1.
Satellite Measurements of
Total Ozone
Satellite measurements of ozone have been made since
the launch of the Backscatter
Ultraviolet (BUV) instrument on the United States Nimbus-4
satellite in 1970 [Heath et al., 1975]. Subsequent instruments,
using the same measurement technique as the Nimbus-4 BUV instrument,
have been providing total ozone measurements nearly continuously since
that time. These instruments provided quantitative information
about the total amount of ozone between the earth’s surface and the
satellite and primarily provide scientists with information about the
distribution of ozone in the stratosphere since ~90% of the ozone in
the atmosphere lies in this region (~15 km – 55 km). The
remaining ~10% is located in the troposphere; the boundary between the
troposphere and the stratosphere, the tropopause, is generally located
at an altitude between 12 km and 18 km and varies as both a function of
latitude and time of year. In the tropics, the tropopause is
located higher in the atmosphere (16-18 km) than at middle latitudes,
where the height of the tropopause can be as low as 8-10 km, or as high
as 15-16 km, with higher tropopause heights generally found in the
summer.
The amount of ozone in a column of air is expressed
in units called Dobson Units (DU) where one DU has a value of 2.69 x
10e16 molecules of ozone cm-2. A representative amount of total
ozone in the atmosphere is 300 DU, of which ~30 DU is in the
troposphere and the remainder is in the stratosphere. In the
unusual occurrence of the “ozone hole” found over Antarctica during
austral spring, total ozone values <100 DU have been measured.
2. Tropospheric Signal in Total
Ozone Measurements
In a series of papers that date back to the
mid-1980s, our research group has pursued the idea that satellite data
sets can be used to glean otherwise unattainable insight into the
distribution of ozone in the troposphere. Although the Total
Ozone Mapping Spectrometer (TOMS) was built to monitor ozone in the
stratosphere, and perhaps provide insight into stratospheric ozone
depletion, Fishman et al. [1986] showed that enhancements in the total
ozone signal over Brazil occurred when widespread biomass burning was
present as shown in Figure 1.
From the monthly average of the
TOMS measurements shown in Figure 2,
enhanced values can be seen over
South America and southern Africa and in a region over the South
Atlantic Ocean, in what appears to be a plume coming off the African
continent during October 1987. During other months, such as
October 1985 (Figure 3), total ozone
enhancements can be found off the
west coast of North America. During this particular month, a
persistent area of anticyclonic circulation was situated off the
northwest coast of the United States, resulting in the plume-like
structure emanating from the highly populated Los Angeles-San Diego
region.
The data depicted in these figures, however, reflect
total ozone measurements, of which only a relatively small percentage
is found in the troposphere. At higher latitudes, meteorological
activity is generally more vigorous and persistent patterns over the
period of a month are rare. Therefore, identification of enhanced
ozone of tropospheric origin is difficult and quasi-persistent plumes
from Europe and northern Asia would be lost beneath the variable
stratospheric ozone amounts that would overwhelm any persistent
tropospheric enhancements in these regions. Thus, persistent
ozone sources in the tropics offered the first indications that
satellite information could be used to identify ozone pollution sources.
3. Separation of Stratospheric
Column Ozone from Total Ozone Measurements
The next step in the research was to find a method
of separating the stratospheric component from the total ozone
measurements. Before that methodology was developed, however, an
important analysis was provided in Fishman and Larsen [1987] where SAGE
(Stratospheric Aerosol and Gas Experiment) ozone profiles were used to
calculate the amount of ozone in the stratosphere, or stratospheric
column ozone (SCO). Analysis of the SCO showed that the amount of
stratospheric ozone at low latitudes was longitudinally
independent. On the other hand, TOMS total ozone measurements
between 15°N and 15°S showed that the meridional distribution
of total ozone was quite different from the SCO distribution and that
the difference between the total column and the SCO was highly
correlated with the total ozone measured by TOMS.
