Emission characteristics of carbonyl compounds from open burning of typical subtropical biomass in South China

Open biomass burning (OBB) is one of the largest primary emission sources for atmospheric carbonyl compounds, key precursors for ozone and secondary organic aerosol pollution. To clarify the carbonyl emissions, the comprehensive characteristics of C1-C10 carbonyl compounds from open burning of seven typical subtropical biomass in China were investigated in this study, which included subtropical plants and agricultural residues. Total 27 carbonyl compounds were detected. The total EFs were 2824 mg kg-1 with 95% confidence interval (CI) [2418, 3322] for burning subtropical plants and 4080 mg kg-1 with 95% CI [3446, 4724] for burning agriculture residues, respectively. The EFs were 2-3 orders of magnitude larger than previous values in China. Aliphatic aldehydes were the largest group of carbonyl groups, with acetaldehyde, as the most abundant carbonyl species (about 30% contribution). Formaldehyde, acetone, acrolein, glyoxal, methylglyoxal, butanone, isovaleraldehyde, and m-tolualdehyde were also found to be abundant and varying with the types of biomass burnt. Formaldehyde emission ratios to acetonitrile and CO were lower than those in previous studies both for burning plants and agricultural residues. There were significant variabilities in the emission ratios and factors among different types of OBBs. Strong positive correlations were found between carbonyl emissions and CO emissions and water content in biomass; furthermore, total carbonyl concentrations measured in the flaming stage were higher than those in the smoldering one. This study provides important fundamental measurement data on carbonyl emissions from burning typical subtropical plants and agricultural residues, which will help improve the quality of emission inventories and better understand the potential impacts of OBB on regional air quality in southern China.

Origin and transformation of volatile organic compounds at a regional background site in Hong Kong: Varied photochemical processes from different source regions

Volatile organic compounds (VOCs) are important gaseous constituents in the troposphere, impacting local and regional air quality, human health, and climate. Oxidation of VOCs, with the participation of nitrogen oxides (NOx), leads to the formation of tropospheric ozone (O3). Accurately apportioning the emission sources and transformation processes of ambient VOCs, and effectively estimation of OH reactivity and ozone formation potential (OFP) will play an important role in reducing O3 pollution in the atmosphere and improving public health. In this study, field measurements were conducted at a regional background site (Hok Tsui; HT) in Hong Kong from October to November 2020 with proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS). VOC data coupled with air mass back trajectory cluster analysis and receptor modelling were applied to reveal the pollution pattern, emission sources and transformation of ambient VOCs at HT in autumn 2020. Seven sources were identified by positive matrix factorization (PMF) analysis, namely vehicular + industrial, solvent usage, primary oxygenated VOCs (OVOCs), secondary OVOCs 1, secondary OVOCs 2 (aged), biogenic emissions, and background + biomass burning. Secondary formation and vehicular + industrial emissions are the vital sources of ambient VOCs at HT supersite, contributing to 20.8 % and 46.7 % of total VOC mixing ratios, respectively. Integrated with backward trajectory analysis and correlations of VOCs with their oxidation products, short-range transport of air masses from inland regions of southeast China brought high levels of total VOCs but longer-range transport of air masses brought more secondary OVOCs in aged air masses. Photolysis of OVOCs was the most important contributor to OH reactivity and OFP, among which aldehyde was the dominant contributor. The results of this study highlight the photochemical processing of VOCs from different source regions which should be considered in strategy making for pollution reduction.

Source apportionment of gaseous Nitrophenols and their contribution to HONO formation in an urban area

Nitrophenols (NPs) have significant impacts on human health, climate, and atmospheric chemistry. Despite numerous measurements of particulate NPs, still little is known about their gaseous atmospheric abundances, sources, and fate. Here, four gaseous NPs [2,4-dinitrophenol (2,4-DNP), 4-nitrophenol (4-NP), 2-nitrophenol (2-NP), and 2-Methyl-4-nitrophenol (2-Me-4-NP)] were continuously monitored during late Spring at an urban site in Houston, Texas. Among the four NPs, 4-NP showed the highest abundance, followed by 2-Me-4-NP, 2-NP, and 2,4-DNP with average concentrations of 1.07 ± 0.19 ppt, 0.47 ± 0.12 ppt, 0.41 ± 0.16 ppt, and 0.27 ± 0.09 ppt, respectively. The positive matrix factorization (PMF) model identified seven sources: industrial NPs, secondary formation, phenol sources, acetonitrile source, natural gas/crude oil, traffic, and petrochemical industries/oil refineries. A zero-dimensional photochemical box model was used to simulate the observed 2-NP and 2,4-DNP. A 50.0% and 70.0% jNO2 was found to be consistent with the measured 2-NP and 2,4-DNP. This yields a nitrous acid (HONO) production of 7.5 ± 2.5 ppt/h from 06:00 to 18:00 Central Standard Time (CST) from both NPs. An extrapolation including other known NPs suggests a maximum HONO formation of 13.8 ppt/h. The results of this study suggest that using PMF analysis supplemented by photochemical box model provides identification of the NPs sources and their atmospheric implication to HONO formation.

