Bibliography - L. W. Horowitz

1. Naik, V., A. M. Fiore, L. W. Horowitz, and H. B. Singh, et al., 2010: Observational constraints on the global atmospheric budget of ethanol. Atomsphere Chemistry and Physics Discussion, http://www.atmos-chem-phys-discuss.net/10/925/2010/acpd-10-925-2010.pdf, 10, 925-945
   [ Abstract ]

   Energy security and climate change concerns have led to the promotion of biomass-derived ethanol, an oxygenated volatile organic compound (OVOC), as a substitute for fossil fuels. Although ethanol is ubiquitous in the troposphere, our knowledge of its current atmospheric budget and distribution is limited. Here, for the first time we use a global chemical transport model in conjunction with atmospheric observations to place constraints on the ethanol budget, noting that additional measurements of ethanol (and its precursors) are still needed to enhance confidence in our estimated budget. Global sources of ethanol in the model include 5.0 Tg yr⁻¹from industrial sources and biofuels, 9.2 Tg yr⁻¹ from terrestrial plants, ~0.5 Tg yr⁻¹ from biomass burning, and 0.05 Tg yr⁻¹ from atmospheric reactions of the ethyl peroxide radical (C₂H₅O₂) with itself and with the methyl peroxide radical (CH₃O₂). The resulting atmospheric lifetime of ethanol in the model is 2.8 days. Gas-phase oxidation by hydroxyl radical (OH) is the primary global sink of ethanol in the model (65%), followed by dry deposition to land (25%), and wet deposition (10%). Over continental areas, ethanol concentrations predominantly reflect direct anthropogenic and biogenic emission sources. Uncertainty in the biogenic ethanol emissions estimated at a factor of three may contribute to the 50% model underestimate of observations in the North American boundary layer. Furthermore, current levels of ethanol measured in remote atmospheres are an order of magnitude larger than those explained by surface sources or by in-situ atmospheric production from observed precursor hydrocarbons in the model, suggesting a major gap in understanding. Stronger constraints on the budget and distribution of ethanol and other VOCs are a critical step towards assessing the impacts of increasing use of ethanol as a fuel.

2. Gloor, M. N., E. Dlugokensky, C. Brenninkmeijer, L. W. Horowitz, D. F. Hurst, G. Dutton, C. Crevoisier, T. Machida, and P. P. Tans, 2007: Three-dimensional SF₆ data and tropospheric transport simulations: Signals, modeling accuracy, and implications for inverse modeling. Journal of Geophysical Research, 112(D15112), doi:10.1029/2006JD007973
   [ Abstract ]

   Surface emissions of SF₆ are closely tied to human activity and thus fairly well known. They therefore can and have been used to evaluate tropospheric transport predicted by models. A range of new atmospheric SF₆ data permit us to expand on earlier studies. The purpose of this first of two papers is to characterize known and new transport constraints provided by the data and to use them to quantify predictive skill of the MOZART-2 atmospheric chemistry and transport model. Main noteworthy observational constraints are (1) a well-known steep N-S gradient at the surface confined to an ≈40° wide latitude band in the tropics; (2) a fairly uniform N-S gradient in the upper troposphere; (3) an increase in the temporal variation in upper troposphere Northern Hemisphere records with increasing latitude; (4) a negative SF₆ gradient in Northern Hemisphere vertical profiles from the surface to 8 km height, but a positive gradient in the Southern Hemisphere; and (5) a clear reflection in surface records of large-scale seasonal atmosphere movements like the undulations of the Intertropical Convergence Zone (ITCZ). Comparison of observations with simulations reveal excellent modeling skills with regards to (1) large-scale annual mean latitudinal gradients at remote surface sites (relative bias of N-S hemisphere difference ≤ 5%) and aloft (≈10 km, relative bias ≤ 25%); (2) seasonality in signals at remote sites caused by large-scale movements of the atmosphere; (3) time variation in upper troposphere records; (4) ‘‘faithfulness’’ of advective transport on timescales up to ≈1 week; and (5) the general shapes and seasonal variation of vertical profiles. The model (1) underestimates the variation in the vertical of profiles, particularly those from locations close to high emissions regions, and (2) overestimates the difference in SF₆ between the planetary boundary layer (PBL) and free troposphere over North America, and thus likely Eurasia, during winter by approximately a factor of 2 (STD ≈ 100%). The comparisons permit estimating lower bounds on representation errors which are large for sites close to continental outflow regions. Given the magnitude of the signals and signal variance, SF₆ provides a strong constraint on interhemispheric transport, PBL ventilation, dispersion pathways of northern midlatitude surface emissions through the upper troposphere, and large-scale movements of the atmosphere.

