Improved continuous measurement system for atmospheric total peroxy and total organic nitrate under the high NOx condition

We improved the thermal dissociation cavity attenuated phase shift spectroscopy (TD-CAPS) instrument to measure atmospheric total peroxy nitrates (PNs) and organic nitrates (ONs) continuously under the condition of high NOx. In TD-CAPS, PNs and ONs are dissociated in heated quartz tubes to form NO2, and the NO2 concentration is measured by cavity attenuated phase shift spectroscopy (CAPS). The original TD-CAPS system overestimates PN and ON concentrations in the presence of high NO concentrations. Our laboratory experiments and numerical simulations showed that the main cause of the overestimation was NO oxidation to NO2 by peroxy radicals generated in the heated quartz tubes. In the improved system, NO was converted to NO2 by adding excess O3 after the quartz tubes so that CAPS detected NOx (NO and NO2) instead of NO2. The uncertainty of the improved system was less than 20% with ∼15 parts per billion by volume (ppbv) NO and ∼80 ppbv NO2. The estimated detection limit (3σ) was 0.018 ppbv with an integration time of 2 min in the presence of 64 ppbv NO2. The improved system was tested for measurement of PNs and ONs in an urban area, and the results indicated that interference from NO was successfully suppressed.

Source characterization of volatile organic compounds at Carlsbad Caverns National Park

Carlsbad Caverns National Park (CAVE), located in southeastern New Mexico, experiences elevated ground-level ozone (O3) exceeding the National Ambient Air Quality Standard (NAAQS) of 70 ppbv. It is situated adjacent to the Permian Basin, one of the largest oil and gas (O&G) producing regions in the US. In 2019, the Carlsbad Caverns Air Quality Study (CarCavAQS) was conducted to examine impacts of different sources on ozone precursors, including nitrogen oxides (NOx) and volatile organic compounds (VOCs). Here, we use positive matrix factorization (PMF) analysis of speciated VOCs to characterize VOC sources at CAVE during the study. Seven factors were identified. Three factors composed largely of alkanes and aromatics with different lifetimes were attributed to O&G development and production activities. VOCs in these factors were typical of those emitted by O&G operations. Associated residence time analyses (RTA) indicated their contributions increased in the park during periods of transport from the Permian Basin. These O&G factors were the largest contributor to VOC reactivity with hydroxyl radicals (62%). Two PMF factors were rich in photochemically generated secondary VOCs; one factor contained species with shorter atmospheric lifetimes and one with species with longer lifetimes. RTA of the secondary factors suggested impacts of O&G emissions from regions farther upwind, such as Eagle Ford Shale and Barnett Shale formations. The last two factors were attributed to alkenes likely emitted from vehicles or other combustion sources in the Permian Basin and regional background VOCs, respectively.Implications: Carlsbad Caverns National Park experiences ground-level ozone exceeding the National Ambient Air Quality Standard. Volatile organic compounds are critical precursors to ozone formation. Measurements in the Park identify oil and gas production and development activities as the major contributors to volatile organic compounds. Emissions from the adjacent Permian Basin contributed to increases in primary species that enhanced local ozone formation. Observations of photochemically generated compounds indicate that ozone was also transported from shale formations and basins farther upwind. Therefore, emission reductions of volatile organic compounds from oil and gas activities are important for mitigating elevated O3 in the region.

Wintertime haze and ozone at Dinosaur National Monument

Dinosaur National Monument (DINO) is located near the northeastern edge of the Uinta Basin and often experiences elevated levels of wintertime ground-level ozone. Previous studies have shown that high ozone mixing ratios in the Uinta Basin are driven by elevated levels of volatile organic compounds (VOCs) and nitrogen oxides (NO_x) from regional oil and gas development coupled with temperature inversions and enhanced photochemistry from persistent snow cover. Here, we show that persistent snow cover and temperature inversions, along with abundant ammonia, also lead to wintertime haze in this region. A study was conducted at DINO from November 2018 through May 2020 where ozone, speciated fine and coarse aerosols, inorganic gases, and VOCs were measured. Three National Ambient Air Quality Standards (NAAQS) ozone exceedances were observed in the first winter, and no exceedances were observed in the second winter. In contrast, elevated levels of particulate matter were observed both winters, with 24-h averaged particle light extinction exceeding 100 Mm^-1. These haze events were dominated by ammonium nitrate, and particulate organics were highly correlated with ammonium nitrate. Ammonium nitrate formation was limited by nitric acid in winter. As such, reductions in regional NO_x emissions should reduce haze levels and improve visibility at DINO in winter. Long-term measurements of particulate matter from nearby Vernal, Utah, suggest that visibility impairment is a persistent issue in the Uinta Basin in winter. From April through October 2019, relatively clean conditions occurred, with average particle extinction of ~10 Mm^-1. During this period, ammonium nitrate concentrations were lower by more than an order of magnitude, and contributions from coarse mass and soil to haze levels increased. VOC markers indicated that the high levels of observed pollutants in winter were likely from local sources related to oil and gas extraction activities.Implications: Elevated ground-level ozone and haze levels were observed at Dinosaur National Monument in winter. Haze episodes were dominated by ammonium nitrate, with 24-h averaged particle light extinction exceeding 100 Mm^-1, reducing visual range near the surface to ~35 km. Despite elevated ammonium nitrate concentrations, additional gas-phase ammonia was available, such that any increase in NO_x emissions in the region is likely to lead to even greater haze levels.