Direct detection of acetonitrile at the pptv level with photoinduced associative ionization time-of-flight mass spectrometry

Photoionization mass spectrometry (PI-MS) has become a versatile tool in the real-time analysis of volatile organic compounds (VOCs) from the atmosphere or exhaled breath. However, some key species, e.g., acetonitrile, are hard to measure due to their higher ionization energies than photon energy. In this study, the direct and sensitive detection of gaseous acetonitrile based on a photoinduced associative ionization (PAI) reaction was investigated with a laboratory-built PAI time-of-flight mass spectrometer (PAI-TOFMS). By doping CH₂Cl₂ in the photoionization ion source, the mass signal of acetonitrile that cannot be effectively obtained by photoionization appeared with an extremely high intensity through the PAI reaction between acetonitrile, CH₂Cl₂, and residual H₂O in the system. Though the moisture in the sample gas has an evident impact on the detection efficiency of acetonitrile, with a relative signal intensity decreasing from 100% under dry conditions to 60% at saturated relative humidity, excellent detection sensitivity was still obtained for gaseous acetonitrile in different matrixes. The sensitivity calibration experiment showed that the detection sensitivities of acetonitrile in N₂ buffer gas, exhaled gas, and outdoor air were 682.4 ± 5.2, 17.0 ± 0.7, and 23.9 ± 0.2 counts pptv^-1, respectively, with an analysis time of 10 s. The corresponding 3σ LODs reached 0.22, 8.82, and 6.28 pptv, which are equivalent to 0.40, 16.0, and 11.4 ng m^-3. The performance of the PAI-TOFMS was first demonstrated by analyzing exhaled acetonitrile from healthy non-smokers and smokers and continuous monitoring of acetonitrile in outdoor air. In summary, this study provides a new and highly sensitive method for the real-time detection of acetonitrile through mass spectrometry.

Atmospheric concentrations and sources of black carbon over tropical Australian waters

Black carbon (BC) aerosols significantly contribute to radiative budgets globally, however their actual contributions remain poorly constrained in many under-sampled ocean regions. The tropical waters north of Australia are a part of the Indo-Pacific warm pool, regarded as a heat engine of global climate, and are in proximity to large terrestrial sources of BC aerosols such as fossil fuel emissions, and biomass burning emissions from northern Australia. Despite this, measurements of marine aerosols, especially BC remain elusive, leading to large uncertainties and discrepancies in current chemistry-climate models for this region. Here, we report the first comprehensive measurements of aerosol properties collected over the tropical warm pool in Australian waters during a voyage in late 2019. The non-marine related aerosol emissions observed in the Arafura Sea region were more intense than in the Timor Sea marine region, as the Arafura Sea was subject to greater continental outflows. The median equivalent BC (eBC) concentration in the Arafura Sea (0.66 μg m^-3) was slightly higher than that in the Timor Sea (0.49 μg m^-3). Source apportionment modelling and back trajectory analysis and tracer studies consistently suggest fossil fuel combustion eBC (eBC_ff) was the dominant contributor to eBC across the entire voyage region, with biomass burning eBC (eBC_bb) making significant additional contributions to eBC in the Arafura Sea. eBC_ff (possibly from ship emissions or oil and gas rigs and their associated activities) and cloud condensation nuclei (CCN) were robustly correlated in the Timor Sea data, whereas eBC_bb positively correlated to CCN in the Arafura Sea, suggesting different sources and atmospheric processing pathways occurred in these two regions. This work demonstrates the substantial impact that fossil fuel and biomass burning emissions can have on the composition of aerosols and cloud processes in the remote tropical marine atmosphere, and their potentially significant contribution to the radiative balance of the rapidly warming Indo-Pacific warm pool.

An Experimental and Master Equation Investigation of Kinetics of the CH2OO + RCN Reactions (R = H, CH3, C2H5) and Their Atmospheric Relevance

We have performed direct kinetic measurements of the CH₂OO + RCN reactions (R = H, CH₃, C₂H₅) in the temperature range 233-360 K and pressure range 10-250 Torr using time-resolved UV-absorption spectroscopy. We have utilized a new photolytic precursor, chloroiodomethane (CH₂ICl), whose photolysis at 193 nm in the presence of O₂ produces CH₂OO. Observed bimolecular rate coefficients for CH₂OO + HCN, CH₂OO + CH₃CN, and CH₂OO + C₂H₅CN reactions at 296 K are (2.22 ± 0.65) × 10^-14 cm³ molecule^-1 s^-1, (1.02 ± 0.10) × 10^-14 cm³ molecule^-1 s^-1, and (2.55 ± 0.13) × 10^-14 cm³ molecule^-1 s^-1, respectively, suggesting that reaction with CH₂OO is a potential atmospheric degradation pathway for nitriles. All the reactions have negligible temperature and pressure dependence in the studied regions. Quantum chemical calculations (ωB97X-D/aug-cc-pVTZ optimization with CCSD(T)-F12a/VDZ-F12 electronic energy correction) of the CH₂OO + RCN reactions indicate that the barrierless lowest-energy reaction path leads to a ring closure, resulting in the formation of a 1,2,4-dioxazole compound. Master equation modeling results suggest that following the ring closure, chemical activation in the case of CH₂OO + HCN and CH₂OO + CH₃CN reactions leads to a rapid decomposition of 1,2,4-dioxazole into a CH₂O + RNCO pair, or by a rearrangement, into a formyl amide (RC(O)NHC(O)H), followed by decomposition into CO and an imidic acid (RC(NH)OH). The 1,2,4-dioxazole, the CH₂O + RNCO pair, and the CO + RC(NH)OH pair are atmospherically significant end products to varying degrees.

