Enigma of Urban Gaseous Oxygenated Organic Molecules: Precursor Type, Role of NOx, and Degree of Oxygenation.

Oxidation of volatile organic compounds (VOCs) forms oxygenated organic molecules (OOMs), which contribute to secondary pollution. Herein, we present measurement results of OOMs using chemical ionization mass spectrometry with nitrate as the reagent ion in Shanghai. Compared to those in forests and laboratory studies, OOMs detected at this urban site were of relatively lower degree of oxygenation. This was attributed to the high NOx concentrations (∼44 ppb), which overall showed a suppression on the propagation reactions. As another result, a large fraction of nitrogenous OOMs (75%) was observed, and this fraction further increased to 84% under a high NO/VOC ratio. By applying a novel framework on OOM categorization and supported by VOC measurements, 50 and 32% OOMs were attributed to aromatic and aliphatic precursors, respectively. Furthermore, aromatic OOMs are more oxygenated (effective oxygen number, nOeff = 4-6) than aliphatic ones (nOeff = 3-4), which can be partly explained by the difference in initiation mechanisms and points to possible discrimination in termination reactions. This study highlights the roles of NOx in OOM formation in urban areas, as well as the formation of nitrogenous products that might show discrimination between aromatic and aliphatic VOCs.

Insufficient Condensable Organic Vapors Lead to Slow Growth of New Particles in an Urban Environment.

Atmospheric new particle formation significantly affects global climate and air quality after newly formed particles grow above ∼50 nm. In polluted urban atmospheres with 1-3 orders of magnitude higher new particle formation rates than those in clean atmospheres, particle growth rates are comparable or even lower for reasons that were previously unclear. Here, we address the slow growth in urban Beijing with advanced measurements of the size-resolved molecular composition of nanoparticles using the thermal desorption chemical ionization mass spectrometer and the gas precursors using the nitrate CI-APi-ToF. A particle growth model combining condensational growth and particle-phase acid-base chemistry was developed to explore the growth mechanisms. The composition of 8-40 nm particles during new particle formation events in urban Beijing is dominated by organics (∼80%) and sulfate (∼13%), and the remainder is from base compounds, nitrate, and chloride. With the increase in particle sizes, the fraction of sulfate decreases, while that of the slow-desorbed organics, organic acids, and nitrate increases. The simulated size-resolved composition and growth rates are consistent with the measured results in most cases, and they both indicate that the condensational growth of organic vapors and H2SO4 is the major growth pathway and the particle-phase acid-base reactions play a minor role. In comparison to the high concentrations of gaseous sulfuric acid and amines that cause high formation rates, the concentration of condensable organic vapors is comparably lower under the high NOx levels, while those of the relatively high-volatility nitrogen-containing oxidation products are higher. The insufficient condensable organic vapors lead to slow growth, which further causes low survival of the newly formed particles in urban environments. Thus, the low growth rates, to some extent, counteract the impact of the high formation rates on air quality and global climate in urban environments.

Secondary Organic Aerosol Formation of Fleet Vehicle Emissions in China: Potential Seasonality of Spatial Distributions.

Vehicle emissions are an important source of urban particular matter. To investigate the secondary organic aerosol (SOA) formation potential of real-world vehicle emissions, we exposed on-road air in Beijing to hydroxyl radicals generated in an oxidation flow reactor (OFR) under high-NOx conditions on-board a mobile laboratory and characterized SOA and their precursors with a suite of state-of-the-art instrumentation. The OFR produced 10-170 µg m-3 of SOA with a maximum SOA formation potential of 39-50 µg m-3 ppmv-1 CO that occurred following an integrated OH exposure of (1.3-2.0) × 1011 molecules cm-3 s. The results indicate relatively shorter photochemical ages for maximum SOA production than previous OFR results obtained under low-NOx conditions. Such timescales represent the balance of functionalization and fragmentation, possibly resulting in different spatial distributions of SOA in different seasons as the oxidant level changes. The detected precursors may explain as much as 13% of the observed SOA with the remaining plausibly contributed by the oxidation of undetected intermediate-volatility organic compounds. Extrapolation of the results suggests an annual SOA production rate of 0.78 Tg yr-1 from mobile gasoline sources in China, highlighting the importance of effective regulation of gaseous vehicular precursors to improve air quality in the future.

Increasing role of phenolic oxidative branch in daytime oxidation process of aromatics in Chinese haze period.

To understand the photooxidation mechanisms of aromatic compounds in the NOx-rich atmosphere, gaseous aromatics and their oxidization products (i.e., methyl glyoxal (MGLY), and nitrated phenols (NPs) including nitrophenols (NPhs) and methylnitrophenols (MNPs)) were measured with a 1-h time resolution on Chongming Island, a downwind region of the Yangtze River Delta (YRD) metropolitans of China in winter 2019 by using a proton-transfer-reaction mass spectrometer (PTR-MS). During the entire observation period, concentrations of the measured VOCs were 9.6 ± 7.1 ppbv for aromatics, 118 ± 59 pptv for MGLY, 36 ± 10 pptv for NPhs, and 9.3 ± 2.8 pptv for MNPs, respectively. Secondary NPs (SNPs) accounted for only 19-24 % of the total nitrated phenols during the clean and transition periods but increased to 44 % of the total on the hazy days. Moreover, the daytime mixing ratios of SNPs increased along with an increasing NO2 concentration during the clean and transition periods, but in the haze period the daytime SNPs first increased along with the increasing NO2 levels and then increased much more sharply when NO2 was >25 ppbv. Such highly proportional and sharply increased daytime SNPs in the haze period indicated an enhanced phenolic oxidation under the high NOx conditions. In addition, the lack of correlations between aromatics and MGLY, increased MGLYaro (MGLY produced by aromatics), and sharply increased ΔSNPs / Δ(benzene + toluene) further suggested that such an increasing role of the phenolic oxidative branch in the daytime oxidation process of aromatics during the YRD haze period was caused by the strong atmospheric oxidation capacity and the high level of NOx.