Formation and Transformation of Atmospheric Brown Carbon (BrC)

Abstract

Atmospheric brown carbon (BrC) aerosol absorbs light in the UV-Vis spectrum and has poorly constrained but potentially large climate forcing impacts. Most current climate models lack detailed chemistry and interlinked properties of atmospheric BrC, that induce large uncertainties in radiative forcing predictions. BrC lifecycle and its atmospheric lifespan have not been fully explored. It is suggested that strongly light absorbing primary BrC (PBrC) may rapidly evolve into weakly light absorbing secondary BrC (SBrC) in the atmosphere, which is considered as bleaching of PBrC. Therefore, formation, transformation and optical properties of PBrC from biomass and fossil-fuel combustion system were investigated in lab and field studies. On the other hand, non-absorbing secondary/primary organic aerosol (SOA/POA) evolve into weakly light absorbing SBrC referred as browning of SOA/POA. The chemical processes underlying SOA aging and the subsequent formation and transformation of SBrC are not well understood. The presence of reactive nitrogen, acidity and water is also thought to further drive the chemistry of SBrC evolution and hence contribute to the global radiative forcing budget. These possibilities, and others, were explored in detail and are presented in this PhD thesis.

Biomass is a major source of PBrC emissions, but also it is an important renewable bioenergy source because of its economic and environmental advantages over fossil fuels. The formation of PBrC and its optical properties in a modern Swedish small-scale biomass burner were explored. The measured parameters include gas and particle concentrations, optical absorption and chemical characteristics of gases and particles. Positive matrix factorization (PMF) was performed to analyze data from a HR-ToF-CIMS equipped with FIGAERO and PASS-3. Results from the factor analysis were linked to the optical properties of the emissions, and lignin and cellulose/hemicellulose pyrolysis products were the most important sources of PBrC under the tested burning conditions.

Further, formation of PBrC and its atmospheric transformation was studied at a remote rural site, the Indo-Gangetic Plains – Centre for Air Research and Education (IGP-CARE) in India,in the Indo Gangetic Plain in India where some of the most severe air pollution episodes occur on Earth. The field measurements of short-lived climate pollutants including BrC, black carbon (BC) and ozone (O3) were conducted over a period of one year. In this study, the elevated concentrations of BrC co-emitted BC were identified, which was mostly PBrC and can largely be attributed to the local biomass burning activities in the neighbouring rural communities. This study's most important finding is that the BrC concentration normalized by BC concentration (BrC/BC ratio) showed a very pronounced diurnal variation throughout the year with distinct morning and evening peaks in general and a minimum at around noon time i.e. boat shape pattern of BrC/BC. The profile of the BrC/BC ratio evolved astonishingly during the day-time. An extremely sharp decline in the BrC/BC ratio at the time of dawn each morning indicates the dominance of photochemical processes in the transformation of PBrC. This is hypothesized to be associated with daytime photochemical bleaching of the PBrC and transforming it into SBrC.

PBrC formation and its optical properties were investigated in three distinct premixed fossil-fuelled i.e. propane, flames. POA containing BrC constituted a high fraction (20–40% by mass) of aerosol mass and was predominantly (i.e., 92–97% by mass) internally mixed with soot particles. In these flames, aerosol mixture containing BrC, POA and BC was found to be highly light absorptive, i.e., an Ångström absorption exponent (AAE) value at 405/781 nm > 1.5. The mass absorption cross-section (MAC - 5 m2g-1) of POA containing BrC at 405 nm under a specific flame i.e. fuel-rich setting, was comparable to MACs of BC particles (8–9 m2 g-1).

SBrC formation, transformation and its optical properties were explored under the influence of reactive nitrogen (NOx, NH3)-, acidity (H2SO4)-, and water-mediated chemistry during the photo-oxidation of toluene and subsequent aging of its SOA. The pattern of toluene SOA formation at [NOx]/[ΔHC] molar ratios 0.15 or below was distinctly different (i.e. constant SOA mass) than that was observed at [NOx]/[ ΔHC] ratios higher than 0.15 (i.e. here, SOA mass decreased). These distinguish between SOA formed under nitrogen-poor (NP) conditions i.e. with low initial NOx concentrations, and nitrogen-rich (NR) SOA formed at higher initial NOx concentrations, which has a higher content of compounds such as organo-nitrates. This distinction is valuable for understanding trends in the formation and properties of SOA containing BrC in the presence of varying concentrations of NOx. Hereafter, NP SOA and NR SOA are referred to SOA formed under nitrogen poor and nitrogen rich conditions, respectively. The light absorption coefficient (Babs) and mass absorption cross-section (MAC) of the SOA increased with [NOx/ΔHC] under both the NP and NR regimes. For NP SOA, the MAC increased with [NOx/ΔHC] independently of the relative humidity (RH). However, the MAC of NR SOA was RH-dependent. Under both NP and NR regimes, NH3 and acidity promoted SOA browning. The highest MAC was observed at the lowest RH (20%) for acidic NR SOA, and it was postulated that the MAC of SOA depends mainly on the pH and the [H+]free/[SOA mass] ratio of the aqueous SOA phase.

In the same preceding experiments, I additionally analyzed data from HR-ToF-CIMS. I found that several m/z (i.e. mass to charge ratio of a chemically ionized molecule detected in mass spectrometry) mimicked the trend of observed bulk Babs – against [NOx/ΔHC] increase, of SOA containing BrC. These m/z hereafter termed BrC molecules containing chromophores. An interesting observation was that the key m/z contributing to the Babs were distinct for each experiment. However, we found m/z 296 as a common dominant BrC chromophore (m/z), under several experimental conditions. The RH played a vital role in determining the BrC composition i.e. m/z distribution. The BrC molecules containing chromophores corresponding to various identified m/z are suggested to be nitro-aromatic compounds (NAC), which are primarily formed via OH oxidation of toluene followed by the nitration processing of the oxidized aromatic ring. The suggested key BrC molecules containing chromophores are the nitro-derivatives of the phenols, catechol, benzoic acid, benzaldehyde and benzonitriles.