Person:

Jacob, Daniel

We evaluate the sensitivity of tropospheric OH, O3, and O3 precursors to photochemical effects of aerosols not usually included in global models: (1) aerosol scattering and absorption of ultraviolet radiation and (2) reactive uptake of HO2, NO2, and NO3. Our approach is to couple a global 3-D model of tropospheric chemistry (GEOS-CHEM) with aerosol fields from a global 3-D aerosol model (GOCART). Reactive uptake by aerosols is computed using reaction probabilities from a recent review (γHO2 = 0.2, γNO2 = 10−4, γNO3 = 10−3). Aerosols decrease the O3 [RIGHTWARDS ARROW] O(1D) photolysis frequency by 5–20% at the surface throughout the Northern Hemisphere (largely due to mineral dust) and by a factor of 2 in biomass burning regions (largely due to black carbon). Aerosol uptake of HO2 accounts for 10–40% of total HOx radical (≡ OH + peroxy) loss in the boundary layer over polluted continental regions (largely due to sulfate and organic carbon) and for more than 70% over tropical biomass burning regions (largely due to organic carbon). Uptake of NO2 and NO3 accounts for 10–20% of total HNO3 production over biomass burning regions and less elsewhere. Annual mean OH concentrations decrease by 9% globally and by 5–35% in the boundary layer over the Northern Hemisphere. Simulated CO increases by 5–15 ppbv in the remote Northern Hemisphere, improving agreement with observations. Simulated boundary layer O3 decreases by 15–45 ppbv over India during the biomass burning season in March and by 5–9 ppbv over northern Europe in August, again improving comparison with observations. We find that particulate matter controls would increase surface O3 over Europe and other industrial regions.

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.”

A unified tropospheric chemistry-aerosol model has been developed within the Goddard Institute for Space Studies general circulation model (GCM). The model includes a detailed simulation of tropospheric ozone-NOx-hydrocarbon chemistry as well as aerosols and aerosol precursors. Predicted aerosol species include sulfate, nitrate, ammonium, black carbon, primary organic carbon, and secondary organic carbon. The partitioning of ammonia and nitrate between gas and aerosol phases is determined by on-line thermodynamic equilibrium, and the formation of secondary organic aerosols is based on equilibrium partitioning and experimentally determined parameters. Two-way coupling between aerosols and chemistry provides consistent chemical fields for aerosol dynamics and aerosol mass for heterogeneous processes and calculations of gas-phase photolysis rates. Although the current version of the unified model does not include a prognostic treatment of mineral dust, we include its effects on photolysis and heterogeneous processes by using three-dimensional off-line fields. We also simulate sulfate and nitrate aerosols that are associated with mineral dust based on currently available chemical understanding. Considering both mineral dust uptake of HNO3 and wet scavenging of HNO3 on ice leads to closer agreement between predicted gas-phase HNO3 concentrations and measurements than in previous global chemical transport model simulations, especially in the middle to upper troposphere. As a result of the coupling between chemistry and aerosols, global burdens of both gas-phase and aerosol species are predicted to respond nonlinearly to changing emissions of NOx, NH3, and sulfur.