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A Review of Surface Ozone Background Levels and Trends

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A survey of the literature was conducted to review historical and current surface ozone data from background stations in Canada, United States and around the world for the purpose of characterizing background levels and trends, present plausible explanations for observed trends and explore projections of future ozone levels. The annual ozone cycle at background sites in the Northern Hemisphere is characterized by a spring maximum, peaking during the month of May. Although presently there is no concensus as to the origin of the spring maximum, evidence supports both enhanced photochemistry in the free troposphere and stratospheric input. Modern day annual average background ozone concentrations over the midlatitudes of the Northern Hemisphere range between approximately 20–45 ppb, with variability being a function of geographic location, elevation and extent of anthropogenic influence. Annual median ozone levels at Canadian background stations fall between 23 and 34 ppb, a range similar to that reported for low-elevation background stations in the United States and around the world. Comparisons of ozone levels with those measured over a century ago indicate that current levels have increased by approximately two times. Although current trends are not uniform, there is some indication that background ozone levels over the midlatitudes of the Northern Hemisphere have continued to rise over the past three decades, and that this rise has been in the range of approximately 0.5–2% per year. Rising trends were steeper in the 1970s and 1980s compared to the 1990s, which have seen either a leveling off or a decline in the magnitude of these trends. Model sensitivity studies indicate that the rise in NOx emissions account for the greatest increase in background ozone levels over the past three decades. A substantial component of the background ozone concentration in western North America may be due to long-range transport of Asian pollution, especially during the spring months. Model projections using IPCC emission scenarios for the 21st century indicate that background ozone may rise to levels that would exceed internationally accepted environmental criteria for human health and the environment.
Historical, current and projected background surface ozone annual concentrations. a-Athens, Greece (Varotsos and Cartalis, 1991). b-Europe (Bojkov, 1986)-avg of daily maxima. c-Montsouris, France (Volz and Kley, 1988). d-Arosa, Switzerland, (Staehelin et al., 1994). e-Arosa, Switzerland, (Staehelin et al., 1994). f-Pt. Barrow, Alaska (CMDL, 2004). g-Virgin Islands National Park, US Virgin Islands (CASTNet, 2004). h-American Samoa (CMDL, 2004). i-South Pole, Antarctica (CMDL, 2004). j-Arrival Heights, Antarctica (CMDL, 2004). k-Ny Alesund, Svalbard, Norway (CMDL, 2004). l-Mauna Loa, Hawaii (CMDL, 2004). m-Mount Rainier National Park, Washington (CASTNet, 2004). n-Denali National Park, Alaska (CASTNet, 2004). oGlacier National Park, Montana (CASTNet, 2004). p-Lassen National Park, California (CASTNet, 2004). q-Rocky Mountain National Park, Colorado (CASTNet, 2004). r-Theodore Roosevelt National Park, North Dakota (CASTNet, 2004). s-Yellowstone National Park, Wyoming (CASTNet, 2004). t-Kejimkujik, Nova Scotia (CAPMoN, 2003). u-Montmorency, Quebec (CAPMoN, 2003). v-Algoma, Ontario (CAPMoN, 2003). w-Chalk River, Ontario, (CAPMoN, 2003). x-Egbert, Ontario (CAPMoN, 2003). y-Experimental Lakes Area (ELA), Ontario, (CAPMoN, 2003). z-Bratt's Lake, Saskatchewan (CAPMoN, 2003). $-Esther, Alberta (CAPMoN, 2003). &-Saturna, British Columbia (CAPMoN, 2003). i-Range of surface O 3 projections for the 2040 (IPCCDDC, 2004). ii-Range of surface O 3 projections for the 2060 (IPCC-DDC, 2004). iii-Range of surface O 3 projections for the 2080 (IPCC-DDC, 2004). iv-Range of surface O 3 projections for 2100 (IPCC-DDC, 2004).
