Comparison of three source apportionment methods based on observed and initial HCHO in Taiyuan, China

Identifying the sources of formaldehyde (HCHO) is key to reducing the pollution of HCHO and ozone (O3) on the ground level. Using the same datasets applied to the positive matrix factorization (PMF) model by (Hua et al., 2023), the initial concentrations of HCHO were estimated using the photochemical age and the sources of observed and initial HCHO were apportioned based on multiple linear regression (MLR) and photochemical age-based parameterization (PCAP) methods. These results suggest that the source of the initial HCHO can better reflect its contribution. The secondary formation contributed to 49.3-69.1 % of initial HCHO at four sites in Taiyuan based on MLR, which was higher (7.4-36.2 %) than the contributions of secondary formation from observed HCHO. The HCHO was mainly affected by anthropogenic secondary (10.8-34.4 %) and background sources (17.4-78.7 %) based on the PCAP method. We compared the results of the HCHO sources from the MLR, PCAP, and PMF models under photochemical loss. There was good agreement among the emission ratios of acetylene-based HCHO obtained by the different methods at the four sites. The correlation analysis of different source apportionment methods illustrated that primary emissions from the PCAP and the MLR model had the greatest correlation (0.22-0.60). Secondary formations from the PMF and MLR models showed good correlations at all four sites, with R values ranging from 0.42 to 0.83. The HCHO peak of diurnal variation simulated by MLR appeared late compared to the other methods, and the difference in daily variation of HCHO from the PMF model was significantly higher than that of PCAP and MLR. The overlapping conclusions of different source apportionment methods should be considered and used to guide efforts to improve air quality.

Source apportionment and ozone formation mechanism of VOCs considering photochemical loss in Guangzhou, China

Understanding the sources and impact of volatile organic compounds (VOCs) on ozone formation is challenging when the traditional method does not account for their photochemical loss. In this study, online monitoring of 56 VOCs was carried out in summer and autumn during high ozone pollution episodes. The photochemical age method was used to evaluate the atmospheric chemical loss of VOCs and to analyze the effects on characteristics, sources, and ozone formation of VOC components. The initial concentrations during daytime were 5.12 ppbv and 4.49 ppbv higher than the observed concentrations in the summer and autumn, respectively. The positive matrix factorization (PMF) model identified 5 major emission sources. However, the omission of the chemical loss of VOCs led to underestimating the contributions of sources associated with highly reactive VOC components, such as those produced by biogenic emissions and solvent usage. Conversely it resulted in overestimating the contributions from VOC components with lower chemical activity such as liquefied petroleum gas (LPG) usage, vehicle emissions, and gasoline evaporation. Furthermore, the estimation of ozone formation may be underestimated when the atmospheric photochemical loss is not taken into account. The ozone formation potential (OFP) method and propylene-equivalent concentration method both underestimated ozone formation by 53.24 ppbv and 47.25 ppbc, respectively, in the summer, and by 40.34 ppbv and 26.37 ppbc, respectively, in the autumn. The determination of the ozone formation regime based on VOC chemical loss was more acceptable. In the summer, the ozone formation regime changed from the VOC-limited regime to the VOC-NOx transition regime, while in the autumn, the ozone formation regime changed from the strong VOC-limited regime to the weak VOC-limited regime. To obtain more thorough and precise conclusions, further monitoring and analysis studies will be conducted in the near future on a wider variety of VOC species such as oxygenated VOCs (OVOCs).

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.

Spatial characterization of HCHO and reapportionment of its secondary sources considering photochemical loss in Taiyuan, China

