Person:

Logan, Jennifer

We analyze the theoretical basis of three different methods to validate and intercompare satellite measurements of atmospheric composition, and apply them to tropospheric ozone retrievals from the Tropospheric Emission Spectrometer (TES) and the Ozone Monitoring Instrument (OMI). The first method (in situ method) uses in situ vertical profiles for absolute instrument validation; it is limited by the sparseness of in situ data. The second method (CTM method) uses a chemical transport model (CTM) as an intercomparison platform; it provides a globally complete intercomparison with relatively small noise from model error. The third method (averaging kernel smoothing method) involves smoothing the retrieved profile from one instrument with the averaging kernel matrix of the other; it also provides a global intercomparison but dampens the actual difference between instruments and adds noise from the a priori. We apply the three methods to a full year (2006) of TES and OMI data. Comparison with in situ data from ozonesondes shows mean positive biases of 5.3 parts per billion volume (ppbv) (10%) for TES and 2.8 ppbv (5%) for OMI at 500 hPa. We show that the CTM method (using the GEOS-Chem CTM) closely approximates results from the in situ method while providing global coverage. It reveals that differences between TES and OMI are generally less than 10 ppbv (18%), except at northern mid-latitudes in summer and over tropical continents. The CTM method further allows for CTM evaluation using both satellite observations. We thus find that GEOS-Chem underestimates tropospheric ozone in the tropics due to possible underestimates of biomass burning, soil, and lightning emissions. It overestimates ozone in the northern subtropics and southern mid-latitudes, likely because of excessive stratospheric influx of ozone.

Nitrogen oxide radicals (NOx) produced by lightning are natural precursors for the production of the dominant tropospheric oxidants, OH and ozone. Observations of the interannual variability (IAV) of tropical ozone and of global mean OH (from the methyl chloroform proxy) offer a window for understanding the sensitivity of ozone and OH to environmental factors. We present the results of simulations for 1998–2006 using the GEOS-Chem chemical transport model (CTM) with IAV in tropical lightning constrained by satellite observations from the Lightning Imaging Sensor. We find that this imposed IAV in lightning NOx improves the ability of the model to reproduce observed IAV in tropical ozone and OH. Lightning is far more important than biomass burning in driving the IAV of tropical ozone, even though the IAV of NOx emissions from fires is greater than that from lightning. Our results indicate that the IAV in tropospheric OH is highly sensitive to lightning relative to other emissions and suggest that lightning contributes an important fraction of the observed IAV in OH inferred from the methyl chloroform proxy. Lightning affects OH through the HO2+ NO reaction, an effect compounded by positive feedback from the resulting increase in ozone production and in CO loss. We can account in the model for the observed increase in OH in 1998–2004 and for its IAV, but the model fails to explain the OH decrease in 2004–2006. We find that stratospheric ozone plays little role in driving IAV in OH during 1998–2006, in contrast to previous studies that examined earlier periods.

Photochemistry occuring in biomass burning plumes over the tropical south Atlantic is analyzed using data collected during the Transport and Atmospheric Chemistry Near the Equator-Atlantic aircraft expedition conducted during the tropical dry season in September 1992 and a photochemical point model. Enhancement ratios (ΔY/ΔX, where Δ indicates the enhancement of a compound in the plume above the local background mixing ratio, Y are individual hydrocarbons, CO, O3, N2O, HNO3, peroxyacetyl nitrate (PAN), CH2O, acetone, H2O2, CH3OOH, HCOOH, CH3COOH or aerosols and X is CO or CO2) are reported as a function of plume age inferred from the progression of Δnon-methane hydrocarbons/ΔCO enhancement ratios. Emission, formation, and loss of species in plumes can be diagnosed from progression of enhancement ratios from fresh to old plumes. O3 is produced in plumes over at least a 1 week period with mean ΔO3/ΔCO = 0.7 in old plumes. However, enhancement ratios in plumes can be influenced by changing background mixing ratios and by photochemical loss of CO. We estimate a downward correction of ∼20% in enhancement ratios in old plumes relative to ΔCO to correct for CO loss. In a case study of a large persistent biomass burning plume at 4-km we found elevated concentrations of PAN in the fresh plume. The degradation of PAN helped maintain NOx mixing ratios in the plume where, over the course of a week, PAN was converted to HNO3. Ozone production in the plume was limited by the availability of NOx, and because of the short lifetime of O3 at 4-km, net ozone production in the plume was negligible. Within the region, the majority of O3 production takes place in air above median CO concentration, indicating that most O3 production occurs in plumes. Scaling up from the mean observed ΔO3/ΔCO in old plumes, we estimate a minimum regional O3 production of 17×1010molecules O3 cm−2 s−1. This O3 production rate is sufficient to fully explain the observed enhancement in tropospheric O3 over the tropical South Atlantic during the dry season.

