PEM West B Results
Overview of PEM-West B Results
In this section, we briefly highlight some of the salient
observations of the PEM-West campaigns, with particular emphasis on
the difference/similarities between phases A and B. More detailed
discussions and additional results are available in the companion
papers found in this special section. As already noted above, the
broad goals of the PEM-West campaigns were to characterize the
transport and chemistry of natural and anthropogenic emissions to the
northwestern Pacific region. From previous studies [Duce
et al., 1980; Prospero et al.,
1985; Merrill, 1989a, b], one expects an
enhanced transport of Asian emissions into the North Pacific Ocean
during the winter and early springtime. Indeed, the time periods for
the two PEM-West studies were selected to maximize the seasonal
contrast based upon these expectations. PEM-West A was conducted in
August/September and represents the northern hemisphere summertime
scenarios, while PEM-West B, conducted in February/March, represents
the winter.
Results from the PEM-West campaigns identify the impact of the major
meteorological features within the region on the quality of
tropospheric air over the northwestern Pacific Ocean region. For
example, the PEM-West data provide seasonal information on the
relative importance of the position/strength of the Japan Jet; the
location of the Pacific High; the influx of cleaner, southern
hemisphere air; and convection and lightning over the Asian continent
on the chemical signature of troposphere air at locations just off the
coast of Asia to mid-Pacific Ocean sites. As a result of the PEM-West
campaigns, data are now available to begin to assess the regional and
global implications of the observations. These data include (1) a
complete suite of chemical measurements at all tropospheric altitudes
and locations throughout the northwestern Pacific for two distinctive
seasons, (2) extensive ground-based longer-term measurements, and (3)
extensive meteorological products. In addition, the PEM-West database
is a baseline tool by which future assessments of the
impact of growing emissions from the Asian continent on remote Pacific
regions can be referenced.
Meteorological Setting
From a large-scale perspective, the meteorological differences
between PEM-West A and B can be characterized by the position and
strength of the Japan Jet and the location of the Pacific High [Merrill
et al., 1997]. During the northern hemisphere late
summer/early fall (i.e., PEM-West A), the Japan Jet tends to be weaker
and positioned more northerly than during the late winter to early
spring (i.e., PEM-West B). For example, during PEM-West A, the maximum
speed of the jet, located at about 12 km altitude, 35·N latitude,
was about 40 m/s, while during PEM-West B, the minimum speed was
observed to be about 70 m/s located at 32·N latitude and at a
lower altitude [Merrill et al., 1997].
In concert with the lower speed and more northerly position of the
Japan Jet, the Pacific High tends to be positioned more eastward and
northerly in the summer to fall period, serving to impede continental
outflow and to enhance inflow of marine air from the south
particularly at the lower altitudes. As a result, the PEM-West A study
period was characterized by more inflow of marine and southern
hemisphere air into the midtropical latitudes, accompanied by
extensive vertical mixing along a typhoon storm track running roughly
parallel with the Asian coast. During PEM-West A, the continental
outflow tended to be limited to north of about 40·N latitude. In
contrast, the PEM-West B period was characterized by enhanced
continental outflow throughout the study region, with a pronounced
continental enhancement north of about 20·N latitude. It is
particularly noteworthy that during the PEM-West B season at latitudes
of >20·N, velocities of strong westerlies resulted in Asian
pollutants being transported to sites thousands of kilometers from the
coast in 1 to 2 days compared to more than 5 days for PEM-West A. An
extremely important implication of quick transport is that Asian
outflow, enhanced in anthropogenic pollutants, arrives at remote ocean
sites relatively fresh in terms of potential for
photochemistry [Merrill et al., 1997;
Talbot et al., 1997 (a, b);
Crawford et al., 1997 (b);
Blake et al., 1997].
Superimposed upon seasonal differences attributed to the position
and strength of the Japan Jet and the Pacific High, are other seasonal
influences associated with the position and strength of Asian highs,
convection and lightning over Asia, and vertical mixing along typhoon
tracks.
Asian Outflow
Detailed analysis of the chemical signatures of trace
gases observed during PEM-West B are discussed by Gregory
et al. [1997], Talbot et al.
[this issue (a, b)], Blake et al.
