Tropospheric Ozone Radiative Forcing Research Articles

Tropospheric ozone radiative forcing uncertainty due to pre-industrial fire and biogenic emissions

Abstract. Tropospheric ozone concentrations are sensitive to natural emissions of precursor compounds. In contrast to existing assumptions, recent evidence indicates that terrestrial vegetation emissions in the pre-industrial era were larger than in the present day. We use a chemical transport model and a radiative transfer model to show that revised inventories of pre-industrial fire and biogenic emissions lead to an increase in simulated pre-industrial ozone concentrations, decreasing the estimated pre-industrial to present-day tropospheric ozone radiative forcing by up to 34 % (0.38 to 0.25 W m−2). We find that this change is sensitive to employing biomass burning and biogenic emissions inventories based on matching vegetation patterns, as the co-location of emission sources enhances the effect on ozone formation. Our forcing estimates are at the lower end of existing uncertainty range estimates (0.2–0.6 W m−2), without accounting for other sources of uncertainty. Thus, future work should focus on reassessing the uncertainty range of tropospheric ozone radiative forcing.

Impact of uncertainties in inorganic chemical rate constants on tropospheric composition and ozone radiative forcing

Abstract. Chemical rate constants determine the composition of the atmosphere and how this composition has changed over time. They are central to our understanding of climate change and air quality degradation. Atmospheric chemistry models, whether online or offline, box, regional or global, use these rate constants. Expert panels evaluate laboratory measurements, making recommendations for the rate constants that should be used. This results in very similar or identical rate constants being used by all models. The inherent uncertainties in these recommendations are, in general, therefore ignored. We explore the impact of these uncertainties on the composition of the troposphere using the GEOS-Chem chemistry transport model. Based on the Jet Propulsion Laboratory (JPL) and International Union of Pure and Applied Chemistry (IUPAC) evaluations we assess the influence of 50 mainly inorganic rate constants and 10 photolysis rates on tropospheric composition through the use of the GEOS-Chem chemistry transport model. We assess the impact on four standard metrics: annual mean tropospheric ozone burden, surface ozone and tropospheric OH concentrations, and tropospheric methane lifetime. Uncertainty in the rate constants for NO2 + OH →M HNO3 and O3 + NO → NO2 + O2 are the two largest sources of uncertainty in these metrics. The absolute magnitude of the change in the metrics is similar if rate constants are increased or decreased by their σ values. We investigate two methods of assessing these uncertainties, addition in quadrature and a Monte Carlo approach, and conclude they give similar outcomes. Combining the uncertainties across the 60 reactions gives overall uncertainties on the annual mean tropospheric ozone burden, surface ozone and tropospheric OH concentrations, and tropospheric methane lifetime of 10, 11, 16 and 16 %, respectively. These are larger than the spread between models in recent model intercomparisons. Remote regions such as the tropics, poles and upper troposphere are most uncertain. This chemical uncertainty is sufficiently large to suggest that rate constant uncertainty should be considered alongside other processes when model results disagree with measurement. Calculations for the pre-industrial simulation allow a tropospheric ozone radiative forcing to be calculated of 0.412 ± 0.062 W m−2. This uncertainty (13 %) is comparable to the inter-model spread in ozone radiative forcing found in previous model–model intercomparison studies where the rate constants used in the models are all identical or very similar. Thus, the uncertainty of tropospheric ozone radiative forcing should expanded to include this additional source of uncertainty. These rate constant uncertainties are significant and suggest that refinement of supposedly well-known chemical rate constants should be considered alongside other improvements to enhance our understanding of atmospheric processes.