Results from a global three-dimensional model for tropospheric O3-NOx-hydrocarbon chemistry are presented and evaluated with surface, ozonesonde, and aircraft measurements. Seasonal variations and regional distributions of ozone, NO, peroxyacetylnitrate (PAN), CO, ethane, acetone, and H2O2 are examined. The model reproduces observed NO and PAN concentrations to within a factor of 2 for a wide range of tropospheric regions including the upper troposphere but tends to overestimate HNO3 concentrations in the remote troposphere (sometimes several fold). This discrepancy implies a missing sink for HNO3 that does not lead to rapid recycling of NOx; only in the upper troposphere over the tropical South Atlantic would a fast conversion of HNO3 to NOx improve the model simulation for NOx. Observed concentrations of acetone are reproduced in the model by including a large biogenic source (15 Tg C yr−1), which accounts for 40% of the estimated global source of acetone (37 Tg C yr−1). Concentrations of H2O2 in various regions of the troposphere are simulated usually to within a factor of 2, providing a test for HOx chemistry in the model. The model reproduces well the observed concentrations and seasonal variations of ozone in the troposphere, with some exceptions including an underestimate of the vertical gradient across the tropical trade wind inversion. A global budget analysis in the model indicates that the supply and loss of tropospheric ozone are dominated by photochemistry within the troposphere and that NOx. emitted in the southern hemisphere is twice as efficient at producing ozone as NOx emitted in the northern hemisphere.

A global three-dimensional model of tropospheric O3-NOx-hydrocarbon chemistry is used to investigate the factors controlling ozone concentrations in the troposphere. Model results indicate a close balance between chemical production and chemical loss of ozone in the tropospheric column at all latitudes (except high latitudes in winter). Using separate tracers for ozone produced in the stratosphere and in different regions of the troposphere, we find that the contribution of transport from the stratosphere to ozone concentrations in the troposphere is about 30% at midlatitudes in winter, 10% in summer, and 5% in the tropics. Production of ozone in the upper, middle, and continental lower troposphere all make significant contributions (10–50%) to ozone concentrations throughout the troposphere. The middle troposphere is a major global source region for ozone even though it is not a region of net production. The springtime maximum of ozone observed at remote sites in the northern extratropics is explained by a phase overlap between ozone transported from the stratosphere which peaks in late winter and ozone produced in the troposphere which peaks in late spring. Our model results do not support previous explanations of the springtime maximum based on wintertime accumulation of ozone or its precursors in the Arctic. The particularly strong springtime maximum at Mauna Loa Observatory (Hawaii) is attributed to long-range transport of Asian pollution over the North Pacific in spring. A sensitivity simulation without nonmethane hydrocarbons (NMHCs) indicates small decreases of ozone concentrations (<15%) in the remote troposphere and a 20% increase in the global mean OH concentration. Without NMHCs as a source of peroxyacetylnitrate, concentrations of NOx decrease by 30% in the remote lower troposphere but increase by 70% in the continental lower troposphere and by 40% in the upper troposphere. Biogenic isoprene accounts for about half of the NMHC effects in the model.

Haze layers with perturbed concentrations of trace gases, believed to originate from tundra and forest wild fires, were observed over extensive areas of Alaska and Canada in 1988. Enhancements of CH, CH, CH, CH, and CH were linearly correlated with CO in haze layers, with mean ratios (mole hydrocarbon/mole CO) of 0.18 (± 0.04 (1 σ)), 0.0019 (± 0.0001), 0.0055 (± 0.0002), 0.0008 (± 0.0001), and 1.2 × 10 (±0.2× 10), respectively. Enhancements of NO, were variable, averaging 0.0056 (± 0.0030) mole NO/mole CO, while perturbations of NO were very small, usually undetectable. At least 1/3 of the NO in the haze layers had been converted to peroxyacetyl nitrate (PAN), representing a potential source of NO to the global atmosphere; much of the balance was oxidized to nitrate (HNO and paniculate). The composition of sub‐Arctic haze layers was consistent with aged emissions from smoldering combustion, except for CH, which appears to be partly biogenic. Inputs from the stratosphere and from biomass fires contributed major fractions of the NO in the remote sub‐Arctic troposphere. Analysis of aircraft and ground data indicates relatively little influence from mid‐latitude industrial NO in this region during summer, possibly excepting transport of PAN. Production of O was inefficient in sub‐Arctic haze layers, less than 0.1 O molecules per molecule of CO, reflecting the low NO/CO emission ratios from smoldering combustion. Mid‐latitude pollution produced much more O, 0.3 – 0.5 O molecules per molecule of CO, a consequence of higher NO/CO emission ratios.

