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

Li et al., in preparation.

Jian et al., in progress.

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]

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.

Fu et al., in progress.

Ye et al., in preparation.

Xu et al., in preparation

Xu, Tian, Chen, Zhang et al., in progress

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

Abstract | We analyzed observations from the Multi-angle Imaging SpectroRadiometer (MISR) to determine the injection heights of biomass-burning smoke plumes over peninsular Southeast Asia (PSEA, here defined as Vietnam, Cambodia, Thailand, Laos, and Myanmar) in the spring, with the goal of evaluating the impacts on long-range pollutant transport. We retrieved the heights of 22 000 MISR smoke pixels from 607 smoke plumes over PSEA during February to April of the years 2001–2010. Forty-five percent of the analyzed smoke pixels were above the local mean boundary layer (1 km) at MISR overpass time (10:30 a.m. local time). We used the GEOS–Chem model to simulate the transport of PSEA biomass-burning pollutants in March 2001. On a monthly mean basis, we found that the direct injection of 40% of the PSEA biomass-burning emissions had little impact on the long-range transport of CO to downwind regions, compared to a control simulation where all biomass-burning emissions were released in the boundary layer. This was because CO at the surface over PSEA was efficiently lifted into the free troposphere by deep convection associated with synoptic-scale weather systems. For pollutants with lifetimes shorter than the synoptic timescale, such as black carbon aerosol (BC), their long-range transport was much more sensitive to the initial plume injection height. The direct injection of NOx from PSEA biomass burning into the free troposphere drove increased formation and transport of peroxyacetyl nitrate (PAN), which in turn led to a small increase in ozone over downwind southern China and the northwestern Pacific. The Pacific subtropical high transported BC emitted from PSEA biomass burning to the marine boundary layer over the tropical northwestern Pacific. We compared our model results to aircraft measurements over the northwestern Pacific during the TRACE-P campaign (March 2001). The direct injection of 40% of the PSEA biomass-burning pollutants into the free troposphere in the model led to a more pronounced BC peak at 3 km over the northwestern Pacific. Our analysis highlights the point that the injection heights of smoke plumes presents great uncertainty over the interpretation of BC measurements downwind of biomass-burning regions.

Publication | Jian, Y., and T.-M. Fu* (2014), Injection heights of springtime biomass burning plumes over the Peninsular Southeast Asia and their impacts on pollutant long-range transport, Atmos. Chem. Phys., 14, 3977-3989, doi:10.5194/acp-14-3977-2014. PDF