Investigation into the differences and relationships between gasSOA and aqSOA in winter haze pollution on Chongming Island, Shanghai, based on VOCs observation

To investigate the formation of secondary organic aerosol (SOA) under current atmospheric conditions, we conducted a field observation of SOA precursors in the downwind region of the Yangtze River Delta (YRD) in winter 2019 using a variety of offline and online instruments. During the entire observation period, the averaged fine particulate SOA was 7.9 ± 2.3 μg m^-3, with precursor concentrations of 31 ± 11 ppbv for the measured volatile organic compounds (VOCs) and 16 ± 12 ppbv for NOx. Compared to those on the clean days, SOA on the haze days increased by a factor of 1.6, while the VOC and NOx increased by a factor of 1.3 and 2.0, respectively. Aerosol liquid water content (ALWC) and oxygenated VOCs (OVOCs, including acetaldehyde, formic acid, acetone, acetic acid, methyl ethyl ketone, and methylglyoxal) relationships suggested that the gasSOA and aqSOA occurred simultaneously on Chongming Island in winter. The gasSOA was primarily formed by the oxidation of aromatics and NOx at low RH (RH < 80%) conditions. In contrast, the aqSOA was formed under higher RH (RH > 80%) conditions via a combination of daytime photochemical aqueous phase processes of water-soluble OVOCs and nocturnal dark aqueous phase processes of primary emissions from biomass. The inversed higher mass ratio of NACs to (benzene + toluene) and nitrogen oxidation ratio (NOR) in the daytime during the gasSOA-dominated haze periods indicated that gasSOA could be transformed to aqSOA at high NOx levels. Our results also suggested the importance of NOx and VOC reduction measures in directly mitigating gasSOA and indirectly mitigating aqSOA during winter haze pollution.

Traffic, transport, and vegetation drive VOC concentrations in a major urban area in Texas

The population of Texas has increased rapidly in the past decade. The San Antonio Field Study (SAFS) was designed to investigate ozone (O₃) production and precursors in this rapidly changing, sprawling metropolitan area. There are still many questions regarding the sources and chemistry of volatile organic compounds (VOCs) in urban areas like San Antonio which are affected by a complex mixture of industry, traffic, biogenic sources and transported pollutants. The goal of the SAFS campaign in May 2017 was to measure inorganic trace gases, VOCs, methane (CH₄), and ethane (C₂H₆). The SAFS field design included two sites to better assess air quality across the metro area: an urban site (Traveler's World; TW) and a downwind/suburban site (University of Texas at San Antonio; UTSA). The results indicated that acetone (2.52 ± 1.17 and 2.39 ± 1.27 ppbv), acetaldehyde (1.45 ± 1.02 and 0.93 ± 0.45 ppbv) and isoprene (0.64 ± 0.49 and 1.21 ± 0.85 ppbv; TW and UTSA, respectively) were the VOCs with the highest concentrations. Additionally, positive matrix factorization showed three dominant factors of VOC emissions: biogenic, aged urban mixed source, and acetone. Methyl vinyl ketone and methacrolein (MVK + MACR) exhibited contributions from both secondary photooxidation of isoprene and direct emissions from traffic. The C₂H₆:CH₄ demonstrated potential influence of oil and gas activities in San Antonio. Moreover, the high O₃ days during the campaign were in the NO_x-limited O₃ formation regime and were preceded by evening peaks in select VOCs, NO_x and CO. Overall, quantification of the concentration and trends of VOCs and trace gases in a major city in Texas offers vital information for general air quality management and supports strategies for reducing O₃ pollution. The SAFS campaign VOC results will also add to the growing body of literature on urban sources and concentrations of VOCs in major urban areas.

