Secondary Production of Organic Aerosols from Biogenic VOCs over Mt. Fuji, Japan

We investigated organic molecular compositions of summertime aerosols collected at the summit of Mt. Fuji (3776 m a.s.l.) in July–August 2009. More than 120 organic species were identified using GC/MS. Concentrations of both primary and secondary organic aerosol (SOA) tracers in whole-day samples were 4–20 times higher than those in nighttime samples, suggesting that valley breeze is an efficient mechanism to uplift the aerosols and precursors from the ground surface to mountaintop in daytime. Using a tracer-based method, we estimated the concentrations of secondary organic carbon (SOC) derived from isoprene, α/β-pinene, and β-caryophyllene to be 2.2–51.2 ngC m^–3 in nighttime and 227–1120 ngC m^–3 during whole-day. These biogenic SOCs correspond to 0.80–31.9% and 26.8–57.4% of aerosol organic carbon in nighttime and whole-day samples, respectively. This study demonstrates that biogenic SOA, which is controlled by the valley breeze, is a significant fraction of free tropospheric aerosols over Mt. Fuji in summer

Strong Impacts of Regional Atmospheric Transport on the Vertical Distribution of Aerosol Ammonium over Beijing

Ammonium (NH4+) is a significant component of fine aerosol particles (PM2.5), and its behavior in the atmosphere is crucial to air pollution. We present a novel study that analyzes the vertical distribution and temporal trends of NH4+ in the urban boundary layer of Beijing, tracking hourly concentrations throughout a complete haze episode. Our results unveil a surprising single-peak profile of NH4+ at heights of 300–700 m in the urban boundary layer with its hourly concentration reaching ∼50 μg m–3, which is 3 times higher than that at the ground level, in contrast to the conventional patterns of decreasing concentrations with height. The vertical structure is closely related to the observed escape of ammonia (NH3) or NH4+ from upwind industrial sources via elevated chimneys. The NH4+ plumes emitted through these sources are prone to transport at an altitude of 270–750 m for approximately 6 h, covering >250 km to Beijing. This study reveals that non-agricultural point emissions of NH4+ impact the vertical patterns of aerosol NH4+ in the urban boundary layer, demonstrating potential opportunities for limiting such emission sources to curb PM2.5 pollution in the North China Plain

Strong Impacts of Regional Atmospheric Transport on the Vertical Distribution of Aerosol Ammonium over Beijing

Ammonium (NH4+) is a significant component of fine aerosol particles (PM2.5), and its behavior in the atmosphere is crucial to air pollution. We present a novel study that analyzes the vertical distribution and temporal trends of NH4+ in the urban boundary layer of Beijing, tracking hourly concentrations throughout a complete haze episode. Our results unveil a surprising single-peak profile of NH4+ at heights of 300–700 m in the urban boundary layer with its hourly concentration reaching ∼50 μg m–3, which is 3 times higher than that at the ground level, in contrast to the conventional patterns of decreasing concentrations with height. The vertical structure is closely related to the observed escape of ammonia (NH3) or NH4+ from upwind industrial sources via elevated chimneys. The NH4+ plumes emitted through these sources are prone to transport at an altitude of 270–750 m for approximately 6 h, covering >250 km to Beijing. This study reveals that non-agricultural point emissions of NH4+ impact the vertical patterns of aerosol NH4+ in the urban boundary layer, demonstrating potential opportunities for limiting such emission sources to curb PM2.5 pollution in the North China Plain

Humic-Like Substances (HULIS) in Aerosols of Central Tibetan Plateau (Nam Co, 4730 m asl): Abundance, Light Absorption Properties, and Sources

Humic-like substances (HULIS) are major components of light-absorbing brown carbon that play an important role in Earth’s radiative balance. However, their concentration, optical properties, and sources are least understood over Tibetan Plateau (TP). In this study, the analysis of total suspended particulate (TSP) samples from central of TP (i.e., Nam Co) reveal that atmospheric HULIS are more abundant in summer than that in winter without obvious diurnal variations. The light absorption ability of HULIS in winter is 2–3 times higher than that in summer. In winter, HULIS are mainly derived from biomass burning emissions in South Asia by long-range transport. In contrast, the oxidation of anthropogenic and biogenic precursors from northeast part of India and southeast of TP are major sources of HULIS in summer

