Cong Cao

Investigation of inhibition of 2-mercaptobenzoxazole for copper corrosion

The inhibition by 2-mercaptobenzoxazole (MBO) of copper corrosion in NaCl solution was investigated by means of potentiodynamic polarization, STM: QCM, XPS, FTIR and AES. The inhibition efficiency depended on MBO concentration, existence of NaCl and the immersion period of the copper sample. The inhibition efficiency of about 99% could be obtained by immersing for above 20 h in 3% NaCl + 1 mM MBO solution. The surface of the copper sample immersed in NaCl + MBO solution possessed FTIR characteristics different from those of MBO solution, irrespective of immersion time. The in-situ STM morphologies showed the rapid formation of a three-dimensional compact inhibition film on the surface of copper in MBO solution containing NaCl. XPS indicated that the inhibition film was cuprous and the chemical environments of S and N atoms were evidently changed. It is concluded that MBO reacted with the cuprous species from the corrosion process, and produced a water-insoluble cuprous complex to form the inhibition film and to retard the corrosion of copper

Characterization of isoprene-derived secondary organic aerosols at a rural site in North China Plain with implications for anthropogenic pollution effects

Isoprene is the most abundant non-methane volatile organic compound (VOC) and the largest contributor to secondary organic aerosol (SOA) burden on a global scale. In order to examine the influence of high concentrations of anthropogenic pollutants on isoprene-derived SOA (SOAi) formation, summertime PM2.5 filter samples were collected with a three-hour sampling interval at a rural site in the North China Plain (NCP), and determined for SOAi tracers and other chemical species. RO2+NO pathway derived 2-methylglyceric acid presented a relatively higher contribution to the SOAi due to the high-NOx (~20 ppb) conditions in the NCP that suppressed the reactive uptake of RO2+HO2 reaction derived isoprene epoxydiols. Compared to particle acidity and water content, sulfate plays a dominant role in the heterogeneous formation process of SOAi. Diurnal variation and correlation of 2-methyltetrols with ozone suggested an important effect of isoprene ozonolysis on SOAi formation. SOAi increased linearly with levoglucosan during June 10–18, which can be attributed to an increasing emission of isoprene caused by the field burning of wheat straw and a favorable aqueous SOA formation during the aging process of the biomass burning plume. Our results suggested that isoprene oxidation is highly influenced by intensive anthropogenic activities in the NCP.

Particle acidity and sulfate production during severe haze events in China cannot be reliably inferred by assuming a mixture of inorganic salts

Atmospheric measurements showed rapid sulfate formation during severe haze episodes in China, with fine particulate matter (PM) consisting of a multi-component mixture that is dominated by organic species. Several recent studies using the thermodynamic model estimated the particle acidity and sulfate production rate, by treating the PM exclusively as a mixture of inorganic salts dominated by ammonium sulfate and neglecting the effects of organic compounds. Noticeably, the estimated pH and sulfate formation rate during pollution periods in China were highly conflicting among the previous studies. Here we show that a particle mixture of inorganic salts adopted by the previous studies does not represent a suitable model system and that the acidity and sulfate formation cannot be reliably inferred without accounting for the effects of multi-aerosol compositions during severe haze events in China. Our laboratory experiments show that SO2 oxidation by NO2 with NH3 neutralization on fine aerosols is dependent on the particle hygroscopicity, phase-state, and acidity. Ammonium sulfate and oxalic acid seed particles exposed to vapors of SO2, NO2, and NH3 at high relative humidity (RH) exhibit distinct size growth and sulfate formation. Aqueous ammonium sulfate particles exPublished by Copernicus Publications on behalf of the European Geosciences Union. 10124 G. Wang et al.: Rapid formation of sulfate in Chinese haze periods hibit little sulfate production, in contrast to aqueous oxalic acid particles with significant sulfate production. Our field measurements demonstrate significant contribution of watersoluble organic matter to fine PM in China and indicate that the use of oxalic acid in laboratory experiments is representative of ambient organic dominant aerosols. While the particle acidity cannot be accurately determined from field measurements or calculated using the thermodynamic model, our results reveal that the pH value of ambient organics-dominated aerosols is sufficiently high to promote efficient SO2 oxidation by NO2 with NH3 neutralization under polluted conditions in China.

