Research
Contents
Chemistry-Meteorology Interactions and Chemistry-Meteorology Model Development
Team members: Xu FENG, Yumin LI, Wenlu WU, Tzung-May FU
Abstract | We present the WRF-GC model v2.0, an online two-way coupling of the Weather Research and Forecasting (WRF) meteorological model (v3.9.1.1) and the GEOS-Chem model (v12.7.2). WRF-GC v2.0 is built on the modular framework of WRF-GC v1.0 and further includes aerosol–radiation interaction (ARI) and aerosol–cloud interaction (ACI) based on bulk aerosol mass and composition, as well as the capability to nest multiple domains for high-resolution simulations. WRF-GC v2.0 is the first implementation of the GEOS-Chem model in an open-source dynamic model with chemical feedbacks to meteorology. In WRF-GC, meteorological and chemical calculations are performed on the exact same 3-D grid system; grid-scale advection of meteorological variables and chemical species uses the same transport scheme and time steps to ensure mass conservation. Prescribed size distributions are applied to the aerosol types simulated by GEOS-Chem to diagnose aerosol optical properties and activated cloud droplet numbers; the results are passed to the WRF model for radiative and cloud microphysics calculations. WRF-GC is computationally efficient and scalable to massively parallel architectures. We use WRF-GC v2.0 to conduct sensitivity simulations with different combinations of ARI and ACI over China during January 2015 and July 2016. Our sensitivity simulations show that including ARI and ACI improves the model's performance in simulating regional meteorology and air quality. WRF-GC generally reproduces the magnitudes and spatial variability of observed aerosol and cloud properties and surface meteorological variables over East Asia during January 2015 and July 2016, although WRF-GC consistently shows a low bias against observed aerosol optical depths over China. WRF-GC simulations including both ARI and ACI reproduce the observed surface concentrations of PM2.5 in January 2015 (normalized mean bias of −9.3 %, spatial correlation r of 0.77) and afternoon ozone in July 2016 (normalized mean bias of 25.6 %, spatial correlation r of 0.56) over eastern China. WRF-GC v2.0 is open source and freely available from http://wrf.geos-chem.org (last access: 20 June 2021).
Publication | Feng, X., Lin, H., Fu, T.-M.*, Sulprizio, M. P., Zhuang, J., Jacob, D. J., Tian, H., Ma, Y., Zhang, L., Wang, X., Chen, Q., Han, Z. (2021), WRF-GC (v2.0): online two-way coupling of WRF (v3.9.1.1) and GEOS-Chem (v12.7.2) for modeling regional atmospheric chemistry-meteorology interactions, Geosci. Model. Dev., 14，3741-3768，doi:10.5194/gmd-14-3741-2021. PDF Full text
Abstract | We developed the WRF-GC model, an online coupling of the Weather Research and Forecasting (WRF) mesoscale meteorological model and the GEOS-Chem atmospheric chemistry model, for regional atmospheric chemistry and air quality modeling. WRF and GEOS-Chem are both open-source community models. WRF-GC offers regional modellers access to the latest GEOS-Chem chemical module, which is state of the science, well documented, traceable, benchmarked, actively developed by a large international user base, and centrally managed by a dedicated support team. At the same time, WRF-GC enables GEOS-Chem users to perform high-resolution forecasts and hindcasts for any region and time of interest. WRF-GC uses unmodified copies of WRF and GEOS-Chem from their respective sources; the coupling structure allows future versions of either one of the two parent models to be integrated into WRF-GC with relative ease. Within WRF-GC, the physical and chemical state variables are managed in distributed memory and translated between WRF and GEOS-Chem by the WRF-GC coupler at runtime. We used the WRF-GC model to simulate surface PM2.5 concentrations over China during 22 to 27 January 2015 and compared the results to surface observations and the outcomes from a GEOS-Chem Classic nested-China simulation. Both models were able to reproduce the observed spatiotemporal variations of regional PM2.5, but the WRF-GC model (r=0.68, bias =29 %) reproduced the observed daily PM2.5 concentrations over eastern China better than the GEOS-Chem Classic model did (r=0.72, bias =55 %). This was because the WRF-GC simulation, nudged with surface and upper-level meteorological observations, was able to better represent the pollution meteorology during the study period. The WRF-GC model is parallelized across computational cores and scales well on massively parallel architectures. In our tests where the two models were similarly configured, the WRF-GC simulation was 3 times more efficient than the GEOS-Chem Classic nested-grid simulation due to the efficient transport algorithm and the Message Passing Interface (MPI)-based parallelization provided by the WRF software framework. WRF-GC v1.0 supports one-way coupling only, using WRF-simulated meteorological fields to drive GEOS-Chem with no chemical feedbacks. The development of two-way coupling capabilities, i.e., the ability to simulate radiative and microphysical feedbacks of chemistry to meteorology, is under way. The WRF-GC model is open source and freely available from http://wrf.geos-chem.org (last access: 10 July 2020).