Proteins and Amino Acids in Fine Particulate Matter in Rural Guangzhou, Southern China: Seasonal Cycles, Sources, and Atmospheric Processes

Water-soluble proteinaceous matter including proteins and free amino acids (FAAs) as well as some other chemical components was analyzed in fine particulate matter (PM_2.5) samples collected over a period of one year in rural Guangzhou. Annual averaged protein and total FAAs concentrations were 0.79 ± 0.47 μg m^–3 and 0.13 ± 0.05 μg m^–3, accounting for 1.9 ± 0.7% and 0.3 ± 0.1% of PM_2.5, respectively. Among FAAs, glycine was the most abundant species (19.9%), followed by valine (18.5%), methionine (16.1%), and phenylalanine (13.5%). Both proteins and FAAs exhibited distinct seasonal variations with higher concentrations in autumn and winter than those in spring and summer. Correlation analysis suggests that aerosol proteinaceous matter was mainly derived from intensive agricultural activities, biomass burning, and fugitive dust/soil resuspension. Significant correlations between proteins/FAAs and atmospheric oxidant (O₃) indicate that proteins/FAAs may be involved in O₃ related atmospheric processes. Our observation confirms that ambient FAAs could be degraded from proteins under the influence of O₃, and the stoichiometric coefficients of the reactions were estimated for FAAs and glycine. This finding provides a possible pathway for the production of aerosol FAAs in the atmosphere, which will improve the current understanding on atmospheric processes of proteinaceous matter

Real-Time Characterization of Aerosol Particle Composition above the Urban Canopy in Beijing: Insights into the Interactions between the Atmospheric Boundary Layer and Aerosol Chemistry

Despite extensive efforts into the characterization of air pollution during the past decade, real-time characterization of aerosol particle composition above the urban canopy in the megacity Beijing has never been performed to date. Here we conducted the first simultaneous real-time measurements of aerosol composition at two different heights at the same location in urban Beijing from December 19, 2013 to January 2, 2014. The nonrefractory submicron aerosol (NR-PM₁) species were measured in situ by a high-resolution aerosol mass spectrometer at near-ground level and an aerosol chemical speciation monitor at 260 m on a 325 m meteorological tower in Beijing. Secondary aerosol showed similar temporal variations between ground level and 260 m, whereas much weaker correlations were found for the primary aerosol. The diurnal evolution of the ratios and correlations of aerosol species between 260 m and the ground level further illustrated a complex interaction between vertical mixing processes and local source emissions on aerosol chemistry in the atmospheric boundary layer. As a result, the aerosol compositions at the two heights were substantially different. Organic aerosol (OA), mainly composed of primary OA (62%), at the ground level showed a higher contribution to NR-PM₁ (65%) than at 260 m (54%), whereas a higher concentration and contribution (15%) of nitrate was observed at 260 m, probably due to the favorable gas–particle partitioning under lower temperature conditions. In addition, two different boundary layer structures were observed, each interacting differently with the evolution processes of aerosol chemistry

Effects of Aqueous-Phase and Photochemical Processing on Secondary Organic Aerosol Formation and Evolution in Beijing, China

Secondary organic aerosol (SOA) constitutes a large fraction of OA, yet remains a source of significant uncertainties in climate models due to incomplete understanding of its formation mechanisms and evolutionary processes. Here we evaluated the effects of photochemical and aqueous-phase processing on SOA composition and oxidation degrees in three seasons in Beijing, China, using high-resolution aerosol mass spectrometer measurements along with positive matrix factorization. Our results show that aqueous-phase processing has a dominant impact on the formation of more oxidized SOA (MO–OOA), and the contribution of MO–OOA to OA increases substantially as a function of relative humidity or liquid water content. In contrast, photochemical processing plays a major role in the formation of less oxidized SOA (LO–OOA), as indicated by the strong correlations between LO–OOA and odd oxygen (O_<i>x</i> = O₃ + NO₂) during periods of photochemical production (R² = 0.59–0.80). Higher oxygen-to-carbon ratios of SOA during periods with higher RH were also found indicating a major role of aqueous-phase processing in changing the oxidation degree of SOA in Beijing. Episodes analyses further highlight that LO–OOA plays a more important role during the early stage of the formation of autumn/winter haze episodes while MO–OOA is more significant during the later evolution period