During the 1990s, our research group focused on
trying to extract information about the troposphere from satellite
measurements by assuming that ozone variability in the stratosphere is
defined on relatively large spatial scales compared with the
troposphere and that information could be obtained about the
troposphere if this larger scale stratospheric component could be
isolated. Once the stratospheric ozone distribution has been
established, we can then examine the “residual” information contained
in the TOMS total ozone measurements to infer information about the
troposphere (see Figure 4). The
resultant distribution derived
from this method was referred to as the “tropospheric ozone residual”
(TOR). The stratospheric ozone distribution that was subtracted
from the concurrent TOMS total ozone measurement was first derived
using co-located SAGE measurements. SAGE has been providing ozone
information since 1979 when it was first launched on the Atmospheric
Explorer Mission 2 (AEM-2) satellite, which functioned for nearly three
years. In 1984, SAGE II was launched on the Earth Radiation
Budget Satellite (ERBS) and provided measurements 21 years before being
turned off in September 2005.
Using standard tropopause height information from
the National Center for Environmental Prediction (NCEP) in conjunction
with the SAGE profiles, Fishman et al. [1990] demonstrated that a
fairly accurate climatological depiction of tropospheric ozone could be
obtained to include a domain that included both the tropics and middle
latitudes (~50˚ N to ~50˚ S). The seasonal climatology of the TOR
is depicted in Figure 5 [Fishman et
al., 1990] and shows the elevated
amounts of tropospheric ozone present in the middle latitudes during
northern hemisphere summer; elevated TOR values are also present in the
tropics and subtropics in the southern hemisphere during austral spring
(September-November), a consequence of widespread biomass burning
during that time of the year. This somewhat surprising finding in
the South Atlantic became the focus of a major field campaign in
1992: TRACE-A (Transport and Atmospheric Chemistry near the
Equator—Atlantic; Fishman et al., 1996b).
4. Deriving TOR from TOMS and
SBUV: The Empirical Correction
The TOR technique using SAGE data has generally been
limited to climatological studies [e.g., Fishman et al., 1991; Fishman
and Brackett, 1997]. A more rigorous analysis of the data
indicates that the TOM/SAGE TOR is not useful to study tropospheric
ozone variations at non-climatological scales because the TOMS/SAGE TOR
is subject to sampling aliasing for periods of seasons or less, a
consequence of the ~40-day period needed to obtain complete latitudinal
coverage [Vukovich et al., 1996].
To achieve better temporal resolution, concurrent
measurements from TOMS and SBUV
(Solar Backscatter Ultraviolet)
instruments have been used to derive daily TOR distributions. The
procedure to derive the TOMS/SBUV TOR also uses knowledge of tropopause
height obtained from gridded fields determined from the NCEP
assimilation/modeling data base [Kalnay et al., 1996]. The grid
that is used for the
calculations is a 100 by 288 matrix, having a resolution of 1°
latitude by 1.25° longitude and covering the region between the
latitudes of 50° S to 50° N. In the region 50°S to
50°N, the SBUV instrument provides approximately 750 - 800 vertical
profiles on a daily basis (SAGE, on the other hand provides ~30
vertical profiles per day). After the stratospheric ozone is
determined at each grid point where SBUV data are available, an
interpolation procedure is used to fill in all missing values within
the daily matrix. A five-day running average is applied using the
daily matrices of the SBUV stratospheric ozone and the resulting matrix
is used to represent the stratospheric ozone distribution for the
central day (i.e., day 3) used in the 5-day running average. The
TOMS data, on the other hand, are not temporally averaged. The
global TOMS/SBUV TOR distribution is determined for a given day by
subtracting the 5-day running averaged SBUV stratospheric ozone data
from the TOMS total ozone data at each grid point.
