorption in aged limonene SOA using high resolution mass spectrometry [66, 99]. These compounds are similar to the products of Maillard reactions between sugars and amino acids that cause browning in food, for example, when toasting bread. Organic aerosols appear to get “toasted” on time scales of hours to days, which is a relevant atmospheric time scale. In collaboration with our Purdue collaborators, we are currently investigating the colored compounds in different secondary brown carbon species with HPLC-PDA-HRMS methods [109, 126]. We are also investigating the kinetics of reactions between selected dicarbonyls and nitrogen-containing compounds in order to better understand the reaction mechanism. Dicarbonyls that we have worked with include methylglyoxal [109, 126] and 4-oxopentanal .
We found that browning chemistry is accelerated quite dramatically by evaporative cloud/fog processing of aerosols; the “brown carbon” compounds are produced much faster during evaporation of droplets containing dissolved SOA and ammonium sulfate [72, 124]. The fact that simple droplet evaporation can dramatically change the optical properties of the compounds dissolved in it has significant implications for direct forcing by aerosols.
An exciting new development has been observation of formation of very strongly light-absorbing compounds in reactions between Fe(III) and catechol/guaiacol  and dicarboxylic acids . This work was done in collaboration with the group of Prof. Hind Al-Abadleh from Wilfrid Laurier University. The formation of colored insoluble polymeric particles reported in these papers could account for new pathways that lead to SOA and brown carbon formation mediated by transition metals.