Publication | Lin, H., Feng, X., Fu, T.-M.*, Tian, H., Ma, Y., Zhang, L., Jacob, D.J., Yantosca, R.M., Sulprizio, M.P., Lundgren, E.W., Zhuang, J., Zhang, Q., Lu, X., Zhang, L., Shen, L., Guo, J., Eastham, S.D., Keller, C.A. (2020), WRF-GC (v1.0): online coupling of WRF (v3.9.1.1) and GEOS-Chem (v12.2.1) for regional atmospheric chemistry modeling – Part 1: Description of the one-way model, Geosci. Model Dev., doi:10.5194/gmd-13-3241-2020. PDF Full text
Abstract | Precipitation over Southern China in April, largely associated with mesoscale convective systems (MCSs), has declined significantly in recent decades. It is unclear how this decline in precipitation may be related to the concurrent increase of anthropogenic aerosols over this region. Here, using observation analyses and model simulations, we showed that increased levels of anthropogenic aerosols can significantly reduce MCS occurrences by 21% to 32% over Southern China in April, leading to less rainfall. Half of this MCS occurrence reduction was due to the direct radiative scattering of aerosols and the indirect enhancement of non‐MCS liquid cloud reflectance by aerosols, which stabilized the regional atmosphere. The other half of the MCS occurrence reduction was due to the microphysical and dynamical responses of the MCS to aerosols. Our results demonstrated the complex effects of aerosols on MCSs via impacts on both the convective systems and on the regional atmosphere.
Publication | Zhang, L., Fu, T.-M.*, Tian, H., Ma, Y., Chen, J.-P., Tsai, T.-C., Tsai, I.-C., Meng, Z., Yang, X. (2020), Anthropogenic aerosols significantly reduce mesoscale convective system occurrences and precipitation over Southern China in April, Geophys. Res. Lett., doi: 10.1029/2019GL086204. Full text
Abstract | Surface ozone, a major air pollutant toxic to human and ecosystems, is produced by the oxidation of volatile organic compounds (VOCs) in the presence of nitrogen oxides (NOx = NO + NO2) and sunlight. Climate warming may affect future surface ozone levels even in the absence of anthropogenic emission changes, but the direction of ozone change due to climate warming remains uncertain over the southeast U.S. (SEUS) and other polluted forested areas. Here we use observations and simulations to diagnose the sensitivity of SEUS August surface ozone to large-scale temperature variations during 1988-2011. We show that the enhanced biogenic emissions and the accelerated photochemical reaction rates associated with warmer temperatures both act to increases surface ozone. However, the sensitivity of surface ozone to large-scale warming is highly variable on interannual and interdecadal timescales owing to variation in regional ozone advection. Our results have important implications for the prediction and management of future ozone air quality.
Publication | Fu, T.-M.*, Y. Zheng, F. Paulot, J. Mao, and R. M. Yantosca (2015), Positive but variable sensitivity of August surface ozone to large-scale warming in the southeast United States, Nature Climate Change, doi: 10.1038/nclimate2567. Full text
Abstract | Organic aerosols (OA) are important components of aerosols. They affect the hygroscopicity of aerosols and exert direct and indirect radiative forcing on global and regional climate. We used the kappa-Köhler theory and a simulation of the aerosol mass concentrations in China for the year 2006 to calculate the number concentration of cloud condensation nuclei (CCN) in China and evaluate the contribution of OA. The number concentrations of CCN in China show a west-to-east gradient, due to the stronger anthropogenic emissions in eastern China. Assuming that all aerosol chemical components are externally mixed, the number concentrations of CCN are 0.9-1.2×1e3 cm-3 seasonally, and OA contributes 30% to the annual mean. Assuming that OA components are internally mixed, while inorganic aerosols are externally mixed, the number concentrations of CCN are 0.9-1.1×1e3 cm-3, and OA contributes 28% to the annual mean. Though the differernce of annual average contribution of OA to CCN number is small when OA is external and internal mixed, there is large different spatially in summer and winter due to more secondary OA in summer and more primary OA in winter. Our results show that OA are important sources of CCN in China.
