Microfluidic Electrochemical Sensor for On-Line Monitoring of Aerosol Oxidative Activity

Particulate matter (PM) air pollution has a significant impact on human morbidity and mortality; however, the mechanisms of PM-induced toxicity are poorly defined. A leading hypothesis states that airborne PM induces harm by generating reactive oxygen species in and around human tissues, leading to oxidative stress. We report here a system employing a microfluidic electrochemical sensor coupled directly to a particle-into-liquid sampler (PILS) system to measure aerosol oxidative activity in an on-line format. The oxidative activity measurement is based on the dithiothreitol (DTT) assay, where, after being oxidized by PM, the remaining reduced DTT is analyzed by the microfluidic sensor. The sensor consists of an array of working, reference, and auxiliary electrodes fabricated in a poly­(dimethylsiloxane)-based microfluidic device. Cobalt­(II) phthalocyanine-modified carbon paste was used as the working electrode material, allowing selective detection of reduced DTT. The electrochemical sensor was validated off-line against the traditional DTT assay using filter samples taken from urban environments and biomass burning events. After off-line characterization, the sensor was coupled to a PILS to enable on-line sampling/analysis of aerosol oxidative activity. Urban dust and industrial incinerator ash samples were aerosolized in an aerosol chamber and analyzed for their oxidative activity. The on-line sensor reported DTT consumption rates (oxidative activity) in good correlation with aerosol concentration (<i>R</i>² from 0.86 to 0.97) with a time resolution of approximately 3 min

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

Trimethylamine from Subtropical Forests Rival Total Farmland Emissions in China

Many types of living plants release gaseous trimethylamine (TMA), making it a potentially important contributor to new particle formation (NPF) in remote areas. However, a panoramic view of the importance of forest biogenic TMA at the regional scale is lacking. Here, we pioneered nationwide mobile measurements of TMA across a transect of contiguous farmland in eastern China and a transect of subtropical forests in southern China. In contrast to the farmland route, TMA concentrations measured during the subtropical forest route correlated significantly with isoprene, suggesting potential TMA emissions from leaves. Our high time-resolved concentrations obtained from a weak photo-oxidizing atmosphere reflected freshly emitted TMA, indicating the highest emission intensity from irrigated dryland (set as the baseline of 10), followed by paddy field (7.1), subtropical evergreen forests (5.9), and subtropical broadleaf and mixed forests (4.3). Extrapolating their proportions roughly to China, subtropical forests alone, which constitute half of the total forest area, account for nearly 70% of the TMA emissions from the nation’s total farmland. Our estimates, despite the uncertainties, take the first step toward large-scale assessment of forest biogenic amines, highlighting the need for observational and modeling studies to consider this hitherto overlooked source of TMA

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

Observation of Fullerene Soot in Eastern China

This work reports the observation of a series of fullerene ions, indicating the occurrence of fullerene soot (FS) in ambient air for the first time using an Aerodyne soot particle-aerosol mass spectrometer (SP-AMS) deployed in eastern China. We found the distribution of these ions showed a pattern almost identical with that of an Alfa Aesar FS standard. Although the SP-AMS may provide only a semiquantitative measurement of the FS, the measured concentrations can still reflect the temporal variations of airborne fullerenes. Combining results from factor analyses and meteorological data, we identified the petrochemical plants situated northeast of the site as the major source responsible for the FS-like ions. Our findings indicate the general presence of FS in ambient air, especially in oil and gas production regions. The SP-AMS technique may offer new insights into characterizing fullerene-related species in other environmental samples, as well

Significant Reductions in Secondary Aerosols after the Three-Year Action Plan in Beijing Summer

Air quality in China has continuously improved during the Three-Year Action Plan (2018–2020); however, the changes in aerosol composition, properties, and sources in Beijing summer remain poorly understood. Here, we conducted real-time measurements of aerosol composition in five summers from 2018 to 2022 along with WRF-Community Multiscale Air Quality simulations to characterize the changes in aerosol chemistry and the roles of meteorology and emission reductions. Largely different from winter, secondary inorganic aerosol and photochemical-related secondary organic aerosol (SOA) showed significant decreases by 55–67% in summer, and the most decreases occurred in 2021. Comparatively, the decreases in the primary aerosol species and gaseous precursors were comparably small. While decreased atmospheric oxidation capacity as indicated by ozone changes played an important role in changing SOA composition, the large decrease in aerosol liquid water and small increase in particle acidity were critical for nitrate changes by decreasing gas-particle partitioning substantially (∼28%). Analysis of meteorological influences demonstrated clear and similar transitions in aerosol composition and formation mechanisms at a relative humidity of 50–60% in five summers. Model simulations revealed that emission controls played the decisive role in reducing sulfate, primary OA, and anthropogenic SOA during the Three-Year Action Plan, while meteorology affected more nitrate and biogenic SOA

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

First Chemical Characterization of Refractory Black Carbon Aerosols and Associated Coatings over the Tibetan Plateau (4730 m a.s.l)

Refractory black carbon (<i>r</i>BC) aerosol is an important climate forcer, and its impacts are greatly influenced by the species associated with <i>r</i>BC cores. However, relevant knowledge is particularly lacking at the Tibetan Plateau (TP). Here we report, for the first time, highly time-resolved measurement results of <i>r</i>BC and its coating species in central TP (4730 m a.s.l), using an Aerodyne soot particle aerosol mass spectrometer (SP-AMS), which selectively measured <i>r</i>BC-containing particles. We found that the <i>r</i>BC was overall thickly coated with an average mass ratio of coating to <i>r</i>BC (<i>R</i>_BC) of ∼7.7, and the coating species were predominantly secondarily formed by photochemical reactions. Interestingly, the thickly coated <i>r</i>BC was less oxygenated than the thinly coated <i>r</i>BC, mainly due to influence of the transported biomass burning organic aerosol (BBOA). This BBOA was relatively fresh but formed very thick coating on <i>r</i>BC. We further estimated the “lensing effect” of coating semiquantitatively by comparing the measurement data from a multiangle absorption photometer and SP-AMS, and found it could lead to up to 40% light absorption enhancement at <i>R</i>_BC > 10. Our findings highlight that BBOA can significantly affect the “lensing effect”, in addition to its relatively well-known role as light-absorbing “brown carbon.

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>⁺