Kinetic Study of the Gas-Phase O(1D) + CH3OH and O(1D) + CH3CN Reactions: Low-Temperature Rate Constants and Atomic Hydrogen Product Yields

Atomic oxygen in its first excited singlet state, O(¹D), is an important species in the photochemistry of several planetary atmospheres and has been predicted to be a potentially important reactive species on interstellar ices. Here, we report the results of a kinetic study of the reactions of O(¹D) with methanol, CH₃OH, and acetonitrile, CH₃CN, over the 50-296 K temperature range. A continuous supersonic flow reactor is used to attain these low temperatures coupled with pulsed laser photolysis and pulsed laser-induced fluorescence to generate and monitor O(¹D) atoms, respectively. Secondary experiments examining the atomic hydrogen product channels of these reactions are also performed, through laser-induced fluorescence measurements of H(²S) atom formation. On the kinetic side, the rate constants for these reactions are seen to be large (>2 × 10^-10 cm³ s^-1) and consistent with barrierless reactions, although they display contrasting dependences as a function of temperature. On the product formation side, both reactions are seen to yield non-negligible quantities of atomic hydrogen. For the O(¹D) + CH₃OH reaction, the derived yields are in good agreement with the conclusions of previous experimental and theoretical works. For the O(¹D) + CH₃CN reaction, whose H-atom formation channels had not previously been investigated, electronic structure calculations of several new product formation channels are performed to explain the observed H-atom yields. These calculations demonstrate the barrierless and exothermic nature of the relevant exit channels, confirming that atomic hydrogen is also an important product of the O(¹D) + CH₃CN reaction.

Persistent Influence of Wildfire Emissions in the Western United States and Characteristics of Aged Biomass Burning Organic Aerosols under Clean Air Conditions

Wildfire-influenced air masses under regional background conditions were characterized at the Mt. Bachelor Observatory (∼2800 m a.s.l.) in summer 2019 to provide a better understanding of the aging of biomass burning organic aerosols (BBOAs) and their impacts on the remote troposphere in the western United States. Submicron aerosol (PM₁) concentrations were low (average ± 1σ = 2.2 ± 1.9 μg sm^-3), but oxidized BBOAs (average O/C = 0.84) were constantly detected throughout the study. The BBOA correlated well with black carbon, furfural, and acetonitrile and comprised above 50% of PM₁ during plume events when the peak PM₁ concentration reached 18.0 μg sm^-3. Wildfire plumes with estimated transport times varying from ∼10 h to >10 days were identified. The plumes showed ΔOA/ΔCO values ranging from 0.038 to 0.122 ppb ppb^-1 with a significant negative relation to plume age, indicating BBOA loss relative to CO during long-range transport. Additionally, increases of average O/C and aerosol sizes were seen in more aged plumes. The mass-based size mode was approximately 700 nm (D_va) in the most oxidized plume that likely originated in Siberia, suggesting aqueous-phase processing during transport. This work highlights the widespread impacts that wildfire emissions have on aerosol concentration and properties, and thus climate, in the western United States.

Large contribution of biomass burning emissions to ozone throughout the global remote troposphere

Ozone is the third most important anthropogenic greenhouse gas after carbon dioxide and methane but has a larger uncertainty in its radiative forcing, in part because of uncertainty in the source characteristics of ozone precursors, nitrogen oxides, and volatile organic carbon that directly affect ozone formation chemistry. Tropospheric ozone also negatively affects human and ecosystem health. Biomass burning (BB) and urban emissions are significant but uncertain sources of ozone precursors. Here, we report global-scale, in situ airborne measurements of ozone and precursor source tracers from the NASA Atmospheric Tomography mission. Measurements from the remote troposphere showed that tropospheric ozone is regularly enhanced above background in polluted air masses in all regions of the globe. Ozone enhancements in air with high BB and urban emission tracers (2.1 to 23.8 ppbv [parts per billion by volume]) were generally similar to those in BB-influenced air (2.2 to 21.0 ppbv) but larger than those in urban-influenced air (-7.7 to 6.9 ppbv). Ozone attributed to BB was 2 to 10 times higher than that from urban sources in the Southern Hemisphere and the tropical Atlantic and roughly equal to that from urban sources in the Northern Hemisphere and the tropical Pacific. Three independent global chemical transport models systematically underpredict the observed influence of BB on tropospheric ozone. Potential reasons include uncertainties in modeled BB injection heights and emission inventories, export efficiency of BB emissions to the free troposphere, and chemical mechanisms of ozone production in smoke. Accurately accounting for intermittent but large and widespread BB emissions is required to understand the global tropospheric ozone burden.

Observations Confirm that Volatile Chemical Products Are a Major Source of Petrochemical Emissions in U.S. Cities

Despite decades of declining air pollution, urban U.S. areas are still affected by summertime ozone and wintertime particulate matter exceedance events. Volatile organic compounds (VOCs) are known precursors of secondary organic aerosol (SOA) and photochemically produced ozone. Urban VOC emission sources, including on-road transportation emissions, have decreased significantly over the past few decades through successful regulatory measures. These drastic reductions in VOC emissions have led to a change in the distribution of urban emissions and noncombustion sources of VOCs such as those from volatile chemical products (VCPs), which now account for a higher fraction of the urban VOC burden. Given this shift in emission sources, it is essential to quantify the relative contribution of VCP and mobile source emissions to urban pollution. Herein, ground site and mobile laboratory measurements of VOCs were performed. Two ground site locations with different population densities, Boulder, CO, and New York City (NYC), NY, were chosen in order to evaluate the influence of VCPs in cities with varying mixtures of VCPs and mobile source emissions. Positive matrix factorization was used to attribute hundreds of compounds to mobile- and VCP-dominated sources. VCP-dominated emissions contributed to 42 and 78% of anthropogenic VOC emissions for Boulder and NYC, respectively, while mobile source emissions contributed 58 and 22%. Apportioned VOC emissions were compared to those estimated from the Fuel-based Inventory of Vehicle Emissions and VCPs and agreed to within 25% for the bulk comparison and within 30% for more than half of individual compounds. The evaluated inventory was extended to other U.S. cities and it suggests that 50 to 80% of emissions, reactivity, and the SOA-forming potential of urban anthropogenic VOCs are associated with VCP-dominated sources, demonstrating their important role in urban U.S. air quality.

