ANALYSIS AND MODELING OF PEM-TROPICS (B) DATA

Daniel J. Jacob (P.I.), Jennifer A. Logan (Co-I.), Martin G. Schultz (Co-I.)

Harvard University

OBJECTIVES

Our analysis of the PEM-Tropics (B) data will focus on answering the following questions:

* What factors control the concentrations of OH and HO2 in the Equatorial and South Pacific troposphere?

* What factors control the concentrations of ozone in the region?

* What are the sources and sinks of related trace gases (NOx, CO, hydrocarbons, acetone, peroxides)?

Our strategy will draw on a combination of data analysis, point photochemical modeling, and global 3-D modeling, as described in the tasks below. It will build on our experience with PEM-Tropics (A) and with earlier GTE missions.

SPECIFIC TASKS

* Compile merged time series of aircraft measurements for distribution to the Science Team.

In PEM-Tropics (A) we were responsible for compiling merged time series of the DC-8 and the P-3B aircraft data along with companion files of photochemical model products. We will do the same in PEM-Tropics (B). Our merging of the data will follow the protocol agreed to by the PEM-Tropics (A) Science Team, to be reviewed by the PEM-Tropics (B) Science Team. As in PEM-Tropics (A), we will produce merged time series matched to different time intervals and to the measurement frequencies of different instruments. Also as in PEM-Tropics (A), we will provide three generations of merged data sets: (1) a preliminary merge two months after completion of the mission, based on the "quick-look" data reported by the investigators in the field; (2) an intermediate merge one month after the first post-mission Science Team meeting, to be usedin the preparation of papers and for the public release of the data; and (3) a final merge for archival by GTE.

* Conduct point photochemical model calculations along the aircraft flight tracks to interpret observed concentrations of HO, NO, and ozone in terms of chemical sources and sinks. We will conduct point photochemical model calculations along the aircraft flight tracks using as constraints the ensemble of measurements aboard the aircraft. The approach will be similar to that used previously in our analyses of data from TRACE-A, PEM-Tropics (A), STRAT, and SONEX. Our goal will be to interpret the observed concentrations of HOx species and to construct regional chemical budgets of O3 and NO. Specific issues to be addressed will include

(1) the balance between chemical production and loss of ozone at different altitudes, (2) the chemistry responsible for the ozone minimum over the Equatorial Pacific, and (3) the degree to which NOx is maintained by chemical recycling from its NOy reservoirs.

* Characterize regional air masses and perturbed layers sampled by the aircraft.

The suite of species measured on PEM-Tropics B should allow us to understand the variability found in ozone and its precursors. We will examine the influence of several factors: biomass burning in the northern hemisphere, industrial pollution from Asia and America, production of NO from lightning, convective transport of marine boundary layer air, transport from southern mid-latitudes, and input from the stratosphere. We will examine the spatial patterns seen in the different species, and analyze correlations between species over a range of spatial scales, from small-scale plumes to large-scale gradients. The chemical data will be analyzed in conjunction with observed circulation patterns and trajectory analyses, recognizing the limitations of the latter in regions of convective activity.

* Interpret the PEM-Tropics (B) data in the context of longer-term records.

PEM-Tropics (B) will provide the first detailed survey of atmospheric composition over the South Pacific in austral summer. However, an aircraft mission offers only a snapshot. It is important to place the data in the context of other, longer-term measurements. Continuous observations of CO2, CH4, CO, O3, and halocarbons are made from several island sites over the Pacific, and these gases are also sampled on regular ship cruises across the Pacific. We will compare the surface data for January-April 1999 with climatological data for this season, and with low-altitude aircraft measurements during PEM-Tropics (B).

We will also compare the aircraft data from PEM-Tropics (B) with those from PEM-West (B), which operated in the same season along the western rim of the Pacific. We will compare the sonde data obtained during PEM-Tropics (B) in 1999 with those from the same season in 1995-98, and with the earlier Samoa data. We will analyze the trajectories associated with the ozone profiles obtained during PEM-Tropics (B), and use the sonde data to put the more limited (in time) aircraft data in context.

* Conduct global 3-D model simulations to investigate the budgets of ozone, OH, and NOx during PEM-Tropics (B). We are presently developing a global 3-D tropospheric chemistry modeling capability in support of GTE missions. Our approach is to drive the Harvard global 3-D model for tropospheric chemistry with assimilated meteorological data from the Global Spectral Model of Florida State University (FSU). We are using a version of the FSU model that has ~2.8o horizontal resolution, 14 sigma vertical levels in the troposphere, and 2-hour temporal resolution with data assimilation every 12 hours. We will apply the Harvard/FSU model to analyze the combination of dynamical and chemical factors controlling ozone and related species in the Pacific troposphere during PEM-Tropics (B). This work will be conducted in collaboration with T.N. Krishnamurti (FSU). The simulation will be initialized with global fields of concentrations two months prior to the beginning of the PEM-Tropics (B) mission, as obtained from the Harvard 3-D model driven with GCM meteorology. We will then conduct a 4-month simulation with the FSU data , the first two months being used to achieve proper initialization of transport patterns. Model results for ozone and other species will be compared to aircraft observations for the individual mission days. Results from the global 3-D model will be used to: (1) interpret the sources and chemical histories of the air masses sampled in PEM-Tropics (B); (2) generalize from the PEM-Tropics (B) observations to construct regional budgets of ozone, NOx, and related species over the tropical Pacific; and (3) assess the implications of the PEM-Tropics (B) observations for improved understanding of the oxidizing power of the atmosphere.