Using this technique, the daily TOR were compared
with data obtained from the TRACE-A field mission in 1992. In
general, except when heavy aerosols were present due to widespread
biomass burning, the TOR generally captured the spatial gradient of
tropospheric ozone, but usually underestimated the magnitude of the
gradient, and nominally captured only ~60% of the spatial difference
between two points when compared with gradients determined from
aircraft measurements [Fishman et al., 1996a]. Further analysis
by Fishman and Balok [1999] found that the climatological distribution
of the archived SBUV data was critically inaccurate in the lowest three
levels of the atmosphere. In turn, these inaccurate values were
the primary contributing factor to the poor agreement found between the
TOMS/SBUV TOR distributions and the TOMS/SAGE TOR noted by Vukovich et
al. [1997]. After analyzing thousands of soundings from more than
20 ozonesonde stations, Fishman and Balok employed an empirical
correction to the archived SBUV measurements to derive stratospheric
integrals that could be subtracted from the concurrent TOMS
observations. These data were then used to characterize the
development of an air pollution episode over the eastern United
States.
As part of our research, we have developed monthly
distributions of TOR fields using concurrent measurements from TOMS and
SBUV instruments. Initially, the use of archived SBUV profiles
had been shown to result in inaccuracies in layer amounts below the
stratospheric ozone peak [Vukovich et al., 1996; Ziemke et al.,
1998]. However, through the use of an empirical correction
applied to the lowest three SBUV archived layers [Fishman and Balok,
1999], a global TOR dataset that greatly enhances the regional detail
that can be obtained using the bounty of the TOMS total ozone database
[Fishman et al., 2003] is available. This methodology has used
empirically corrected SBUV data (see Figure
6) to derive the amount of
ozone in the stratosphere, a quantity called stratospheric column ozone
(SCO) which is computed daily and then subtracted from the concurrent
measurement of total ozone derived from TOMS.
5. Scientific Findings from
the TOMS/SBUV TOR Database
The unique attribute of our approach is the use of
as many TOMS measurements as possible to derive a tropospheric
product. In simplest terms, a daily global distribution of the
stratospheric component of total ozone is constructed which should
contain only large-scale structure. As a result, regional
enhancements are significantly better resolved than in the analyses
derived using TOMS and SAGE (see Figure 7).
In the
June-July-August (JJA) NH summertime depiction, for example, highest
TOR values are located throughout the eastern United States and
throughout eastern Asia. Prominent high values are also seen
emanating off the west coast of the United States as well as along the
Ganges River Valley in northeastern India.. In fact, the original
TOMS/SAGE TOR over northwest India showed a relative minimum, which was
interpreted to be associated with the relatively higher elevations and
lack of population in the Tibetan Plateau. The greater detail in
the present analysis shows a better-defined, relatively small region of
low ozone over the higher elevations. However, just south of that
region in the Ganges River Valley, extending west of Delhi and eastward
through Bangladesh and northern Burma, much higher values of ozone are
observed. High values are now seen throughout central and eastern
China.
6. Validation of TOR Database
Validation of the TOR fields using this methodology
is extremely difficult. An initial comparison of TOR values with
a number of ozonesonde stations showing the improvement of the
application of the empirical correction to the lowest three levels of
the Version 6 SBUV archive was presented in Fishman et al.
[2003]. Another validation study showing how the TOR accurately
captures the horizontal gradient of the monthly climatological values
of tropospheric column ozone derived from long-term ozonesonde stations
was presented in Creilson et al. [2003; see Figure
8].
The most comprehensive validation study was recently
published in Wozniak et al. [2005].
Although the validation of
TOR fields is extremely difficult without intensive dedicated field
missions, the other product generated by the TOR methodology, namely
the
stratospheric column ozone (SCO), can be compared against available
measurements derived from both in situ and satellite techniques.
In turn, these satellite measurements have undergone intensive scrutiny
since they have been used to assess how much ozone has been
destroyed due to the release of chlorofluorocarbons [WMO, 1999;
WMO, 2003]. A comparison between the SCO values derived from our
empirically corrected SBUV methodology and the SCO derived from SAGE is
shown in Figure 9.
7. The Erroneous Allegations of de
Laat and Aben
Shortly after the publication of Fishman et al.
