Terje K. Berntsen

Transport of Asian air pollution to North America

Using observations from the Cheeka Peak Observatory in northwestern Washington State during March-April, 1997, we show that Asian anthropogenic emissions significantly impact the concentrations of a large number of atmospheric species in the air arriving to North America during spring. Isentropic back-trajectories can be used to identify possible times when this impact will be felt, however trajectories alone are not sufficient to indicate the presence of Asian pollutants. Detailed chemical and meteorological data from one of these periods (March 29th, 1997) indicates that the surface emissions were lifted into the free troposphere over Asia and then transported to North America in ∼6 days.

Radiative forcing in the 21st century due to ozone changes in the troposphere and the lower stratosphere

ranging from 0.40 to 0.78 W m 2 on a global and annual average. The lower stratosphere contributes an additional 7.5–9.3 DU to the calculated increase in the ozone column, increasing radiative forcing by 0.15–0.17 W m 2 . The modeled radiative forcing depends on the height distribution and geographical pattern of predicted ozone changes and shows a distinct seasonal variation. Despite the large variations between the 11 participating models, the calculated range for normalized radiative forcing is within 25%, indicating the ability to scale radiative forcing to global-mean ozone column change. INDEX TERMS: 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0341 Atmospheric Composition and Structure: Middle atmosphere—constituent transport and chemistry (3334) Citation: Gauss, M., et al., Radiative forcing in the 21st century due to ozone changes in the troposphere and the lower stratosphere, J. Geophys. Res., 108(D9), 4292, doi:10.1029/2002JD002624, 2003.

CO2 emissions from biomass combustion for bioenergy: atmospheric decay and contribution to global warming.

Carbon dioxide (CO2) emissions from biomass combustion are traditionally assumed climate neutral if the bioenergy system is carbon (C) flux neutral, i.e. the CO2 released from biofuel combustion approximately equals the amount of CO2 sequestered in biomass. This convention, widely adopted in life cycle assessment (LCA) studies of bioenergy systems, underestimates the climate impact of bioenergy. Besides CO2 emissions from permanent C losses, CO2 emissions from C flux neutral systems (that is from temporary C losses) also contribute to climate change: before being captured by biomass regrowth, CO2 molecules spend time in the atmosphere and contribute to global warming. In this paper, a method to estimate the climate impact of CO2 emissions from biomass combustion is proposed. Our method uses CO2 impulse response functions (IRF) from C cycle models in the elaboration of atmospheric decay functions for biomass‐derived CO2 emissions. Their contributions to global warming are then quantified with a unit‐based index, the GWPbio. Since this index is expressed as a function of the rotation period of the biomass, our results can be applied to CO2 emissions from combustion of all the different biomass species, from annual row crops to slower growing boreal forest.

METRICS OF CLIMATE CHANGE: ASSESSING RADIATIVE FORCING AND EMISSION INDICES

In this paper, we review existing and alternative metrics of climate change, with particular emphasis on radiative forcing and global warming potentials (GWPs), in terms of their scientific performance. Radiative forcing is assessed in terms of questions such as the utility of the concept, uncertainties and sensitivity to key assumptions. The assessment of emission indices focuses on the climate and other resulting impacts (end points) against which emissions are weighted; the extent to which (and how) time dependence is included, with regard to both emission control and impact; how cost issues are dealt with; and the sensitivity of the metrics to various assumptions. It is concluded that the radiative forcing concept is a robust and useful metric of the potential climatic impact of various agents and that there are prospects for improvement by weighing different forcings according to their effectiveness. We also find that although the GWP concept is associated with serious shortcomings, it retains advantages over any of the proposed alternatives in terms of political feasibility. Alternative metrics, however, make a significant contribution to addressing important issues, and this contribution should be taken into account in the further development of refined metrics of climate change.

Climate forcing from the transport sectors

Although the transport sector is responsible for a large and growing share of global emissions affecting climate, its overall contribution has not been quantified. We provide a comprehensive analysis of radiative forcing from the road transport, shipping, aviation, and rail subsectors, using both past- and forward-looking perspectives. We find that, since preindustrial times, transport has contributed ≈15% and 31% of the total man-made CO2 and O3 forcing, respectively. A forward-looking perspective shows that the current emissions from transport are responsible for ≈16% of the integrated net forcing over 100 years from all current man-made emissions. The dominating contributor to positive forcing (warming) is CO2, followed by tropospheric O3. By subsector, road transport is the largest contributor to warming. The transport sector also exerts cooling through reduced methane lifetime and atmospheric aerosol effects. Shipping causes net cooling, except on future time scales of several centuries. Much of the forcing from transport comes from emissions not covered by the Kyoto Protocol.

Fresh air in the 21st century

Ozone is an air quality problem today for much of the worlds population. Regions can exceed the ozone air quality standards (AQS) through a combination of local emissions, meteorology favoring pollution episodes, and the clean-air baseline levels of ozone upon which pollution builds. The IPCC 2001 assessment studied a range of global emission scenarios and found that all but one projects increases in global tropospheric ozone during the 21st century. By 2030, near-surface increases over much of the northern hemisphere are estimated to be about 5 ppb (+2 to +7 ppb over the range of scenarios). By 2100 the two more extreme scenarios project baseline ozone increases of >20 ppb, while the other four scenarios give changes of -4 to +10 ppb. Even modest increases in the background abundance of tropospheric ozone might defeat current AQS strategies. The larger increases, however, would gravely threaten both urban and rural air quality over most of the northern hemisphere.