Long-term variations of C1-C5 alkyl nitrates and their sources in Hong Kong

Investigating the long-term trends of alkyl nitrates (RONO₂) is of great importance for evaluating the variations of photochemical pollution. Mixing ratios of C₁-C₅ RONO₂ were measured in autumn Hong Kong from 2002 to 2016, and the average level of 2-butyl nitrate (2-BuONO₂) always ranked first. The C₁-C₄ RONO₂ all showed increasing trends (p < 0.05), and 2-BuONO₂ had the largest increase rate. The enhancement in C₃ RONO₂ was partially related to elevated propane, and dramatic decreases (p < 0.05) in both nitrogen monoxide (NO) and nitrogen dioxide (NO₂) also led to the increased RONO₂ formation. In addition, an increase of hydroxyl (OH) and hydroperoxyl (HO₂) radicals (p < 0.05) suggested enhanced atmospheric oxidative capacity, further resulting in the increases of RONO₂. Source apportionment of C₁-C₄ RONO₂ specified three typical sources of RONO₂, including biomass burning emission, oceanic emission, and secondary formation, of which secondary formation was the largest contributor to ambient RONO₂ levels. Mixing ratios of total RONO₂ from each source were quantified and their temporal variations were investigated. Elevated RONO₂ from secondary formation and biomass burning emission were two likely causes of increased ambient RONO₂. By looking into the spatial distributions of C₁-C₅ RONO₂, regional transport from the Pearl River Delta (PRD) was inferred to build up RONO₂ levels in Hong Kong, especially in the northwestern part. In addition, more serious RONO₂ pollution was found in western PRD region. This study helps build a comprehensive understanding of RONO₂ pollution in Hong Kong and even the entire PRD.

Observations of C1-C5 alkyl nitrates in the Yellow River Delta, northern China: Effects of biomass burning and oil field emissions

Alkyl nitrates (RONO₂) are important reservoirs of nitrogen oxides and play key roles in the tropospheric chemistry. Two phases of intensive campaigns were conducted during February-April and June-July of 2017 at a rural coastal site and in open oil fields of the Yellow River Delta region, northern China. C₁-C₅ alkyl nitrates showed higher concentration levels in summer than in winter-spring (p < 0.01), whilst their parent hydrocarbons showed an opposite seasonal variation pattern. The C₃-C₅ RONO₂ levels in the oil fields were significantly higher than those in the ambient rural air. Alkyl nitrates showed well-defined diurnal variations, elucidating the effects of in-situ photochemical production and regional transport of aged polluted plumes. Backward trajectory analysis and fire maps revealed the significant contribution of biomass burning to the observed alkyl nitrates and hydrocarbons. A simplified sequential reaction model and an observation-based chemical box model were deployed to diagnose the formation mechanisms of C₁-C₅ RONO₂. The C₃-C₅ RONO₂ were mainly produced from the photochemical oxidation of their parent hydrocarbons (i.e., C₃-C₅ alkanes), whilst C₁-C₂ RONO₂ compounds have additional sources. In addition to parent hydrocarbons, longer alkanes with >4 carbon atoms were also important precursors of alkyl nitrates in the oil fields. This study demonstrates the significant effects of oil field emissions and biomass burning on the volatile organic compounds and alkyl nitrate formation, and provides scientific support for the formulation of control strategies against photochemical air pollution in the Yellow River Delta region.