3. Naik, V., D. L. Mauzerall, L. W. Horowitz, M. D. Schwarzkopf, V. Ramaswamy, and Michael Oppenheimer, 2007: On the sensitivity of radiative forcing from biomass burning aerosols and ozone to emission location. Geophysical Research Letters, 34(L03818), doi:10.1029/2006GL028149
   [ Abstract ]

   Biomass burning is a major source of air pollutants, some of which are also climate forcing agents. We investigate the sensitivity of direct radiative forcing due to tropospheric ozone and aerosols (carbonaceous and sulfate) to a marginal reduction in their (or their precursor) emissions from major biomass burning regions. We find that the largest negative global forcing is for 10% emission reductions in tropical regions, including Africa (-4.1 mWm-2 from gas and -4.1 mWm-2 from aerosols), and South America (-3.0 mWmfrom gas and -2.8 mWmfrom aerosols). We estimate that a unit reduction in the amount of biomass burned in India produces the largest negative ozone and aerosol forcing. Our analysis indicates that reducing biomass burning emissions causes negative global radiative forcing due to ozone and aerosols; however, regional differences need to be considered when evaluating controls on biomass burning to mitigate global climate change.

4. Crevoisier, C., M. N. Gloor, E. Gloaguen, L. W. Horowitz, Jorge Sarmiento, C. Sweeney, and P. P. Tans, 2006: A direct carbon budgeting approach to infer carbon sources and sinks. Design and synthetic application to complement the NACP observation network. Tellus, 58(5), doi:10.1111/j.1600-0889.2006.00214.x 366-375
   [ Abstract ]

   In order to exploit the upcoming regular measurements of vertical carbon dioxide (CO₂) profiles over North America implemented in the framework of the North American Carbon Program (NACP), we design a direct carbon budgeting approach to infer carbon sources and sinks over the continent using model simulations. Direct budgeting puts a control volume on top of North America, balances air mass in- and outflows into the volume and solves for the surface fluxes. The flows are derived from the observations through a geostatistical interpolation technique called Kriging combined with transport fields from weather analysis. The use of CO₂ vertical profiles simulated by the atmospheric transport model MOZART-2 at the planned 19 stations of the NACP network has given an estimation of the error of 0.39 GtC yr^-1 within the model world. Reducing this error may be achieved through a better estimation of mass fluxes associated with convective processes affecting North America. Complementary stations in the north-west and the north-east are also needed to resolve the variability of CO₂ in these regions. For instance, the addition of a single station near 52°N; 110°W is shown to decrease the estimation error to 0.34 GtC yr^-1.

5. Naik, V., D. L. Mauzerall, L. W. Horowitz, M. D. Schwarzkopf, V. Ramaswamy, and Michael Oppenheimer, 2006: Net radiative forcing due to changes in regional emissions of tropospheric ozone precursors. Journal of Geophysical Research, 110(D24306), doi:10.1029/2005JD005908
   [ Abstract ]