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Atmospheric Environment 38 (2004) 3431–3442
A review of surface ozone background levels and trends
Roxanne Vingarzan
Environment Canada, Aquatic and Atmospheric Sciences Division, 201-401 Burrard Street, Vancouver BC, Canada, V6C 3S5
Received 21 July 2003; accepted 2 March 2004
Abstract
A survey of the literature was conducted to review historical and current surface ozone data from background
stations in Canada, United States and around the world for the purpose of characterizing background levels and trends,
present plausible explanations for observed trends and explore projections of future ozone levels. The annual ozone
cycle at background sites in the Northern Hemisphere is characterized by a spring maximum, peaking during the month
of May. Although presently there is no concensus as to the origin of the spring maximum, evidence supports both
enhanced photochemistry in the free troposphere and stratospheric input. Modern day annual average background
ozone concentrations over the midlatitudes of the Northern Hemisphere range between approximately 20–45 ppb, with
variability being a function of geographic location, elevation and extent of anthropogenic influence. Annual median
ozone levels at Canadian background stations fall between 23 and 34 ppb, a range similar to that reported for low-
elevation background stations in the United States and around the world. Comparisons of ozone levels with those
measured over a century ago indicate that current levels have increased by approximately two times. Although current
trends are not uniform, there is some indication that background ozone levels over the midlatitudes of the Northern
Hemisphere have continued to rise over the past three decades, and that this rise has been in the range of approximately
0.5–2% per year. Rising trends were steeper in the 1970s and 1980s compared to the 1990s, which have seen either a
leveling off or a decline in the magnitude of these trends. Model sensitivity studies indicate that the rise in NO
x
emissions account for the greatest increase in background ozone levels over the past three decades. A substantial
component of the background ozone concentration in western North America may be due to long-range transport of
Asian pollution, especially during the spring months. Model projections using IPCC emission scenarios for the 21st
century indicate that background ozone may rise to levels that would exceed internationally accepted environmental
criteria for human health and the environment.
Crown Copyright r2004 Published by Elsevier Ltd. All rights reserved.
Keywords: Background ozone; Ozone trends; Spring maximum; Tropospheric ozone; Ozone projections
1. Introduction
The existence of a background level of ozone in the
atmosphere is well established. There is considerable
interest in quantifying surface background ozone con-
centrations and associated trends, as they serve to define
a lower boundary with respect to reductions of ozone by
control of anthropogenic precursors. Background ozone
is generally defined as the fraction of ozone present in a
given area that is not attributed to anthropogenic
sources of local origin. As such, background ozone has
several well-documented sources, both natural and
anthropogenic. These include: (1) downward transport
of stratospheric ozone through the free troposphere to
near ground level, (2) in situ ozone production from
methane emitted from swamps and wetlands reacting
with natural NO
x
(from soils, lightning strikes and
downward transport of NO from the stratosphere), (3)
in situ production of ozone from reactions of biogenic
ARTICLE IN PRESS
E-mail address: roxanne.vingarzan@ec.gc.ca
(R. Vingarzan).
1352-2310/$ - see front matter Crown Copyright r2004 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.atmosenv.2004.03.030
VOCs with natural NO
x
and (4) long-range transport of
ozone from distant pollutant sources (EPA, 1993).
The ozone concentration in any given area results
from a combination of formation, transport, destruction
and deposition. Elevation also affects surface ozone
concentrations, with higher concentrations typically
seen at sites located in the free troposphere. Given the
many complex natural and anthropogenically induced
factors that influence current surface ozone levels
and the variability resulting from these factors, it is
difficult to assign a single background ozone concentra-
tion. It is also unlikely that any area of the Earth is
completely free from anthropogenic influences. How-
ever, measurements taken at sites least affected by these
influences can give an indication of background surface
ozone levels.
Recent investigations indicate that the concentration
of ozone in the Earth’s atmosphere is changing.
Although there is good agreement regarding a rise in
background levels over the past century, in recent
decades divergent trends in tropospheric ozone have
been observed over different regions of the globe. This
document summarizes the seasonal characteristics of
background ozone in the Northern Hemisphere,
historical and current background levels and trends
and possible reasons for these observed trends.
Model projections of future ozone levels are presented
and discussed in the context of historical and current
levels.
2. Seasonal cycles
The background level of ozone is not static, but has
been shown to exhibit pronounced seasonal cycles that
have different shapes at different latitudes and altitudes
(Logan, 1985,1999;Monks, 2000). These cycles are
controlled by a number of processes including photo-
chemistry, deposition, and transport, acting at local,
regional and global scales.