Publication | Xing, L., and T.-M. Fu* (2014), Contributions of organic aerosols to cloud condensation nuclei numbers in China, submitted to Acta Scientiarum Naturalium Universitatis Pekinensis. In press. (In Chinese)
Attribution of Chinese air pollution
Team members: Nan LI, Yue JIAN, Heng TIAN, Hansen CAO, Tzung-May FU
Abstract | We used the GEOS-Chem model and its adjoint to quantify Chinese non-methane volatile organic compound (NMVOC) emissions for the year 2007, using the tropospheric column concentrations of formaldehyde and glyoxal observed by the Global Ozone Monitoring Experiment 2A (GOME-2A) instrument and the Ozone Monitoring Instrument (OMI) as quantitative constraints. We conducted a series of inversion experiments using different combinations of satellite observations to explore their impacts on the top-down emission estimates. Our top-down estimates for Chinese annual total NMVOC emissions were 30.7 to 49.5 (average 41.9) Tgyr−1, including 16.4 to 23.6 (average 20.2) Tgyr−1 from anthropogenic sources, 12.2 to 22.8 (average 19.2) Tgyr−1 from biogenic sources, and 2.08 to 3.13 (average 2.48) Tgyr−1 from biomass burning. In comparison, the a priori estimate for Chinese annual total NMVOC emissions was 38.3Tgyr−1, including 18.8Tgyr−1 from anthropogenic sources, 17.3Tgyr−1 from biogenic sources, and 2.27Tgyr−1 from biomass burning. The simultaneous use of glyoxal and formaldehyde observations helped distinguish the NMVOC species from different sources and was essential in constraining anthropogenic emissions. Our four inversion experiments consistently showed that the Chinese anthropogenic emissions of NMVOC precursors of glyoxal were larger than the a priori estimates. Our top-down estimates for Chinese annual emission of anthropogenic aromatics (benzene, toluene, and xylene) ranged from 5.5 to 7.9Tgyr−1, 2% to 46% larger than the estimate of the a priori emission inventory (5.4Tgyr−1). Three out of our four inversion experiments indicated that the seasonal variation in Chinese NMVOC emissions was significantly stronger than indicated in the a priori inventory. Model simulations driven by the average of our top-down NMVOC emission estimates (which had a stronger seasonal variation than the a priori) showed that surface afternoon ozone concentrations over eastern China increased by 1–8ppb in June and decreased by 1–10ppb in December relative to the simulations using the a priori emissions and were in better agreement with measurements. We concluded that the satellite observations of formaldehyde and glyoxal together provided quantitative constraints on the emissions and source types of NMVOCs over China and improved our understanding on regional chemistry.
Publication | Cao, H., T.-M. Fu*, L. Zhang, D. K. Henze, C. Chan Miller, C. Lerot, G. Gonzalex Abad, I. De Smedt, Q. Zhang, M. van Roosendael, K. Chance, J. Li, J. Zheng, and Y. Zhao (2018), Adjoint inversion of Chinese non-methane volatile organic compound emissions using space-based observations of formaldehyde and glyoxal, Atmospheric Physics and Chemistry, 18, 15017-15046, doi:10.5194/acp-18-15017-2018. Full text
Abstract | We simulated elemental carbon (EC) and organic carbon (OC) aerosols in China and compared model results to surface measurements at Chinese rural and background sites, with the goal of deriving “top-down” emission estimates of EC and OC, as well as better quantifying the secondary sources of OC. We included in the model state-of-the-science Chinese “bottom-up” emission inventories for EC (1.92 TgC/yr) and OC (3.95 TgC/yr), as well as updated secondary OC formation pathways. The average simulated annual mean EC concentration at rural and background sites was 1.1 μgC/m3, 56% lower than the observed 2.5 μgC/m3. The average simulated annual mean OC concentration at rural and background sites was 3.4 μgC/m3, 76% lower than the observed 14 μgC/m3. Multiple regression to fit surface monthly mean EC observations at rural and background sites yielded the best estimate of Chinese EC source of 3.05±0.78 TgC/yr. Based on the topdown EC emission estimate and observed seasonal primary OC/EC ratios, we estimated Chinese OC emissions to be 6.67±1.30 6.67±1.30 TgC/yr. Using these top-down estimates, the simulated average annual mean EC concentration at rural and background sites was significantly improved to 1.9 μgC/m3. However, the model still significantly underestimated observed OC in all seasons (simulated average annual mean OC at rural and background sites was 5.4 μgC/m3), with little skill in capturing the spatiotemporal variability. Secondary formation accounts for 21% of Chinese annual mean surface OC in the model, with isoprene being the most important precursor. In summer, as high as 62% of the observed surface OC may be due to secondary formation in eastern China. Our analysis points to four shortcomings in the current bottom-up inventories of Chinese carbonaceous aerosols: (1) the anthropogenic source is underestimated on a national scale, particularly for OC; (2) the spatiotemporal distributions of emissions are misrepresented; (3) there is a missing source in western China, likely associated with the use of biofuels or other low-quality fuels for heating; and (4) sources in fall are not well represented, either because the seasonal shifting of. emissions and/or secondary formation are poorly captured or because specific fall emission events are missing. In addition, secondary production of OC in China is severely underestimated. More regional measurements with better spatiotemporal coverage are needed to resolve these shortcomings.