Sensitivity of midnineteenth century tropospheric ozone to atmospheric chemistry‐vegetation interactions

AbstractWe use an Earth System model (HadGEM2‐ES) to investigate the sensitivity of midnineteenth century tropospheric ozone to vegetation distribution and atmospheric chemistry‐vegetation interaction processes. We conduct model experiments to isolate the response of midnineteenth century tropospheric ozone to vegetation cover changes between the 1860s and present day and to CO2‐induced changes in isoprene emissions and dry deposition over the same period. Changes in vegetation distribution and CO2 suppression of isoprene emissions between midnineteenth century and present day lead to decreases in global isoprene emissions of 19% and 21%, respectively. This results in increases in surface ozone over the continents of up to 2 ppbv and of 2–6 ppbv in the tropical upper troposphere. The effects of CO2 increases on suppression of isoprene emissions and suppression of dry deposition to vegetation are small compared with the effects of vegetation cover change. Accounting for present‐day climate in addition to present‐day vegetation cover and atmospheric CO2 concentrations leads to increases in surface ozone concentrations of up to 5 ppbv over the entire northern hemisphere (NH) and of up to 8 ppbv in the NH free troposphere, compared with a midnineteenth century control simulation. Ozone changes are dominated by the following: (1) the role of isoprene as an ozone sink in the low NOx midnineteenth century atmosphere and (2) the redistribution of NOx to remote regions and the free troposphere via PAN (peroxyacetyl nitrate) formed from isoprene oxidation. We estimate a tropospheric ozone radiative forcing of 0.264 W m−2 and a sensitivity in ozone radiative forcing to midnineteenth century to present‐day vegetation cover change of −0.012 W m−2.

The contribution of greenhouse gases to the recent slowdown in global-mean temperature trends

The recent slowdown in the rate of increase in global-mean surface temperature (GMST) has generated extensive discussion, but little attention has been given to the contribution of time-varying trends in greenhouse gas concentrations. We use a simple model approach to quantify this contribution. Between 1985 and 2003, greenhouse gases (including well-mixed greenhouse gases, tropospheric and stratospheric ozone, and stratospheric water vapour from methane oxidation) caused a reduction in GMST trend of around 0.03–0.05 K decade−1 which is around 18%–25% of the observed trend over that period. The main contributors to this reduction are the rapid change in the growth rates of ozone-depleting gases (with this contribution slightly opposed by stratospheric ozone depletion itself) and the weakening in growth rates of methane and tropospheric ozone radiative forcing. Although CO2 is the dominant greenhouse gas contributor to GMST trends, the continued increase in CO2 concentrations offsets only about 30% of the simulated trend reduction due to these other contributors. These results emphasize that trends in non-CO2 greenhouse gas concentrations can make significant positive and negative contributions to changes in the rate of warming, and that they need to be considered more closely in analyses of the causes of such variations.

Ozone database in support of CMIP5 simulations: results and corresponding radiative forcing