A global inventory of gaseous sulfur emissions with 1°×1° resolution is described. Emissions from fuel combustion and industrial activities are estimated for countries where no detailed inventories are available by using economic data for individual sulfur‐emitting activities, sulfur emission factors, and information on sulfur recovery. Fuel sulfur contents are specified as a function of fuel type and country of origin and are conserved during international trading. This procedure for estimating emissions reproduces well existing inventories for countries in Europe and North America, suggesting that it can be applied with some confidence to other countries. Emissions from biomass burning, volcanoes, and oceans are derived from existing data bases and are distributed with fine spatial resolution. Emissions from terrestrial vegetation are computed as a function of leaf area index, temperature, and solar radiation. The global emission of sulfur gases in 1980 is estimated to be 102 Tg S yr, apportioned among fuel combustion and industrial activities (76%), marine biosphere (12%), volcanoes (9%), biomass burning (2%), and terrestrial biosphere (1%). Detailed breakdowns of anthropogenic and natural sources are given for individual countries and regions. Anthropogenic sources account for 84% of total sulfur emissions in the northern hemisphere and for 50% in the southern hemisphere. Biomass burning dominates emissions in central Africa during the dry season but is of minor importance elsewhere. Smelters dominate anthropogenic emissions in the Arctic and in the southern hemisphere. Volcanoes are significant contributors to the sulfur budget in Central America, the East Indies, and some subarctic regions.

This paper explores some of the mechanisms governing the accumulation of passive tracers over the tropical southern Atlantic Ocean during the northern hemisphere fall season. There has been a pioneering observation regarding ozone maxima over the South Atlantic during austral spring. The understanding of the formation of this maxima has been the prime motivation for this study. Using a global model as a frame of reference, we have carried out three kinds of experiments during the period of the Transport and Atmospheric Chemistry Near the Equator-Atlantic (TRACE A) project of 1992. The first of these is a simple advection of total ozone (a passive tracer) in time using the Florida State University global spectral model. Integration over the period of roughly 1 week showed that the model quite closely replicates the behavior of the observed total ozone from the total ozone mapping spectrometer (TOMS). This includes many of the changes in the features of total ozone over the tropical and subtropical region of the southern Atlantic Ocean. These studies suggest a correlation of 0.8 between the observed ozone over this region and ozone modeled from “dynamics alone,” i.e., without recourse to any photochemistry. The second series of experiments invoke sustained sources of a tracer over the biomass burn region of Africa and Brazil. Furthermore, sustained sources were also introduced in the active frontal “descending air” region of the southern hemisphere and over the Asian monsoon's east-west circulation. These experiments strongly suggest that air motions help to accumulate tracer elements over the tropical southern Atlantic Ocean. A third series of experiments address what may be required to improve the deficiencies of the vertical stratification of ozone predicted by the model over the flight region of the tropical southern Atlantic during TRACE A. Here we use the global model to optimally derive plausible accumulation of burn elements over the fire count regions of Brazil and Africa to provide passive tracer advections to closely match what was observed from reconnaissance aircraft-based measurements of ozone over the tropical southern Atlantic Ocean.

We examine the sources and chemistry affecting nitrogen oxides (NOx = NO + NO2) over the tropical Pacific (30°S–20°N) using observations from the Pacific Exploratory Mission to the Tropics B (PEM-Tropics B) aircraft mission conducted in March–April 1999. A global model of tropospheric chemistry driven by assimilated meteorological data is used to interpret the observations. Median concentrations observed over the South Pacific during PEM-Tropics B were 7 pptv NO, 16 pptv peroxyacetyl nitrate (PAN), and 34 pptv nitric acid (HNO3); the model generally reproduces these observations but overestimates those over the North Pacific. Lightning was the largest source of these species in the equatorial and South Pacific tropospheric column and in the tropical North Pacific upper troposphere. The oceanic source of acetone implied by high observations of acetone concentrations (mean 431 pptv) allows an improved simulation of PAN/NOx chemistry. However, the high acetaldehyde concentrations (mean 78 pptv) measured throughout the troposphere are inconsistent with our understanding of acetaldehyde and PAN chemistry. Simulated concentrations of HNO3 and HNO3/NOx are highly sensitive to the model representation of deep convection and associated HNO3 scavenging. Chemical losses of NOx during PEM-Tropics B exceed chemical sources by a factor of 2 in the South Pacific upper troposphere. The chemical imbalance, also apparent in the low observed HNO3/NOx ratio, is explained by NOx injection from lightning and by frequent convective overturning which depletes HNO3. The observed imbalance was less during the PEM-Tropics A campaign in September 1996, when aged biomass burning effluents over the South Pacific pushed the NOx budget toward chemical steady state.

We define the radiative forcings used in climate simulations with the SI2000 version of the Goddard Institute for Space Studies (GISS) global climate model. These include temporal variations of well-mixed greenhouse gases, stratospheric aerosols, solar irradiance, ozone, stratospheric water vapor, and tropospheric aerosols. Our illustrations focus on the period 1951–2050, but we make the full data sets available for those forcings for which we have earlier data. We illustrate the global response to these forcings for the SI2000 model with specified sea surface temperature and with a simple Q-flux ocean, thus helping to characterize the efficacy of each forcing. The model yields good agreement with observed global temperature change and heat storage in the ocean. This agreement does not yield an improved assessment of climate sensitivity or a confirmation of the net climate forcing because of possible compensations with opposite changes of these quantities. Nevertheless, the results imply that observed global temperature change during the past 50 years is primarily a response to radiative forcings. It is also inferred that the planet is now out of radiation balance by 0.5 to 1 W/m2 and that additional global warming of about 0.5°C is already “in the pipeline.”