Long-term observations of oxygenated volatile organic compounds (OVOCs) in an urban atmosphere in southern China, 2014-2019

Oxygenated volatile organic compounds (OVOCs) are important precursors and intermediate products of atmospheric photochemical reactions, which can promote the formation of secondary pollutants such as ozone (O₃) and secondary organic aerosol (SOA). However, there have been few studies on the sources of and long-term variation in ambient OVOCs. This study combined sensitive, near real-time measurements of VOCs by proton transfer reaction-mass spectrometry (PTR-MS) with an improved photochemical age parameterization method to quantify daytime sources of OVOCs in an urban atmosphere in China from 2014 to 2019, permitting the observation of the impacts of emission control strategies that were implemented during this period. Temporal variation in six key OVOCs (methanol, acetaldehyde, acetone, methyl ethyl ketone (MEK), formic acid, and acetic acid) were observed. The sum of concentrations of OVOCs was averagely 13% higher during the dry season (November to April), when winds transported polluted air masses to Shenzhen from the continent, than during the wet season, and peak diurnal levels occurred during the daytime year-round due to photochemical production and higher daytime anthropogenic emissions. The average dry season concentration of OVOCs declined from a peak of 30.3 ppb in 2015 to 18.7 ppb in 2019. The results of source apportionment showed that primary anthropogenic sources contributed the most to methanol, MEK, and acetic acid (32-51%); the dominant sources of acetaldehyde and formic acid were both primary and secondary anthropogenic sources; and biomass burning contributed a small fraction (5-11%) to the six OVOCs. From 2014 to 2019, contributions from primary anthropogenic sources of OVOCs decreased significantly by 50-60% due to intensive pollution control measures in Shenzhen, whereas pollution control measures had no observable impact on secondary OVOCs, indicating their formation was not limited by availability of their primary VOC precursors.

Identifying Volatile Chemical Product Tracer Compounds in U.S. Cities

With traffic emissions of volatile organic compounds (VOCs) decreasing rapidly over the last decades, the contributions of the emissions from other source categories, such as volatile chemical products (VCPs), have become more apparent in urban air. In this work, in situ measurements of various VOCs are reported for New York City, Pittsburgh, Chicago, and Denver. The magnitude of different emission sources relative to traffic is determined by measuring the urban enhancement of individual compounds relative to the enhancement of benzene, a known tracer of fossil fuel in the United States. The enhancement ratios of several VCP compounds to benzene correlate well with population density (R² ∼ 0.6-0.8). These observations are consistent with the expectation that some human activity should correlate better with the population density than transportation emissions, due to the lower per capita rate of driving in denser cities. Using these data, together with a bottom-up fuel-based inventory of vehicle emissions and volatile chemical products (FIVE-VCP) inventory, we identify tracer compounds for different VCP categories: decamethylcyclopentasiloxane (D5-siloxane) for personal care products, monoterpenes for fragrances, p-dichlorobenzene for insecticides, D4-siloxane for adhesives, para-chlorobenzotrifluoride (PCBTF) for solvent-based coatings, and Texanol for water-based coatings. Furthermore, several other compounds are identified (e.g., ethanol) that correlate with population density and originate from multiple VCP sources. Ethanol and fragrances are among the most abundant and reactive VOCs associated with VCP emissions.

Wildfire and prescribed burning impacts on air quality in the United States

Air quality impacts from wildfires have been dramatic in recent years, with millions of people exposed to elevated and sometimes hazardous fine particulate matter (PM _2.5 ) concentrations for extended periods. Fires emit particulate matter (PM) and gaseous compounds that can negatively impact human health and reduce visibility. While the overall trend in U.S. air quality has been improving for decades, largely due to implementation of the Clean Air Act, seasonal wildfires threaten to undo this in some regions of the United States. Our understanding of the health effects of smoke is growing with regard to respiratory and cardiovascular consequences and mortality. The costs of these health outcomes can exceed the billions already spent on wildfire suppression. In this critical review, we examine each of the processes that influence wildland fires and the effects of fires, including the natural role of wildland fire, forest management, ignitions, emissions, transport, chemistry, and human health impacts. We highlight key data gaps and examine the complexity and scope and scale of fire occurrence, estimated emissions, and resulting effects on regional air quality across the United States. The goal is to clarify which areas are well understood and which need more study. We conclude with a set of recommendations for future research.