Abstract. A continuous tropospheric and stratospheric vertically resolved ozone time series, from 1850 to 2099, has been generated to be used as forcing in global climate models that do not include interactive chemistry. A multiple linear regression analysis of SAGE I+II satellite observations and polar ozonesonde measurements is used for the stratospheric zonal mean dataset during the well-observed period from 1979 to 2009. In addition to terms describing the mean annual cycle, the regression includes terms representing equivalent effective stratospheric chlorine (EESC) and the 11-yr solar cycle variability. The EESC regression fit coefficients, together with pre-1979 EESC values, are used to extrapolate the stratospheric ozone time series backward to 1850. While a similar procedure could be used to extrapolate into the future, coupled chemistry climate model (CCM) simulations indicate that future stratospheric ozone abundances are likely to be significantly affected by climate change, and capturing such effects through a regression model approach is not feasible. Therefore, the stratospheric ozone dataset is extended into the future (merged in 2009) with multi-model mean projections from 13 CCMs that performed a simulation until 2099 under the SRES (Special Report on Emission Scenarios) A1B greenhouse gas scenario and the A1 adjusted halogen scenario in the second round of the Chemistry-Climate Model Validation (CCMVal-2) Activity. The stratospheric zonal mean ozone time series is merged with a three-dimensional tropospheric data set extracted from simulations of the past by two CCMs (CAM3.5 and GISS-PUCCINI) and of the future by one CCM (CAM3.5). The future tropospheric ozone time series continues the historical CAM3.5 simulation until 2099 following the four different Representative Concentration Pathways (RCPs). Generally good agreement is found between the historical segment of the ozone database and satellite observations, although it should be noted that total column ozone is overestimated in the southern polar latitudes during spring and tropospheric column ozone is slightly underestimated. Vertical profiles of tropospheric ozone are broadly consistent with ozonesondes and in-situ measurements, with some deviations in regions of biomass burning. The tropospheric ozone radiative forcing (RF) from the 1850s to the 2000s is 0.23 W m−2, lower than previous results. The lower value is mainly due to (i) a smaller increase in biomass burning emissions; (ii) a larger influence of stratospheric ozone depletion on upper tropospheric ozone at high southern latitudes; and possibly (iii) a larger influence of clouds (which act to reduce the net forcing) compared to previous radiative forcing calculations. Over the same period, decreases in stratospheric ozone, mainly at high latitudes, produce a RF of −0.08 W m−2, which is more negative than the central Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) value of −0.05 W m−2, but which is within the stated range of −0.15 to +0.05 W m−2. The more negative value is explained by the fact that the regression model simulates significant ozone depletion prior to 1979, in line with the increase in EESC and as confirmed by CCMs, while the AR4 assumed no change in stratospheric RF prior to 1979. A negative RF of similar magnitude persists into the future, although its location shifts from high latitudes to the tropics. This shift is due to increases in polar stratospheric ozone, but decreases in tropical lower stratospheric ozone, related to a strengthening of the Brewer-Dobson circulation, particularly through the latter half of the 21st century. Differences in trends in tropospheric ozone among the four RCPs are mainly driven by different methane concentrations, resulting in a range of tropospheric ozone RFs between 0.4 and 0.1 W m−2 by 2100. The ozone dataset described here has been released for the Coupled Model Intercomparison Project (CMIP5) model simulations in netCDF Climate and Forecast (CF) Metadata Convention at the PCMDI website (http://cmip-pcmdi.llnl.gov/).

Future tropospheric ozone simulated with a climate‐chemistry‐biosphere model

A climate‐chemistry model and a biogenic emission model are used to investigate the relative impact of anthropogenic and biogenic emissions of ozone precursors and global warming on the evolution of ozone in 2100. A warmer and wetter climate leads to enhanced ozone photochemical destruction in the lower troposphere, a more intense Brewer‐Dobson circulation in the stratosphere and a lightning NOx emission increased from 5 to 7.5 Tg(N)/yr. Over Europe and the eastern US climate change locally causes surface ozone to increase because of enhanced PAN thermal decomposition and more stagnant meteorological conditions. The global and annual mean OH concentration remains quite stable and the methane lifetime is unchanged in the future. Increased biogenic emissions contribute by 30–50% to surface ozone summer formation in northern continental regions. The feedback of climate change and of biogenic emissions increases the 2000 to 2100 tropospheric ozone radiative forcing by 12%, up to a global mean value of 0.58 Wm−2.

Evolution of tropospheric ozone under anthropogenic activities and associated radiative forcing of climate

The budget of ozone and its evolution associated with anthropogenic activities are simulated with the Model for Ozone and Related Chemical Tracers (MOZART) (version 1). We present the changes in tropospheric ozone and its precursors (CH4, NMHCs, CO, NOx) since the preindustrial period. The ozone change at the surface exhibits a maximum increase at midlatitudes in the northern hemisphere reaching more than a factor of 3 over Europe, North America, and Southeast Asia during summer. The calculated preindustrial ozone levels are particularly sensitive to assumptions about natural and biomass burning emissions of precursors. The possible future evolution of ozone to the year 2050 is also simulated, using the Intergovernmental Panel on Climate Change IS92a scenario to estimate the global and geographical changes in surface emissions. The future evolution of ozone stresses the important role played by the tropics and the subtropics. In this case a maximum ozone increase is calculated in the northern subtropical region and is associated with increased emissions in Southeast Asia and Central America. The ozone future evolution also affects the more remote regions of the troposphere, and an increase of 10–20% in background ozone levels is calculated over marine regions in the southern hemisphere. Our best estimate of the global and annual mean radiative forcing associated with tropospheric ozone increase since the preindustrial era is 0.43 W m−2. This value represents about 20% of the forcing associated with well‐mixed greenhouse gases. The normalized tropospheric ozone radiative forcing is 0.048 W m−2 DU−1. An upper estimate on our forcing of 0.77 W m−2 is calculated when a stratospheric tracer is used to approximate background ozone levels. In 2050 an additional ozone forcing of 0.26 W m−2 is calculated, providing a forcing from preindustrial to 2050 of 0.69 W m−2.