Formaldehyde (HCHO) plays an important role in atmospheric ozone (O₃) formation. To accurately identify the sources of HCHO, carbonyls and volatile organic compounds (VOCs) were measured at three urban sites (Taoyuan, TY-U; Jinyuan, JY-U; Xiaodian, XD-U) and a suburban site (Shanglan, SL-B) in Taiyuan during a high O₃ period (from July 20 to August 3, 2020). The average mixing ratio of HCHO at XD-U (8.1 ± 2.8 ppbv) was comparable to those at TY-U (7.4 ± 2.1 ppbv) and JY-U (7.0 ± 2.3 ppbv) but higher (p < 0.01) than that at SL-B (4.9 ± 2.3 ppbv). HCHO contributed to 54.3-59.9 % of the total ozone formation potentials (OFPs) of non-methane hydrocarbons (NMHCs) at four sites. The diurnal variation of HCHO concentrations reached a peak value at 12:00-15:00, which may be attributed to the strong photochemical reaction. To obtain more accurate source results of HCHO under the condition of photochemical loss, the initial concentrations of NMHCs were estimated based on photochemical age parameterization and incorporated into the positive matrix factorization (PMF) model (termed IC-PMF). According to the IC-PMF results, secondary formation (SF) contributed the most to HCHO at XD-U (35.6 %) and SL-B (25.1 %), whereas solvent usage (SU) (40.9 %) and coking sources (CS) (36.0 %) were the major sources at TY-U and JY-U, respectively. Compared to the IC-PMF, the conventional PMF analysis based on the observed data underestimated the contributions of SU (100.5-154.2 %) and biogenic sources (BS) (28.5-324.7 %). Further reapportionment of secondary HCHO by multiple linear regression indicated that SU dominated the sources of HCHO at SL-B (28.3 %) and TY-U (41.7 %), while industrial emissions (IE) and CS contributed the most to XD-U (26.6 %) and JY-U (43.0 %) in Taiyuan from north to south, respectively.

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.

Ozone and SOA formation potential based on photochemical loss of VOCs during the Beijing summer

Volatile organic compounds (VOCs) are easily degraded by oxidants during atmospheric transport. Therefore, the contribution of VOCs to ozone (O₃) and secondary organic aerosol (SOA) formation at a receptor site is different from that in a source area. In this study, hourly concentrations of VOCs and other pollutants, such as O₃, NOx, HONO, CO, and PM_2.5, were measured in the suburbs (Daxing district) of Beijing in August 2019. The photochemical initial concentrations (PICs), in which the photochemical losses of VOCs were accounted for, were calculated to evaluate the contribution of the VOCs to O₃ and SOA formation. The mean (±standard deviation) measured VOC concentrations and the PICs were 11.2 ± 5.7 and 14.6 ± 8.4 ppbv, respectively, which correspond to O₃ formation potentials (OFP) of 57.8 ± 26.3 and 103.9 ± 109.4 ppbv and SOA formation potentials (SOAP) of 8.4 ± 4.1 and 10.3 ± 7.4 μg m^-3, respectively. Alkenes contributed 80.5% of the consumed VOCs, followed by aromatics (13.3%) and alkanes (6.2%). The contributions of the alkenes and aromatics to the OFP_PICs were 56.8% and 30.3%, respectively; while their corresponding contributions to the SOAP_PICs were 1.9% and 97.3%, respectively. The OFP_PICs was linearly correlated with the observed O₃ concentrations (OFP_PICs = 41.5 + 1.40 × c_O3, R² = 0.87). The O₃ formation was associated with a VOC-limited regime at the receptor site based on the measured VOCs and changed to a transition regime and a NOx sensitive regime based on the PIC. Our results suggest that more attention should be paid to biogenic VOCs when studying O₃ formation in summer in Beijing, while the control of anthropogenic aromatic compounds should be given priority in terms of SOA formation.

Secondary Organic Aerosol Formation Potential from Ambient Air in Beijing: Effects of Atmospheric Oxidation Capacity at Different Pollution Levels

Secondary organic aerosol (SOA) plays a critical role in sustained haze pollution in megacities. Traditional observation of atmospheric aerosols usually analyzes the ambient organic aerosol (OA) but neglects the SOA formation potential (SOAFP) of precursors remaining in ambient air. Knowledge on SOAFP is still limited, especially in megacities suffering from frequent haze. In this study, the SOAFP of ambient air in urban Beijing was characterized at different pollution levels based on a two-year field observation using an oxidation flow reactor (OFR) system. Both OA and SOAFP increased as a function of ambient pollution level, in which increasing concentrations of precursor volatile organic compounds (VOCs) and decreasing atmospheric oxidation capacity were found to be the two main influencing factors. To address the role of the atmospheric oxidation capacity in SOAFP, a relative OA enhancement ratio (ER_OA = 1 + SOAFP/OA) and the elemental composition of the OA were investigated in this study. The results indicated that the atmospheric oxidation capacity was weakened and resulted in higher SOAFP on more polluted days. The relationship found between SOAFP and the atmospheric oxidation capacity could be helpful in understanding changes in SOA pollution with improving air quality in the megacities of developing countries.

