Time-Resolved Intermediate-Volatility and Semivolatile Organic

Jul 9, 2019 - Time-Resolved Intermediate-Volatility and Semivolatile Organic Compound Emissions from Household Coal Combustion in Northern China ...
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Time-Resolved Intermediate-Volatility and Semivolatile Organic Compound Emissions from Household Coal Combustion in Northern China Siyi Cai,†,‡,⊥ Liang Zhu,§,⊥ Shuxiao Wang,*,†,‡ Armin Wisthaler,§ Qing Li,∥ Jingkun Jiang,†,‡ and Jiming Hao†,‡

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State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, P. R. China ‡ State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing 100084, P. R. China § Department of Chemistry, University of Oslo, Postboks 1033 Blindern, NO-0315 Oslo, Norway ∥ Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, P. R. China S Supporting Information *

ABSTRACT: Coal combustion in low-efficiency household stoves results in the emission of large amounts of nonmethane organic compounds (NMOCs), including intermediatevolatility compounds (IVOCs) and semivolatile organic compounds (SVOCs). This conceptual picture is reasonably well established, however, quantitative assessment of I/SVOC emissions from household stoves is rare. We used a protontransfer-reaction time-of-flight mass spectrometer (PTR-ToFMS) to quantify the emissions of organic gases from a typical Chinese household coal stove operated with anthracite and bituminous coals. Most NMOCs (approximately 64−88%) were dominated by hydrocarbons and emitted during the ignition and flaming phases. The ratio of oxidized hydrocarbons increased during the flaming and smoldering stages due to the elevated combustion efficiency. The average emission factors of NMOCs were 121 ± 25.7 and 3690 ± 930 mg/kg for anthracite and bituminous coals, respectively. I/SVOCs contributed to approximately 30% of the total emitted NMOC mass during bituminous coal combustion, much higher than the contribution of biomass burning (approximately 1.5%). Furthermore, I/SVOCs may contribute over 70% of the secondary organic aerosol (SOA) mass formed from gaseous organic species emitted as a result of bituminous coal combustion. This study highlights the importance of inventorying coal-originated I/SVOCs when conducting SOA formation simulation studies.

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INTRODUCTION

coal is widely used in rural China because of its low price and easy ignition. In addition to carbonaceous particles, household coal combustion also results in the emission of a wide range of gaseous organic species, especially nonmethane organic compounds (NMOCs), including volatile organic compounds (VOCs), intermediate-volatility organic compounds (IVOCs), and semivolatile organic compounds (SVOCs).12−15 IVOCs and SVOCs refer to organic compounds with a saturation concentration (C*) between 103 and 106 μg/m316 and between 10−1 and 103 μg/m3,14 respectively. I/SVOCs are known to undergo rapid oxidation in the atmosphere, form condensable byproducts and contribute to the formation and growth of secondary organic aerosol (SOA). Recent studies have

China currently accounts for approximately half of the world’s annual coal consumption.1 Despite negative impacts on the climate and public health attributed to coal combustion,2−5 coal is likely to remain the dominant energy source in China’s power, industrial, and residential sectors for the coming decades. In recent years, significant progress has been made in reducing pollutant emissions from large-scale coal-fired power plants.6 Consequently, residential coal combustion is now gaining increased attention from the air quality research community.7 Coal is the dominant energy source for household cooking and heating in rural China.8−10 The use of poor quality coal along with the low combustion efficiency of household stoves results in high emission levels of carbonaceous particles. Previous studies indicated that emission factors (EFs) from household stoves were approximately 100 times higher than those from industrial coal boilers with regard to bituminous coal combustion.2,11 Bituminous © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

February 5, 2019 July 9, 2019 July 10, 2019 July 10, 2019 DOI: 10.1021/acs.est.9b00734 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

