Effect of bromine chemistry on natural tropospheric ozone: improved simulation of observations from the turn of the 20 th century

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1 1 Effect of bromine chemistry on natural tropospheric ozone: improved simulation of observations from the turn of the 20 th century Justin P. Parrella 1, Mathew J. Evans 2, Daniel J. Jacob 1, Qing Liang 3, Loretta J. Mickely 1, Benjamin Miller 1, John A. Pyle 4,5, Xin Yang 4,5 1 School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA. 2 School of Earth and the Environment, University of Leeds, Leeds, UK. 3 Goddard Earth Sciences & Technology Center, University of Maryland, Baltimore County, MD, United States. 4 National Centre for Atmospheric Science (NCAS), Cambridge, CB2 1EW, UK. 5 Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW, UK. Ozone is a toxic gas in surface air and a major anthropogenic greenhouse gas in the troposphere. Current models systematically overestimate the low surface ozone concentrations measured in the late 19 th and early 20 th century 1,2,3,4. This suggests that current values of the natural ozone background used as baseline for air quality and climate policy are too high. Here we show that inclusion of tropospheric bromine chemistry in two different global models for the preindustrial atmosphere significantly improves agreement with ozone measurements of a century ago, both in terms of magnitude and seasonal variation. A natural surface ozone background of 13 ± 4 ppb is computed for the US, lower than current estimates and implying a greater margin for ozone reduction by emission controls. Tropospheric ozone (O 3 ) plays a central role in Earth s atmospheric photochemistry and climate as an oxidant and greenhouse gas 5. Ozone is generated in the troposphere by photochemical oxidation of CO and volatile organic compounds (VOCs) in the presence

2 2 of NO x (NO x NO + NO 2 ). These precursors have large anthropogenic as well as natural sources. Quantifying the contributions from different sources to ozone requires 3-D chemical transport models (CTMs) with detailed representations of emissions, transport, and chemistry. Natural background ozone can be inferred from these models by zeroing anthropogenic emissions, and this has provided the standard reference point for assessing the radiative forcing from anthropogenic ozone 5 and the potential benefits of air quality regulations 6. However, the modelled ozone background concentrations computed in this manner are typically % higher than values measured at surface sites at the turn of the 20 th century (5-15 ppb), and also do not reproduce the observed seasonal variations 1,2,3,4. Most of these old measurements were made using the Schönbein method 7, which is susceptible to interference from relative humidity and reducing gases 8. However, measurements taken at Montsouris near Paris with a quantitative and reliable arsenate method show values in the same range as the others 7,8,9. We propose here that the apparent model overestimates of the natural ozone background could be caused by tropospheric bromine chemistry missing from the standard CTMs. Bromine radicals (BrO x Br + BrO) are well-known catalysts for ozone destruction in the stratosphere 10,11, and also drive ozone depletion in polar surface air 12. In the stratosphere, they originate from photolysis and oxidation of halons and bromocarbons 10,11. In polar surface air, photochemical reactions involving sea salt in aerosols or on sea ice are the primary sources 12. Bromine radicals are also generated throughout the troposphere by oxidation and photolysis of very short-lived (VSL) bromocarbons, the most important being bromoform (CHBr 3 ) and dibromomethane (CH 2 Br 2 ) of marine origin 11. In the marine boundary layer (MBL), debromination of sea-salt aerosols is an additional source 13. Spectral measurements by differential optical absorption spectroscopy (DOAS) made from ground-based and balloon platforms suggest a global BrO tropospheric background of ppt 14, which can be explained

3 3 by global models with known sources and photochemistry 15. Bromine present at these trace concentrations would have important consequences for global tropospheric ozone in the present-day atmosphere, mostly through NO x loss due to BrONO 2 hydrolysis and to direct O 3 loss by the catalytic cycle involving HOBr formation and photolysis 15,16,17. As we discuss here, bromine chemistry could also play an important role in reconciling models and observations for the pre-industrial atmosphere. We use here two 3-D global tropospheric chemical transport models, GEOS- Chem and p-tomcat, to investigate the natural background ozone. Bromine chemistry in p-tomcat is described in previous work 15,18 and treats all sources of tropospheric bromine discussed below for GEOS-Chem, with, in addition, a source from blowing snow in polar regions 18,19. We developed a tropospheric bromine simulation capability in the GEOS-Chem global CTM 20, v , coupled with the standard ozone-no x -VOC-aerosol chemistry in that model. The tropospheric ozone simulation in GEOS-Chem has been evaluated against ozonesonde and satellite observations in a number of recent studies 21,22,23. Significant recent improvements in the model include the Linoz parameterization of stratospheric ozone chemistry 24, and non-local boundary layer mixing 25. All GEOS- Chem simulations shown here are conducted with 4 o latitude x 5 o longitude resolution, using GEOS-5 assimilated meteorological fields for 2007 from the NASA Global Modelling and Assimilation Office (GMAO). p-tomcat simulations shown here are performed on a 2.8 x 2.8 (T42) spatial grid with meteorological fields from the European Centre for Medium-Range Weather Forecasts (ECMWF). In both GEOS- Chem and p-tomcat, our simulations of the preindustrial atmosphere differ from present-day only in emissions. In GEOS-Chem, we remove all anthropogenic sources, including NO x from fertilizer use, and reduce methane from 1700 ppb to 700 ppb. Very similar conditions are applied for p-tomcat preindustrial simulations. Biomass