Publication | Fu, T.-M.*, J.J. Cao, X.Y. Zhang, S.C. Lee, Q. Zhang, Y.M. Han, W.J. Qu, Z. Han, R. Zhang, Y.X. Wang, D. Chen, and D.K. Henze (2012), Carbonaceous aerosols in China: top-down constraints on primary sources and estimation of secondary contribution, Atmos. Chem. Phys., 12, 2725-2746, doi:10.5194/acp-12-2725-2012. PDF
Volatile organic compounds (VOCs): global and regional emissions and impacts
Team members: Wenlu WU, Xiaolin WANG
Volatile organic compounds (VOC) impact the oxidizing power of the atmosphere and produce ozone and secondary organic aerosols. VOCs are emitted into the atmosphere from both natural and anthropogenic activities, and quantifying these many overlapping sources can be a challenge. We use remote sensing (satellite) and in situ observations to make 'top-down' estimates of VOC emissions from different sources. We use chemical transport models to evaluate the impact of VOCs on tropospheric chemistry.
Abstract | We used the GEOS-Chem model and its adjoint to quantify Chinese non-methane volatile organic compound (NMVOC) emissions for the year 2007, using the tropospheric column concentrations of formaldehyde and glyoxal observed by the Global Ozone Monitoring Experiment 2A (GOME-2A) instrument and the Ozone Monitoring Instrument (OMI) as quantitative constraints. We conducted a series of inversion experiments using different combinations of satellite observations to explore their impacts on the top-down emission estimates. Our top-down estimates for Chinese annual total NMVOC emissions were 30.7 to 49.5 (average 41.9) Tgyr−1, including 16.4 to 23.6 (average 20.2) Tgyr−1 from anthropogenic sources, 12.2 to 22.8 (average 19.2) Tgyr−1 from biogenic sources, and 2.08 to 3.13 (average 2.48) Tgyr−1 from biomass burning. In comparison, the a priori estimate for Chinese annual total NMVOC emissions was 38.3Tgyr−1, including 18.8Tgyr−1 from anthropogenic sources, 17.3Tgyr−1 from biogenic sources, and 2.27Tgyr−1 from biomass burning. The simultaneous use of glyoxal and formaldehyde observations helped distinguish the NMVOC species from different sources and was essential in constraining anthropogenic emissions. Our four inversion experiments consistently showed that the Chinese anthropogenic emissions of NMVOC precursors of glyoxal were larger than the a priori estimates. Our top-down estimates for Chinese annual emission of anthropogenic aromatics (benzene, toluene, and xylene) ranged from 5.5 to 7.9Tgyr−1, 2% to 46% larger than the estimate of the a priori emission inventory (5.4Tgyr−1). Three out of our four inversion experiments indicated that the seasonal variation in Chinese NMVOC emissions was significantly stronger than indicated in the a priori inventory. Model simulations driven by the average of our top-down NMVOC emission estimates (which had a stronger seasonal variation than the a priori) showed that surface afternoon ozone concentrations over eastern China increased by 1–8ppb in June and decreased by 1–10ppb in December relative to the simulations using the a priori emissions and were in better agreement with measurements. We concluded that the satellite observations of formaldehyde and glyoxal together provided quantitative constraints on the emissions and source types of NMVOCs over China and improved our understanding on regional chemistry.
