Progress 06/15/00 to 09/30/04
Outputs
(1) Springtime photochemistry. The relatively long-term in situ observations of the TOPSE experiment allow for analysis of the seasonal characteristics of photochemistry at northern mid and high latitudes. The in situ chemistry analysis has yielded a number of new and perhaps surprising findings, among which are (1) the radical cycling is slowed down drastically because of rapid heterogeneous loss of H2O2; (2) springtime NOx concentrations show the same seasonal trend as HOx sources; and (3) no evidence is found that the potential accumulation of reactive nitrogen at northern high latitudes in winter contributed significantly to the springtime O3/NOx photochemistry. In collaboration with Dr. Alan Fried, we examined CH2O measurements during TOPSE. The measurement-model agreement is much improved compared to the previous mid-latitude missions. (2) Diagnosing sources of tropospheric O3 using tracer correlations. Using various tracers including O3, total reactive nitrogen
(NOy), peroxyacetylnitrate (PAN), CO, CH4, C2H2, C3H8, CH3Cl, CH3Br, C2Cl4, CFC-11 (CCl3F), HCFC-141B (CH3CCl2F), Halon-1211 (CBrClF2), 7Be, and potential temperature, we analyzed TOPSE observations for the air masses that contributed most to the observed variability and seasonal trend of O3 at northern mid and high latitudes. We find that stratospherically influenced air accounts for 35-45% of the observed O3 variability. It accounts for about 40% of the seasonal O3 trend at mid latitudes but <20% at high latitudes. At mid latitudes, reactive nitrogen rich air masses transported through Asia are much more significant than other tropospheric contributors. At high latitudes, the O3 variability is significantly influenced by air masses transported from mid latitudes that are poor in reactive nitrogen. The O3 trend, in contrast, is largely defined by air masses rich in reactive nitrogen transported through Asia and Europe across the Pacific or the Arctic (3) Global simulations of CH3I.
It is the most abundant halogen species in the atmosphere and contribute to the halogen loading in the stratosphere. Current understanding of its sources is poor. In addition, our analysis of the TOPSE observations (February-May 2000) indicates that they are unique tracers for tropospheric transport. We have implemented various known sources of CH3Cl in the GEOS-CHEM model. Separate tracers are used to track CH3Cl from different sources. The model simulation has been compared with surface observations. Our primary results suggest that a large missing source (~50% of the total known source) is required to match observed levels in the model. We have compared our model simulations with surface and aircraft observations. The surface seasonal trend and vertical profiles from aircraft observations are examined in details to evaluate the sources and transport of the model. We find that the missing source is largely limited in the tropics.
Impacts
While the model captures well a large portion of the observations we compiled, there are many regions where the model simulations have very different characteristics from the observations.
Publications
- Wang, Y. et al., Intercontinental transport of pollution manifested in the variability and seasonal trend of springtime O3 at northern mid and high latitudes, submitted to Science, 2002.
- Wang, Y., et al., Springtime photochemistry at northern mid and high latitudes, J. Geophys. Res., in press, 2002.
- Fried, A., et al., Tunable diode laser measurements of formaldehyde during the TOPSE 2000 study: Distributions, trends, and model comparisons, J. Geophys. Res., in press, 2002.
- Hansen, J., et al., Climate forcing in GISS SI2000 simulations, J. Geophys. Res., in press, 2002.
|
Progress 01/01/01 to 12/31/01
Outputs
In the past year, we have finished examining observations over the tropical Pacific from NASA PEM-Tropics B mission. Highlights of the work include identifying the pathways of anthropogenic emissions from the northern hemisphere into the tropics, examining the effects of convection on the oxidation capacity of the atmosphere, estimating the amount of NO production from marine lightning, and investigating the sources of SO2 in the tropical free troposphere. We have continued our work on analyzing observations from TOPSE. The analysis of photochemistry is almost finished. We have examined the details of HOx, O3, and reactive nitrogen chemistry at northern mid and high latitudes. Furthermore, we have adopted positive matrix factorization method to identify the different air mass observed during TOPSE and the sources of ozone increases during the campaign. The 3-D model simulation for the TOPSE is under way. We have begun work on simulating global distributions of
nonmethane hydrocarbons, oxygenated hydrocarbons, and halocarbons. A global 3-D model driven by assimilated meteorological fields has been adopted in my group. We are currently conducting simulations using the model. We have obtained satellite observations of CH2O from GOME. Retrieval and analysis work are under way.
Impacts
PEM-tropics B work illustrates that human activities can influence remote tropical Pacific and consequently change the oxidation capacity of the atmosphere. We compared the effects of long-range transport from the northern hemisphere with those from tropical biomass burning and the effects are drastically different. The former decreases the oxidation capacity of the atmosphere while the latter increases the oxidation capacity. While the natural source of DMS from the ocean is sufficient to explain observed SO2 concentrations, we found that oxidation of DMS appears to be much faster suggesting unknown pathways of oxidation in the free troposphere. During TOPSE, the radical chemistry is driven primarily by photolysis of O3 and the subsequent reaction of O(1D) and H2O, which increase rapidly during spring. Photolysis of CH2O, either transported into the region or produced by unknown chemical pathways, appears to provide a significant HOx source at 6-8 at high latitudes.