IMPLICATIONS

In the recent decade the area of wildfires in the United States has increased dramatically and the resulting smoke has exposed millions of people to unhealthy air quality. In this critical review we examine the key factors and impacts from fires including natural role of wildland fire, forest management, ignitions, emissions, transport, chemistry and human health.

Analysis of Volatile Organic Compounds during the OCTAVE Campaign: Sources and Distributions of Formaldehyde on Reunion Island

The Oxygenated Compounds in the Tropical Atmosphere: Variability and Exchanges (OCTAVE) campaign aimed to improve the assessment of the budget and role of oxygenated volatile organic compounds (OVOCs) in tropical regions, and especially over oceans, relying on an integrated approach combining in situ measurements, satellite retrievals, and modeling. As part of OCTAVE, volatile organic compounds (VOCs) were measured using a comprehensive suite of instruments on Reunion Island (21.07° S, 55.38° E) from 7 March to 2 May 2018. VOCs were measured at a receptor site at the Maïdo observatory during the entire campaign and at two source sites: Le Port from 19 to 24 April 2018 (source of anthropogenic emissions) and Bélouve from 25 April to 2 May 2018 (source of biogenic emissions) within a mobile lab. The Maïdo observatory is a remote background site located at an altitude of 2200 m, whereas Bélouve is located in a tropical forest to the east of Maïdo and Le Port is an urban area located northwest of Maïdo. The major objective of this study was to understand the sources and distributions of atmospheric formaldehyde (HCHO) in the Maïdo observatory on Reunion Island. To address this objective, two different approaches were used to quantify and determine the main drivers of HCHO at Maïdo. First, a chemical-kinetics-based (CKB) calculation method was used to determine the sources and sinks (biogenic, anthropogenic/primary, or secondary) of HCHO at the Maïdo site. The CKB method shows that 9% of the formaldehyde formed from biogenic emissions and 89% of HCHO had an unknown source; that is, the sources cannot be explicitly described by this method. Next, a positive matrix factorization (PMF) model was applied to characterize the VOC source contributions at Maïdo. The PMF analysis including VOCs measured at the Maïdo observatory shows that the most robust solution was obtained with five factors: secondary biogenic accounting for 17%, primary anthropogenic/solvents (24%), primary biogenic (14%), primary anthropogenic/combustion (22%), and background (23%). The main contributions to formaldehyde sources as described by the PMF model are secondary biogenic (oxidation of biogenic VOCs with 37%) and background (32%). Some assumptions were necessary concerning the high percentage of unknown HCHO sources of the CKB calculation method such as the biogenic emission factor resulting in large discrepancies between the two methods.

Relationships between Particulate Matter, Ozone, and Nitrogen Oxides during Urban Smoke Events in the Western US

Urban ozone (O₃) pollution is influenced by the transport of wildfire smoke but observed impacts are highly variable. We investigate O₃ impacts from smoke in 18 western US cities during July-September, 2013-2017, with ground-based monitoring data from air quality system sites, using satellite-based hazard mapping system (HMS) fire and smoke product to identify overhead smoke. We present four key findings. First, O₃ and PM_2.5 (particulate matter <2.5 μm in diameter) are elevated at nearly all sites on days influenced by smoke, with the greatest mean enhancement occurring during multiday smoke events; nitrogen oxides (NO_x) are not consistently elevated across all sites. Second, PM_2.5 and O₃ exhibit a nonlinear relationship such that O₃ increases with PM_2.5 at low to moderate 24 h PM_2.5, peaks around 30-50 μg m^-3, and declines at higher PM_2.5. Third, the rate of increase of morning O₃ is higher and NO/NO₂ ratios are lower on smoke-influenced days, which could result from additional atmospheric oxidants in smoke. Fourth, while the HMS product is a useful tool for identifying smoke, O₃ and PM_2.5 are elevated on days before and after HMS-identified smoke events implying that a significant fraction of smoke events is not detected.

VOC characteristics, sources and contributions to SOA formation during haze events in Wuhan, Central China

Based on detailed data on 102 volatile organic compounds (VOCs) measured continuously from 2016.10.9 to 2016.11.17 in Wuhan, the VOC characteristics, secondary organic aerosol (SOA) characteristics, SOA formation potential (SOAP), potential source regions, sources and contributions during different haze episodes were analyzed. The total VOC (TVOC) concentrations on clear days (visibility >10 km), slight haze days (visibility of 5-10 km), and severe haze days (visibility <5 km) were 34.87 ± 14.89 ppbv, 45.06 ± 26.69 ppbv, and 49.55 ± 24.82 ppbv, respectively. The SOAP on haze days (447.04 ± 253.85 ppbv) was significantly higher than that on clear days (300.62 ± 138.48 ppbv), and aromatics were the dominant contributors to SOA formation under different visibility conditions, accounting for approximately 97% of the total SOAP. The ratio of ethylbenzene to m/p-xylene (E/X) indicated that atmospheric photochemical reactions were slightly stronger on haze days. The ratio of toluene to benzene (T/B) indicated that vehicle exhaust had significant effects on VOCs, but no significant changes occurred during different haze episodes. The ratio of benzene, toluene, ethylbenzene and xylenes (BTEX) to CO indicated that VOCs from solvent usage in painting/coating and industrial emissions increased with increasing haze pollution. Based on backward trajectories and the potential source contribution function (PSCF), short-distance transport was the main source influencing VOC pollution, especially transport from the southwest. Seven sources were identified by positive matrix factorization (PMF): industrial sources, vehicular exhaust, solvent usage in painting/coating, fuel evaporation, liquefied petroleum gas (LPG) usage, biogenic sources and biomass burning. Moreover, solvent usage in painting/coating, vehicle exhaust and LPG usage were the most important sources that significantly aggravated VOC pollution during haze events. The results can provide references for local governments developing control strategies of VOCs during haze pollution events.