Publication | Cao, H., T.-M. Fu*, L. Zhang, D. K. Henze, C. Chan Miller, C. Lerot, G. Gonzalex Abad, I. De Smedt, Q. Zhang, M. van Roosendael, K. Chance, J. Li, J. Zheng, and Y. Zhao (2018), Adjoint inversion of Chinese non-methane volatile organic compound emissions using space-based observations of formaldehyde and glyoxal, Atmospheric Physics and Chemistry, 18, 15017-15046, doi:10.5194/acp-18-15017-2018. Full text
Abstract | We construct global budgets of atmospheric glyoxal and methylglyoxal with the goal of quantifying their potential for global secondary organic aerosol (SOA) formation via irreversible uptake by aqueous aerosols and clouds. We conduct a detailed simulation of glyoxal and methylglyoxal in the GEOS-Chem global 3-D chemical transport model including our best knowledge of source and sink processes. Our resulting best estimates of the global sources of glyoxal and methylglyoxal are 45 Tg/a and 140 Tg/a, respectively. Oxidation of biogenic isoprene contributes globally 47% of glyoxal and 79% of methylglyoxal. The second most important precursors are acetylene (mostly anthropogenic) for glyoxal and acetone (mostly biogenic) for methylglyoxal. Both acetylene and acetone have long lifetimes and provide a source of dicarbonyls in the free troposphere. Atmospheric lifetimes of glyoxal and methylglyoxal in the model are 2.9 h and 1.6 h, respectively, mostly determined by photolysis. Simulated dicarbonyl concentrations in continental surface air at northern midlatitudes are in the range 10–100 ppt, consistent with in situ measurements. On a global scale, the highest concentrations are over biomass burning regions, in agreement with glyoxal column observations from the SCIAMACHY satellite instrument. SCIAMACHY and a few ship cruises also suggest a large marine source of dicarbonyls missing from our model. The global source of SOA from the irreversible uptake of dicarbonyls in GEOS-Chem is 11 Tg C/a, including 2.6 Tg C/a from glyoxal and 8 Tg C/a from methylglyoxal; 90% of this source takes place in clouds. The magnitude of the global SOA source from dicarbonyls is comparable to that computed in GEOS-Chem from the standard mechanism involving reversible partitioning of semivolatile products from the oxidation of monoterpenes, sesquiterpenes, isoprene, and aromatics.
Publication | Fu, T.-M.*, D. J. Jacob, F. Wittrock, J. P. Burrows, M. Vrekoussis, and D. K. Henze (2008), Global budgets of atmospheric glyoxal and methylglyoxal, and implications for formation of secondary organic aerosols, J. Geophys. Res., 113, D15303, doi:10.1026/2007JD009505. PDF
Abstract | We use a continuous 6-year record (1996–2001) of GOME satellite measurements of formaldehyde (HCHO) columns over east and south Asia to improve regional emission estimates of reactive nonmethane volatile organic compounds (NMVOCs), including isoprene, alkenes, HCHO, and xylenes. Mean monthly HCHO observations are compared to simulated HCHO columns from the GEOS-Chem chemical transport model using state-of-science, ‘‘bottom-up’’ emission inventories from Streets et al. (2003a) for anthropogenic and biomass burning emissions and Guenther et al. (2006) for biogenic emissions (MEGAN). We find that wintertime GOME observations can diagnose anthropogenic reactive NMVOC emissions from China, leading to an estimate 25% higher than Streets et al. (2003a). We attribute the difference to vehicular emissions. The biomass burning source for east and south Asia is almost 5 times the estimate of Streets et al. (2003a). GOME reveals a large source from agricultural burning in the North China Plain in June missing from current inventories. This source may reflect a recent trend toward in-field burning of crop residues as the need for biofuels diminishes. Biogenic isoprene emission in east and south Asia derived from GOME is 56 ± 30 Tg/yr, similar to 52 Tg/yr from MEGAN. We find, however, that MEGAN underestimates emissions in China and overestimates emissions in the tropics. The higher Chinese biogenic and biomass burning emissions revealed by GOME have important implications for ozone pollution. We find 5 to 20 ppb seasonal increases in surface ozone in GEOS-Chem for central and northern China when using GOME-derived versus bottom-up emissions. Our methodology can be adapted for other regions of the world to provide top-down constraints on NMVOC emissions where multiple emission source types overlap in space and time.
