Light Absorption Properties Of Brown Carbon Research Articles

Insight into the Role of NH3/NH4+ and NOx/NO3- in the Formation of Nitrogen-Containing Brown Carbon in Chinese Megacities.

Particulate brown carbon (BrC) plays a crucial role in the global radiative balance due to its ability to absorb light. However, the effect of molecular formation on the light absorption properties of BrC remains poorly understood. In this study, atmospheric BrC samples collected from six Chinese megacities in winter and summer were characterized through ultrahigh-performance liquid chromatography coupled with Orbitrap mass spectrometry (UHPLC-Orbitrap MS) and light absorption measurements. The average values of BrC light absorption coefficient at a wavelength of 365 nm (babs365) in winter were approximately 4.0 times higher than those in summer. Nitrogen-containing organic molecules (CHNO) were identified as critical components of light-absorbing substances in both seasons, underscoring the importance of N-addition in BrC. These nitrogen-containing BrC chromophores were more closely related to nitro-containing compounds originating from biomass burning and nitrogen oxides (NOx)/nitrate (NO3-) reactions in winter. In summer, they were related to reduced N-containing compounds formed in ammonia (NH3)/ammonium (NH4+) reactions. The NH3/NH4+-mediated reactions contributed more to secondary BrC in summer than winter, particularly in southern cities. Compared with winter, the higher O/Cw, lower molecule conjugation indicator (double bond equivalent, DBE), and reduced BrC babs365 in summer suggest a possible bleaching mechanism during the oxidation process. These findings strengthen the connection between molecular composition and the light-absorbing properties of BrC, providing insights into the formation mechanisms of BrC chromophores across northern and southern Chinese cities in different seasons.

Measurement report: Optical properties and sources of water-soluble brown carbon in Tianjin, North China – insights from organic molecular compositions

Abstract. Brown carbon (BrC) aerosols exert vital impacts on climate change and atmospheric photochemistry due to their light absorption in the wavelength range from near-ultraviolet (UV) to visible light. However, the optical properties and formation mechanisms of ambient BrC remain poorly understood, limiting the estimation of their radiative forcing. In the present study, fine particles (PM2.5) were collected during 2016–2017 on a day/night basis over urban Tianjin, a megacity in northern China. Light absorption and fluorescence properties of water extracts of PM2.5 were investigated to obtain seasonal and diurnal patterns of atmospheric water-soluble BrC. There were obvious seasonal, but no evident diurnal, variations in the light absorption properties of BrC. In winter, BrC showed much stronger light-absorbing ability, with a mass absorption efficiency at 365 nm (MAE365) in winter (1.54±0.33 m2 gC−1) that was 1.8 times larger than MAE365 in summer (0.84±0.22 m2 gC−1). Direct radiative effects by BrC absorption relative to black carbon in the UV range were 54.3±16.9 % and 44.6±13.9 % in winter and summer, respectively. In addition, five fluorescent components in BrC, including three humic-like fluorophores and two protein-like fluorophores were identified with excitation–emission matrix fluorescence spectrometry and parallel factor (PARAFAC) analysis. The less oxygenated components contributed more to winter and nighttime samples, while more oxygenated components increased in summer and daytime samples. The higher humification index (HIX), together with lower biological index (BIX) and fluorescence index (FI), suggests that the chemical compositions of BrC were associated with a high aromaticity degree in summer and daytime due to photobleaching. Fluorescent properties indicate that wintertime BrC were predominantly affected by primary emissions and fresh secondary organic aerosol (SOA), while summer ones were more influenced by aging processes. Results of source apportionments using organic molecular compositions of the same set of aerosols reveal that fossil fuel combustion and aging processes, primary bioaerosol emission, biomass burning, and biogenic and anthropogenic SOA formation were the main sources of BrC. Biomass burning contributed much more to BrC in winter and at nighttime, while biogenic SOA contributed more in summer and during the daytime. In particular, our study highlights that primary bioaerosol emission is an important source of BrC in urban Tianjin in summer.

Biomass burning organic aerosols significantly influence the light absorption properties of polarity-dependent organic compounds in the Pearl River Delta Region, China

Atmospheric brown carbon (BrC) is an important constituent of light-absorbing organic aerosols with many unclear issues. Here, the light-absorption properties of BrC with different polarity characteristics at a regional site of Pearl River Delta Region during 2016–2017, influenced by sources and molecular compositions, were revealed using radiocarbon analysis and Fourier transform ion cyclotron resonance mass spectrometry. Humic-like substance (HULIS), middle polar (MP), and low polar (LP) carbon fractions constitute 46 ± 17%, 30 ± 7%, and 7 ± 3% of total absorption coefficient from bulk extracts, respectively. Our results show that the absorption proportions of HULIS and MP to the total BrC absorption are higher than their mass proportions to organic carbon mass, indicating that HULIS and MP are the main light-absorbing components in water-soluble and water-insoluble organic carbon fractions, respectively. With decreases in non-fossil HULIS, MP, and LP carbon fractions (66 ± 2%, 52 ± 2%, and 36 ± 3%, respectively), the abundances of unsaturated compounds and mass absorption efficiency at 365 nm of three fractions decreased synchronously. Increases in both non-fossil carbon and levoglucosan in winter imply that the enhanced light-absorption could be attributed to elevated levels of biomass burning organic aerosols (BBOA), which increases the number of light-absorbing nitrogen-containing compounds. Moreover, the major type of potential BrC in HULIS and MP carbon fractions are oxidized BBOA, but the potential BrC chromophores in LP are mainly associated with primary BBOA. This study reveals that biomass burning has adverse effects on radiative forcing and air quality, and probably indicates the significant influences of atmospheric oxidation reactions on the forms of chromophores.