[1997], Kondo et al. [1997 (a, b)],
and Dibb et al. [1997]. It is
significant to note that the methodologies employed to classify the
outflow regions varied from the use of backward trajectories to
selected hydrocarbon ratios, yet the result presents a consistent
perspective of the PEM-West B outflow regions. In general, their
conclusions are that (1) the PEM-West B period exhibited a greater
degree of fresh continental influence throughout the
northwestern Pacific region than during PEM-West A and (2) that the
southern boundary of the region having the more pronounced continental
influence observed during each season moved southward from about 40·N
in PEM-West A to about 20·N during PEM-West B. Blake
et al. [1997], for example, notes that during PEM-West B,
significantly higher mixing ratios for the NMHCs and C2Cl4 were
observed for a larger portion of the PEM-West B region as compared to
the PEM-West A, particularly in the low and midtroposphere at
latitudes greater than about 20·N latitude. [S. Smyth et al.
(unpublished manuscript), 1997] also notes that larger values of
C2H2/CO were observed over the western North Pacific during PEM-West B
compared to PEM-West A, indicative of less processed anthropogenic
emissions during PEM-West B. Kondo et al.
[1997 (b)], observes that in the continental air mass below 4
km, the levels of reactive nitrogen, as well as other continental
species were found to be significantly higher during PEM-West B than
those of PEM-West A. However, above 7 km, Kondo
et al. [1997 (b)] note that the converse was true because
convective activities were much weaker during early spring than in the
late summer to early fall time period. The above cited studies also
conclude that during PEM-West B, unlike PEM-West A, the chemical
signatures of trace gases indicative of aged marine air were
infrequently observed, and then only at latitudes below about 20·N.
The influence of clean or aged marine air or regional chemical budgets
is illustrated by the acidic gas data (HNO3, HCOOH, and CH3COOH) of
Talbot et al., [1997 (a, b)] showing
mixing ratios <100 parts per trillion by volume (pptv) for aged
marine air contrasted to 10 parts per billion by volume (ppbv) in
discrete plumes measured off the coast of Asia.
Photochemical Observations
An indicator of the strong influence that transport from markedly
different sources, such as aged marine air and Asian outflow, can have
on the photochemistry in the tropics is discussed by Crawford
et al. [1997 (a, b)]. Their analysis suggest three distinct
photochemical/geographical regions during PEM-West B (1) tropical,
less than 20·N latitude; (2) extratropical between 20· and
30·N latitude and (3) extratropical, greater than 30·N
latitude. They further divide the tropical region into a "high NOx"
and a "low NOx" regime based upon a shift in the air
mass source into the tropics, which resulted in a dramatic shift in
the observed NOx mixing ratio.
The implications of the ozone photochemisty associated with the
high/low NOx regimes in the tropical region are discussed by
Crawford et al. [1997 (a)]. During the
high NOx regime, encompassing the transit flight to Guam and
the first three local flights from Guam, NO mixing ratios at high
altitudes, often exceeded 100 pptv. In contrast, during the low NOx
regime, encompassing the final local Guam flight and the transit
flight to Hong Kong, NO values never exceeded 20 pptv.
Crawford et al. [1997 (a)] note that the
low NOx regime was found to be composed of air from the remote
north/south Pacific that had experienced significant convective
processing as evidenced by elevated CH3I levels at high altitude and
uniform ozone levels from the surface to 10 km. By contrast, the
regime designated as high NOx showed elevated levels of C3H8
and Pb210, suggesting air transported from the outflow of deep
convection over the continent. Interestingly, Crawford
et al. [1997 (a)], and also Kawakami
et al. [1997], suggest that DMSP satellite observations
recorded during the PEM-West B sampling period point strongly toward
lightning as contributing to the continental source of NOx
during the high NOx period. The photochemical analysis by
Crawford et al. [1997 (a)] suggest that
the net effect of photochemistry on ozone in the tropics was that of
net destruction at all altitudes within the low NOx regime.
Their analysis also indicates the tropospheric ozone column decreasing
by nearly 4% per day. For the high NOx regime, net production
of ozone for altitudes above 4 km approximately offset net destruction
at the lower altitudes. By comparison, the PEM-West A tropical data
was found to most resemble the low NOx regime.