The estimated lag time (the time required for flue gas to be transported from the stove’s chimney to the sampling point) for all instruments was approximately 2 s, which is expected to be sufficient to reach gas-particulate partitioning equilibration.41 An electronic thermocouple, which was installed upstream of the sampling point, indicated a temperature of approximately 30−40 °C during the experiments, which was close to the ambient temperature. Ashes were removed from the stove bottom after each measurement. Ambient air that was filtered by particulate filters (Hebida Company, Beijing, China) was used to flush the combustion chamber and sampling pipes between measurements. Measurement of the levels of NMOCs and other air pollutants prior to ignition was conducted as the baseline and subsequently subtracted from the data recorded. During real world residential coal combustion operations for heating activity, new coal is commonly added to maintain the combustion instead of waiting for its extinction and reigniting.40 This type of consecutive combustion and isolated combustion cycle (referred as a complete combustion cycle) are both employed during cooking activities. Therefore, consecutive and isolated coal-stoking operations were both conducted in this study to represent the residential coal combustion operations in real world. Consecutive coal-stoking practices were simulated by adding 1 kg of coal approximately 5−6 times to sustain the flaming stage instead of reigniting. Sampling Setup and Analysis Procedure of NMOCs and Other Air Pollutants. The concentrations of CO and CO2 in the dilution tunnel were monitored by online detectors (Model 48i, Thermo Scientific, U.S.A.). The concentration of NMOCs was measured by a portable analyzer (JiShunAn JK40, Shenzhen, China) equipped with a photoionization detector (PID) module, which provided NMOC concentrations without speciation (noted as NMOC-PID). It should be noted that PID is sensitive to humidity and not suitable for the analysis of oxygenated compounds and nitrogen-containing VOCs. Fine particle (PM2.5) samples were collected on quartz fiber filters (47 mm, Whatman, U.K.) and analyzed for organic carbon (OC) using the thermal optical reflectance (TOR) method (DRI 2001A, U.S.A.). A PTR-ToF-MS (PTR-TOF 8000, Ionicon Analytik, Austria) was employed to examine gaseous organic compounds possessing a higher proton affinity than that of water. A backward-facing sampling inlet was implemented to sample only gaseous (and possibly ultrafine particulate) substances (see Figure S2). The sampling line consisted of a piece of 1-m long, 1/4′ thermally isolated Siltek SS line backed with an exhaust pump (Leynow, Germany) running at a rate of 25 L/ min. This temperature setting aimed to minimize the evaporation of aerosols during the transport from dilution tunnel to the sampling point. Upstream of the pump, flow at a rate of 0.8 L/min was branched via a piece of 1.2-m long, 120 °C, 1/16′ Siltek SS line to the PTR-ToF-MS inlet. The operation mechanism of the H3O+ PTR-ToF-MS is briefly described here, detail information can be found elsewhere.42−46 The drift tube was operated at 100 °C to minimize possible condensation of medium/low volatility compounds. The drift tube pressure was maintained at 2.30 mbar, and the drift tube voltage was maintained at 470 V (i.e., operation at 116 Td (1 Td = 10−17 V cm2)). The measured m/ z span was between 15 and 500 Da, and the interval between data acquisition points was set to 10 s. A constant flow of 1,4diiodobenzene was infused into the drift tube. [M·H]+ and [M-

indicated that I/SVOCs emitted from vehicles and biomass combustion are highly efficient SOA precursors.17−19 Zhao et al. (2016)20 estimated that up to 80% of the total SOA mass in eastern China was derived from photooxidized I/SVOCs. I/ SVOC emissions from household coal combustion were not included in Zhao’s study owing to the lack of EF data. A qualitative and quantitative assessment of I/SVOC emissions has been analytically challenging. The chemical analysis is commonly based on offline chromatographic analysis of samples collected on a quartz fiber filter or sorbent.21−30 Often, IVOC emissions are estimated according to IVOCs-to-POA (primary organic aerosol) or IVOCs-toNMOC ratios.17 IVOCs thus remain unspeciated, which makes it impossible to incorporate IVOCs into SOA models, including the volatility basis set (VBS) approach.31−34 In addition, offline analytical techniques lack the temporal resolution that can reveal the evolution of I/SVOC emissions within combustion cycles. To better characterize I/SVOC emissions from household coal stoves, quantitative measurements of gaseous organic compounds from these stoves via methods with improved speciation and temporal resolution are urgently needed (see Figure S1 of the Supporting Information, SI). In this study, the time-resolved emission of NMOCs from a typical Chinese household stove operated with different types of anthracite and bituminous coals was examined using a highmass-resolution proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS). The use of an online mass spectrometer allowed us to study the temporal variation in emissions during individual combustion cycles and upon repeated stoking. We also estimated the potential contribution of identified substances to SOA formation.