[2003], in Atmospheric Chemistry and Physics (ACP), an article was
published in Atmospheric Chemistry and Physics Discussions (ACPD) by de
Laat and Aben [2003] that challenged some of the findings in Fishman et
al. [2003]. In that paper, they claimed that “it is possible to
obtain a tropospheric O3
column that is very similar to what is being
presented in Fishman et al. [2003], solely based on the Logan [1999]
tropospheric O3 climatology
and an estimate for the tropopause heights
without using satellite data.” Although works appearing in ACPD
go through an initial peer-review screening, “subsequent interactive
discussion and public commenting by the referees, authors and other
members of the scientific community is expected to enhance quality
control for papers published in ACP beyond the limits of the
traditional closed peer-review,” according to the guidelines
established by the European Geophysical Union, the scientific body that
oversees the editorial process for these journals. As can be seen
in Figure 10 and the accompanying table, the tropospheric ozone
distribution of Logan [1999] is considerably different from what
appears in Fishman et al. [2003], especially in northern India, one of
the key regional findings reported in that study. A more detailed
analysis of the fallacies in de Laat and Aben’s reasoning is available
in Fishman [2003].
Furthermore it is notable that de Laat and Aben’s manuscript was never
published in ACP, because of the public comments put forth in the
discussion phase of ACPD.
8. Scientific Studies Using the
Empirically Corrected TOR Data
A.
Relationship between the
North Atlantic Oscillation and Trans-Atlantic Pollution Transport
Creilson et al. [2003] examined the
seasonal and regional distribution of tropospheric ozone across the
North Atlantic and its relationship to the North Atlantic Oscillation
(NAO) as a possible transport mechanism across the North
Atlantic. The study found distinct springtime interannual
variability over the North Atlantic in the TOR fields that is
correlated with the interannual variability of the NAO. Positive
phases of the NAO are indicative of a stronger Bermuda/Azores high and
a stronger Icelandic low and thus faster more zonal flow across the
North Atlantic from west to east. This flow regime appears to be
causing the transport of tropospheric ozone across the North Atlantic
and onto Europe. An example of the highly variable nature of the
TOR over Europe and the flow patterns that usually accompany these
patterns is provided in Figure 11.
The entire article is found at
Creilson et al. [2003].
B. Arctic
Oscillation–induced
variability in satellite-derived tropospheric ozone
Creilson et al. [2005] showed that
there is a statistically significant correlation between the Arctic
Oscillation (AO) and the
springtime tropospheric ozone distribution over the northeastern
Atlantic, but not over the Pacific. This finding is consistent
with our understanding of the differing effect that the AO has on the
Atlantic versus Pacific basins and the strong influence that the El
Niño phenomenon has in the Pacific. The insight gained
from this study contributes to the growing use of teleconnections as a
forecast tool, providing insight into the interaction between
prevailing meteorological conditions and the formation of significant
pollution events. On the other hand, correlations across the
Pacific were stronger with the Southern Oscillation Index (SOI) than
with the AO. An analysis of the 500 hPa flow patterns across the
North Pacific shows that the core of the strongest anomalies are
located to the north of our study area and also to the south where the
influence of ENSO is strongest, suggesting that enhanced tropospheric
ozone in the eastern Pacific/western United States is due to a
different set of processes and relationships. These findings are
summarized in Figure 12; the complete
discussion can be found in Creilson et al.
[2005].
C.
Interannual Variability of
Stratospheric and Tropospheric Ozone
Fishman et al. [2005] show that the
interannual variability (IAV) of the stratospheric column ozone (SCO)
from the TOR data base is consistent with previous findings for total
ozone that show a strong correlation with the quasi-biennial
oscillation (QBO) at low latitudes (Figure
13). For tropospheric
ozone, there are strong regional enhancements due to in situ generation
from large emissions (Figure 14); the
IAV of some of these regional
enhancements, on the other hand, are strongly correlated with the phase
of El Niño/Southern Oscillation (ENSO) and are consistent with
our understanding of how regions of subsidence are more conducive to
the in situ production of ozone pollution (Figure
15). The
insight gained from these analyses will hopefully provide a better
understanding between prevailing meteorological conditions and the
evolution of widespread ozone episodes on shorter time scales with the
eventual goal of producing an air quality forecasting capability so
that exposure of the human population to elevated levels of ozone can
be reduced. The complete paper has been published as Fishman et
al. [2005].
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