Resolving Ultraviolet-Visible Spectra for Complex Dissolved Mixtures of Multitudinous Organic Matters in Aerosols.

Light-absorbing organic aerosols, referred to as brown carbon (BrC), play a vital role in the global climate and air quality. Due to the complexity of BrC chromophores, the identified absorbing substances in the ambient atmosphere are very limited. However, without comprehensive knowledge of the complex absorbing compounds in BrC, our understanding of its sources, formation, and evolution mechanisms remains superficial, leading to great uncertainty in climatic and atmospheric models. To address this gap, we developed a constrained non-negative matrix factorization (NMF) model to resolve the individual ultraviolet-visible spectrum for each substance in dissolved organic aerosols, with the power of ultrahigh-performance liquid chromatography-diode array detector-ultrahigh-resolution mass spectrometry (UHPLC-DAD-UHRMS). The resolved spectra were validated by selected standard substances and validation samples. Approximately 40,000 light-absorbing substances were recognized at the MS1 level. It turns out that BrC is composed of a vast number of substances rather than a few prominent chromophores in the urban atmosphere. Previous understanding of the absorbing feature of BrC based on a few identified compounds could be biased. Weak-absorbing substances missed previously play an important role in BrC absorption when they are integrated due to their overwhelming number. This model brings the property exploration of complex dissolved organic mixtures to a molecular level, laying a foundation for identifying potentially significant compositions and obtaining a comprehensive chemical picture.

Aqueous secondary organic aerosol formation attributed to phenols from biomass burning.

Biomass burning emits large quantities of phenols, which readily partition into the atmospheric aqueous phase and subsequently may react to produce aqueous secondary organic aerosol (aqSOA). For the first time, we quantitatively explored the influence of phenols emitted from biomass burning on aqSOA formation in the winter of Beijing. A typical haze episode associated with significant aqSOA formation was captured. During this episode, aqueous-phase processing of biomass burning promoted aqSOA formation was identified. Furthermore, high-resolution mass spectrum analysis provided molecular-level evidence of the phenolic aqSOA tracers. Estimation of aqSOA formation rate (RaqSOA) with compiled laboratory kinetic data indicated that biomass-burning phenols can efficiently produce aqSOA at midday, with RaqSOA of 0.42 µg m-3 h-1 accounting for 15 % of total aqSOA formation rate. The results highlight that aqSOA formation of phenols contributes the haze pollution. This implies the importance of regional joint control of biomass burning to mitigate the heavy haze.

Particle hygroscopicity inhomogeneity and its impact on reactive uptake.

Atmospheric particles are important reaction vessels for multiphase chemistry. We conducted a meta-analysis of previous field observations in various environments (includes ocean, urban and rural regions), showing that particle hygroscopicity inhomogeneity (PHI) is ubiquitous for the continental atmospheric particles, in which a considerable part of the particulate matters is hydrophobic (10%-33% on average). However, the effects of PHI in quantifying the uptake process of reactive gases are still unclear. Here, taking N2O5 uptake as an example, we showed that using a laboratory-based parameterization scheme without considering the PHI might result in a misestimation of uptake rate coefficient, especially under low ambient relative humidity (RH). Such misestimation may be caused by the differences of the uptake coefficients, as well as the proportion of surface area concentration (SA) between hydrophilic and hydrophobic particles. We suggested that the PHI should be well-considered in establishing the reactive traces gases heterogeneous uptake parameterizations.

Current challenges of improving visibility due to increasing nitrate fraction in PM2.5 during the haze days in Beijing, China.

