Abstract. Brown carbon (BrC) plays an essential impact on radiative forcing due to its ability to absorb sunlight. In this study, the optical properties and molecular characteristics of water-soluble and methanol-soluble organic carbon (OC; MSOC) emitted from the simulated combustion of biomass and coal fuels and vehicle emissions were investigated using ultraviolet–visible (UV–vis) spectroscopy, excitation–emission matrix (EEM) spectroscopy, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with electrospray ionization (ESI). The results showed that these smoke aerosol samples from biomass burning (BB) and coal combustion (CC) had a higher mass absorption efficiency at 365 nm (MAE365) than vehicle emission samples. A stronger MAE365 value was also found in MSOC than water-soluble organic carbon (WSOC), indicating low polar compounds would possess a higher light absorption capacity. Parallel factor (PARAFAC) analysis identified six types of fluorophores (P1–6) in WSOC including two humic-like substances (HULIS-1) (P1 and P6), three protein-like substances (PLOM) (P2, P3, and P5), and one undefined substance (P4). HULIS-1 was mainly from aging vehicle exhaust particles; P2 was only abundant in BB aerosols; P3 was ubiquitous in all tested aerosols; P4 was abundant in fossil burning aerosols; and P5 was more intense in fresh vehicle exhaust particles. The MSOC chromophores (six components; C1–6) exhibited consistent characteristics with WSOC, suggesting the method could be used to indicate the origins of chromophores. FT-ICR mass spectra showed that CHO and CHON were the most abundant components of WSOC, but S-containing compounds appeared in a higher abundance in CC aerosols and vehicle emissions than BB aerosols, while considerably fewer S-containing compounds largely with CHO and CHON were detected in MSOC. The unique formulas of different sources in the van Krevelen (VK) diagram presented different molecular distributions. To be specific, BB aerosols with largely CHO and CHON had a medium H ∕ C and low O ∕ C ratio, while CC aerosols and vehicle emissions largely with S-containing compounds had an opposite H ∕ C and O ∕ C ratio. Moreover, the light absorption capacity of WSOC and MSOC was positively associated with the unsaturation degree and molecular weight in the source aerosols. The above results are potentially applicable to further studies on the EEM-based or molecular-characteristic-based source apportionment of chromophores in atmospheric aerosols.

Abstract. Brown carbon (BrC) draws increasing attention due to its effects on climate and other environmental factors. In China, household coal burned for heating and cooking purposes releases huge amounts of carbonaceous particles every year; however, BrC emissions have rarely been estimated in a persuasive manner due to the unavailable emission characteristics. Here, seven coals jointly covering geological maturity from low to high were burned in four typical stoves as both chunk and briquette styles. The optical integrating sphere (IS) method was applied to measure the emission factors (EFs) of BrC and black carbon (BC) via an iterative process using the different spectral dependence of light absorption for BrC and BC and using humic acid sodium salt (HASS) and carbon black (CarB) as reference materials. The following results have been found: (i) the average EFs of BrC for anthracite coal chunks and briquettes are 1.08 ± 0.80 and 1.52 ± 0.16 g kg−1, respectively, and those for bituminous coal chunks and briquettes are 8.59 ± 2.70 and 4.01 ± 2.19 g kg−1, respectively, reflecting a more significant decline in BrC EFs for bituminous coals than for anthracites due to briquetting. (ii) The BrC EF peaks at the middle of coal's geological maturity, displaying a bell-shaped curve between EF and volatile matter (Vdaf). (iii) The calculated BrC emissions from China's residential coal burning amounted to 592 Gg (1 Gg  =  109 g) in 2013, which is nearly half of China's total BC emissions. (iv) The absorption Ångström exponents (AAEs) of all coal briquettes are higher than those of coal chunks, indicating that the measure of coal briquetting increases the BrC ∕ BC emission ratio and thus offsets some of the climate cooling effect of briquetting. (v) In the scenario of current household coal burning in China, solar light absorption by BrC (350–850 nm in this study) accounts for more than a quarter (0.265) of the total absorption. This implies the significance of BrC to climate modeling.

Humic-like substances (HULIS) are a class of high molecular weight, light-absorbing compounds that are highly related to brown carbon (BrC). In this study, the sources and compositions of HULIS isolated from fine particles collected in Beijing, China during the 2014 Asia-Pacific Economic Cooperation (APEC) summit were characterized based on carbon isotope (C and C) and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses, respectively. HULIS were the main light-absorbing components of water-soluble organic carbon (WSOC), accounting for 80.2 ± 6.1% of the WSOC absorption capacity at 365 nm. The carbon isotope data showed that HULIS had a lower non-fossil contribution (53 ± 4%) and were less enriched with C (-24.2 ± 0.6‰) relative to non-HULIS (62 ± 8% and -20.8 ± 0.3‰, respectively). The higher relative intensity fraction of sulfur-containing compounds in HULIS before and after APEC was attributed to higher sulfur dioxide levels emitted from fossil fuel combustion, whereas the higher fraction of nitrogen-containing compounds during APEC may have been due to the relatively greater contribution of non-fossil compounds or the influence of nitrate radical chemistry. The results of investigating the relationships among the sources, elemental compositions, and optical properties of HULIS demonstrated that the light absorption of HULIS appeared to increase with increasing unsaturation degree, but decrease with increasing oxidation level. The unsaturation of HULIS was affected by both sources and aging level.