Grantee Research Project Results
Final Report: Evaluating chemical mechanisms with recent field data to account for the contributions of volatile chemical product emissions to urban ozone pollution
EPA Grant Number: R840010Title: Evaluating chemical mechanisms with recent field data to account for the contributions of volatile chemical product emissions to urban ozone pollution
Investigators: Bates, Kelvin , Stockwell, Chelsea , Coggon, Matthew , Gkatzelis, Georgios , Li, Meng , McDonald, Brian
Institution: University of Colorado at Boulder , NOAA Earth System Research Laboratories
EPA Project Officer: Chung, Serena
Project Period: August 1, 2020 through July 31, 2023 (Extended to April 30, 2024)
Project Amount: $396,135
RFA: Chemical Mechanisms to Address New Challenges in Air Quality Modeling (2019) RFA Text | Recipients Lists
Research Category: Early Career Awards , Air Quality and Air Toxics , Air
Objective:
The objective of this research is to improve the mechanisms that describe the chemical oxidation of volatile chemical product (VCP) emissions. Reduced mechanisms used in chemical transport models have been traditionally designed to represent the chemistry of hydrocarbons, such as alkanes and aromatics emitted from fossil fuel sources. Our focus was to improve the chemistry that represents the OH oxidation of oxygenated volatile organic compounds (oVOCs), which are important ingredients in VCPs. We used field data to identify key molecules emitted from VCPs, then modified chemical mechanisms to better represent the OH degradation pathways for these molecules, then evaluated the impact of these updates on model simulations of ozone and other key secondary products, such as peroxy acyl nitrates. Over the course of our research, we identified the importance of cooking on urban oVOC emissions. Similar to VCPs, cooking is not well represented in urban air quality models and we updated and evaluated cooking oVOC chemistry.
Summary/Accomplishments (Outputs/Outcomes):
1. Identification of missing emissions and chemistry in models
In Coggon et al. (2021), we analyzed field data and emissions inventories representative of New York City (NYC) during the 2018 NY-ICE campaign. We found that chemical mechanisms used to simulate air quality misrepresent the OH oxidation pathways of alcohols, glycols, and glycol ethers. These species accounted for more than 50% of the emissions and OH reactivity associated with VCPs. Furthermore, we identified the significant contribution of fragranced VCPs to the urban flux of monoterpenes. The emissions of these highly reactive VOCs were updated in our emissions inventories (e.g., FIVE-VCP, Coggon et al., 2021) and used to evaluate VCP impacts on ozone formation.
Over the course of our research, we found that field observations of long-chain aldehydes (i.e., aldehydes with C > 6) could not be explained by the emissions of VCPs, fossil fuels, or biogenic sources. In Coggon et al. (2024), we analyzed mobile laboratory data collected during the 2021 SUNVEx campaign in Las Vegas, NV. We show that cooking is a major source of long-chain aldehydes, fatty acids, and ethanol to urban air and that the atmospheric abundance of these VOCs is strongly impacted by the spatial density of restaurants. We conducted a source apportionment analysis and found that ~20% of the anthropogenic VOCs observed from a ground site downwind of the Las Vegas Strip were attributable to commercial and residential cooking. These aldehydes were significantly underrepresented in typical emissions inventories (e.g., the National Emissions Inventory (NEI)) and their chemistry was missing from commonly used mechanisms.
2. Updates to Chemical Mechanisms
Our observations highlighted the need to update reactions that describe the atmospheric degradation of alcohols, glycols, glycol ethers, and aldehydes in urban air quality models. We focused on incorporating chemical reactions into version 2 of the Regional Atmospheric Chemistry Mechanism (RACM2, Goliff et al., 2013), which is a reduced mechanism used in WRF-Chem for regional air quality modeling, and the Master Chemical Mechanism (MCM, Jenkin et al., 2003), which is an explicit mechanism commonly used in box modeling.
In Coggon et al. (2021), Zhu et al. (2024a), and Stockwell et al. (2024), we describe our updates to the RACM2_Berkeley2.0 mechanism, which is an extension to RACM2 that accounts for nitrates formed from biogenic VOCs (Browne et al., 2014; Zare et al., 2018). The new mechanism, termed “RACM2B-VCP”, includes reduced reactions for alcohol and glycol surrogates that represent the bulk reactivity of oVOCs emitted from volatile chemical products. These surrogates include isopropanol, propylene glycol, and glycerol. Lower abundant alcohols, glycols, and glycol ethers are lumped to these surrogates based on structural similarities. The goal in representing these emissions is to improve the pathways that lead to peroxy acyl nitrate formation, which was shown to be an important product of oVOC chemistry (Coggon et al., 2021). We include reactions for VCP tracers, including D4-siloxane for adhesives, D5-siloxane for personal care products, parachlorobenzotriflouride (PCBTF) for industrial coatings, and paradichlorobenzene (PDCBZ) for insecticides. These tracers are useful to compare against VOC observations and validate model simulations of VCP emissions and chemistry.