   The global distribution of tropospheric ozone (O₃) depends on the emission of precursors, chemistry, and transport. For small perturbations to emissions, the global radiative forcing resulting from changes in O₃ can be expressed as a sum of forcings from emission changes in different regions. Tropospheric O₃ is considered in present climate policies only through the inclusion of indirect effect of CH₄ on radiative forcing through its impact on O₃ concentrations. The short-lived O₃ precursors (NO_x, CO, and NMHCs) are not directly included in the Kyoto Protocol or any similar climate mitigation agreement. In this study, we quantify the global radiative forcing resulting from a marginal reduction (10%) in anthropogenic emissions of NO_x alone from nine geographic regions and a combined marginal reduction in NO_x, CO, and NMHCs emissions from three regions. We simulate, using the global chemistry transport model MOZART-2, the change in the distribution of global O₃ resulting from these emission reductions. In addition to the short-term reduction in O₃, these emission reductions also increase CH₄ concentrations (by decreasing OH); this increase in CH₄ in turn counteracts part of the initial reduction in O₃ concentrations. We calculate the global radiative forcing resulting from the regional emission reductions, accounting for changes in both O₃ and CH₄. Our results show that changes in O₃ production and resulting distribution depend strongly on the geographical location of the reduction in precursor emissions. We find that the global O₃ distribution and radiative forcing are most sensitive to changes in precursor emissions from tropical regions and least sensitive to changes from midlatitude and high-latitude regions. Changes in CH₄ and O₃ concentrations resulting from NO_x emission reductions alone produce offsetting changes in radiative forcing, leaving a small positive residual forcing (warming) for all regions. In contrast, for combined reductions of anthropogenic emissions of NO_x, CO, and NMHCs, changes in O₃ and CH₄ concentrations result in a net negative radiative forcing (cooling). Thus we conclude that simultaneous reductions of CO, NMHCs, and NO_x lead to a net reduction in radiative forcing due to resulting changes in tropospheric O₃ and CH₄ while reductions in NO_x emissions alone do not.

6. Fiore, A. M., L. W. Horowitz, D. W. Purves, H. Levy II, M. J. Evans, Y. Wang, Y. Li, and R. M. Yantosca, 2005: Evaluating the contribution of changes in isoprene emissions to surface ozone trends over the eastern United States. Journal of Geophysical Research, 110(D12303), doi:10.1029/2004JD005485
   [ Abstract ]

   Reducing surface ozone (O₃) to concentrations in compliance with the national air quality standard has proven to be challenging, despite tighter controls on O₃ precursor emissions over the past few decades. New evidence indicates that isoprene emissions changed considerably from the mid-1980s to the mid-1990s owing to land-use changes in the eastern United States (Purves et al., 2004). Over this period, U.S. anthropogenic VOC (AVOC) emissions decreased substantially. Here we apply two chemical transport models (GEOS-CHEM and MOZART-2) to test the hypothesis, put forth by Purves et al. (2004), that the absence of decreasing O₃ trends over much of the eastern United States may reflect a balance between increases in isoprene emissions and decreases in AVOC emissions. We find little evidence for this hypothesis; over most of the domain, mean July afternoon (1300–1700 local time) surface O₃ is more responsive (ranging from -9 to +7 ppbv) to the reported changes in anthropogenic NO_x emissions than to the concurrent isoprene (-2 to +2 ppbv) or AVOC (-2 to 0 ppbv) emission changes. The estimated magnitude of the O₃ response to anthropogenic NO_x emission changes, however, depends on the base isoprene emission inventory used in the model. The combined effect of the reported changes in eastern U.S. anthropogenic plus biogenic emissions is insufficient to explain observed changes in mean July afternoon surface O₃ concentrations, suggesting a possible role for decadal changes in meteorology, hemispheric background O₃, or subgrid-scale chemistry. We demonstrate that two major uncertainties, the base isoprene emission inventory and the fate of isoprene nitrates (which influence surface O₃ in the model by -15 to +4 and +4 to +12 ppbv, respectively), preclude a well-constrained quantification of the present-day contribution of biogenic or anthropogenic emissions to surface O₃ concentrations, particularly in the high-isopreneemitting southeastern United States. Better constraints on isoprene emissions and chemistry are needed to quantitatively address the role of isoprene in eastern U.S. air quality.

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