Joyce E. Penner

A global three‐dimensional model study of carbonaceous aerosols

We have developed detailed emission inventories for the amount of both black and organic carbon particles from biomass burning sources (wood fuel, charcoal burning, dung, charcoal production, agricultural, savanna and forest fires). We have also estimated an inventory for organic carbon particles from fossil fuel burning and urban activities from an existing inventory for fossil fuel sources of black carbon. We also provide an estimate for the natural source of organic matter. These emissions have been used together with our global aerosol model to study the global distribution of carbonaceous aerosols. The accuracy of the inventories and the model formulation has been tested by comparing the model simulations of carbonaceous aerosols in the atmosphere and in precipitation with observations reported in the literature. For most locations and seasons, the predicted concentrations are in reasonable agreement with the observations, although the model underpredicts black carbon concentrations in polar regions. The predicted concentrations in remote areas are extremely sensitive to both the rate of removal by wet deposition and the height of injection of the aerosols. Finally, a global map of the aerosol single scattering albedo was developed from the simulated carbonaceous particle distribution and a previously developed model for aerosol sulfates. The computed aerosol single scattering albedos compare well with observations, suggesting that most of the important aerosol species have been included in the model. For most locations and seasons, the single scattering albedo is larger than 0.85, indicating that these aerosols, in general, lead to a net cooling.

Effects of aerosol from biomass burning on the global radiation budget.

An analysis is made of the likely contribution of smoke particles from biomass burning to the global radiation balance. These particles act to reflect solar radiation directly; they also can act as cloud condensation nuclei, increasing the reflectivity of clouds. Together these effects, although uncertain, may add up globally to a cooling effect as large as 2 watts per square meter, comparable to the estimated contribution of sulfate aerosols. Anthropogenic increases of smoke emission thus may have helped weaken the net greenhouse warming from anthropogenic trace gases.

Contribution of different aerosol species to the global aerosol extinction optical thickness: Estimates from model results

We combine global distributions of aerosol loading resulting from transport models for soil dust, sulfate, sea salt, and carbonaceous aerosol. From the aerosol distributions we estimate optical thicknesses and compare them with Sun photometer measurements and satellite retrievals, thereby revealing problems with both model results and comparisons with such measurements. Globally, sulfate, dust, and carbonaceous particles appear to contribute equally to the total aerosol optical thickness. Owing to the different optical properties of different aerosol types, aerosol composition should be taken into consideration for estimating the aerosol climate effect as well as for aerosol retrievals from satellite measurements.

NOx from lightning: 1. Global distribution based on lightning physics

This paper begins a study on the role of lightning in maintaining the global distribution of nitrogen oxides (NOx) in the troposphere. It presents the first global and seasonal distributions of lightning-produced NOx (LNOx) based on the observed distribution of electrical storms and the physical properties of lightning strokes. We derive a global rate for cloud-to-ground (CG) flashes of 20–30 flashes/s with a mean energy per flash of 6.7×109 J. Intracloud (IC) flashes are more frequent, 50–70 flashes/s but have 10% of the energy of CG strokes and, consequently, produce significantly less NOx. It appears to us that the majority of previous studies have mistakenly assumed that all lightning flashes produce the same amount of NOx, thus overestimating the NOx production by a factor of 3. On the other hand, we feel these same studies have underestimated the energy released in CG flashes, resulting in two negating assumptions. For CG energies we adopt a production rate of 10×1016 molecules NO/J based on the current literature. Using a method to simulate global lightning frequencies from satellite-observed cloud data, we have calculated the LNOx on various spatial (regional, zonal, meridional, and global) and temporal scales (daily, monthly, seasonal, and interannual). Regionally, the production of LNOx is concentrated over tropical continental regions, predominantly in the summer hemisphere. The annual mean production rate is calculated to be 12.2 Tg N/yr, and we believe it extremely unlikely that this number is less than 5 or more than 20 Tg N/yr. Although most of LNOx, is produced in the lowest 5 km by CG lightning, convective mixing in the thunderstorms is likely to deposit large amounts of NOx, in the upper troposphere where it is important in ozone production. On an annual basis, 64% of the LNOx, is produced in the northern hemisphere, implying that the northern hemisphere should have natural ozone levels as much as 2 times greater than the southern hemisphere, even before anthropogenic influences. The amount of O3 produced from this NOx is expected to exceed the stratospheric source by a factor of 1.5, and thus the hemispheric asymmetry in LNOx would lead to a significant excess of northern hemisphere O3 even in the preindustrial troposphere. (The monthly climatologies for LNOx on a 1°×1° latitude-longitude grid can be obtained by e-mail to cprice@flash.tau.ac.il).

Quantifying and minimizing uncertainty of climate forcing by anthropogenic aerosols

The clear-sky climate forcing by anthropogenic aerosols has been shown to be of sufficient magnitude to mask the effects of anthropogenic greenhouse gases over large regions. Anthropogenic aerosols are composed of a variety of aerosol types including water-soluble inorganic species (e.g., sulfate, nitrate, ammonium), organic condensed species, elemental or black carbon, and mineral dust. Estimates of the clear-sky forcing by anthropogenic sulfate aerosols and by organic biomass-burning aerosols have been published previously. Here we estimate the uncertainty in the forcing by these aerosol types. Estimates of the clear-sky forcing by other anthropogenic aerosol types do not even exist though the forcing by these aerosol types is thought to be smaller than that by sulfate and biomass burning aerosols.