4 4 burning is decreased to 10% of its present value; a sensitivity simulation with biomass burning maintained at its present value shows little difference for ozone. Our representation of bromine chemistry in GEOS-Chem includes 10 species: Br 2, Br, BrO, HBr, HOBr, BrONO 2, BrNO 2, CH 3 Br, CH 2 Br 2, and CHBr 3, largely following Yang et al. [2005] 15 with updated rate constants 26. Inorganic bromine radicals (Br, BrO) are produced from CHBr 3 (mean tropospheric lifetime of 21 days against oxidation by OH and photolysis), CH 2 Br 2 (90 days), CH 3 Br (1.1 years), and sea-salt bromide. Br and BrO rapidly cycle between each other and with their non-radical reservoirs, defining the collective inorganic bromine (Br y ) family. HBr, the most stable non-radical reservoir, has a mean tropospheric lifetime of only 8 hours in GEOS-Chem against oxidation by OH to regenerate Br atoms. Eventually, Br y is removed from the atmosphere by wet and dry deposition of HBr, HOBr, and BrONO 2. Recycling of Br y by aerosol chemistry in the model is limited to hydrolysis of BrONO 2 producing HOBr and HNO 15,27 3. Figure 1 compares observed mean vertical profiles of CHBr 3 and CH 2 Br 2 over the North Pacific from the TRACE-P (2001) and INTEX-B (2006) field campaigns 28,29 to concentrations simulated using GEOS-Chem. Marine emissions of these bromocarbons are from Liang et al. [2010] 30, with emissions for CHBr 3 scaled seasonally at northern latitudes greater than 30 N to better match observations from a decade of NASA field campaigns 28,29,31,32,33, as is shown in the figure 1 insets. The resulting global sources are 407 Gg Br a -1 for CHBr 3 and 57 Gg Br a -1 for CH 2 Br 2. The successful simulation of the vertical gradients as illustrated in Figure 1 provides some confidence in the modelcomputed loss rates producing bromine radicals. CH 3 Br has a sufficiently long lifetime to be well-mixed in the troposphere and we impose a surface air concentration of 7-8 ppt in the present-day atmosphere based on observations 34, for an implied source of 52 Gg Br a -1, in good agreement with literature estimates for loss to tropospheric OH 35. In the preindustrial atmosphere the CH 3 Br surface air concentration is reduced to 5 ppt

5 5 based on ice core records 10,36 and the implied source is 34 Gg Br a -1. The sea-salt source of gaseous inorganic bromine is computed by applying observed sea-salt debromination fractions 15 to the sea salt source in GEOS-Chem 37. The resulting sea-salt source of 2800 Gg Br a -1 is large relative to the bromocarbons but mainly confined to the MBL where deposition is fast. We find in the model that CHBr 3 dominates the source of inorganic bromine in the free troposphere where the effect on global ozone is most important. Comparisons between bromocarbon observations and p-tomcat have been discussed previously 38. Figure 2 shows the annual zonal mean BrO concentration for the present-day atmosphere as simulated by GEOS-Chem. Our tropospheric concentrations are typically in the ppt range, at the low end of the ppt observation range reported by Platt and Hönninger [2003] 14, and comparable with p-tomcat 15. Higher concentrations could be achieved in the model by including aerosol-phase recycling of HBr and HOBr to bromine radicals, as in other CTM studies 16,18, but that chemistry remains hypothetical on the scale of the global troposphere 39. Model BrO concentrations increase with altitude, reflecting the decreasing efficiency of the Br y deposition sink. Preindustrial bromine in the model is on average 0.5 ppt higher than present-day at midlatitudes and 0.2 ppt higher in the tropics, reflecting the slower rate of BrO x loss by the CH 2 O + Br reaction (due to the reduced methane source of CH 2 O) and BrO + NO 2. We now compare our simulations of ozone for the pre-industrial atmosphere with and without bromine chemistry. Figure 3 shows simulated and observed seasonal variations for six representative locations around the turn of the 20 th century 8,9,40. Our simulations without bromine chemistry are largely consistent with Mickley et al. [2001] 2 and too high by up to 10 ppb (14 ppb) for GEOS-Chem (p-tomcat) compared with observations. Including tropospheric bromine chemistry decreases simulated monthly ozone by 1 6 ppb, largely correcting the overestimate. The impact

