Tropospheric Diagnostic Modeling Studies of PEM-Tropics B Field Data: Investigation

of the HO_x/NO_x/O₃ Photochemical System and its Coupling to Sulfur Chemistry

 
Doug D. Davis (PI)

Gao Chen (Co-I)

Georgia Institute of Technology

221 Bobby Dodd Way

Atlanta, GA 30332

(404) 894-9565

dd16@prism.gatech.edu

Ozone Studies

Reflecting the insights gained from analysis of the PEM-West and PEM-Tropics A data sets, three basic questions will be investigated during PEM-Tropics B as related to ozone and atmospheric oxidants:

1. What is the impact of photochemistry on the ozone budget in the tropical Pacific?
   
2. What are the relative contributions of primary and secondary NO_x sources to the active
   pool of photochemical NO_x, and how do these vary with altitude and geographical location
   
3. What are the controlling factors in regulating HO_x levels in the tropical Pacific?

The approach to be taken in the proposed research will be similar to that previously used to analyze the PEM-West and PEM-Tropics A data. A photochemical box model capable of producing both photostationary state and time dependent results will be employed. This model has been recently revised, but is still basically very similar to models previously described by Davis et al. [1993, 1996] and Crawford et al. [1996, 1997b,c]. Typical constraining species/parameters include: O₃, CO, NO, H₂O, NMHCs, H₂O₂, CH₃OOH, HNO₃, PAN and the physical parameters temperature, pressure, and the UV solar flux.

PSS model output will be most useful in the comparison of model predicted and observed values. This exercise constitutes the single most important method for gaining insight into possible elements of Aincompleteness@ in the model=s chemical mechanism. For PEM-Tropics B, our goal will be to compare with observed values of OH, peroxy radicals, H₂O₂, CH₃OOH, CH₂O, and NO₂. In particular, this effort will focus on several of the sorties to be flown on the P-3B aircraft while based at Christmas Island. As noted in the NRA two separate sequences of sunrise and sunset flights will be carried out from this tropical environment thus permitting near 24 hour coverage of the diel profiles of the family of HO_x related species listed above as well as numerous sulfur species whose concentration levels are critically tied to OH. This will represent one of the most extensive tests of photochemical mechanisms to date.

The time-dependent (TD) mode of the model will be used to provide diel profiles as well as diurnal averages for all transient radical species as well as output products for the photochemical budget of O₃ (i.e., F(O₃), D(O₃), and P(O₃)). This model will also be used to evaluate the photochemical budget of NO_x. Analysis of NO_x sources is important both because of its controlling influence on O₃ photochemistry and because of its impact on the partitioning of HO_x radicals. This is a particularly important issue in the upper free troposphere since this is a major photochemical source region for O₃. The purpose of model calculations as related to the NO_x budget will be to determine what fraction of the NO_x might be explicable via the recycling of HNO₃ and PAN. Previous studies have shown this value to generally be less than about 30% [Davis et al., 1996; Jacob et al., 1996; Crawford et al., 1997b,c]. The assumption then is that the remaining portion of NO_x is due to some additional unknown recycling of NO_y species or is contributed from primary sources.

A semi-quantitative evaluation of primary sources will be carried out through a statistical analysis of the measured NO and calculated NO_x observations. Primary source injections are both episodic and often elevate NO_x concentrations well above levels in surrounding air. By contrast, recycling is a relatively slow process serving to sustain background NO_x levels. Thus large, short-duration enhancements in observed NO can often be attributed to primary source injections of NO_x. As previously shown by Crawford [1997a], statistical removal of these primary source enhancements can lead to estimates of primary source NO_x contributions to the total pool of observed NO_x. Detailed and comprehensive evaluation of correlations between NO and all other available parameters during these periods of enhancement can often lead to an identification of the source type (e.g., lightning, aircraft, convection of surface emissions, stratospheric intrusion). These methods have been applied previously to PEM-West data [Davis et al., 1996; and Crawford, 1997a] and will also be used in the evaluation of the PEM-Tropics data.

Sulfur Studies

PEM-Tropics A provided a tantalizing but very limited sulfur data base. An examination of a full solar cycle of evolving sulfur chemistry was not possible. Very limited buffer layer and lower free tropospheric observations of sulfur were made. Critical DMS oxidation products such as DMSO were not measured. Perhaps most important, due to the overall limitations of the PEM-Tropics A data base, no comprehensive assessment could be made of the BL sulfur budget. As outlined in the PEM-Tropics B NRA, the planned set of 7 local flights from Christmas Island, with observations being made at nearly all times of the solar cycle, and with a high priority being given to measurements of both gas phase and aerosol sulfur species as well as the critical oxidizing agents like OH, an exceptionally rich data base should follow. We propose to use this data base to address two general questions:

1. For a tropical marine BL environment what are the major sources and sinks of sulfur and what
   are the major chemical forms of sulfur that participate in these?
   
   
2. What are the major changes that occur in the DMS oxidation mechanism when shifting from a tropical
   marine BL environment to the overhead buffer layer or still higher in elevation to the lower-free troposphere?