Enhanced Light Scattering of Secondary Organic Aerosols by Multiphase Reactions

Secondary organic aerosol (SOA) plays a pivotal role in visibility and radiative forcing, both of which are intrinsically linked to the refractive index (RI). While previous studies have focused on the RI of SOA from traditional formation processes, the effect of multiphase reactions on the RI has not been considered. Here, we investigate the effects of multiphase processes on the RI and light-extinction of <i>m</i>-xylene-derived SOA, a common type of anthropogenic SOA. We find that multiphase reactions in the presence of liquid water lead to the formation of oligomers from intermediate products such as glyoxal and methylglyoxal, resulting in a large enhancement in the RI and light-scattering of this SOA. These reactions will result in increases in light-scattering efficiency and direct radiative forcing of approximately 20%–90%. These findings improve our understanding of SOA optical properties and have significant implications for evaluating the impacts of SOA on the rapid formation of regional haze, global radiative balance, and climate change

Seasonal Characterization of Organic Nitrogen in Atmospheric Aerosols Using High Resolution Aerosol Mass Spectrometry in Beijing, China

Despite extensive efforts to characterize organic nitrogen (ON) in atmospheric aerosols, knowledge of the sources and processes of ON in the megacity of Beijing is still limited, mainly due to the complexity of ON species and the absence of highly time-resolved measurements. Here we demonstrate the applications of Aerodyne high-resolution time-of-flight aerosol mass spectrometer combined with positive matrix factorization in characterization of ON in submicron aerosols. Our results show that the average nitrogen-to-carbon ratios (N/C) vary from 0.021 to 0.028, and the average ON concentrations range from 0.26 to 0.59 μg m^–3 during four seasons in Beijing. ON accounts for 7–10% of the total nitrogen (TN) on average, yet the sources vary differently across different seasons. We found that 56–65% of ON was secondary during three seasons except winter when 59–67% was related to primary emissions. Particularly, more oxidized secondary organic aerosol contributes the dominant fraction of ON (39–44%) in spring, summer and autumn, while biomass burning is a more important source of ON in winter (23–44%). These results are consistent with the better positive correlations between N/C and oxygen-to-carbon ratio, a surrogate of organic aerosol aging, during these three seasons than that in winter. N/C also shows a clear increase as a function of relative humidity during all seasons, suggesting that aqueous-phase processing likely played an important role in formation of nitrogen-containing compounds. In addition, the uncertainties and limitations in quantification of ON with aerosol mass spectrometry are illustrated, particularly, ON could be underestimated by ∼20–42% by ignoring the fragment contributions in NH_<i>x</i>⁺ and NO_<i>x</i>⁺

Isotopic Composition of Atmospheric Mercury in China: New Evidence for Sources and Transformation Processes in Air and in Vegetation

The isotopic composition of atmospheric total gaseous mercury (TGM) and particle-bound mercury (PBM) and mercury (Hg) in litterfall samples have been determined at urban/industrialized and rural sites distributed over mainland China for identifying Hg sources and transformation processes. TGM and PBM near anthropogenic emission sources display negative δ²⁰²Hg and near-zero Δ¹⁹⁹Hg in contrast to relatively positive δ²⁰²Hg and negative Δ¹⁹⁹Hg observed in remote regions, suggesting that different sources and atmospheric processes force the mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) in the air samples. Both MDF and MIF occur during the uptake of atmospheric Hg by plants, resulting in negative δ²⁰²Hg and Δ¹⁹⁹Hg observed in litter-bound Hg. The linear regression resulting from the scatter plot relating the δ²⁰²Hg to Δ¹⁹⁹Hg data in the TGM samples indicates distinct anthropogenic or natural influences at the three study sites. A similar trend was also observed for Hg accumulated in broadleaved deciduous forest foliage grown in areas influenced by anthropogenic emissions. The relatively negative MIF in litter-bound Hg compared to TGM is likely a result of the photochemical reactions of Hg²⁺ in foliage. This study demonstrates the diagnostic stable Hg isotopic composition characteristics for separating atmospheric Hg of different source origins in China and provides the isotopic fractionation clues for the study of Hg bioaccumulation