The rapid increase of in situ O3 production in spring is fueled by concurrent increases of primary HOx sources and NO concentrations. No evidence is found that long-lived reactive nitrogen species accumulated at mid or high latitudes. At mid latitudes, net in situ chemical production rates of O3 are calculated from February to May. The rate peaks in April coinciding with the observed peak of column O3 (0-8 km). Much of the column O3 increase at mid latitudes can be explained by net in situ O3 production. In contrast, there is a net in situ O3 loss from February to April at high latitudes.
Publications
- Wang et al., Springtime photochemistry at northern mid and high latitudes, submitted to J. Geophys. Res., 2002.
- Fried et al., Tunable diode laser measurements of formaldehyde during the TOPSE 2000 study: Distributions, trends, and model comparisons, submitted to J. Geophys. Res., 2002.
- Hansen, J., et al., Climate forcing in GISS SI2000 simulations, J. Geophys. Res., in press, 2002.
- Wang, Y., et al., Factors Controlling Tropospheric O3, OH, NOx, and SO2 over the Tropical Pacific during PEM-Tropics B, J. Geophys. Res., 106, 32,733-32,748, 2001.
- Davis, D., et al., Marine latitude/altitude OH distributions: Comparison of Pacific Ocean observations with models, J. Geophys. Res., 106, 32,691-32,708, 2001.
|
Progress 01/01/00 to 12/31/00
Outputs
Observations over the tropics from the Pacific Exploratory Mission-Tropics A Experiment are analyzed using a one-dimensional model with an explicit formulation for convective transport. Adopting tropical convective mass fluxes from a general circulation model (GCM) yields a large discrepancy between observed and simulated CH3I concentrations. Observations of CH3I imply the convective mass outflux to be more evenly distributed with altitude over the tropical ocean than suggested by the GCM. We find that using a uniform convective turnover lifetime of 20 days in the upper and middle troposphere enables the model to reproduce CH3I observations. The model reproduces observed concentrations of H2O2 and CH3OOH. Convective transport of CH3OOH from the lower troposphere is estimated to account for 40-80% of CH3OOH concentrations in the upper troposphere. Photolysis of CH3OOH transported by convection more than doubles the primary HOx source and increases OH concentrations and
O3 production by 10-50% and 0.4 ppbv d1, respectively, above 11 km. Its effect on the OH concentration and O3 production integrated over the tropospheric column is, however, small. The effects of pollutant import from biomass burning regions are much more dominant. Using C2H2 as a tracer, we estimate that biomass burning outflow enhances O3 concentrations, O3 production, and concentrations of NOx and OH by 60%, 45%, 75%, and 7%, respectively. The model overestimates HNO3 concentrations by about a factor of 2 above 4 km for the upper one-third quantile of C2H2 data while it generally reproduces HNO3 concentrations for the lower and middle one-third quantiles of C2H2 data. We examine concurrent measurements of CN (diameter > 8 nm), NO, and NOy in the upper troposphere over the North Atlantic during the SONEX Experiment (Oct.-Nov., 1997). High CN and NOy observations are attributed largely to the enhancement in convective outflow. Using the ratio of NO/NOy as a chemical clock, we
estimate that dilution of convective high-CN plumes is rapid (on a time scale of < 2 days) and accounts for a large fraction of elevated CN concentrations above the background. We estimate that less than 7% of observed high-CN (> 10000 cm3) plumes may be attributed to aircraft emissions. The contribution by aircraft emissions to upper tropospheric CN concentrations is estimated to be significantly higher than 7% because aircraft plumes dilute much faster than convective plumes and hence are sampled less frequently.
Impacts
Our work suggests that anthropogenic emissions from biomass burning significantly change the chemical compositions of the troposphere over the tropical Pacific and that convection is a large source of condensation nuclei in the upper troposphere over the North Atlantic.
Publications
- Wang, Y., Global tropospheric OH: Observational constraints and model simulations, 21, 18-21, IGACtivities Newsletter, 2000 (Invited; Reprinted in IGBP Global Change Newsletter, 43, 2000).
- Davis, D., et al., Marine Latitude/Altitude OH Distributions: Comparison of Pacific Ocean Observations with Models, submitted to J. Geophys. Res., 2001.
- Wang, Y., et al., Factors Controlling Tropospheric O3, OH, NOx, and SO2 over the Tropical Pacific during PEM-Tropics B, submitted to J. Geophys. Res., 20001.
- Wang Y., S. C. Liu, H. Yu, S. Sandholm, T.-Y. Chen, and D. R. Blake, Influence of convection and biomass burning on tropospheric chemistry over the tropical Pacific, J. Geophys. Res., 105, 9321-9333, 2000.
- Wang Y., S. C. Liu, B. E. Anderson, Y. Kondo, G. L. Gregory, G. W. Sachse, S. A. Vay, D. Blake, H. B. Singh, A. M. Thompson, Evidence of convection as a major source of condensation nuclei in the northern midlatitude upper troposphere, Geophys. Res. Lett., 27, 369-372, 2000.
|
|