Atmospheric evolution of molecular-weight-separated brown carbon from biomass burning

Abstract. Biomass burning is a major source of atmospheric brown carbon (BrC), and through its absorption of UV/VIS radiation, it can play an important role in the planetary radiative balance and atmospheric photochemistry. The considerable uncertainty of BrC impacts is associated with its poorly constrained sources, transformations, and atmospheric lifetime. Here we report laboratory experiments that examined changes in the optical properties of the water-soluble (WS) BrC fraction of laboratory-generated biomass burning particles from hardwood pyrolysis. Effects of direct UVB photolysis and OH oxidation in the aqueous phase on molecular-weight-separated BrC were studied. Results indicated that the majority of low-molecular-weight (MW) BrC (<400 Da) was rapidly photobleached by both direct photolysis and OH oxidation on an atmospheric timescale of approximately 1 h. High MW BrC (≥400 Da) underwent initial photoenhancement up to ∼15 h, followed by slow photobleaching over ∼10 h. The laboratory experiments were supported by observations from ambient BrC samples that were collected during the fire seasons in Greece. These samples, containing freshly emitted to aged biomass burning aerosol, were analyzed for both water- and methanol-soluble BrC. Consistent with the laboratory experiments, high-MW BrC dominated the total light absorption at 365 nm for both methanol and water-soluble fractions of ambient samples with atmospheric transport times of 1 to 68 h. These ambient observations indicate that overall, biomass burning BrC across all molecular weights has an atmospheric lifetime of 15 to 28 h, consistent with estimates from previous field studies – although the BrC associated with the high-MW fraction remains relatively stable and is responsible for light absorption properties of BrC throughout most of its atmospheric lifetime. For ambient samples of aged (>10 h) biomass burning emissions, poor linear correlations were found between light absorptivity and levoglucosan, consistent with other studies suggesting a short atmospheric lifetime for levoglucosan. However, a much stronger correlation between light absorptivity and total hydrous sugars was observed, suggesting that they may serve as more robust tracers for aged biomass burning emissions. Overall, the results from this study suggest that robust model estimates of BrC radiative impacts require consideration of the atmospheric aging of BrC and the stability of high-MW BrC.

Molecular Chemistry of Atmospheric Brown Carbon Inferred from a Nationwide Biomass Burning Event.

Lag Ba'Omer, a nationwide bonfire festival in Israel, was chosen as a case study to investigate the influence of a major biomass burning event on the light absorption properties of atmospheric brown carbon (BrC). The chemical composition and optical properties of BrC chromophores were investigated using a high performance liquid chromatography (HPLC) platform coupled to photo diode array (PDA) and high resolution mass spectrometry (HRMS) detectors. Substantial increase of BrC light absorption coefficient was observed during the night-long biomass burning event. Most chromophores observed during the event were attributed to nitroaromatic compounds (NAC), comprising 28 elemental formulas of at least 63 structural isomers. The NAC, in combination, accounted for 50-80% of the total visible light absorption (>400 nm) by solvent extractable BrC. The results highlight that NAC, in particular nitrophenols, are important light absorption contributors of biomass burning organic aerosol (BBOA), suggesting that night time chemistry of •NO3 and N2O5 with particles may play a significant role in atmospheric transformations of BrC. Nitrophenols and related compounds were especially important chromophores of BBOA. The absorption spectra of the BrC chromophores are influenced by the extraction solvent and solution pH, implying that the aerosol acidity is an important factor controlling the light absorption properties of BrC.

The influence of solvent and pH on determination of the light absorption properties of water-soluble brown carbon

Brown carbon (BrC) is a class of unidentified organic compounds that efficiently absorb solar radiation in the ultraviolet (UV) wavelengths, and its effects on climate are poorly understood. Measurement of the light absorption properties of BrC in liquid extracts is a commonly used BrC analytical method, but the optical characteristics of water-soluble BrC may be affected by pH and solvent. In this work, we investigated the effects of concentration, pH, and solvent on water-soluble BrC from ambient aerosols, biomass burning, diesel exhaust, and a humic substance standard. The results showed that pH can affect the light absorption properties of water-soluble BrC, whereas concentration had little effect, except low concentrations dissolved in methanol. Therefore, the pH of humic-like substances (HULIS) should be adjusted to the same value as water-soluble carbon (WSOC) for calculating the light-absorption contribution of HULIS to WSOC. The light absorptivity of water-soluble BrC dissolved in methanol was higher than that in water. Considering the pH and concentration effects, extraction of WSOC with a particle: water ratio of 0.25 mg/mL is proposed as well as to get a reference pH for light absorption analysis.