Publication | Fu, T.-M.*, D. J. Jacob, P. I. Palmer, K. Chance, Y. X. Wang, B. Barletta, D. R. Blake, J. C. Stanton, M. J. Pilling (2007), Space-based formaldehyde measurements as constraints on volatile organic compound emissions in East and South Asia, J. Geophys. Res., 112, D06312, doi:10.1029/2006JD007853. PDF
Publications : Millet et al. [2007], Palmer et al. [2006]
Cao et al., in progress.
Organic aerosols (OA)
Team members: Yumin Li, Li XING, Tzung-May FU
Secondary organic aerosols (SOA) are the organic mass transferred into the particulate phase in the atmosphere. Many recent observations have found SOA concentrations to be much higher than can be explained by current models in most parts of the atmosphere.
Using a global 3-D atmospheric chemistry model, we investigate the missing source of SOA. In particular, we find that the heteorogeneous uptake of dicarbonyls in aeorsols and clouds can help explained the observed SOA concentrations and variability.
Abstract | We developed a parametrizable box model to empirically derive the yields of semivolatile products from VOC oxidation using chamber measurements, while explicitly accounting for the multigenerational chemical aging processes (such as the gas-phase fragmentation and functionalization and aerosol-phase oligomerization and photolysis) under different NOx levels and the loss of particles and gases to chamber walls. Using the oxidation of isoprene as an example, we showed that the assumptions regarding the NOx-sensitive, multigenerational aging processes of VOC oxidation products have large impacts on the parametrized product yields and SOA formation. We derived sets of semivolatile product yields from isoprene oxidation under different NOx levels. However, we stress that these product yields must be used in conjunction with the corresponding multigenerational aging schemes in chemical transport models. As more mechanistic insights regarding SOA formation from VOC oxidation emerge, our box model can be expanded to include more explicit chemical aging processes and help ultimately bridge the gap between the process-based understanding of SOA formation from VOC oxidation and the bulk-yield parametrizations used in chemical transport models.
Publication | Xing, L., M. Shrivastava*, T.-M. Fu*, P. Roldin, Y. Qian, L. Xu, N. L. Ng, J. Shilling, A. Zelenyuk, and C. Cappa (2018), Parameterized yields of semi-volatile products from isoprene oxidation under different NOx levels:impacts of chemical aging and wall-loss of reactive gases, Environmental Science and Technology, doi: 10.1021/acs.est.8b00373. Full text SI
Abstract | We construct global budgets of atmospheric glyoxal and methylglyoxal with the goal of quantifying their potential for global secondary organic aerosol (SOA) formation via irreversible uptake by aqueous aerosols and clouds. We conduct a detailed simulation of glyoxal and methylglyoxal in the GEOS-Chem global 3-D chemical transport model including our best knowledge of source and sink processes. Our resulting best estimates of the global sources of glyoxal and methylglyoxal are 45 Tg/a and 140 Tg/a, respectively. Oxidation of biogenic isoprene contributes globally 47% of glyoxal and 79% of methylglyoxal. The second most important precursors are acetylene (mostly anthropogenic) for glyoxal and acetone (mostly biogenic) for methylglyoxal. Both acetylene and acetone have long lifetimes and provide a source of dicarbonyls in the free troposphere. Atmospheric lifetimes of glyoxal and methylglyoxal in the model are 2.9 h and 1.6 h, respectively, mostly determined by photolysis. Simulated dicarbonyl concentrations in continental surface air at northern midlatitudes are in the range 10–100 ppt, consistent with in situ measurements. On a global scale, the highest concentrations are over biomass burning regions, in agreement with glyoxal column observations from the SCIAMACHY satellite instrument. SCIAMACHY and a few ship cruises also suggest a large marine source of dicarbonyls missing from our model. The global source of SOA from the irreversible uptake of dicarbonyls in GEOS-Chem is 11 Tg C/a, including 2.6 Tg C/a from glyoxal and 8 Tg C/a from methylglyoxal; 90% of this source takes place in clouds. The magnitude of the global SOA source from dicarbonyls is comparable to that computed in GEOS-Chem from the standard mechanism involving reversible partitioning of semivolatile products from the oxidation of monoterpenes, sesquiterpenes, isoprene, and aromatics.