We also include reactions to RACM2B-VCP that represent the OH degradation of saturated and unsaturated long-chain aldehydes emitted from cooking (Stockwell et al., 2024). The emissions and chemistry of cooking VOCs substantially increases modeled anthropogenic VOCs and brings WRF-Chem simulations of OH reactivity into closer agreement with observations (Zhu et al., 2024b).
Many of the key VCP emissions that contribute to OH reactivity already have reaction schemes in the MCM. Two important VOCs - glycerol and tetrahydrofuran – were missing and their OH chemistry was included based on schemes derived from GECKO-A (Aumont et al., 2005). For cooking VOCs, we include reactions schemes for C7 – C12 aldehydes based on published mechanisms and GECKO-A reaction rates for RO2 radicals (Barua et al., 2023; Bowman et al., 2003; Chacon-Madrid et al., 2010). A key goal was to represent the formation of organic nitrates formed as a result of high-NOx OH chemistry. Many of these products, including aldehyde and hydroxy aldehyde nitrates, were observed by our instrumentation at a ground site in Pasadena, CA during the 2021 RECAP-CA campaign.
3. Mechanism Intercomparisons
We validate the mechanism updates to RACM2B-VCP by comparing the product distributions formed in box model simulations against those resulting from models using the updated MCM. We focus on the ability of RACM2B-VCP to mirror the distributions of ozone, formaldehyde, and peroxyacyl nitrates (PAN) formed from different anthropogenic VOC sectors (e.g., VCPs, mobile sources, and cooking). We determine product distributions by conducting VOC sensitivity analyses using the emissions-based box model described by Stockwell et al. (2024). We include an additional mechanism comparison against the Community Regional Atmospheric Chemistry Multiphase Mechanism (CRACMM, Pye et al., 2023). Our updates to VCP chemistry are incorporated into CRACMM.
Our analysis shows that the VCP and cooking updates to RACM2B-VCP produce product distributions that closely resemble those from the MCM. Both mechanisms also compare well against simulations using CRACMM. Broadly, all three mechanisms show that VCP and cooking emissions are the dominant contributors to ozone and PAN, which is consistent with results from simulations conducted in NYC by Coggon et al. (2021). Overall, these comparisons show that updates to reduced chemistry in RACM2B-VCP result in simulations that are comparable to those using the explicit chemistry of the MCM.
4. Quantitatively compare models against field observations
Following our work to validate RACM2B-VCP, we apply our updates to box model and WRF-Chem simulations of air quality in the Los Angeles basin during the 2021 RECAP-CA campaign (Stockwell et al., 2024; Zhu et al., 2024a). We compare model output to the detailed chemical measurements performed at a ground site in Pasadena, CA. The two models are complementary and use the FIVE-VCP-NEI17NRT emissions inventory, which includes updates to the VCP emissions first described by McDonald et al. (2018) and increased emissions of cooking based on the source apportionment analysis conducted in Las Vegas (Coggon et al., 2024). Our updates to VCP emissions are described by Zhu et al. (2024a) and our updates to cooking are outlined by Stockwell et al. (2024) and Zhu et al. (in prep).
Both WRF-Chem and the box model exhibit good agreement with ozone and NOx measured at the Pasadena ground site. Additional datasets from aircraft and the regulatory monitoring network show that WRF-Chem recreates the vertical and spatial distribution of ozone throughout the LA Basin (Zhu et al., 2024a). Importantly, both models capture the temporal and spatial distribution of VCP and cooking tracers, suggesting that the emissions and OH chemistry of these sources are well captured by the updates applied to RACM2B-VCP and FIVE-VCP-NEI17NRT. Both models show that improved representation of oVOC emissions and chemistry results in closer agreement with PAN observed at the Pasadena ground site.
We exchanged RACM2B-VCP in the box model with the updated MCM mechanism described in Section 2. A goal of our research was to use this detailed mechanism to identify products of atmospheric chemistry measured by our mass spectrometers. In Pasadena, our iodide and NH4+ adduct chemical ionization mass spectrometers observed a series of C7-C11 nitrates that were formed from OH oxidation. The model showed that these products – identified as aldehyde and hydroxy aldehyde nitrates – resulted from the high-NOx chemistry of long-chain aldehydes. The model accurately simulated the magnitude and day-to-day variability of these products and showed that they primarily originated from cooking emissions. The model agreement with our observations demonstrates that our updates to aldehyde emissions and chemistry improves simulations of a significantly underestimated emissions source. These marker compounds may be used in future research to better understand the fate of cooking VOCs in urban air.