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MATERIALS AND METHODS Tested Coal Samples and Stove. Two anthracite and three bituminous coal types were examined. The moisture, ash, volatile matter, and fixed carbon content of coal samples were measured according to a standard method (GB/T 30732− 2014) by the China Coal Research Institute.35 Table S1 lists the compositions and properties of the coal samples. Values of the volatile matter content on a dry-ash-free basis (Vdaf) of anthracite 1 (AC1), anthracite 2 (AC2), bituminous coal 1 (BC1), bituminous coal 2 (BC2), and bituminous coal 3 (BC3) were 8.3, 8.4, 34.6, 33.2, and 37.0%, respectively. Experiments for all coal samples were conducted in the same stove (Xinhuaxin Company, Hebei, China). The structure of stove used and fuels tested here are widely used in northern China, additional details on their representatives are detailed elsewhere.36−40 Experimental Setup. The combustion facility and sampling setup are depicted in Figures S2 and S3, respectively, and they are described in our previous studies.36−40 The experiments were conducted in the Changping District, a suburb of Beijing. The experimental stove was placed inside a stainless-steel chamber (with an approximate volume of 10 m3). Flue gas, mixed with particle-free ambient air, was drawn into the dilution pipe region (diameter = 0.22 m) by both inlet and outlet blowers. The chamber was maintained at a slightly positive pressure. The gas flow rate in the dilution pipe was 0.3 m3/s. Since the ventilation of the chamber, determined by the dilution ratio in the pipe, was controlled to simulate the realworld household combustion, the dilution ratio could not be high enough to simulate flue gas diluted by real atmosphere. B

DOI: 10.1021/acs.est.9b00734 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology I·H]+ signals of the 1,4-diiodobenzene together with H3O+ peaks were used to generate the relationship between the ToF time and m/z, which was later employed to realign the recorded mass spectra. Prior to the campaign, the PTR-ToFMS was calibrated against a calibration cylinder (Apel Riemer Environmental Inc., Broomfield, CO, U.S.A.) containing VOCs (acetonitrile, propene, benzene, toluene, xylene, and trimethylbenzene) and oxygenated VOCs (formaldehyde, acetaldehyde, acetone, and methyl vinyl ketone (MVK)) at various humidities (see Table S2). Additional calibrations using a certified To-14 Aromatics Mix cylinder (Restek, U.S.A.) were performed to ensure the best instrumental performance during the campaign. Humidity-dependent calibration factors for propene and formaldehyde were acquired. The calibration factor of formaldehyde was also used for quantifying the hydrogen cyanide (HCN) and isocyanic acid (HNCO) contents.47,48 Raw spectral data analysis, including dead-time correction, mass calibration, peak fitting, area extraction, and signal averaging, was performed using a PTR-ToF data analyzer.49 The peak assignment was derived based on accurate m/z values, isotopic patterns, previous study results,46 and twodimensional gas chromatography (LECO Pegasus 4D GC×GC-ToF-MS, LECO Instrument GmbH., Germany) results of the PM2.5 samples collected on quartz fiber filters (47 mm, Whatman, U.K.) (see Table S3). It should be noted that PTR is not measuring all species and so there is an unidentified/uncharacterized component of the profile. Short-chain alkanes and alkenes are known to be nonionizable under H 3 O + operation mode. Since O 2 + ionization mode was not practiced due to the lack of oxygen cylinder during the campaign, flue gas samples were collected in SUMMA canisters (3.2 L, Entech Inc.) and hydrocarbons were analyzed by EPA TO-15 using GC/MS (US Agilent Technologies Inc. 7100A-7890A-5975C). Ambient air that was filtered by particulate filters was used for background measurements between sampling measurements. Despite the existence of various highly volatile species, the background level of mostly interested I/VOCs are generally less than 0.1 ppbV, close or lower than the detection limit of the PTR-ToFMS instrument used. The estimated uncertainty of organic compounds with external calibrants (e.g., propene, acetonitrile, formaldehyde, acetaldehyde, acetone, benzene, toluene, and xylene) was within ±20%. The uncertainty of quantified compounds without external calibrants was ±40% given the error propagation of the mass discrimination curve (±25%) and calculated reaction rate of the coefficients (±30%).47 For the compounds with unknown reaction rate coefficients, the calibration factor of acetone was employed as a proxy with an estimation uncertainty of ±55%. We note that the values of formic and acetic acids were underestimated since humidity dependence was not considered due to the lack of calibrations. Due to complex mixture of samples, characterization of line losses during room temperature transport from the dilution tunnel to PTR-ToF-MS sampling point was not performed. Previous literatures employing similar sampling setup reported no significant losses on both C10 and C15 species.54,55 Calculation of the EFs. EFs of NMOCs and other air pollutants (CO and CO2) were calculated as follows: EF (mg/kg) =