The existence of seasonal cycles in ozone concentra-
tions was noted as early as a century ago. Surface ozone
measurements from the late 19th and early 20th
centuries show a spring to early summer ozone
maximum (Lisac and Grubisic, 1991;Feister and
Warmbt, 1987;Mukammal et al., 1985;Meagher et al.,
1987;Bojkov, 1986). This spring maximum is well
documented in current surface observations from back-
ground sites in the Northern Hemisphere (Scheel et al.,
1990;Angle and Sandhu, 1986;Meagher et al., 1987;
Singh et al., 1978). However, at some background sites,
a summer maximum is more evident, due to the
influence of local photochemical ozone production from
precursor emissions (Hough and Derwent, 1990). The
spring maximum is reported to peak in May in the
latitudinal range 10–60and increase in concentration in
a south to north direction. North of 60latitude it
declines in concentration and shifts to later months
(Winkler, 1988). Longitudinal gradients in the seasonal
cycle have also been reported, with concentrations
increasing in a west to east direction in continental
United States (Logan, 1989) and in a northwest to
southeast direction in Europe, where the peak also shifts
to late summer (Scheel et al., 1997).
In a recent review of the available literature, Monks
(2000) concluded that the spring maximum is a North-
ern Hemispheric phenomenon, and that the magnitude
of the spring ozone maximum at ground level seems to
have increased over the last century. Presently, there is
no over-arching concensus as to the mechanisms that
lead to the formation of the spring ozone maximum.
Early evidence linked the spring ozone peak to strato-
spheric intrusion episodes (Oltmans, 1981;Levy et al.,
1985;Logan, 1985), a phenomenon resulting from a
combination of storm track presence and a low
tropopause allowing for vertical down-mixing of ozone.
However, recent research from the US NSF TOPSE
experiment over the North American Arctic suggests
that although stratospheric injections of ozone appear to
be the dominant source of ozone in the troposphere at
high northern latitudes in the spring, they do not cause
the spring peak. Rather it appears that photochemical
production resulting from increased solar radiation
acting upon a pool of accumulated NO
x
and hydro-
carbons built up during the winter period is the major
cause of the spring time increase in ozone (Dibb et al.,
2003). These findings are supported by evidence of other
gases (e.g. PAN, NO
x
), with tropospheric only sources,
showing a similar spring maximum in the Northern
Hemisphere at the surface (Bottenheim et al., 1994;
Sirois and Bottenheim, 1995;Sorteberg et al., 1998;
Solberg et al., 1997;Fenneteaux et al., 1999). An
alternative explanation for the ozone maximum has
been presented by Liu et al. (1987), who proposed that
the long photochemical lifetime of ozone in winter,
which is approximately 200 days, allows anthropogeni-
cally produced ozone to accumulate and contribute to
the observed spring peak. A factor more recently linked
to the annual ozone cycle is the intercontinental
transport of pollution (Marenco et al., 1994;Wang
et al., 1998;Jaffe et al., 2003). The same atmospheric
mechanism responsible for the intercontinental trans-
port of Asian desert dust during the spring has been
shown to transport primary emissions and ozone at least
as far as North America (Parrish et al., 1992;Jaffe et al.,
1999;Kotchenruther et al., 2001).
3. Pre-industrial levels
In the 19th century, ozone was the focus of many
scientific studies to prove its existence and discover its
ARTICLE IN PRESS
R. Vingarzan / Atmospheric Environment 38 (2004) 3431–34423432
functions in the atmosphere and in human health. The
earliest ozone measurements began in the mid 1800s
when more than 300 stations recorded ozone concentra-
tions in Europe and the United States. However, only a
few stations observed ozone continuously for more than
a few years, and hence long-term data are limited to
these stations. Data obtained from these early observa-
tions is only semi-quantitative in nature because of
difficulties related to the method’s sensitivity to humid-
ity and antioxidants in the air. In spite of these
uncertainties, these data give a general indication of
what the natural background level of ozone would be in
the absence of significant anthropogenic influences. For
example, a re-evaluation of the daily ozone concentra-
tion over Athens for the period 1901–1940 gives a range
of around 20 ppb (Varotsos and Cartalis, 1991). Bojkov
(1986) concluded that late 19th century measurements
from the Great Lakes area of North America yielded an
average daily maximum of approximately 19 ppb and
that European measurements between the 1850s and
1900 were mostly in the range of approximately 17–23
ppb. Using ozone data collected at Montsouris, France,
between 1876 and 1910, Volz and Kley (1988) reported
an annual average ranging from 5 to 16 ppb with an
average over the period of 11 ppb.