5. Emission contributions to simulated ozone.
Our work to constrain models with observations has resulted in improved chemical representation of VCP and cooking emissions. Our final task was to determine the contribution of these emissions to ozone formation processes. We focus on results from Stockwell et al. (2024) where VCP, mobile source, and cooking emissions were varied in order to determine source sector contributions to maximum daily 8-hr average ozone (MDA8 O3) in the LA Basin during RECAP-CA. These tests show that anthropogenic VOCs contribute ~12 ppb to MDA8 O3 in regions where ozone production is VOC-limited. Approximately 44% is attributed to VCPs, 28% to fossil fuels, and 28% to cooking VOCs. Biogenic VOCs contribute ~40% (8 ppb) of the total ozone formed in Pasadena. We note that Stockwell et al. (2024) is the first study to evaluate cooking contributions to ozone in LA. The equivalent contributions of cooking and fossil fuel VOCs to ozone formation emphasizes the declining contribution of mobile sources to urban ozone pollution.
We also vary NOx emissions and show that Pasadena is nearing the transition point for NOx-sensitive ozone production. NOx is expected to be significantly reduced in the near future due to vehicle electrification imposed by California regulations (e.g., the Advanced Clean Cars II regulation approved in 2022). Our modeling suggests that the LA Basin is at the point where continued NOx reductions will be very effective in reducing O3 pollution.
References:
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Barua S, Iyer S, Kumar A, Seal P, and Rissanen M. An aldehyde as a rapid source of secondary aerosol precursors: theoretical and experimental study of hexanal autoxidation, Atmospheric Chemistry & Physics, 23, 10517-10532, 10.5194/acp-23-10517-2023, 2023.
Bowman JH, Barket DJ, and Shepson PB. Atmospheric Chemistry of Nonanal, Environmental Science & Technology, 37, 2218-2225, 10.1021/es026220p, 2003.
Chacon-Madrid HJ, Presto, AA, and Donahue, NM. Functionalization vs. fragmentation: n-aldehyde oxidation mechanisms and secondary organic aerosol formation, Physical Chemistry Chemical Physics, 12, 13975-13982, 10.1039/C0CP00200C, 2010.
Goliff WS, Stockwell, WR, and Lawson CV. The regional atmospheric chemistry mechanism, version 2, Atmospheric Environment, 68, 174-185, 10.1016/j.atmosenv.2012.11.038, 2013.
Jenkin ME, Saunders SM, Wagner V, and Pilling MJ. Protocol for the development of the Master Chemical Mechanism, MCM v3 (Part B): tropospheric degradation of aromatic volatile organic compounds, Atmospheric Chemistry & Physics, 3, 181-193, 10.5194/acp-3-181-2003, 2003.
McDonald BC, de Gouw JA, Gilman JB, Jathar SH, Akherati A, Cappa CD, Jimenez JL, Lee-Taylor J, Hayes PL, McKeen SA, Cui YY, Kim SW, Gentner, DR, Isaacman-VanWertz G, Goldstein AH, Harley RA, Frost GJ, Roberts JM, Ryerson TB, and Trainer M. Volatile chemical products emerging as largest petrochemical source of urban organic emissions, Science, 359, 760-764, 10.1126/science.aaq0524, 2018.
Stockwell CE, Coggon MM, Schwantes RH, Harkins C, Verryken B, Lyu C, Zhu Q, Xu L, Gilman JB, Lamplugh A. et al. Urban ozone formation and sensitivities to volatile chemical products, cooking emissions, and NOx across the Los Angeles Basin. Atmospheric Chemistry & Physics. 2024, In review.
Stockwell CE, Coggon MM et al. Atmospheric chemistry of long-chain aldehydes emitted from cooking. In prep, 2024.
Zare A, Romer PS, Nguyen T, Keutsch FN, Skog K, and Cohen RC. A comprehensive organic nitrate chemistry: insights into the lifetime of atmospheric organic nitrates, Atmospheric Chemistry & Physics, 18, 15419-15436, 10.5194/acp-18-15419-2018, 2018.
Zhu Q, Schwantes RH, Stockwell CE, Harkins C, Lyu C, Coggon MM, Warneke C, Schnell J, He J, Pye HOT, et al. Co-benefits of the Zero-Emission Vehicle Adoption on CO2 Emissions and O3 Pollution in Los Angeles. In prep, 2024.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
| Other project views: | All 13 publications | 8 publications in selected types | All 8 journal articles |
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Coggon M, Stockwell C, Claflin M, Phannerstill E, Xu L, Gilman J, Marcantonio J, Cao C, Bates K, Gkatzelis G, Lamplaugh A, Ketz E, Arata C, Hornbrook R, Piel F, Majluf F, Blake D, Whisthaler A, Cangaratna M, Lerner B, Goldstein A. Identifying and correcting interferences to PTR-ToF-MS measurements of isoprene and other urban volatile organic compounds. ATMOSPHERIC MEASUREMENT TECHNIQUES 2024;17(2):801-825 |
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Pfannerstill EY, Arata C, Zhu Q, Schulze BC, Ward R, Woods R, et al. Temperature-dependent emissions dominate aerosol and ozone formation in Los Angeles. Science. 2024;384(6702):1324-9. |
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Supplemental Keywords:
Urban ozone, air quality, atmospheric chemistry, organic nitrates, emission tracers
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Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.