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.

Constraints on Aerosol Nitrate Photolysis as a Potential Source of HONO and NO x

The concentration of nitrogen oxides (NO _x) plays a central role in controlling air quality. On a global scale, the primary sink of NO _x is oxidation to form HNO₃. Gas-phase HNO₃ photolyses slowly with a lifetime in the troposphere of 10 days or more. However, several recent studies examining HONO chemistry have proposed that particle-phase HNO₃ undergoes photolysis 10-300 times more rapidly than gas-phase HNO₃. We present here constraints on the rate of particle-phase HNO₃ photolysis based on observations of NO _x and HNO₃ collected over the Yellow Sea during the KORUS-AQ study in summer 2016. The fastest proposed photolysis rates are inconsistent with the observed NO _x to HNO₃ ratios. Negligible to moderate enhancements of the HNO₃ photolysis rate in particles, 1-30 times faster than in the gas phase, are most consistent with the observations. Small or moderate enhancement of particle-phase HNO₃ photolysis would not significantly affect the HNO₃ budget but could help explain observations of HONO and NO _x in highly aged air.

Comparative study of volatile organic compounds in ambient air using observed mixing ratios and initial mixing ratios taking chemical loss into account - A case study in a typical urban area in Beijing

Volatile organic compounds (VOCs) can react with atmospheric radicals while being transported after being emitted, resulting in substantial losses. Using only observed VOC mixing ratios to assess VOC pollution, is therefore problematic. The observed mixing ratios and initial mixing ratios taking chemical loss into consideration were performed using data for 90 VOCs in the atmosphere in a typical urban area in Beijing in winter 2013 to gain a more accurate view of VOC pollution. The VOC sources, ambient VOC mixing ratios and compositions, variability and influencing factors, contributions to near-ground-ozone and health risks posed were assessed. Source apportionment should be conducted using initial mixing ratios, but health risks should be assessed using observed mixing ratios. The daytime daily mean initial mixing ratio (72.62ppbv) was 7.72ppbv higher than the daytime daily mean observed mixing ratio (64.90ppbv). Alkenes contributed >70% of the consumed VOCs. The nighttime daily mean observed mixing ratio was 71.66ppbv, 6.76ppbv higher than the daytime mixing ratio. The observed mixing ratio for 66 VOCs was 40.31% higher in Beijing than New York. The OFPs of Ini-D (266.54ppbv) was underestimated 23.41% compared to the OFP of Obs-D (204.14ppbv), improving emission control of ethylene and propene would be an effective way of controlling O₃. Health risk assessments performed for 28 hazardous VOCs show that benzene, chloroform, 1,2-dichloroethane, and acetaldehyde pose carcinogenic risk and acrolein poses non-carcinogenic risks. Source apportionment results indicated that vehicle exhausts, solvent usage and industrial processes were the main VOC source during the study.

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.

Investigation of carbonyl compound sources at a rural site in the Yangtze River Delta region of China

Carbonyl compounds are important intermediates in atmospheric photochemistry, but their primary sources are still not understood well. In this work, carbonyls, hydrocarbons, and alkyl nitrates were continuously measured during November 2011 at a rural site in the Yangtze River Delta region of China. Mixing ratios of carbonyls and hydrocarbons showed large fluctuations during the entire measurement. The average level for total measured volatile organic compounds during the pollution episode from 25th to 27th November, 2011 was 91.6 ppb, about 7 times the value for the clean period of 7th-8th, November, 2011. To preliminarily identify toluene sources at this site, the emission ratio of toluene to benzene (T/B) during the pollution episode was determined based on photochemical ages derived from the relationship of alkyl nitrates to their parent alkanes. The calculated T/B was 5.8 ppb/ppb, significantly higher than the values of 0.2-1.7 ppb/ppb for vehicular exhaust and other combustion sources, indicating the dominant influence of industrial emissions on ambient toluene. The contributions of industrial sources to ambient carbonyls were then calculated using a multiple linear regression fit model that used toluene and alkyl nitrates as respective tracers for industrial emission and secondary production. During the pollution episode, 18.5%, 69.0%, and 52.9% of measured formaldehyde, acetaldehyde, and acetone were considered to be attributable to industrial emissions. The emission ratios relative to toluene for formaldehyde, acetaldehyde, and acetone were determined to be 0.10, 0.20 and 0.40 ppb/ppb, respectively. More research on industrial carbonyl emission characteristics is needed to understand carbonyl sources better.