6 6 of bromine is most pronounced in winter and spring, when competition from other photochemical sinks of ozone is less. This helps to reproduce the aseasonal behaviour of ozone observed at Montsouris and most other sites, which was previously regarded as a major discrepancy since stratospheric influx combined with slow photochemical loss generally causes a strong natural winter-spring maximum in models 1. Including bromine emissions from blowing snow at polar latitudes 18, as in the p-tomcat integrations, further reduces preindustrial ozone in springtime and improves the modelled aseasonality, especially at Montsouris. The pronounced summer minimum of ozone at Tokyo is due to the monsoon, and bromine has minimal impact then because of fast scavenging. In contrast, the impact of bromine at Tokyo is particularly large in winter (and corrects the previous model discrepancy), as sea-salt emissions of Br y are enhanced in the local MBL and the winter monsoon brings dry air masses from the Asian interior. Bromine does not completely remove ozone overestimates at all of the discussed surface sites, and further improvements might include treating iodine chemistry, which could increase ozone loss due to halogens by up to a factor of two in the MBL 41. Lower natural ozone due to bromine chemistry implies a much larger present-day radiative forcing from anthropogenic tropospheric ozone than the current best estimate of 0.35 W m -2 from IPCC [2007] 5. The forcing could be as large as 0.80 W m -2, 2 which can be compared to1.6 W m -2 for CO 5 2. Lower natural ozone also has implications for air quality policy as it means that more surface ozone in the present-day atmosphere is amenable to control. Figure 4 shows cumulative frequency distributions of simulated US natural background concentrations with vs. without bromine chemistry using GEOS-Chem. Values are shown as the maximum daily 8-hour average (MDA8) ozone concentration, which is the standard metric used in air quality policy. The simulation without bromine yields a mean natural ozone background of 17 ± 5 ppb, with a range from 5 to 50 ppb. The corresponding mean in the simulation with bromine is 13 ± 4 ppb

7 7 with a range from 5 to 38 ppb. In comparison, the current US air quality standard is 75 ppb (4 th highest yearly MDA8 value). A caveat to our results is that we find similar absolute decreases of ozone in the model for the present-day simulation when including tropospheric bromine chemistry. Although BrO concentrations are lower in the present-day atmosphere, ozone concentrations are higher so that absolute effects are comparable. Zhang et al. [2010] 23 recently presented an extensive global comparison of GEOS-Chem to ozonesonde and satellite measurements of tropospheric ozone. They found that the model was too high in southern mid-latitudes and the northern subtropics, but too low in the tropics. Including bromine chemistry improves the simulation in these former regions but aggravates the underestimate in the tropics and generates some new underestimates in northern mid-latitudes. This problem requires further investigation and could possibly be addressed by increasing the lightning, soil, and biomass burning sources of NO x. Acknowledgements. This work was funded by the NASA Atmospheric Composition Modeling and Analysis Program. J.A.P. and X.Y. thank NERC and NCAS for funding. Author Contributions. J.P.P. developed bromine simulation in GEOS-Chem, provided all GEOS-Chem output shown, contributed to comparisons between VSL observations and GEOS-Chem output, and contributed to interpretation of model results. M.J.E. contributed the paper concept and to interpretation of model results. D.J.J. contributed to scientific direction and interpretation of model results. Q.L. provided emissions of VSL bromocarbons. L.J.M. contributed to scientific direction and interpretation of results. B.M. performed comparisons between GEOS-Chem and observed VSL vertical profiles and calculated seasonal scaling for CHBr 3 emissions to improve agreement. J.A.P. and X.Y. contributed p- TOMCAT simulations and to interpretation of model results; they have led development of the bromine simulation in p-tomcat.

8 8 Author information. Correspondence and requests for materials should be addressed to J.P.P Figure 1 Comparison between GEOS-Chem and observed VSL bromocarbons. Main figures: Mean vertical profiles of CHBr 3 and CH 2 Br 2 over the North Pacific (30 o - 60 o N) through March, April, and May. Aircraft observations from the NASA TRACE-P (2001) and INTEX-B (2006) field campaigns 28,31 are compared to the model for concurrent locations in Horizontal bars are standard deviations in the observations. Insets: the seasonal variations in tropospheric columns north of 30 N compiled from NASA aircraft field campaigns (TRACE-P, INTEX-A, INTEX-B, ARCTAS) 28,29,31,32,33 and compared to model values. CHBr 3 emissions in the model were scaled down relative to the original fluxes from Liang et al. [2010] 30 to better fit the aircraft observations as shown inset. No adjustment was needed for CH 2 Br 2. Figure 2 Bromine monoxide as simulated by GEOS-Chem. Annual zonal mean BrO mixing ratio in parts per trillion (ppt) simulated by GEOS-Chem for the present-day atmosphere. Figure 3 Preindustrial surface ozone measurements compared to models. Monthly mean surface ozone around the turn of the 20 th century. Observations from representative sites worldwide 8,9,40 are compared to GEOS- Chem and p-tomcat model simulations without and with tropospheric bromine chemistry. Figure 4 Bromine s impact on maximum daily 8-hour average (MDA8) surface ozone. Annual cumulative probability distributions of MDA8 natural ozone concentrations in US surface air, for simulations with and without

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