PTR-MS and GC-MS as complementary techniques for analysis of volatiles: A tutorial review

This tutorial review is a critical commentary on the combined use of two instrumental analytical techniques, namely GC-MS and PTR-MS. The first mention of such an analytical approach likely appeared after the year 2000 and despite many advantages, it has not been applied very often. Therefore, the aim of this article is to elaborate on the concept of their combined use and to provide a curse tutorial for those considering taking such an approach. The issue of complementarity was raised in a broad sense of this term. Special emphasis was placed on indicating the possibilities of complementary utilization of GC-MS and PTR-MS and presenting the advantages and disadvantages as well as the current application of these techniques when used together.

The contributions of biomass burning to primary and secondary organics: A case study in Pearl River Delta (PRD), China

Synchronized online measurements of gas- and particle- phase organics including non-methane hydrocarbons (NMHCs), oxygenated volatile organic compounds (OVOCs) and submicron organic matters (OM) were conducted in November 2010 at Heshan, Guangdong provincial supersite, China. Several biomass burning events were identified by using acetonitrile as a tracer, and enhancement ratios (EnRs) of organics to carbon monoxide (CO) obtained from this work generally agree with those from rice straw burning in previous studies. The influences of biomass burning on NMHCs, OVOCs and OM were explored by comparing biomass burning impacted plumes (BB plumes) and non-biomass burning plumes (non-BB plumes). A photochemical age-based parameterization method was used to characterize primary emission and chemical behavior of those three organic groups. The emission ratios (EmRs) of NMHCs, OVOCs and OM to CO increased by 27-71%, 34-55% and 67% in BB plumes, respectively, in comparison with non-BB plumes. The estimated formation rate of secondary organic aerosol (SOA) in BB plumes was found to be 24% faster than non-BB plumes. By applying the above emission ratios to the whole PRD, the annual emissions of VOCs and OM from open burning of crop residues would be 56.4 and 3.8Gg in 2010 in PRD, respectively.

Evolution process and sources of ambient volatile organic compounds during a severe haze event in Beijing, China

108 ambient volatile organic compounds (VOCs) were measured continuously at a time resolution of an hour using an online gas chromatography-frame ionization detector/mass spectrometry (GC-FID/MS) in October 2014 in Beijing, and positive matrix factorization (PMF) was performed with online data. The evolution process and causes for high levels of VOCs during a haze event were investigated through comprehensive analysis. Results show that mixing ratios of VOCs during the haze event (89.29 ppbv) were 2 to 5 times as that in non-haze days, There was a distinct accumulation process of VOCs at the beginning of the haze event, and the mixing ratios of VOCs maintained at the high levels until to the end of pollution when the mixing ratios of ambient VOCs recovered to the normal concentration levels in a few hours. Some reactive and toxic species increased remarkably as well, which indicates a potential health risk to the public in terms of VOCs. Eight sources were resolved by PMF, and results revealed gasoline exhaust was the largest contributor (32-46%) to the ambient VOCs in Beijing. Emissions of gasoline exhaust surged from 13.46 to 40.36 ppbv, with a similar variation pattern to total VOCs, indicating that high levels of VOCs were largely driven to by expanded vehicular emissions. Emissions of biomass burning also increased noticeably (from 2.32 to 11.12 ppbv), and backward trajectories analysis indicated regional transport of biomass burning emissions. Our findings suggested that extremely high levels of VOCs during the haze event was primarily attributed to vehicular emissions, biomass burning and regional transport, as well as stationary synoptic conditions.

Limitation of the Use of the Absorption Angstrom Exponent for Source Apportionment of Equivalent Black Carbon: a Case Study from the North West Indo-Gangetic Plain

Angstrom exponent measurements of equivalent black carbon (BCeq) have recently been introduced as a novel tool to apportion the contribution of biomass burning sources to the BCeq mass. The BCeq is the mass of ideal BC with defined optical properties that, upon deposition on the aethalometer filter tape, would cause equal optical attenuation of light to the actual PM2.5 aerosol deposited. The BCeq mass hence is identical to the mass of the total light-absorbing carbon deposited on the filter tape. Here, we use simultaneously collected data from a seven-wavelength aethalometer and a high-sensitivity proton-transfer reaction mass spectrometer installed at a suburban site in Mohali (Punjab), India, to identify a number of biomass combustion plumes. The identified types of biomass combustion include paddy- and wheat-residue burning, leaf litter, and garbage burning. Traffic plumes were selected for comparison. We find that the combustion efficiency, rather than the fuel used, determines αabs, and consequently, the αabs can be ∼1 for flaming biomass combustion and >1 for older vehicles that operate with poorly optimized engines. Thus, the absorption angstrom exponent is not representative of the fuel used and, therefore, cannot be used as a generic tracer to constrain source contributions.