Effects of 1997 Indonesian forest fires on tropospheric ozone enhancement, radiative forcing, and temperature change over the Hong Kong region

Tropospheric ozone enhancements were measured over Hong Kong (22.2°N, 114.3°E) by electrochemical concentration cell ozonesondes during the 1997 period when many forest fires were burning in Indonesia. The enhancements have a maximum ozone concentration of up to 130 ppbv and an ozone‐enhanced layer depth of 10 km. We used Total Ozone Mapping Spectrometer, advanced very high resolution radiometer satellite image, and back air trajectory to identify the source region and the transport pattern of ozone. The strong tropospheric ozone enhancements covered all of tropical Southeast Asia and subtropical south China. They were due to photochemical ozone buildup from the biomass burning emissions from the Indonesian fires. The ozone‐rich air mass was transported to Hong Kong following the east Asia local Hadley circulation and an abnormal anticyclonic flow related to the El Niño phenomenon in the tropical western Pacific. A rough estimation of the radiative forcing due to the ozone enhancement was carried out for two cases in October and December in 1997 using a normalized tropospheric ozone radiative forcing parameter derived from the Unified Chemistry‐Climate model [Mickley et al., 1999]. The ozone enhancements induced an additional radiative forcing of 0.26 and 0.48 Wm−2 compared to the normal total forcing of 0.48 and 0.39 Wm−2 in October and December over the Hong Kong region. Estimation of the associated surface temperature change suggests that enhanced ozone from biomass burning on the scale of the 1997 Indonesian fires may have significant impact on regional surface temperature change.

Future estimates of tropospheric ozone radiative forcing and methane turnover — The impact of climate change

We present a range of estimates for future radiative forcings due to changes in tropospheric ozone (O3T). Ozone distributions were generated by the UKMO 3‐D chemistry‐transport model for 1990, 2030, 2060, and 2100, using four sets of boundary conditions. Anthropogenic emissions evolved following either the IPCC SRES “high” (A2) or “central” (B2) case. Each scenario was run with both a fixed (1990) climate, and with a changing climate, as generated by a coupled ocean‐atmosphere GCM, forced with IS92a emissions. Calculated global mean O3T radiative forcings for the A2 (B2) cases for 1990–2100 were +0.43 (+0.22) W m−2 when climate change was ignored; these fell to +0.27 (+0.09) W m−2 when climate change was included. Without climate change, CH4 lifetimes (τCH4) lengthened by 7–12% between 1990 and 2100; however, when climate change was included, τCH4 fell by 0–5%. Hence climate warming exerts a negative feedback on itself by enhancing O3T and CH4 destruction.

Evolution of tropospheric ozone radiative forcing

We present the first estimate of the evolution of tropospheric ozone (O3(T)) radiative forcing since 1860 and into the future. The UKMO 3‐D chemistry‐transport model (STOCHEM) was used to simulate the tropospheric composition in 1860, 1950, 1970, 1990 and 2100, by changing trace gas emissions. The future scenario used a doubled CO2 climate. STOCHEM includes extensive non‐methane hydrocarbon (NMHC) chemistry, and produces a reasonable simulation of present‐day O3(T). Radiative forcings caused by the modelled changes in O3(T) since 1860 were calculated using the UKMO radiation code, and included clouds and stratospheric temperature adjustment. Calculated changes in the global annual mean forcing since 1860 were 0.13, 0.22, 0.29 and 0.48 W m−2 for the four years. Up to 1990 this forcing scales linearly with the change in total NOx emissions since 1860; this linearity breaks down in 2100. The 1990 forcing is at the lower end of the range from previous modelling studies (0.28–0.51 W m−2), but is still significant, enhancing the well‐mixed greenhouse gas forcing by over 10%.