Tunable Diode Laser Measurements of Formaldehyde and Potentially Hydrogen
Peroxide on NASA's WP3 Aircraft During TRACE-P.

This proposal describes a collaborative two-year effort between scientists
at the National Center for Atmospheric Research (NCAR), the University of
Toronto, and the University of Tulsa to participate in NASA's Transport and
Chemical Evolution Over the Pacific (TRACE-P) study. This research group
specifically proposes to deploy a Dual Channel Airborne tunable diode Laser
Spectrometer (DCALS), jointly developed by scientists at NCAR and the
University of Toronto, for ambient measurements of formaldehyde (CH2O) and
potentially hydrogen peroxide (H2O2) onboard NASA's WP-3B aircraft during
TRACE-P. 

The TRACE-P study will examine the chemical composition, transport, and
chemical evolution of air as it moves from Asia out across the Pacific
Ocean. Formaldehyde (CH2O) and hydrogen peroxide (H2O2) are two of the many
reactive intermediates whose concentrations need to be accurately measured
during TRACE-P. Formaldehyde is formed from the oxidation of hydrocarbons
emitted from the continent, and its decomposition provides an important
source of reactive hydrogen radicals over relatively remote regions of the
ocean. Hydrogen peroxide, by contrast, serves as a major radical reservoir
over these same regions. Accurate knowledge of the concentrations of both
gases is critical for testing our understanding of the processes involved
in the chemical evolution of the Asian outflow. 

The NCAR/University of Toronto collaborative team has developed a
longstanding history of carrying out accurate ambient measurements of CH2O
on both ground-based and aircraft platforms employing tunable diode laser
absorption spectroscopy (TDLAS); this collaborative team has successfully
acquired CH2O data in 7 different field campaigns (3 ground-based and 4
aircraft-based) during the past 8 years. With each campaign, the
NCAR/University of Toronto team has continued to advance improvements in
the measurement methodology, and this has culminated in the development of
the DCALS system, which is presently being operated onboard NCAR's C-130
aircraft during the TOPSE campaign. This system is successfully acquiring
simultaneous measurements of CH2O and H2O2. During this mission, ambient
measurements for both gases are being acquired in 10-second increments, and
these measurements can be further averaged for improved precision. This
approach thus provides fast (10-second) data, which are important for
studying the chemical evolution of continental outflow events, as well as
lower time resolution data where higher measurement precisions are
required. During the TOPSE campaign, the high altitude flight legs, where
the ambient levels of both gases are low and stable, provides an upper
limit for the inherent instrument precision. In the case of CH2O, the
10-second precision (at the 2s level) averages 156-parts-per-trillion by
volume (pptv), and the 1-minute and 2-minute averages of such data further
improve the measurement precision to 60-pptv and 46-pptv, respectively. At
present, the ambient measurement precision for the H2O2 channel is
approximately an order of magnitude worse, and this is one of the main foci
this proposal intends to address. 

We are proposing to build upon our expertise by deploying the DCALS system
with its unique capabilities for simultaneous fast measurements of CH2O and
potentially H2O2 during the TRACE-P field campaign. We are specifically
proposing to employ the present DCALS system with a number of improvements
aimed at enhancing the performance of the H2O2 channel, which is affected
by the lasing spatial beam quality and its associated pointing stability.
In addition, we also plan to purchase redundancies for some key instrument
components as well as repackage some existing components to reduce system
weight and size. Upon completion of the field campaign, the second proposal
year will be devoted exclusively to data analysis and interpretation as
well as any necessary laboratory and field instrument tests that may be
required.