where Ci (mg/m3) is the concentration of NMOCs or air pollutants (CO and CO2) in the sampling tunnel, V (m3) is the flue gas volume produced in each combustion process, and Mc (kg) represents the weight of the burned coal as received. V was derived from the tunnel diameter, flue gas velocity, and overall combustion period. Mc of the coal was weighed before combustion. The presented EFs were calculated as mean values of two or three replicates. CNMOC was obtained from PTRToF-MS data. CCO and CCO2 were obtained from Thermo (or JiShunAn) online monitors. Emissions of NMOCs and I/SVOCs from household coal combustion in north China were calculated by the following equation: Ei =

∑ Ei ,j = ∑ (Aj × EF)i j

j

where E represents the emissions, and A is the activity level (the amount of household coal consumption). Additionally, i, and j are parameters that represent the type of pollutant (NMOCs or I/SVOCs) and province. Estimation of the SOA Formation Potential. We used the following equation to estimate the SOA formation potential: SOAFP =

∑ EFi × Yi

where SOAFP is the SOA formation potential, EFi is the emission factor for species i, and Yi is the SOA yield for species i. EFi data for the I/SVOC species were obtained in this study (see Table S5). Yi data for the I/SVOC species are presented in Table S8.

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RESULTS AND DISCUSSION Mass Spectral Analysis and Peak Assignments. A quantitative overview of the mass spectra of two typical coal types is shown in Figure S4. A total of 89 peaks were observed by PTR-ToF-MS during the measurement of BC1, while 79 peaks were observed for AC1 (see Figure S4a and Table S3). The peaks accounted for approximately 90−96% of the overall peak intensities across the full mass spectra. The compounds with a value of m/z > 260 were not recorded, mainly due to the low partitioning of large molecules into the gaseous phase. The observed peaks were classified into 7 groups depending on the functional groups: aliphatics, carbonyls, alcohols, acids, nitrogen-containing VOC, aromatics, and oxygenated aromatics. A double-bond equivalent plot against the carbon number (#C), as shown in Figure S4b, suggests a highly conjugated structure for most organic molecules with #C > 10. Most of these substances were designated as either aromatics or oxygenated aromatics that originated directly from unburnt coal and/or the cracking of higher-order aromatics during the combustion process. We note that such aromatic substances as well as oxygenated aromatics do not fragment upon protonation under the mild drift tube conditions of the PTR-ToF-MS.48 Ethyl- or propyl-substituted aromatics are the exceptions, and they are known to fragment.48 Sulfurcontaining substances were not listed due to their low contents. The peak identifications were further confirmed by GC×GC-ToF-MS measurements of the particles collected on filters (see Table S3). SIMPOL.1, a simple group contribution method, was used to predict the vapor pressure of substances according to their molecular structure.50 The estimated vapor pressures were