Determination of Semivolatile Organic Nitrates in Ambient Atmosphere by Gas Chromatography/Electron Ionization–Mass Spectrometry

Semivolatile organic nitrates (SVONs) contribute a large proportion of total organic nitrates and play an important role in the tropospheric chemistry. However, the composition and concentrations of SVONs in the atmosphere remain unclear due to the lack of reliable analytical techniques for specific organic nitrates. In this study, a method based on gas chromatography and electron ionization–mass spectrometry was developed to detect ambient SVONs that were collected via polyurethane foam disk enrichment. Three SVONs were identified in the semivolatile samples from urban Jinan during spring based on the characteristic fragment ions of [NO2]+ and [CH2NO3]+ and the characteristic fragment loss of NO2 and NO3: 1-pentyl nitrate (molecular weight [MW] = 133), 4-hydroxy-isoprene nitrate (MW = 147), and (3,4)-di-hydroxy-isoprene nitrate (MW = 163). The latter two isoprene nitrates were rarely detected in the real atmosphere in previous studies. The contents of 1-pentyl nitrate, 4-hydroxy-isoprene nitrate, and (3,4)-di-hydroxy-isoprene nitrate were roughly quantified based on the standard of 1-pentyl nitrate, with a detection limit of 50 μg L−1. In addition, Fourier transform infrared spectrometry was used to determine the total SVONs content. The average concentrations of 1-pentyl nitrate, 4-hydroxy-isoprene nitrate, (3,4)-di-hydroxy-isoprene nitrate, and total SVONs in Jinan during spring were 20.2 ± 7.2, 13.2 ± 7.2, 36.5 ± 8.4, and 380.0 ± 190.8 ng m−3, respectively. The three identified SVONs contributed only 20.2 ± 5.5% to the total SVONs, which suggests that some unidentified SVONs are present in the ambient atmosphere and that studies with improved or advanced analytical techniques will be required to identify them.

Photochemical Formation of C1-C5 Alkyl Nitrates in Suburban Hong Kong and over the South China Sea

Alkyl nitrates (RONO₂) are important reservoirs of atmospheric nitrogen, regulating nitrogen cycling and ozone (O₃) formation. In this study, we found that propane and n-butane were significantly lower at the offshore site (WSI) in Hong Kong ( p < 0.05), whereas C₃-C₄ RONO₂ were comparable to the suburban site (TC) ( p > 0.05). Stronger oxidative capacity at WSI led to more efficient RONO₂ formation. Relative incremental reactivity (RIR) was for the first time used to evaluate RONO₂-precursor relationships. In contrast to a consistently volatile organic compounds (VOC)-limited regime at TC, RONO₂ formation at WSI switched from VOC-limited regime during O₃ episodes to VOC and nitrogen oxides (NO _x) colimited regime during nonepisodes. Furthermore, unlike the predominant contributions of parent hydrocarbons to C₄-C₅ RONO₂, the production of C₁-C₃ RONO₂ was more sensitive to other VOCs like aromatics and carbonyls, which accounted for ∼40-90% of the productions of C₁-C₃ alkylperoxy (RO₂) and alkoxy radicals (RO) at both sites. This resulted from the decomposition of larger RO₂/RO and the change of OH abundance under the photochemistry of other VOCs. This study advanced our understanding of the photochemical formation of RONO₂, particularly the relationships between RONO₂ and their precursors, which were not confined to the parent hydrocarbons.

A quantitative assessment of distributions and sources of tropospheric halocarbons measured in Singapore

This work reports the first ground-based atmospheric measurements of 26 halocarbons in Singapore, an urban-industrial city-state in Southeast (SE) Asia. A total of 166 whole air canister samples collected during two intensive 7 Southeast Asian Studies (7SEAS) campaigns (August-October 2011 and 2012) were analyzed for C₁-C₂ halocarbons using gas chromatography-electron capture/mass spectrometric detection. The halocarbon dataset was supplemented with measurements of selected non-methane hydrocarbons (NMHCs), C₁-C₅ alkyl nitrates, sulfur gases and carbon monoxide to better understand sources and atmospheric processes. The median observed atmospheric mixing ratios of CFCs, halons, CCl₄ and CH₃CCl₃ were close to global tropospheric background levels, with enhancements in the 1-17% range. This provided the first measurement evidence from SE Asia of the effectiveness of Montreal Protocol and related national-scale regulations instituted in the 1990s to phase-out ozone depleting substances (ODS). First- and second-generation CFC replacements (HCFCs and HFCs) dominated the atmospheric halocarbon burden with HFC-134a, HCFC-22 and HCFC-141b exhibiting enhancements of 39-67%. By combining near-source measurements in Indonesia with receptor data in Singapore, regionally transported peat-forest burning smoke was found to impact levels of several NMHCs (ethane, ethyne, benzene, and propane) and short-lived halocarbons (CH₃I, CH₃Cl, and CH₃Br) in a subset of the receptor samples. The strong signatures of these species near peat-forest fires were potentially affected by atmospheric dilution/mixing during transport and by mixing with substantial urban/regional backgrounds at the receptor. Quantitative source apportionment was carried out using positive matrix factorization (PMF), which identified industrial emissions related to refrigeration, foam blowing, and solvent use in chemical, pharmaceutical and electronics industries as the major source of halocarbons (34%) in Singapore. This was followed by marine and terrestrial biogenic activity (28%), residual levels of ODS from pre-Montreal Protocol operations (16%), seasonal incidences of peat-forest smoke (13%), and fumigation related to quarantine and pre-shipment (QPS) applications (7%).