Effects of Beijing Olympics control measures on reducing reactive hydrocarbon species

Stringent air-quality control measures were implemented for the 2008 Beijing Olympic Games. This large-scale manmade experiment provided an opportunity to evaluate the effectiveness of measures to reduce the reactivity of hydrocarbons (HCs) from emission sources, which is important for ground-level ozone abatement. Photochemical initial concentrations (PICs), i.e., the levels of HCs from sources before undergoing chemical reactions, were calculated from ambient measurements. PICs obtained using the ratio method for HCs and the sequential reaction model for alkyl nitrates were in good agreement. Propene, 1-butene, iso-butene, trans-2-butene, cis-2-butene, trans-2-pentene, and m,p-xylene were identified as key reactive species in terms of their photochemical consumptions and correspondent ozone formation potentials (OFPs). During the Olympics and Paralympics, the PICs of these seven species were reduced by 27-66%, contributing 20% to the reduction in total PICs and 60% to the reduction in total OFP compared with June levels. Source apportionments from the chemical mass balance model indicated that gasoline vehicle exhaust was the predominant contributor to the key reactive species (45-78%). Reductions of gasoline vehicle exhaust during the Olympics and Paralympics explained 53-77% and 59-68% of the reductions in PICs of the key reactive HCs and total OFP, respectively.

Response of an aerosol mass spectrometer to organonitrates and organosulfates and implications for atmospheric chemistry

Organonitrates (ON) are important products of gas-phase oxidation of volatile organic compounds in the troposphere; some models predict, and laboratory studies show, the formation of large, multifunctional ON with vapor pressures low enough to partition to the particle phase. Organosulfates (OS) have also been recently detected in secondary organic aerosol. Despite their potential importance, ON and OS remain a nearly unexplored aspect of atmospheric chemistry because few studies have quantified particulate ON or OS in ambient air. We report the response of a high-resolution time-of-flight aerosol mass spectrometer (AMS) to aerosol ON and OS standards and mixtures. We quantify the potentially substantial underestimation of organic aerosol O/C, commonly used as a metric for aging, and N/C. Most of the ON-nitrogen appears as NO(x)+ ions in the AMS, which are typically dominated by inorganic nitrate. Minor organonitrogen ions are observed although their identity and intensity vary between standards. We evaluate the potential for using NO(x)+ fragment ratios, organonitrogen ions, HNO(3)+ ions, the ammonium balance of the nominally inorganic ions, and comparison to ion-chromatography instruments to constrain the concentrations of ON for ambient datasets, and apply these techniques to a field study in Riverside, CA. OS manifests as separate organic and sulfate components in the AMS with minimal organosulfur fragments and little difference in fragmentation from inorganic sulfate. The low thermal stability of ON and OS likely causes similar detection difficulties for other aerosol mass spectrometers using vaporization and/or ionization techniques with similar or larger energy, which has likely led to an underappreciation of these species.

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.

Extensive regional atmospheric hydrocarbon pollution in the southwestern United States

Light alkane hydrocarbons are present in major quantities in the near-surface atmosphere of Texas, Oklahoma, and Kansas during both autumn and spring seasons. In spring 2002, maximum mixing ratios of ethane [34 parts per 109 by volume (ppbv)], propane (20 ppbv), and n-butane (13 ppbv) were observed in north-central Texas. The elevated alkane mixing ratios are attributed to emissions from the oil and natural gas industry. Measured alkyl nitrate mixing ratios were comparable to urban smog values, indicating active photochemistry in the presence of nitrogen oxides, and therefore with abundant formation of tropospheric ozone. We estimate that 4-6 teragrams of methane are released annually within the region and represents a significant fraction of the estimated total U.S. emissions. This result suggests that total U.S. natural gas emissions may have been underestimated. Annual ethane emissions from the study region are estimated to be 0.3-0.5 teragrams.