Publication | Fu, T.-M.*, D. J. Jacob, F. Wittrock, J. P. Burrows, M. Vrekoussis, and D. K. Henze (2008), Global budgets of atmospheric glyoxal and methylglyoxal, and implications for formation of secondary organic aerosols, J. Geophys. Res., 113, D15303, doi:10.1026/2007JD009505. PDF
Abstract | We simulated elemental carbon (EC) and organic carbon (OC) aerosols in China and compared model results to surface measurements at Chinese rural and background sites, with the goal of deriving “top-down” emission estimates of EC and OC, as well as better quantifying the secondary sources of OC. We included in the model state-of-the-science Chinese “bottom-up” emission inventories for EC (1.92 TgC/yr) and OC (3.95 TgC/yr), as well as updated secondary OC formation pathways. The average simulated annual mean EC concentration at rural and background sites was 1.1 μgC/m3, 56% lower than the observed 2.5 μgC/m3. The average simulated annual mean OC concentration at rural and background sites was 3.4 μgC/m3, 76% lower than the observed 14 μgC/m3. Multiple regression to fit surface monthly mean EC observations at rural and background sites yielded the best estimate of Chinese EC source of 3.05±0.78 TgC/yr. Based on the topdown EC emission estimate and observed seasonal primary OC/EC ratios, we estimated Chinese OC emissions to be 6.67±1.30 6.67±1.30 TgC/yr. Using these top-down estimates, the simulated average annual mean EC concentration at rural and background sites was significantly improved to 1.9 μgC/m3. However, the model still significantly underestimated observed OC in all seasons (simulated average annual mean OC at rural and background sites was 5.4 μgC/m3), with little skill in capturing the spatiotemporal variability. Secondary formation accounts for 21% of Chinese annual mean surface OC in the model, with isoprene being the most important precursor. In summer, as high as 62% of the observed surface OC may be due to secondary formation in eastern China. Our analysis points to four shortcomings in the current bottom-up inventories of Chinese carbonaceous aerosols: (1) the anthropogenic source is underestimated on a national scale, particularly for OC; (2) the spatiotemporal distributions of emissions are misrepresented; (3) there is a missing source in western China, likely associated with the use of biofuels or other low-quality fuels for heating; and (4) sources in fall are not well represented, either because the seasonal shifting of. emissions and/or secondary formation are poorly captured or because specific fall emission events are missing. In addition, secondary production of OC in China is severely underestimated. More regional measurements with better spatiotemporal coverage are needed to resolve these shortcomings.
Publication | Fu, T.-M.*, J.J. Cao, X.Y. Zhang, S.C. Lee, Q. Zhang, Y.M. Han, W.J. Qu, Z. Han, R. Zhang, Y.X. Wang, D. Chen, and D.K. Henze (2012), Carbonaceous aerosols in China: top-down constraints on primary sources and estimation of secondary contribution, Atmos. Chem. Phys., 12, 2725-2746, doi:10.5194/acp-12-2725-2012. PDF
Abstract | We used the regional air quality model CMAQ to simulate organic aerosol (OA) concentrations over the Pearl River Delta region (PRD) and compared model results to measurements. Our goals were (1) to evaluate the potential contribution of the aqueous reactive uptake of dicarbonyls (glyoxal and methylglyoxal) as a source of secondary organic aerosol (SOA) in an urban environment, and (2) to quantify the sources of SOA in the PRD in fall. We improved the representation of dicarbonyl gas phase chemistry in CMAQ, as well as added SOA formation via the irreversible uptake of dicarbonyls by aqueous aerosols and cloud droplets, characterized by a reactive uptake coefficient gamma = 2.9e3 based on laboratory studies. Our model results were compared to aerosol mass spectrometry (AMS) measurements in Shenzhen during a photochemical smog event in fall 2009. Including the new dicarbonyl SOA source in CMAQ led to an increase in the simulated mean SOA concentration at the sampling site from 4.1 μg/m3 to 9.0 μg/m3 during the smog event, in better agreement with the mean observed oxygenated OA (OOA) concentration (8.0 μg/m3). The simulated SOA reproduced the variability of observed OOA (r = 0.89). Moreover, simulated dicarbonyl SOA was highly correlated with simulated sulfate (r = 0.72), consistent with the observed high correlation between OOA and sulfate (r = 0.84). Including the dicarbonyl SOA source also increased the mean simulated concentrations of total OA from 8.2 μg/m3 to 13.1 μg/m3, closer to the mean observed OA concentration (16.5 μg/m3). The remaining difference between the observed and simulated OA was largely due to impacts from episodic biomass burning emissions, but the model did not capture this variability. We concluded that, for the PRD in fall and outside of major biomass burning events, 75% of the total SOA was biogenic. Isoprene was the most important precursor, accounting for 41% of the total SOA. Aromatics accounted for 13% of the total SOA. Our results show that the aqueous chemistry of dicarbonyls can be an important SOA source, potentially accounting for 53% of the total surface SOA in the PRD in fall.