Seasonal variations of C1-C4 alkyl nitrates at a coastal site in Hong Kong: Influence of photochemical formation and oceanic emissions

Five C₁-C₄ alkyl nitrates (RONO₂) were measured at a coastal site in Hong Kong in four selected months of 2011 and 2012. The total mixing ratios of C₁-C₄ RONO₂ (Σ₅RONO₂) ranged from 15.4 to 143.7 pptv with an average of 65.9 ± 33.0 pptv. C₃-C₄ RONO₂ (2-butyl nitrate and 2-propyl nitrate) were the most abundant RONO₂ during the entire sampling period. The mixing ratios of C₃-C₄ RONO₂ were higher in winter than those in summer, while the ones of methyl nitrate (MeONO₂) were higher in summer than those in winter. Source analysis suggests that C₂-C₄ RONO₂ were mainly derived from photochemical formation along with biomass burning (58.3-71.6%), while ocean was a major contributor to MeONO₂ (53.8%) during the whole sampling period. The photochemical evolution of C₂-C₄ RONO₂ was investigated, and found to be dominantly produced by the parent hydrocarbon oxidation. The notable enrichment of MeONO₂ over C₃-C₄ RONO₂ was observed in a summer episode when the air masses originating from the South China Sea (SCS) and MeONO₂ was dominantly derived from oceanic emissions. In order to improve the accuracy of ozone (O₃) prediction in coastal environment, the relative contribution of RONO₂ from oceanic emissions versus photochemical formation and their coupling effects on O₃ production should be taken into account in future studies.

Air toxic emissions from snowmobiles in Yellowstone National Park

A study on emissions associated with oversnow travel in Yellowstone National Park (YNP) was conducted for the time period of February 13-16, 2002 and February 12-16, 2003. Whole air and exhaust samples were characterized for 85 volatile organic compounds using gas chromatography. The toxics including benzene, toluene, ethylbenzene, xylenes (p-, m-, and o-xylene), and n-hexane, which are major components of two-stroke engine exhaust, show large enhancements during sampling periods resulting from increased snowmobile traffic. Evaluation of the photochemical history of air masses sampled in YNP revealed that emissions of these air toxics were (i) recent, (ii) persistent throughout the region, and (iii) consistent with the two-stroke engine exhaust sample fingerprints. The annual fluxes were estimated to be 0.35, 1.12, 0.24, 1.45, and 0.36 Gg yr(-1) for benzene, toluene, ethylbenzene, xylenes, and n-hexane, respectively, from snowmobile usage in YNP. These results are comparable to the flux estimates of 0.23, 0.77, 0.17, and 0.70 Gg yr(-1) for benzene, toluene, ethylbenzene, and xylenes, respectively, that were derived on the basis of (i) actual snowmobile counts in the Park and (ii) our ambient measurements conducted in 2003. Extrapolating these results, annual emissions from snowmobiles in the U.S. appear to be significantly higher than the values from the EPA National Emissions Inventory (1999). Snowmobile emissions represent a significant fraction ( approximately 14-21%) of air toxics with respect to EPA estimates of emissions by nonroad vehicles. Further investigation is warranted to more rigorously quantify the difference between our estimates and emission inventories.

Blowing smoke in Yellowstone: air quality impacts of oversnow motorized recreation in the park

Snowmobile use in Yellowstone National Park has been shown to impact air quality, with implications for the safety and welfare of Park staff and other Park resource values. Localized impacts have been documented at several high-use sites in the Park, but the broader spatial variability of snowmobile emissions and air quality was not understood. Measurements of 87 volatile organic compounds (VOCs) were made for ambient air sampled across the Park and West Yellowstone, Montana, during 2 days of the 2002-2003 winter use season, 1 year before the implementation of a new snowmobile policy. The data were compared with similar data from pristine West Coast sites at similar latitudes. Backward trajectories of local air masses, alkyl nitrate-parent alkane ratios, and atmospheric soundings were used to identify the VOC sources and assess their impact. Different oversnow vehicle types used in the Park were sampled to determine their relative influence on air mass pollutant composition. VOCs were of local origin and demonstrated strong spatiotemporal variability that is primarily influenced by levels of snowmobile traffic on given road segments at different times of day. High levels of snowmobile traffic in and around West Yellowstone produced consistently high levels of benzene, toluene, and carbon monoxide.