∑ Ci × V /Mc C

DOI: 10.1021/acs.est.9b00734 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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transformed to produce gases at elevated temperatures). Alternatively, the more effective conversion of devolatilized organic matter to CO2 during the flaming stage reduced the emission of NMOCs. As a result, the emissions of NMOCs first diminished and then plateaued at a relatively high level. The NMOC emissions during the first two stages accounted for nearly 70% of the total emissions throughout the entire combustion process. During the transition from the flaming stage to the flameless smoldering stage, CO emissions increased, again indicating incomplete combustion and poor conversion to CO2. The remaining high stove temperature resulted in the occurrence of the second NMOC peak, which was much less profound than the previous peak during the ignition stage. This result was mainly because a smaller amount of combustible materials was present in the coal during this stage than that during the first two stages. Toward the end of the cycle, the combustion process was dominated by char oxidation. Other characteristics of the ember stage included nearly depleted combustible materials and a low combustion temperature. Accordingly, the concentrations of NMOCs and CO2 continued to decrease, and CO concentrations plateaued for a period of hours. Figure 1b shows the time-resolved profile of identified m/z values detected by the PTR-ToF-MS in a complete combustion cycle. The NMOC profile exhibited a slower decay in the smoldering and ember stages, possibly due to the complex formation pathways of N-containing VOC. Timeresolved profiles of NMOCs for the other four coal types are shown in Figure S6. Similar to what was observed during BC1 combustion, most NMOCs (approximately 64−88%) emerged during the ignition and flaming phases for both anthracite and bituminous coals. Temporal Evolution of the Chemical Compositions. Combustion conditions (e.g., the availability of combustible materials and combustion temperature) are expected to affect the composition of NMOCs during the various combustion stages. Figure 1c presents the averaged NMOC compositions throughout a complete combustion cycle. The hydrocarbons (CxHy, including both aliphatics and aromatics), which were primarily generated by pyrolysis of the volatile matter in coal, dominated the first two phases (see Figure 1c). With the increasing combustion efficiency in both the flaming and smoldering phases, the number of oxidized forms of hydrocarbons increased (see Figure 1c).51 The percentages of carbonyls and acids increased throughout the combustion cycle, while the fraction of oxygenated aromatics decreased when approaching the late stage of combustion (see Figure 1c). The oxygenated products of macromolecular aromatics may have possessed lower volatilities and were expected to partition into the coexisting organic aerosols in the flue gas. The latter was consistent with the previously reported increasing oxidation degree of POAs from coal-fired stoves during a combustion cycle.40 The emission of organic acids during the combustion cycle was reported to be closely correlated with CO emissions.43 Furthermore, long-chain fatty acids, detected in organic aerosols,40 are nonvolatile and supposed to be particle-bound; therefore, they were not examined in the current study. The fraction of N-containing VOC increased throughout the combustion process (see Figure 1c). In this study, the Ncontaining VOC were HCN, acetonitrile, HNCO, and a compound tentatively termed nitromethane (CH3NO2). The release of HCN and HNCO has been previously reported

then transformed to saturation concentrations of the identified compounds at room temperature. Most aromatic and oxygenated compounds with large carbon numbers fall into the groups of IVOCs and SVOCs. As mentioned earlier, ethyl- or propyl-substituted aromatics tend to lose their alkyl group upon protonation. The elimination of ethyl or propyl groups does not dramatically alter the saturation concentrations of the identified organic compounds; therefore, alkyl group elimination does not affect the distribution of volatility bins. Trace Dynamics During the Complete Combustion Cycle. A combustion cycle mainly comprised four stages: ignition, flaming, smoldering, and ember. Trends of CO and CO2 were used to separate the aforementioned 4 stages.40 Figure 1a shows CO, CO2, and NMOC emissions during a

Figure 1. Emissions during a complete coal combustion cycle for BC1. (a) CO, CO2 and NMOC emissions from a complete combustion cycle of BC1 (modified combustion efficiency: MCE = ΔCO2/(ΔCO2+ΔCO)). (b) Time-resolved profile for identified m/z values in a complete combustion cycle. (c) Average NMOC compositions for two repeated experiments at each stage.

complete combustion cycle of BC1. Upon ignition, the CO concentration peaked, indicating that the combustion was incomplete. NMOC levels detected by the PTR-ToF-MS and those detected by the PID (NMOC-PID) are compared qualitatively in the Supporting Information (SI) (see Figure S5). The flaming stage was characterized by diminished CO and increased CO2 emissions and the highest temperature due to the improved combustion efficiency. A high temperature promotes coal devolatilization (a process in which coal is D

DOI: 10.1021/acs.est.9b00734 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 2. (a) Average NMOC compositions in each batch. (b) Time-resolved profile of corresponding m/z values during the consecutive coalstoking processes for BC1.