In the context of these two general questions several "specific issues" will be explored. Among these are:

a) Nighttime assessment of the BL loss rate for SO₂, e.g., sea surface and heterogeneous reaction losses.

b) Re-assessment of the quantitative yield of SO₂ from OH/DMS oxidation.

c) Re-assessment of the relative yields of SO₂ from the OH/DMS addition and abstraction channels.

d) Evaluation of the addition channel formation efficiency as well as the major loss processes for DMSO.

e) Evaluation of the major production and loss pathways for DMSO₂.

f) Re-assessment of the yield of MSA(g) and MS based on DMSO observations.

g) Evaluation of "(H₂SO₄) for the BL, buffer layer, and lower free troposphere, based on measured total wet surface areas versus humidity corrected measured dry aerosol surface areas.

The answers to these plus other issues, together with extensive meteorological input on the mixing characteristics of the Christmas Island environment, should provide the necessary quantitative framework within which a sulfur budget assessment can be carried out.

The general approach to be taken in our analysis will be similar to that used by our group in the analysis of the PEM-Tropics A data [Davis et al., 1998b] as well as other recent sulfur field data sets [Chen et al., 1998b; Davis et al., 1998b]. Model simulations will be carried out using a sulfur box model based on 14 sulfur processes. Four of these processes encompass two or more branches. The respective rate coefficients, BR's, and/or over-all-conversion-efficiencies (OCEF's) have been previously presented by Davis et al. [1998a,b].

Mission Scientist Program

The principal investigator, D. Davis, will also participate in the PEM-Tropics B program as a mission scientist for the P-3B aircraft program. As mission scientist for NASA's P-3B aircraft, he will work toward fulfilling the following responsibilities:

• Serve as a scientific advisor to the project manager.

• Based on input from the mission meteorologist, make recommendations to the P-3B science team and project manager concerning flight plans and science objectives for each mission.

• Help coordinate the P-3B flight plans with the DC-8 mission plan.

• Based on an agreed upon flight plan, work with the mission meteorologist and NASA flight planners to lay out detailed flight profiles for each mission.

• Review flight data from each mission as it comes available and, in coordination with the project manager, update the science team as to the progress being made in satisfying the overall science objectives of the P-3 PEM-Tropics B program.

• Together with the designated co-mission scientist on the DC-8 serve as a scientific spokesperson for the PEM-Tropics B science team.

Referencs:

Chen, G., D. Davis, P. Kasibhatla, A. Bandy, and D. Thornton, A photochemical assessment of DMS sea-to-air flux as inferred from PEM-West A and B observations, J. Geophys. Res., accepted, 1998a.

Chen G., D. D. Davis, P. Kasibhatla, A. R. Bandy, D. C. Thornton, B. J. Huebert, and A. D. Clarke, A Study of Tropical DMS Oxidation Chemistry: Comparison of Christmas Island Field Observations of SO₂ and DMS with Model Simulations, in preparation, 1998b.

Crawford, J. H., et al., Photostationary state analysis of the NO₂-NO system based on airborne observations from the western and central North Pacific, J. Geophys. Res., 101, 2053-2072, 1996.

Crawford, J. H., An analysis of the photochemical environment over the western, North Pacific based on aircraft observations, Dissertation,1997a.

Crawford, J. H., et al., Implications of large scale shifts in tropospheric NO_x levels in the remote tropical Pacific, J. Geophys. Res., in press, 1997b.

Crawford, J. H., et al., An assessment of ozone photochemistry in the extratropical western north Pacific: Impact of continental outflow during the late winter/earlier spring, J. Geophys. Res., in press, 1997c.

Davis D. D., G. Chen, W. Chameides, J. Bradshaw, S. Sandholm, M. Rodgers, J. Schendal, S. Madronich, G. Sachse, G. Gregory, B. Anderson, J. Barrick, M. Shipham, J. Collins, L. Wade, and D. Blake, A Photostationary state analysis of the NO₂-NO system based on airborne observations from the subtropical/tropical North and South Atlantic, J. Geophys. Res., 98, 23501-23523, 1993.

Davis, D. D., J. Crawford, G. Chen, W. Chameides, S. Liu, J. Bradshaw, S. Sandholm, G. Sachse, G. Gregory, B. Anderson, J. Barrick, A. Bachmeier, J. Collins, E. Browell, D. Blake, S. Rowland, Y. Kondo, H. Singh, R. Talbot, B. Heikes, J. Merrill, J. Rodriguez, and R. E. Newell, Assessment of the ozone photochemistry tendency in the western North Pacific as inferred from PEM-West A observations during the fall of 1991, J. Geophys. Res., 101, 2111-2134, 1996.

Davis, D. D., G. Chen, P. Kasibhatla, H. Berresheim, F. L. Eisele, D. Tanner, and A. Jefferson, DMS oxidation in the Antarctic marine boundary layer: Comparison of model simulations with observations for DMSO(g), DMSO₂(g), MSA(g), and H₂SO₄(g), J. Geophys. Res., in press, 1998a.

Davis D. D., G. Chen, F. Eisele, B. Huebert, D. Tanner, L. Mauldin, A. Bandy, D. Thornton, and D. Lenschow, DMS oxidation in the equatorial Pacific: Comparison of model simulations with field observations for DMS, SO₂, H₂SO₄(g), SMA(g), MS, and NSS, J. Geophys. Res, Submitted, 1998b.

Jacob, D. J., B. G. Heikes, S.-M. Fan, J. A. Logan, D. L. Mauzerall, J. D. Bradshaw, H. B. Singh, G. L. Gregory, R. W. Talbot, D. R. Blake, and G. W. Sachse, The origin of ozone and NO_x in the tropical troposphere: A photochemical analysis of aircraft observations over the south Atlantic basin, J. Geophys. Res.101, 24,235-24,250, 1996.