4. Arosa, Switzerland, mid-20th century
Surface ozone data collected at the alpine location of
Arosa, Switzerland (elevation of 1800 m) during the
1950s, provide valuable insight in the progression
of the rise in ozone concentrations over the past
century. Based on measurements taken between June
1950 and May 1951, Staehelin et al. (1994) report a
median annual ozone concentration of approximately
18 ppb. Measurements taken at the same location
between 1989–1991, indicate an approximate doubling
of the median ozone concentration over a period of
three decades.
5. Current levels
Background ozone measurements from remote sites
around the world are routinely measured by the Climate
Monitoring and Diagnostics Laboratory (CMDL) of
NOAA, an agency responsible for monitoring in situ
greenhouse gas and ozone depleting substances on a
global scale. Table 1 presents the range of annual mean
surface ozone concentrations at six remote sites around
the world. Although the range of annual means appears
to be relatively broad, four of the six stations listed in
Table 1 report annual means ranging between 19 and 33
ppb. Falling outside of this range is the high elevation
site at Mauna Loa, Hawaii, which is located in the free
troposphere, and the station in American Samoa which
reports levels similar to those of pre-industrial times.
Additional long-term data on background ozone is
available from the Clean Air Status and Trends
Network (CASTNet, 2004) from sites designated as
protected areas. Table 2 presents a summary of annual
medians and maxima compiled for 11 national parks in
the United States. Annual medians at US parks range
from 13 to 47 ppb, while annual maxima range from 49
to 109 ppb. Ozone levels are generally higher at high-
elevation sites, reflecting inputs from the free tropo-
sphere. The lowest annual medians are reported for the
North Cascades National Park in northwestern Wa-
shington state, while Lassen National Park (1756 m) has
the highest range of annual maxima.
In Canada, the Canadian Air and Precipitation
Network (CAPMoN) has been recording surface ozone
concentrations at rural sites over the past two decades.
Table 3 presents a summary of annual ozone statistics
from these sites. Annual median ozone concentrations at
Canadian background sites range from 23 to 34 ppb,
while annual maxima range from 63 to 108 ppb. Overall,
the Saturna Island station in British Columbia has the
lowest concentration, while stations in Ontario, Quebec
and Nova Scotia generally have higher concentrations.
Annual medians at CAPMoN stations are similar to
ARTICLE IN PRESS
Table 1
Range of annual (January–December) hourly ozone concentrations (ppb) at background sites around the world (CMDL, 2004)
Location Elevation (m) Period of record Range of annual means
Pt. Barrow, Alaska 11 1992–2001 23–29
Ny Alesund, Svalbard, Spitsbergen
a
475 1989–1993 28–33
b
Mauna Loa, Hawaii
c
3397 1992–2001 37–46
d
American Samoa 77 1995–2001 11–14
South Pole, Antarctica 2835 1992–2001 26–30
d
Arrival Heights, Antarctica n/a 1997–1999 23–26
a
University of Stockholm Meteorological Institute.
b
Annual medians.
c
10:00–18:00 UTC.
d
High elevation site.
R. Vingarzan / Atmospheric Environment 38 (2004) 3431–3442 3433
those measured at low-elevation background stations in
the US and around the world. The relatively high range
of annual maxima at background stations in south
eastern Canada (73–116 ppb) indicates that the Canada
Wide Standard for Ozone, currently set at 65 ppb
(fourth highest 8 h daily maximum averaged over 3
years, CCME, 2002), may be difficult to achieve at these
stations.
6. Trends
Comparisons of ozone background levels with those
measured in the late 19th–early 20th centuries indicates
that current ozone levels have risen by approximately
two times (Stevenson, 2001;Bozo and Weidinger, 1995;
Staehelin et al., 1994;Cartalis and Varotsos, 1994). The
fact that this rise has occurred in parallel with industrial
development indicates that present day background
ozone includes a substantial anthropogenic component.