Seasonal variation and size distribution of biogenic secondary organic aerosols at urban and continental background sites of China

Size-resolved biogenic secondary organic aerosols (BSOA) derived from isoprene and monoterpene photooxidation in Qinghai Lake, Tibetan Plateau (a continental background site) and five cities of China were measured using gas chromatography/mass spectrometry (GC/MS). Concentrations of the determined BSOA are higher in the cities than in the background and are also higher in summer than in winter. Moreover, strong positive correlations (R2=0.44-0.90) between BSOA and sulfate were found at the six sites, suggesting that anthropogenic pollution (i.e., sulfate) could enhance SOA formation, because sulfate provides a surface favorable for acid-catalyzed formation of BSOA. Size distribution measurements showed that most of the determined SOA tracers are enriched in the fine mode (<3.3μm) except for cis-pinic and cis-pinonic acids, both presented a comparable mass in the fine and coarse (>3.3μm) modes, respectively. Mass ratio of oxidation products derived from isoprene to those from monoterpene in the five urban regions during summer are much less than those in Qinghai Lake region. In addition, in the five urban regions relative abundances of monoterpene oxidation products to SOA are much higher than those of isoprene. Such phenomena suggest that BSOA derived from monoterpenes are more abundant than those from isoprene in Chinese urban areas.

Molecular distribution and stable carbon isotopic compositions of dicarboxylic acids and related SOA from biogenic sources in the summertime atmosphere of Mt. Tai in the North China Plain

Molecular distributions and stable carbon isotopic (δ13C values) compositions of dicarboxylic acids and related secondary organic aerosols (SOA) in PM2.5 aerosols collected on a day/night basis at the summit of Mt. Tai (1534 m a.s.l.) in the summer of 2016 were analyzed to investigate the sources and photochemical aging process of organic aerosols in the forested highland region of the North China Plain. The molecular distributions of dicarboxylic acids and related SOA are characterized by the dominance of oxalic acid (C2), followed by malonic (C3), succinic (C4) and azelaic (C9) acids. The concentration ratios of C2 /C4, diacid-C /OC and C2 / total diacids are larger in the daytime than in the nighttime, suggesting that the daytime aerosols are more photochemically aged than those in the nighttime due to the higher temperature and stronger solar radiation. Both ratios of C2 /C4 (R2 > 0.5) and C3 /C4 (R2 > 0.5) correlated strongly with the ambient temperatures, indicating that SOA in the mountaintop atmosphere are mainly derived from the photochemical oxidation of local emissions rather than long-range transport. The mass ratios of azelaic acid to adipic acid (C9 /C6), azelaic acid to phthalic aid (C9 /Ph) and glyoxal to methylglyoxal (Gly /mGly) and the strong linear correlations of major dicarboxylic acids and related SOA (i.e., C2, C3, C4, ωC2, Pyr, Gly and mGly) with biogenic precursors (SOA tracers derived from isoprene, α/β-pinene and β-caryophyllene) further suggest that aerosols in this region are mainly originated from biogenic sources (i.e., tree emissions). C2 concentrations correlated well with aerosol pH, indicating that particle acidity favors the organic acid formation. The stable carbon isotopic compositions (δ13C) of the dicarboxylic acids are higher in the daytime than in the nighttime, with the highest value (−16.5± 1.9 ‰) found for C2 and the lowest value (−25.2± 2.7 ‰) found for C9. An increase in δ13C values of C2 along with increases in C2 /Gly and C2 /mGly ratios was observed, largely due to the isotopic fractionation effect during the precursor oxidation process.