A pervasive role for biomass burning in tropical high ozone/low water structures

Air parcels with mixing ratios of high O3 and low H2O (HOLW) are common features in the tropical western Pacific (TWP) mid-troposphere (300-700 hPa). Here, using data collected during aircraft sampling of the TWP in winter 2014, we find strong, positive correlations of O3 with multiple biomass burning tracers in these HOLW structures. Ozone levels in these structures are about a factor of three larger than background. Models, satellite data and aircraft observations are used to show fires in tropical Africa and Southeast Asia are the dominant source of high O3 and that low H2O results from large-scale descent within the tropical troposphere. Previous explanations that attribute HOLW structures to transport from the stratosphere or mid-latitude troposphere are inconsistent with our observations. This study suggest a larger role for biomass burning in the radiative forcing of climate in the remote TWP than is commonly appreciated.

Breath testing and personal exposure--SIFT-MS detection of breath acetonitrile for exposure monitoring

Breath testing has potential for the rapid assessment of the source and impact of exposure to air pollutants. During the development of a breath test for acetonitrile using selected ion flow tube mass spectrometry (SIFT-MS) raised acetonitrile concentrations in the breath of volunteers were observed that could not be explained by known sources of exposure. Workplace/laboratory exposure to acetonitrile was proposed since this was common to the volunteers with increased breath concentrations. SIFT-MS measurements of acetonitrile in breath and air were used to confirm that an academic chemistry laboratory was the source of exposure to acetonitrile, and quantify the changes that occurred to exhaled acetonitrile after exposure. High concentrations of acetonitrile were detected in the air of the chemistry laboratory. However, concentrations in the offices were not significantly different across the campus. There was a significant difference in the exhaled acetonitrile concentrations of people who worked in the chemistry laboratories (exposed) and those who did not (non-exposed). SIFT-MS testing of air and breath made it possible to determine that occupational exposure to acetonitrile in the chemistry laboratory was the cause of increased exhaled acetonitrile. Additionally, the sensitivity was adequate to measure the changes to exhaled amounts and found that breath concentrations increased quickly with short exposure and remained increased even after periods of non-exposure. There is potential to add acetonitrile to a suite of VOCs to investigate source and impact of poor air quality.

Halocarbon emissions from the United States and Mexico and their global warming potential

We use recent aircraft measurements of a comprehensive suite of anthropogenic halocarbons, carbon monoxide (CO), and related tracers to place new constraints on North American halocarbon emissions and quantify their global warming potential. Using a chemical transport model (GEOS-Chem) we find that the ensemble of observations are consistent with our prior best estimate of the U.S. anthropogenic CO source, but suggest a 30% underestimate of Mexican emissions. We develop an optimized CO emission inventory on this basis and quantify halocarbon emissions from their measured enhancements relative to CO. Emissions continue for many compounds restricted under the Montreal Protocol, and we show that halocarbons make up an important fraction of the total greenhouse gas source for both countries: our best estimate is 9% (uncertainty range 6-12%) and 32% (21-52%) of equivalent CO2 emissions for the U.S. and Mexico, respectively, on a 20 year time scale. Performance of bottom-up emission inventories is variable, with underestimates for some compounds and overestimates for others. Ongoing methylchloroform emissions are significant in the U.S. (2.8 Gg/y in 2004-2006), in contrast to bottom-up estimates (< 0.05 Gg), with implications for tropospheric OH calculations. Mexican methylchloroform emissions are minor.

Measurements of volatile organic compounds in the earth's atmosphere using proton-transfer-reaction mass spectrometry

Proton-transfer-reaction mass spectrometry (PTR-MS) allows real-time measurements of volatile organic compounds (VOCs) in air with a high sensitivity and a fast time response. The use of PTR-MS in atmospheric research has expanded rapidly in recent years, and much has been learned about the instrument response and specificity of the technique in the analysis of air from different regions of the atmosphere. This paper aims to review the progress that has been made. The theory of operation is described and allows the response of the instrument to be described for different operating conditions. More accurate determinations of the instrument response involve calibrations using standard mixtures, and some results are shown. Much has been learned about the specificity of PTR-MS from inter-comparison studies as well the coupling of PTR-MS with a gas chromatographic interface. The literature on this issue is reviewed and summarized for many VOCs of atmospheric interest. Some highlights of airborne measurements by PTR-MS are presented, including the results obtained in fresh and aged forest-fire and urban plumes. Finally, the recent work that is focused on improving the technique is discussed.