Anthropogenic monoterpenes aggravating ozone pollution

Monoterpenes have been known to have a critical influence on air quality and climate change through their impact on the formation of fine particles. Here we present field evidence that monoterpene oxidations largely enhanced local ozone production in a regional site in eastern China. The observed monoterpene was most likely from biomass burning rather than biogenic emissions, as indicated by the high correlation with CO at night-time, and the observed ratio of these two species was consistent with previously determined values from biomass burning experiments. Fast monoterpene oxidations were determined experimentally based on direct radical measurements, leading to a daily ozone enhancement of 4-18 parts per billion by volume (ppb), which was 6%-16% of the total ozone production, depending on the speciation of monoterpenes. It demonstrates that the previously overlooked anthropogenic monoterpenes make an important contribution to O3 production in eastern China. The role could possibly be important at similar locations across China and other parts of the world that are characterized by massive emissions, especially where there are high NO x levels. Our results highlight that anthropogenic monoterpenes should be taken into account when proceeding with the coordinated mitigation of O3 and particulate matter pollution.

Nocturnal atmospheric chemistry of NO3 and N2O5 over Changzhou in the Yangtze River Delta in China

Comprehensive observations of the nocturnal atmospheric oxidation of NO₃ and N₂O₅ were conducted at a suburban site in Changzhou in the YRD using cavity ring-down spectroscopy (CRDS) from 27 May to 24 June, 2019. High concentrations of NO₃ precursors were observed, and the nocturnal production rate of NO₃ was determined to be 1.7 ± 1.2 ppbv/hr. However, the nighttime NO₃ and N₂O₅ concentrations were relatively low, with maximum values of 17.7 and 304.7 pptv, respectively, illustrating the rapid loss of NO₃ and N₂O₅. It was found that NO₃ dominated the nighttime atmospheric oxidation, accounting for 50.7%, while O₃ and OH only contributed 34.1% and 15.2%, respectively. For the reactions of NO₃ with volatile organic compounds (VOCs), styrene was found to account for 60.3%, highlighting its dominant role in the NO₃ reactivity. In general, the contributions of the reactions between NO₃ and VOCs and the N₂O₅ uptake to NO₃ loss were found to be about 39.5% and 60.5%, respectively, indicating that N₂O₅ uptake also played an important role in the loss of NO₃ and N₂O₅, especially under the high humidity conditions in China. The formation of nitrate at night mainly originated from N₂O₅ uptake, and the maximum production rate of NO₃^- reached 6.5 ppbv/hr. The average NO_x consumption rate via NO₃ and N₂O₅ chemistry was found to be 0.4 ppbv/h, accounting for 47.9% of the total NO_x removal. The predominant roles of NO₃ and N₂O₅ in nitrate formation and NOx removal in the YRD region was highlighted in this study.

Characterizing nitrate radical budget trends in Beijing during 2013-2019

Nitrate (NO₃) radical is an important oxidant in the atmosphere as it regulates the NO_x budget and impacts secondary pollutant formation. Here, a long-term observational dataset of NO₃-related species at an urban site in Beijing was used to investigate changes in the NO₃ budget and their atmospheric impacts during 2013-2019, in this period the Clean Air Actions Plan was carried out in China. We found that (1) changes in NO₃ precursors (NO₂ and O₃) led to a significant increase in NO₃ formation in the surface layer in winter but a decrease in summer; (2) a reduction in NO_x promoted thermal equilibrium, favoring the formation of NO₃ rather than dinitrogen pentoxide (N₂O₅). The simultaneous decrease in PM_2.5, during these years, further weakened the N₂O₅ heterogeneous uptake; (3) a box model simulation revealed that both the reactions of NO₃ with volatile organic compounds (VOC) and N₂O₅ uptake were weakened in summer, implying that the policy actions implemented help to moderate secondary aerosol formation caused by NO₃ and N₂O₅ chemistry in summer; and (4) during winter, both NO₃ + VOC and N₂O₅ uptake were enhanced. Specifically, for the N₂O₅ uptake, the rapid increase in NO₃ production, or to some extent, NO₃ oxidation capacity, far outweighed the negative shift effect, leading to a net enhancement of N₂O₅ uptake in winter, which indicates that the action policy implemented led to an adverse effect on particulate nitrate formation via N₂O₅ uptake in winter. This may explain the persistent winter particulate nitrate pollution in recent years. Our results highlight the systematic changes in the NO₃ budget between 2013 and 2019 in Beijing, which subsequently affect secondary aerosol formation in different seasons.