The geographical division of the tropical and extratropical
component of the PEM-West B study region by Crawford
et al. [1997 (b)] along the 20·N latitude is consistent
with the results from the air mass characterization papers noted in
the previous section. The additional subdivision of the extratropical
component along 30·N latitude by Crawford
et al. [1997 (b)] was based on the observation of an abrupt
drop in the tropopause height from 16 to 89 km, and a
corresponding increase in the total O3 column of about 150 Dobson
units. While significant outflow of ozone and its precursors were
observed both to the north and south of 30·N, the dramatic
increase in overhead O3 north of 30·N led to a significant
decrease in photochemical activity. In spite of these decreases,
calculations by Crawford et al. [1997 (b)]
showed that both extratropical regions produced net ozone at all
altitudes. This finding is especially important in that it marks the
first time that net ozone production has been observed in the lower
marine troposphere. That this condition is observed in the late
winter/early spring also emphasizes the critical role of meteorology
in transporting ozone precursors into the western, North Pacific.
While the calculated rates of increase for the tropospheric O3 column
appeared to be unusually large (e.g., 2% per day south of 30·N
and 1% per day to the north), they serve as a potential source for
offsetting the large net destruction observed in some regions of the
tropics.
Crawford et al. [1997 (b)] note that
the importance of meteorological flow patterns to the photochemical
environment of the extratropical western North Pacific becomes even
clearer when comparing PEM-West A with PEM-West B. While differences
between the two for altitudes above 4 km were small, the impact of
continental outflow for the lower troposphere during PEM-West B was
enough that if similar levels had been present during PEM-West A it
would have doubled the rate of tropospheric O3 column formation during
PEM-West A. Given the actual levels of precursors for both field
studies and the fact that they had similar rates of O3 destruction,
the enhanced formation for PEM-West B was sufficient to create net
column ozone production in PEM-West B; whereas, for PEM-West A an
approximate balance was observed.
Sulfur Dioxide
An additional important focus of the PEM-West campaigns was
evaluation of the sources of tropospheric sulfur species. During
PEM-West A, Thornton et al. [1996]
reported that SO2 exhibited a marked increase in mixing ratio with
altitude. This strong dependence of SO2 on altitude was attributed to
(1) long-range transport of anthropogenic emissions injected into the
free troposphere via continental convection, (2) the influence of
direct emissions of SO2 from the Pinatubo eruption of June 1991 into
the troposphere, and (3) inputs of stratospheric air affected by
Pinatubo into the free troposphere. In contrast to PEM-West A, no
clear gradient in SO2 was observed in the upper troposphere during
PEM-West B, and furthermore, the mixing ratios of SO2 were
significantly lower [Thornton et al., 1997
(a)]. Thornton et al. [1997 (a)]
also conclude that during PEM-West A and B the oxidation of
dimethylsulfide (DMS) was a relatively insignificant source of free
troposphere SO2. Of particular interest is the observation by
Thornton et al. [1997 (a, b)] and
Talbot et al. [1997 (a, b)] of the
coincidence of high mixing ratios of SO2, NO, and ultrafine CN
observed above 9 km in the tropical convergence zone during PEM-West
B. These observations support those of Clarke
[1992, 1993] relative to the production of new particles at high
altitude.
Ground-Based Observations
Ground-based observations were incorporated in the design of the
PEM-West campaigns to provide a longer temporal record of the Asian
outflow than possible with the short intensive airborne measurements
from the DC-8. Analyses reported by Arimoto et
al. [1997], comparing chemical data from ground stations in
Asia and the North Pacific with the data from DC-8, provide important
insight into the conclusions emerging from the ground and airborne
measurements. Aerosol sampling from ground stations on Hong Kong,
Taiwan, Okinawa, and Cheju; and at three islands in the North Pacific
(Shemya, Midway, and Oahu) are compared with aircraft samples obtained
during flights over the western Pacific. Interestingly,
Arimoto et al. [1997] note that
conditions representative of a strong, or even moderate dust storms,
were never encountered by the DC-8. Observations at the ground sites
did indicate a significant stronger continental outflow during
PEM-West B than PEM-West A which is consistent with the aircraft
observations [Kajii et al., 1997;
Liu et al., 1997; Wang
et al., 1997 ; K. S. Lam et al., unpublished
manuscript, 1997]. In addition, a study of the transport of O3, CO,
HNO3, SO2, and HCl through trajectory analysis at Oki Island, Japan,
showed substantially elevated levels of all species other than O3 for
air masses from the west compared to those from the north [Kajii
et al., 1997]. The corresponding enhancement of O3 was only
about 20%, which was attributed to relatively slow photochemical
production in the fast moving air masses. This is substantiated by
observations in Hong Kong and Taiwan, which showed similar levels of
O3 as Oki Island [Liu et al., 1997;
Wang et al., 1997; K. S. Lam et al.,
unpublished manuscript, 1997].