during the initial stage of coal combustion. Ledesma et al. (1998) suggested that the formation of N-containing VOC was attributed to several mechanisms ranging from unassisted ring rupture to assisted ring rupture involving radicals attacking the heterocyclic structures of tar-N compounds.52 This mechanism may explain the rather persistent presence of N-containing VOC throughout the combustion cycle. Similar temporal variations in NMOCs were observed for anthracite coals, even though they had much lower overall EFs (see Figure S6). Analysis of Consecutive Coal-Stoking Practices. Table S4 summarizes the NMOC EFs of complete combustion cycles and consecutive batches. The average EFs of a complete combustion cycle were 18.8 and 15.6% higher than those of the consecutive stoking batches for AC1 and BC1, respectively. This observation resembled the cold-start scenario in vehicle emission measurements in a certain way.53 The elevated starting temperature and more efficient combustion efficiency explained the lower EFs of NMOCs during the consecutive stoking processes. Figures 2a and S7 plots the averaged NMOC compositions during consecutive stoking practices. Batch 1 was ignited with propane gas, whereas batches 2−6 represented the stoking procedures. The bulk composition of NMOCs is shown as a function of the batches during consecutive coal-stoking combustion processes. The results suggested that the chemical compositions of NMOCs were similar during coal-stoking practices. Slight differences among the EFs and composition and variation trends of NMOCs were observed between the complete combustion cycle and continuous batch process. This indicated that the two combustion operations do not affect the final EFs. EFs and Chemical Compositions of NMOCs. Integrated across the complete combustion process, the EFs of NMOCs for AC1, AC2, BC1, BC2, and BC3 were 113.6 ± 20.7, 129.0 ± 31.0, 3004.0 ± 999.9, 3747.3 ± 484.3, and 4308.0 ± 196.0 mg/kg, respectively. Table S5 summarizes the EFs for the

listed NMOCs. The EFs of anthracite coals were significantly lower than those of bituminous coals. Among the three bituminous coals, the EF for BC3 was the highest (see Table S4), which was probably because it had the highest content of Vdaf. We further compared the EFs of benzene and toluene with those reported in previous studies employing offline techniques. The comparison results for benzene and toluene reached a reasonable level of agreement (see Table S6). Klein et al. (2018)54 also measured levels of gaseous organics from household coal combustion using a PTR-ToF-MS, and rather consistent emission EFs for both bituminous and anthracite coals were reported. Other studies on vehicular and biomass burning often projected EFs of IVOCs from POA mass concentrations using empirical factors.16 Taking bituminous coal as an example, the EF of OC for BC1 was 2875 mg/kg, and the ratio between IVOCs and POAs (based on OC measurements) is approximately 0.3−0.4, which is similar to those reported for biomass burning and biofuel burning.15 The IVOC-to-POA ratio in this study is lower than those estimated for emissions from diesel engines, which are normally in the range of 1.5− 12,14,56 and those from gasoline engines, which are in the range of 5−7.57 The mass spectra of VOC and I/SVOC emissions from the five different types of coals are shown in Figure 3a−e. The discrepancies between the two anthracite coals could be attributed to differences in their coal compositions. Among the three bituminous coals, similar emission patterns were observed despite various production origins (see Figure S8). This result suggested that the NMOC mass spectra presented in this study could be tentatively representative of bituminous coal emissions, even at larger geographic scales. In combination with the aforementioned IVOC-to-POA ratios and available coal POA measurements, it is feasible to estimate IVOC EFs for bituminous coal by regional and nationwide E

DOI: 10.1021/acs.est.9b00734 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 4. EFs of NMOCs by saturation concentration (C*) for the five different types of anthracite and bituminous coals.