Enhanced wintertime oxidation of VOCs via sustained radical sources in the urban atmosphere

Daytime atmospheric oxidation chemistry is conventionally considered to be driven primarily by the OH radical, formed via photolytic sources. In this paper we examine how, during winter when photolytic processes are slow, chlorine chemistry can have a significant impact on oxidative processes in the urban boundary layer. Photolysis of nitryl chloride (ClNO₂) provides a significant source of chlorine atoms, which enhances the oxidation of volatile organic compounds (VOCs) and the production of atmospheric pollutants. We present a set of observations of ClNO₂ and HONO made at urban locations in central England in December 2014 and February 2016. While direct emissions and in-situ chemical formation of HONO continue throughout the day, ClNO₂ is only formed at night and is usually completely photolyzed by midday. Our data show that, during winter, ClNO₂ often persists through the daylight hours at mixing ratios above 10-20 ppt (on average). In addition, relatively high mixing ratios of daytime HONO (>65 ppt) provide a strong source of OH radicals throughout the day. The combined effects of ClNO₂ and HONO result in sustained sources of Cl and OH radicals from sunrise to sunset, which form additional ozone, PAN, oxygenated VOCs, and secondary organic aerosol. We show that radical sources such as ClNO₂ and HONO can lead to a surprisingly photoactive urban atmosphere during winter and should therefore be included in atmospheric chemical models.

N2O5at water surfaces: binding forces, charge separation, energy accommodation and atmospheric implications

Studying the interactions between N₂O₅and water in nano-sized clusters, in bulk and on the surface of water.

Cavity enhanced spectroscopy for measurement of nitrogen oxides in the Anthropocene: results from the Seoul tower during MAPS 2015

Cavity enhanced spectroscopy, CES, is a high sensitivity direct absorption method that has seen increasing utility in the last decade, a period also marked by increasing requirements for understanding human impacts on atmospheric composition. This paper describes the current NOAA six channel cavity ring-down spectrometer (CRDS, the most common form of CES) for measurement of nitrogen oxides and O₃. It further describes the results from measurements from a tower 300 m above the urban area of Seoul in late spring of 2015. The campaign demonstrates the performance of the CRDS instrument and provides new data on both photochemistry and nighttime chemistry in a major Asian megacity. The instrument provided accurate, high time resolution data for N₂O₅, NO, NO₂, NO_yand O₃, but suffered from large wall loss in the sampling of NO₃, illustrating the requirement for calibration of the NO₃inlet transmission. Both the photochemistry and nighttime chemistry of nitrogen oxides and O₃were rapid in this megacity. Sustained average rates of O₃buildup of 10 ppbv h⁻¹during recurring morning and early afternoon sea breezes led to a 50 ppbv average daily O₃rise. Nitrate radical production rates,P(NO₃), averaged 3–4 ppbv h⁻¹in late afternoon and early evening, much greater than contemporary data from Los Angeles, a comparable U. S. megacity. TheseP(NO₃) were much smaller than historical data from Los Angeles, however. Nighttime data at 300 m above ground showed considerable variability in high time resolution nitrogen oxide and O₃, likely resulting from sampling within gradients in the nighttime boundary layer structure. Apparent nighttime biogenic VOC oxidation rates of several ppbv h⁻¹were also likely influenced by vertical gradients. Finally, daytime N₂O₅mixing ratios of 3–35 pptv were associated with rapid daytimeP(NO₃) and agreed well with a photochemical steady state calculation.

Kinetic Study of the Gas-Phase Reactions of Nitrate Radicals with Methoxyphenol Compounds: Experimental and Theoretical Approaches