A possible correlation between satellite-derived cloud and aerosol microphysical parameters

The column aerosol particle number and low cloud microphysical parameters derived from AVHRR remote sensing are compared over ocean for four months in 1990. There is a positive correlation between cloud optical thickness and aerosol number concentration, whereas the effective particle radius has a negative correlation with aerosol number. The cloud liquid water path (LWP), on the other hand, tends to be constant with no large dependence on aerosol number. This result contrasts with results from recent model simulations which imply that there is a strong positive feedback between LWP and aerosol number concentration. Estimates for indirect forcing over oceans derived from the satellite data/model comparison range from −0.7 to −1.7 Wm−2.

Variations in the predicted spatial distribution of atmospheric nitrogen deposition and their impact on carbon uptake by terrestrial ecosystems

Widespread mobilization of nitrogen into the atmosphere from industry, agriculture, and biomass burning and its subsequent deposition have the potential to alleviate nitrogen limitation of productivity in terrestrial ecosystems, and may contribute to enhanced terrestrial carbon uptake. To evaluate the importance of the spatial distribution of nitrogen deposition for carbon uptake and to better quantify its magnitude and uncertainty NO y -N deposition fields from five different three-dimensional chemical models, GCTM, GRANTOUR, IMAGES, MOGUNTIA, and ECHAM were used to drive NDEP, a perturbation model of terrestrial carbon uptake. Differences in atmospheric sources of NO x -N, transport, resolution, and representation of chemistry, contribute to the distinct spatial patterns of nitrogen deposition on the global land surface; these differences lead to distinct patterns of carbon uptake that vary between 0.7 and 1.3 Gt C yr -1 globally. Less than 10% of the nitrogen was deposited on forests which were most able to respond with increased carbon storage because of the wide C:N ratio of wood as well as its long lifetime. Addition of NH x -N to NO y -N deposition, increased global terrestrial carbon storage to between 1.5 and 2.0 Gt C yr -1 , while the missing terrestrial sink is quite similar in magnitude. Thus global air pollution appears to be an important influence on the global carbon cycle. If N fertilization of the terrestrial biosphere accounts for the missing C sink or a substantial portion of it, we would expect significant reductions in its magnitude over the next century as terrestrial ecosystems become N saturated and O 3 pollution expands.

Prediction of the number of cloud droplets in the ECHAM GCM

In this paper a prognostic equation for the number of cloud droplets (CDNC) is introduced into the ECHAM general circulation model. The initial CDNC is based on the mechanistic model of Chuang and Penner [1995], providing a more realistical prediction of CDNC than the empirical method previously used. Cloud droplet nucleation is parameterized as a function of total aerosol number concentration, updraft velocity, and a shape parameter, which takes the aerosol composition and size distribution into account. The total number of aerosol particles is obtained as the sum of marine sulfate aerosols produced from dimethyl sulfide, hydrophylic organic and black carbon, submicron dust, and sea-salt aerosols. Anthropogenic sulfate aerosols only add mass to the preexisting aerosols but do not form new particles. The simulated annual mean liquid water path, column CDNC, and effective radius agree well with observations, as does the frequency distributions of column CDNC for clouds over oceans and the variations of cloud optical depth with effective radius.

Evaluation and intercomparison of global atmospheric transport models using 222Rn and other short‐lived tracers

Simulations of 222Rn and other short-lived tracers are used to evaluate and intercompare the representations of convective and synoptic processes in 20 global atmospheric transport models. Results show that most established three-dimensional models simulate vertical mixing in the troposphere to within the constraints offered by the observed mean 222Rn concentrations and that subgrid parameterization of convection is essential for this purpose. However, none of the models captures the observed variability of 222Rn concentrations in the upper troposphere, and none reproduces the high 222Rn concentrations measured at 200 hPa over Hawaii. The established three-dimensional models reproduce the frequency and magnitude of high-222Rn episodes observed at Crozet Island in the Indian Ocean, demonstrating that they can resolve the synoptic-scale transport of continental plumes with no significant numerical diffusion. Large differences between models are found in the rates of meridional transport in the upper troposphere (interhemispheric exchange, exchange between tropics and high latitudes). The four two-dimensional models which participated in the intercomparison tend to underestimate the rate of vertical transport from the lower to the upper troposphere but show concentrations of 222Rn in the lower troposphere that are comparable to the zonal mean values in the three-dimensional models.

An assessment of the radiative effects of anthropogenic sulfate

We use a coupled climate/chemistry model with cloud nucleation processes parameterized in terms of local aerosol number, anthropogenic sulfate mass concentration, and updraft velocity to investigate both direct and indirect anthropogenic sulfate radiative forcings. We estimate that the global direct radiative forcing is about {minus}0.4Wm{sup {minus}2} with a maximum over Europe where the strongest anthropogenic sulfur emissions occur. With different approaches for the formation of anthropogenic sulfate and its relation to aerosol size distribution, we estimate that the indirect forcing may range from {minus}0.6 to {minus}1.6Wm{sup {minus}2}. This range reduces to {minus}0.4 to {minus}1.1Wm{sup {minus}2} if a prescribed marine background particle number concentration is universally applied over the ocean. Contrary to the direct effect which is more significant over continents, the calculated maximum of indirect forcing is located over the Atlantic Ocean near the coastline of North America. Our simulations indicate that anthropogenic sulfate may result in important increases in reflected solar radiation, which would mask locally the warming from increased greenhouse gases. We also compare the simulated cloud drop effective radii with those retrieved from satellite data to validate the accuracy of our cloud drop parameterization.{copyright} 1997 American Geophysical Union