traditional offline analyses, which was probably due to analytic losses during sampling and/or pretreatment.21−30 The results in this study represent an important contribution to the source spectra of I/SVOCs from household coal combustion. The EFs for each I/SVOC and proportions of total NMOCs are also listed in Table S5. Note that the NMOCs in this paragraph include short-chain alkanes, ethane, and acetylene, which are not detectable by the PTR-ToF-MS. The EFs of these light hydrocarbons were estimated to be 1.3−5.9 times of that of benzene according to the GC/MS analysis of samples collected in SUMMA canister, within the range of reported ratios.58−61 This approach allowed us to compare the I/SVOC-to-NMOC ratio with reported values.56,57,62 In other studies in which I/SVOC levels from biomass burning were measured, the I/SVOC groups (conventionally listed as m/z > 165) accounted for approximately 1.5% of NMOC emissions.42,62 Previous studies for on-road gasoline vehicle emissions have reported a level of approximately 4% of IVOCs in the total hydrocarbon emissions,57 which is comparable with that from anthracite combustion. Previous studies indicated that for on-road diesel engines, the IVOC-toNMOC ratio was in the range of approximately 20−60%,16,56 most likely due to fuel composition.63 Lu et al. (2018)63 compiled comprehensive organic emission profiles for gasoline and diesel engines including IVOC and SVOC. For gasoline engines, IVOCs and SVOCs contribute ∼4.5% and ∼1.1% of the total organic emission, respectively. On the other hand, IVOCs and SVOCs contribute ∼51.3% and ∼4.6% of total organic emissions from diesel engines. In short, the IVOC-toNMOC ratio of ∼30% from the household coal combustion is between the ones from gasoline and diesel combustion. In combination with the annual amount of coal consumption, household coal combustion (particularly bituminous coal combustion) constitutes a significant I/SVOC source in Northern China, especially during the heating season. Emission Reduction due to the Transition from Bituminous to Clean Energy. The I/SVOC emissions from household coal combustion in northern China (including Beijing, Tianjin, Hebei, Shanxi, Shandong, Henan, and Inner Mongolia provinces) in 2015 were estimated (see Table S7). Household coal consumption data in the aforementioned areas were acquired from the China Energy Statistical Yearbook.64 The EFs for bituminous and anthracite coal were consistent with the average values in the current study. The proportions of bituminous coal in Beijing and Hebei Provinces were 55 and 97%, respectively, as shown in Figure S9. Other provinces were assumed to possess a usage percentage for bituminous coal that was similar to that in Hebei Province (90%). The total

Figure 3. Quantitative source profiles of the five tested coals: (a) Anthracite 1, (b) Anthracite 2, (c) Bituminous coal 1, (d) Bituminous coal 2, and (e) Bituminous coal 3.

SOA formation models (such as the 2D-VBS model).20 Utilization of the reported IVOC-to-NMOC ratio is an alternative approach57 since POA measurements can be dependent on temperature and dilution factor during sampling. Please note that the IVOC-to-POA ratio in this study might be underestimated because the dilution ratio was not high enough to make the partitioning of I/SVOC comparable to real atmosphere. Clear differences, both qualitatively and quantitatively, existed between the NMOC mass spectra for anthracite and bituminous coals. The transition from bituminous to anthracite coals was expected to exhibit a large reduction in hydrocarbon and oxygenated compounds. Alternatively, N-containing VOC (HCN and HNCO) dominated NMOC emissions of anthracite coals (see Figure S6a,b); therefore, changing from bituminous to anthracite coals did not substantially reduce the emission level of N-containing VOC (Table S5). In this respect, the budgets of HCN and HNCO emissions from coal consumption in China are of great interest and demand additional studies. Implications on the S/IVOC Emission and SOA Formation Potential. Figure 4 depicts groups of NMOC EFs as a function of the saturation concentration. These results could serve as I/SVOC emission inputs for advanced SOA formation models equipped with VBS modules. The IVOCs accounted for 11, 7, 26, 26, and 31%, and the SVOCs accounted for 0, 0, 3, 3, and 3% of the total NMOCs for AC1, AC2, BC1, BC2, and BC3, respectively. In other studies on household coal combustion, I/SVOCs were rarely observed by F

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Environmental Science & Technology

possible reasons why SOA production are lower than POA emissions: (1) SOA production in this study was estimated from only gaseous I/SVOCs while diffusive substances from POA also contributed heavily to SOA formation in practical scenarios. (2) SOA production is expected to be underestimated due to the following reasons. High NOx levels, which are typical in China, often result in lower SOA yields.68 The high NOx SOA production scenario, which relies on the minimal SOA yield values reported,65 renders a comparable fractional contribution of I/SVOCs. Furthermore, a recent report suggested that SOA yields increase with increasing carbon numbers of respective molecules.78 The employment of these extrapolated SOA yields is expected to significantly increase the weight of reported I/SVOCs in the budget of SOA production during the heating season of winter. There are two uncertainties linked to the possible underestimation of I/SVOC emissions. The first possible uncertainty is caused by the unidentified minor peaks in PTR-ToF-MS mass spectra in the case of bituminous coals. A lump sum of all unidentified peaks with m/z > 165 increased the I/SVOC EFs by 10−16% (see Figure 5). Accordingly, the sum of unidentified and identified I/SVOCs was responsible for over 70% of the predicted SOA production. In addition, some of the peaks that were listed among the aliphatics, which are not considered SOA precursors in this study, could be fragments of long-chain alkanes; the latter are known to undergo extensive dissociation upon protonation in a PTR-ToF-MS.79 Second, the predicted value of SOA production presented in this study is a simple addition of the observed NMOCs; SOA yields were determined based on chamber measurements.68 Chamber studies often reflect the initial stages of photochemical reactions, which in general, do not last as long as the atmospheric aging period of several days.80 Additionally, in a real atmosphere, the existence of polycyclic aromatic hydrocarbons (PAHs) and other SOA precursors can increase biogenic SOA formation by factors of two to five.81 This observation supports the need for future work on chamber measurements of atmospherically diluted emissions from coal stoves by taking primary organic aerosols into the equation.