Publication | Li, N., T.-M. Fu*, J.J. Cao*, S.C. Lee, X.-F. Huang, L.-Y. He, K.-F. Ho, J. S. Fu, and Y.-F. Lam (2013), Sources of secondary organic aerosols in the Pearl River Delta region in fall: contributions from the aqueous reactive uptake of dicarbonyls, Atmos. Environ., 76, 200-207, doi:10.1016/j.atmosenv.2012.12.005. PDF
Abstract | We calculated the organic matter to organic carbon mass ratios (OM/OC mass ratios) in PM2.5 collected from 14 Chinese cities during summer and winter of 2003 and analyzed the causes for their seasonal and spatial variability. The OM/OC mass ratios were calculated two ways. Using a mass balance method, the calculated OM/OC mass ratios averaged 1.92±0.39 year-round, with no significant seasonal or spatial variation. The second calculation was based on chemical species analyses of the organic compounds extracted from the PM2.5 samples using dichloromethane/methanol and water. The calculated OM/OC mass ratio in summer was relatively high (1.75±0.13) and spatially-invariant due to vigorous photochemistry and secondary organic aerosol (OA) production throughout the country. The calculated OM/OC mass ratio in winter (1.59±0.18) was significantly lower than that in summer, with lower values in northern cities (1.51±0.07) than in southern cities (1.65±0.15). This likely reflects the wider usage of coal for heating purposes in northern China in winter, in contrast to the larger contributions from biofuel and biomass burning in southern China in winter. On average, organic matter constituted 36 % and 34 % of Chinese urban PM2.5 mass in summer and winter, respectively. We report, for the first time, a high regional correlation between Zn and oxalic acid in Chinese urban aerosols in summer. This is consistent with the formation of stable Zn oxalate complex in the aerosol phase previously proposed by Furukawa and Takahashi (2011). We found that many other dicarboxylic acids were also highly correlated with Zn in the summer Chinese urban aerosol samples, suggesting that they may also form stable organic complexes with Zn. Such formation may have profound implications for the atmospheric abundance and hygroscopic properties of aerosol dicarboxylic acids.
Publication | Xing, L., T.-M. Fu*, J.J. Cao, S.C. Lee, G.H. Wang, K.-F. Ho, M.-C. Cheng, C.-F. You, and T.J. Wang (2013), Seasonal and spatial variability of the OM/OC mass ratios and high regional correlation between oxalic acid and zinc in Chinese urban organic aerosols, Atmos. Chem. Phys., 13, 4307-4318, doi:10.5194/acp-13-4307-2013. PDF
Xing et al., in progress.
Air-sea exchange of organic materials
Team members: Cenlin HE, Tzung-May FU
The ocean can act both as a source and a sink of atmospheric organic material. The air/sea exchange of organic materials is complexly regulated by both physical and biological conditions at the interface and poorly understood. We developed a new conceptual model to account for these physical and biological processes, including the presence of microfilms, production/consumption of organic matter by marine life, and other photochemical processes.
Abstract | The authors propose a new “three-layer” conceptual model for the air-sea exchange of organic gases, which includes a dynamic surface microlayer with photochemical and biological processes. A parameterization of this three-layer model is presented, which was used to calculate the air-sea fluxes of acetone over the Pacific Ocean. The air-sea fluxes of acetone calculated by the three-layer model are in the same direction but possess half the magnitude of the fluxes calculated by the traditional two-layer model in the absence of photochemical and biological processes. However, photochemical and biological processes impacting acetone in the microlayer can greatly vary the calculated fluxes in the three-layer model, even reversing their direction under favorable conditions. Our model may help explain the discrepancies between measured and calculated acetone fluxes in previous studies. More measurements are needed to validate our conceptual model and provide constraints on the model parameters.
Publication | He, C.L., and T.-M. Fu* (2012), Air-sea exchange of volatile organic compounds: a new model with microlayer effects, Atmospheric and Oceanic Science Letters, 6(2), 97-102. PDF