Influence of Asian emissions on the composition of air reaching the North Western United states

A global 3-D CTM has been used to study the impact of current and future emissions from Asia on CO, PAN and O3 across the North Pacific. Recent measurements from Washington State have been used to verify the model results with respect to average concentrations as well as amplitude of perturbations during episodic events of strong Asian influence. By performing CTM experiments with and without anthropogenic emissions from Asia, we find that there is a significant contribution from Asian sources in the air arriving to the North Western United States, especially during spring. The mean contribution, which can not easily be inferred from the available measurements, during spring are 34 ppbv, 26 pptv and 4 ppbv for CO, PAN and O3, respectively. The maximum enhancements caused by Asian sources during episodes are 42 ppbv, 75 pptv, and 7.5 ppbv, respectively. The amplitude of the perturbations during short term (2–5 days) episodes of strong Asian influence are quite similar to springtime Asian pollution events which have recently been observed on the west coast of Washington state. A doubling of the current Asian emissions in the model yields significant enhancements in all species, though not necessarily in a linear manner.

A global three‐dimensional chemical transport model for the troposphere: 1. Model description and CO and ozone results

A global three-dimensional photochemical tracer/transport model (CTM) of the troposphere has been developed. The model is based on the NASA/Goddard Institute for Space Studies (GISS) CTM with the incorporation of an extensive photochemical scheme. The model resolution is 8° latitude and 10° longitude with nine vertical layers below 10 hPa. One year of meteorological data from a free running GCM (NASA/GISS), including advective winds and convection frequency with 8-hour time resolution, is used as input. Transport of species by advection, convection, and diffusion is included in the model. The chemical scheme consists of 49 components, 85 thermal reactions, and 16 photolytic reactions. The chemical scheme is solved by the quasi steady state approximation (QSSA) method with iterations and chemical families, with a time step of 30 min. The model simulates well the lower tropospheric distribution of key species like carbon monoxide, nonmethane hydrocarbons (NMHCs), and ozone. The model is also able to simulate the important pattern of background NOx distribution [see Jaffe et al. this issue]. In the upper troposphere, coarse model resolution gives some discrepancies between modeled ozone concentrations and observations, especially at high latitudes. A global tropospheric ozone budget is presented. Net ozone production is found in the boundary layer and in the upper troposphere. In the middle free troposphere there is a close balance between chemical loss and production of ozone, giving a small net ozone loss. Hydroxyl (OH) concentrations are found to be sensitive to parameterization of cloud effects on photolysis rates. A global mean tropospheric OH concentration of 1.1×106 molecules/cm3 is calculated, which is about 15% higher than recent estimates from analysis of CH3CCI3 observations indicate.

Comparing the climate effect of emissions of short- and long-lived climate agents

Multi-gas climate agreements require a metric by which emissions of gases with different lifetimes and radiative properties can be placed on a common scale. The Kyoto Protocol to the United Nations Framework Convention on Climate Change uses the global warming potential (GWP) as such a metric. The GWP has attracted particular criticism as being inappropriate in the context of climate policy which seeks to restrict warming below a given target, because it gives equal weight to emissions irrespective of the target and the proximity to the target. The use of an alternative metric, the time-dependent global temperature change potential (GTP), is examined for its suitability and the prospects for it including very short-lived species. It retains the transparency and relative ease of use, which are attractive features of the GWP, but explicitly includes a dependence on the target of climate policy. The weighting of emissions using the GTP is found to be significantly dependent on the scenarios of future emissions and the sensitivity of the climate system. This may indicate that the use of any GTP-based weighting in future policymaking would necessitate regular revisions, as the global-mean temperature moves towards a specified target.

Application of the CALIOP layer product to evaluate the vertical distribution of aerosols estimated by global models: AeroCom phase i results

[1] The CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) layer product is used for a multimodel evaluation of the vertical distribution of aerosols. Annual and seasonal aerosol extinction profiles are analyzed over 13 sub-continental regions representative of industrial, dust, and biomass burning pollution, from CALIOP 2007–2009 observations and from AeroCom (Aerosol Comparisons between Observations and Models) 2000 simulations. An extinction mean height diagnostic (Za) is defined to quantitatively assess the models’ performance. It is calculated over the 0–6 km and 0–10 km altitude ranges by weighting the altitude of each 100 m altitude layer by its aerosol extinction coefficient. The mean extinction profiles derived from CALIOP layer products provide consistent regional and seasonal specificities and a low inter-annual variability. While the outputs from most models are significantly correlated with the observed Za climatologies, some do better than others, and 2 of the 12 models perform particularly well in all seasons. Over industrial and maritime regions, most models show higher Za than observed by CALIOP, whereas over the African and Chinese dust source regions, Za is underestimated during Northern Hemisphere Spring and Summer. The positive model bias in Za is mainly due to an overestimate of the extinction above 6 km. Potential CALIOP and model limitations, and methodological factors that might contribute to the differences are discussed.