The gas-phase reactions of five methoxyphenols (three disubstituted and two trisubstituted) with nitrate radicals were studied in an 8000 L atmospheric simulation chamber at atmospheric pressure and 294 ± 2 K. The NO3 rate constants were investigated with the relative kinetic method using PTR-ToF-MS and GC-FID to measure the concentrations of the organic compounds. The rate constants (in units of cm(3) molecule(-1) s(-1)) determined were: 2-methoxyphenol (guaiacol; 2-MP), k(2-MP) = (2.69 ± 0.57 × 10(-11); 3-methoxyphenol (3-MP), k(3-MP) = (1.15 ± 0.21) × 10(-11); 4-methoxyphenol (4-MP), k(4-MP) = (13.75 ± 7.97) × 10(-11); 2-methoxy-4-methylphenol, k(2-M-4-MeP) = (8.41 ± 5.58) × 10(-11) and 2,6-dimethoxyphenol (syringol; 2,6-DMP), k(2,6-DMP) = (15.84 ± 8.10) × 10(-11). The NO3 rate constants of the studied methoxyphenols are compared with those of other substituted aromatics, and the differences in the reactivity are construed regarding the substituents (type, number and position) on the aromatic ring. This study was also supplemented by a theoretical approach of the methoxyphenol reactions with nitrate radicals. The upper limits of the NO3 overall rate constants calculated were in the same order of magnitude than those experimentally determined. Theoretical calculations of the minimum energies of the adducts formed from the reaction of NO3 radicals with the methoxyphenols were also performed using a DFT approach (M06-2X/6-31G(d,p)). The results indicate that the NO3 addition reactions on the aromatic ring of the methoxyphenols are exothermic, with energy values ranging between -13 and -21 kcal mol(-1), depending on the environment of the carbon on which the oxygen atom of NO3 is attached. These energy values allowed identifying the most suitable carbon sites for the NO3 addition on the aromatic ring of the methoxyphenols: at the exception of the 3-MP, the NO3 ipso-addition to the hydroxyl group is one of the favored sites for all the studies compounds.

Open-path cavity ring-down spectroscopy for trace gas measurements in ambient air

The present work used a near-infrared methane cavity ring-down spectroscopy (CRDS) sensor to examine performance and limitations of open-path CRDS for atmospheric measurements. A simple purge-enclosure was developed to maintain high mirror reflectivity and allowed >100 hours of operation with mirror reflectivity above 0.99996. We characterized effects of aerosols on ring-down decay signals and found the dominant effect to be fluctuations by large super-micron particles. Simple software filtering approaches were developed to combat these fluctuations allowing noise-equivalent sensitivity of ~6x10^-10 cm^-1HJ Hz^-1/2 within a factor of ~3 of closed-path systems (based on stability of the absorption baseline). Sensor measurements were validated against known methane concentrations in a closed-path configuration, while open-path validation was performed by side-by-side comparison with a commercial closed-path system.

Changes in the optical properties of benzo[a]pyrene-coated aerosols upon heterogeneous reactions with NO2 and NO3

Chemical reactions can alter the chemical, physical, and optical properties of aerosols. It has been postulated that nitration of aerosols can account for atmospheric absorbance over urban areas. To study this potentially important process, the change in optical properties of laboratory-generated benzo[a]pyrene (BaP)-coated aerosols following exposure to NO(2) and NO(3) was investigated at 355 nm and 532 nm by three aerosol analysis techniques. The extinction coefficient was determined at 355 nm and 532 nm from cavity ring-down aerosol spectroscopy (CRD-AS); the absorption coefficient was measured by photoacoustic spectroscopy (PAS) at 532 nm, while an on-line aerosol mass spectrometer (AMS) supplied real-time quantitative information about the chemical composition of aerosols. In this study, 240 nm polystyrene latex (PSL) spheres were thinly coated with BaP to form 300 or 310 nm aerosols that were exposed to high concentrations of NO(2) and NO(3) and measured with CRD-AS, PAS, and the AMS. The extinction efficiencies (Q(ext)) changed after exposure to NO(2) and NO(3) at both wavelengths. Prior to reaction, Q(ext) for the 355 nm and 532 nm wavelengths were 4.36 ± 0.04 and 2.39 ± 0.05, respectively, and Q(ext) increased to 5.26 ± 0.04 and 2.79 ± 0.05 after exposure. The absorption cross-section at 532 nm, determined with PAS, reached σ(abs) = (0.039 ± 0.001) × 10(-8) cm(2), indicating that absorption increased with formation of nitro-BaP, the main reaction product detected by the AMS. The single-scattering albedo (SSA), a measure of particle scattering efficiency, decreased from 1 to 0.85 ± 0.03, showing that changes in the optical properties of BaP-covered aerosols due to nitration may have implications for regional radiation budget and, hence, climate.