emissions of NMOCs and I/SVOCs from household coal combustion in the winter were thus projected to be 153 and 54 kt in north China, respectively. Figure S9 shows the estimated I/SVOC emissions as a function of the usage percentage for bituminous coal. Replacing bituminous coal with anthracite coal allows for reductions in NMOC and I/SVOC emissions by 97 and 99%, respectively. Other clean energy such as electricity and natural gas are also encouraged by local and central authorities. It is reasonable to predict a full implementation of such a shift judging on the fact that Chinese government has made great efforts to promote residential coal replacement action in northern China. Prediction of the SOA Production from the I/SVOC Emission Inventory for Household Coal Combustion. The contribution of coal-originated NMOCs upon completing OH photooxidation to the formation potential of SOA was estimated. The SOA formed from each organic compound was estimated by multiplying the EF of the organic compound by its average SOA yield from the literature (see Table S8).65−75 All compounds were assumed to be completely photo-oxidized (equivalent to approximately 1−2 days of aging in the ambient environment at an OH concentration of 2−17 × 106 molecules cm−3 in China76). The proportional contributions of each compound were estimated based on the sum of SOA formed. Figure 5 exhibits the fractional contribution of individual

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Figure 5. Fractional contributions of individual NMOCs to the predicted SOA formation potential assuming a complete photooxidation.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.9b00734. Compositions and properties of coals used, results of PTR-ToF-MS calibration measurements, peak assignments, EFs, proportions of NMOCs, comparisons with other studies, emissions of I/SVOCs in northern China, SOA yields used in this study, common technologies for organic compound analysis, measurement system photographs, time-resolved emissions, changes in compositions of NMOC with time, and estimated VOC and I/ SVOC emissions (PDF)

NMOCs that are known to be SOA precursors. For all bituminous coals, I/SVOCs accounted for over 70% of the SOA produced from the coal emission inventory. The proportional contribution of I/SVOCs was found to be slightly lower for anthracite coals than for bituminous coals. The latter indicated that despite a dramatic reduction in the overall EF for anthracite coals, the relative contribution of I/SVOCs to SOA formation would not be greatly affected. Drozd et al. (2019)77 characterized I/SVOC from gasoline vehicle and estimated SOA formation after 24 h of oxidation, and found that IVOC emissions contributed 45% of SOA formation. Comparing to the report on gasoline vehicle, the over 70% I/ SVOCs contribution to SOA formation from the residential coal combustion likely contributes significantly to atmospheric SOA formation in Northern China. In other words, failure to include I/SVOCs in either emission inventories or SOA models results in significant underestimation of the contribution of coal-fired stove emissions to SOA production.20 For BC1, the estimated SOA production was 555.41 mg/kg, while the EF of OC was 2875 mg/kg. There are several

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-10-62771466; e-mail: [email protected]. ORCID

Shuxiao Wang: 0000-0001-9727-1963 Armin Wisthaler: 0000-0001-5050-3018 Qing Li: 0000-0003-0587-1748 G

DOI: 10.1021/acs.est.9b00734 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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⊥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21625701), the National Key R&D Program of China (2017YFC0213005), and the Ministry of Environmental Protection of China (DQGG0301 and DQGG0204). The authors would like to thank Dr. Bin Zhao and Dr. Tomás ̌ Mikoviny for their insightful discussions.

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DOI: 10.1021/acs.est.9b00734 Environ. Sci. Technol. XXXX, XXX, XXX−XXX