The annual mean PM2.5 mass concentration has decreased because of the stringent emission controls implemented in Beijing, China in recent years, whereas the nitrate NO3- mass fraction in PM2.5 increases gradually. Low-visibility events occur frequently even though PM2.5 pollution has been mitigated significantly, with the daily mean PM2.5 mass concentration mostly less than 75 µg/m3. In this study, the non-linear relationship was analyzed between atmospheric visibility and PM2.5 based on chemical composition from a two-year field observation. Our results showed that NO3- became the main constituent of PM2.5, especially during the haze pollution episodes. A localized parameterization scheme was proposed between the atmospheric extinction coefficient (σext) and major chemical constituents of PM2.5 by multiple linear regression (MLR). The contribution of NO3- to σext increased with increasing air pollution, and NO3- became the most important contributor for PM2.5 above 75 µg/m3. The visibility decreased with increasing NO3- mass fraction for the same PM2.5 mass concentration when PM2.5 was above 20 µg/m3. The hygroscopicity of PM2.5 increased with increasing mass fraction of hygroscopic NO3-. These results stressed the importance of reducing particulate NO3- and its precursors (for instance, NH3) through effective emission control measures as well as the tightening of PM2.5 standards to further improve air quality and visibility in Beijing.

The particle phase state during the biomass burning events.

The phase state of biomass burning aerosols (BBA) remains largely unclear, impeding our understanding of their effects on air quality, climate and human health, due to its profound roles in mass transfer between gaseous and particulate phase. In this study, the phase state of BBA was investigated by measuring the particle rebound fraction ƒ combining field observations and laboratory experiments. We found that both ambient and laboratory-generated BBA had unexpectedly lower rebound fraction ƒ (<0.6) under the dry conditions (RH = 20-50%), indicating that BBA were in non-solid state at such low RH. This was obviously different from the secondary organic aerosols (SOA) derived from the oxidation of both anthropogenic and biogenic volatile organic compounds, typically with a rebound fraction ƒ larger than 0.8 at RH below 50%. Therefore, we proposed that the diffusion coefficient of gaseous molecular in the bulk of BBA might be much higher than SOA under the dry conditions.

Links between the optical properties and chemical compositions of brown carbon chromophores in different environments: Contributions and formation of functionalized aromatic compounds.

Links between the optical properties and chemical compositions of brown carbon (BrC) are poorly understood because of the complexity of BrC chromophores. We conducted field studies simultaneously at both vehicle-influenced site and biomass burning-affected site in China in polluted winter. The chemical compositions and light absorption values of functionalized aromatic compounds, including phenyl aldehyde, phenyl acid, and nitroaromatic compounds, were measured. P-phthalic acid, nitrophenols and nitrocatechols were dominant BrC species, accounting for over 50% of the concentration of identified chromophores. Nitrophenols and nitrocatechols contributed more than 50% of the identified BrC absorbance between 300 and 400 nm. Oxidation of biomass burning-related products (e.g., pyrocatechol and methylcatechols) and anthropogenic volatile organic compounds (e.g., benzene and toluene) generated similar BrC chromophores, implying that these functionalized aromatic compounds play an important role in both environments. Compared with the biomass burning-affected site (22%), functionalized aromatic compounds at vehicle-influenced site accounted for a higher percentage of BrC absorption (25%). This research improves our understanding of the links between optical properties and composition of BrC, and the difference between BrC chromophores from BB-influenced area and vehicle-affected area under polluted atmospheric conditions.

Humidity-Dependent Phase State of Gasoline Vehicle Emission-Related Aerosols.

The phase states of primarily emitted and secondarily formed aerosols from gasoline vehicle exhausts were investigated by quantifying the particle rebound fraction (f). The rebound behaviors of gasoline vehicle emission-related aerosols varied with engines, fuel types, and photochemical aging time, showing distinguished differences from biogenic secondary organic aerosols. The nonliquid-to-liquid phase transition of primary aerosols emitted from port fuel injection (PFI) and gasoline direct injection (GDI) vehicles started at a relative humidity (RH) = 50 and 60%, and liquefaction was accomplished at 60 and 70%, respectively. The RH at which f declined to 0.5 decreased from 70 to 65% for the PFI case with 92# fuel, corresponding to the photochemical aging time from 0.37 to 4.62 days. For the GDI case, such RH enhanced from 60 to 65%. Our results can be used to imply the phase state of traffic-related aerosols and further understand their roles in urban atmospheric chemistry. Taking Beijing, China, as an example, traffic-related aerosols were mainly nonliquid during winter with the majority ambient RH below 50%, whereas they were mostly liquid during the morning rush hour of summer, and traffic-related secondary aerosols fluctuated between nonliquid and liquid during the daytime and tended to be liquid at night with increased ambient RH.