The observations of surface O3, CO, NO, and SO2 at the southern tip
of Hong Kong Island indicate that the local pollution episodes can be
identified by pollutant tracers such as CO and NO and thus the
regional characteristics of these species can be obtained from the
observations by filtering out the local episodes [Wang
et al., 1997; K. S. Lam et al., unpublished
manuscript, 1997]. An interesting finding from their observations is
that the seasonal variation of O3 in Hong Kong is controlled strongly
by the Asian monsoon system which is characterized by strong
northeasterlies in winter and tropical marine winds in summer. As a
result, the median O3 level in winter was found to be about 50% higher
than that in summer. Furthermore, photochemically derived high mixing
ratios of O3 occurred primarily in the fall when the combination of
continental outflow of O3 precursors and prevailing fair weather
provided conditions conducive to photochemical production of O3.
NOy Intercomparison
During PEM-West (A and B) measurements of NO and NOy aboard
the DC-8 by investigators from the Nagoya University (NU) and the
Georgia Institute of Technology (GIT) provided an opportunity for the
first airborne intercomparison of NOy measurements in the
troposphere, as well as additional intercomparisons of NO measurements
[Gregory et al., 1985, 1987]. The NU
measurements were obtained using a two-channel chemiluminescence
instrument [Kondo et al., 1997 (a)] for
simultaneous detection of atmospheric NO, and the NO resulting from
the catalytic conversion of odd nitrogen compounds (i.e., NOy)
to NO on the surface of a heated gold tube. The GIT measurements were
obtained using a three-channel system for simultaneous measurement of
NO, NO2, and NOy. In each channel, the species detected was NO
by using a two-photon, laser-induced-fluorescence technique [Sandholm
et al., 1990]. In the NO2 channel, NO was generated through
photolysis of NO2 to NO. (Because of concerns over inlet artifacts [J.
D. Bradshaw et al., unpublished manuscript, 1997], the GIT NO2
data were not archived for the PEM-West B campaign.) In the NOy
channel, NO was generated through a catalytic converter. The NOy
converters used by NU and GIT were based on a design by
Fahey et al. [1985], although important
aspects, such as size, mass flow rate, flow stream surface
characteristics, etc. differed in each instrument.
During both the PEM-West A and B missions, the NO measurements from
each instrument agreed very well. During PEM-West A, the two sets of
NOy measurements exhibited virtually no correlation [Crosley,
1996]. While recommendations from the Crosley report were
implemented during PEM-West B by investigators from NU [Kondo
et al., 1997 (a)] and GIT [J. D. Bradshaw et al.,
unpublished manuscript, 1997] and the correlation between the two sets
of measurements was improved somewhat, there continued to be
substantial disagreement between the two sets of NOy
measurements. Investigators from NU, reporting on post PEM-West B
laboratory studies to quantify potential interference and/or artifacts
during the PEM-West B mission, concluded that their inlet system and
application of the techniques for converting odd nitrogen species to
NO is substantially free of inlet losses (e.g., HNO3) and/or
interference or artifacts [Kondo et al.,
1997 (a)]. Investigators from GIT included a statement with
their archived NOy data
(http://www-gte.larc.nasa.gov/gte_arch.htm:
readmeni.git
and
readmeny.git
) describing tests from which they conclude that their inlet system is
also free of loss of HNO3, but their measurements of NOy are
likely to have contributions from non- NOy forms of fixed
nitrogen (e.g., HCN, NH3, etc.) of unknown and variable magnitudes.
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