Although there is good evidence for an increase in the
global background level of ozone over the past century,
there is less certainty regarding trends in the past few
decades. Part of the difficulty in determining a global
trend is due to the relatively small number of background
stations and the difficulty in identifying monitoring sites
which are representative of background conditions. Some
sites designated as background are subject to anthro-
pogenic pollution under the influence of specific synoptic
patterns. Others are affected by intercontinental long-
range transport of pollutants. These anthropogenic
influences complicate the interpretation of data from
many background stations. Additional difficulties arise in
comparing reported trends among sites due to differences
in the time period or season chosen for analysis and
choice of reported statistic. In spite of these difficulties,
there seems to be fairly good indication that background
ozone levels in the Northern Hemisphere have continued
to rise over the past three decades. However, the evidence
for increasing trends in surface ozone is not global in
nature and is not always consistent among monitoring
sites. The following discussion provides a review of
available trend data from background stations around
the world.
ARTICLE IN PRESS
Table 2
Range of annual (January–December) hourly median and maximum ozone concentrations (ppb) at background stations in protected
areas of the United States (CASTNet, 2004)
Location Elevation (m) Period of record Range of annual medians Range of annual maxima
Denali National Park, Alaska 640 1998–2001 29–34 49–68
Glacier National Park, Montana 976 1989–2001 19–27 57–77
Voyageurs NP, Montana 429 1997–2001 28–35 74–83
Theodore Roosevelt NP, North Dakota 850 1983–2001 29–43 61–82
Yellowstone NP, Wyoming 2469 1996–2001 37–45
a
68–79
a
Rocky Mountain NP, Colorado 2743 1994–2001 40–47
a
68–102
a
Olympic National Park, Washington 125 1998–2001 19–22 50–63
North Cascades NP, Washington 109 1996–2001 14–18 48–69
Mount Rainier National Park, Washington 421 1995–2001 13–20 54–98
Lassen National Park, California 1756 1995–2001 38–43
a
81–109
a
Virgin Islands NP, US Virgin Islands 80 1998–2001 19–24 50–64
a
High elevation site.
Table 3
Range of annual (January–December) hourly meadian and maximum ozone concentrations (ppb) at Canadian background stations
(CAPMoN, 2003)
Location Elevation (m) Period of record Range of annual medians Range of annual maxima
Kejimkujik, Nova Scotia
a
127 1989–2001 25–34 76–116
Montmorency, Quebec 640 1989–1996 28–32 73–99
Algoma, Ontario
a
411 1988–2001 27–33 76–108
Chalk River, Ontario
a
184 1988–1996 25–31 79–107
Egbert, Ontario
a
253 1989–2001 27–32 90–113
E.L.A., Ontario 369 1989–2001 28–33 64–87
Bratt’s Lake, Saskatchewan 588 1999–2001 26–29 63–68
Esther, Alberta 707 1995–2001 26–31 63–78
Saturna Island, British Columbia 178 1992–2001 23–27 65–82
a
Stations affected by long-range transport of anthropogenic emissions.
R. Vingarzan / Atmospheric Environment 38 (2004) 3431–34423434
Table 4 lists background stations which have
reported increasing ozone trends over the past three
decades. Note that trend data from Arkona and
Hohenpeissenberg have been questioned due method
inconsistencies over the long period of measurement
(Low et al., 1990, 1991); however, trends from Hohen-
peissenberg have since been shown to be statistically
sound (Staehelin et al., 1994). Also it should be noted
that trends shown in Table 4 span a range of periods and
therefore may not be directly comparable. Nevertheless
all modern day trends are presented for the sake of
completion. Trends ranged between 0.06 and 2.6% per
year with some of the largest increasing trends seen at
stations in Europe and Japan. Trends at most stations
were not constant over the reporting period but were
generally larger through the 1970s to the mid 1980s.
From 1980 onwards, trends have either continued to
increase on a smaller scale or have remained constant
(Oltmans et al., 1998;Logan, 1994). The most widely
accepted explanation for the smaller rate of increase in
background ozone since the mid-1980s has been the
decline in nitrogen oxide emissions in Europe and North
America. Another factor that may have slowed down
the rate of increase is the global decline in carbon
monoxide concentrations (Novelli et al., 1998). In the
Southern Hemisphere, the small increasing trends at
Cape Point, South Africa, and Cape Grim, Australia,
may reflect an increase in African biomass burning
(Fishman et al., 1991). It is interesting to note that with
the exception of Cape Point and Cape Grim, all stations
reporting increasing trends are in the Northern Hemi-
sphere. Although this may suggest a hemispheric
difference, it should be noted that there are relatively
few observations in the Southern Hemisphere, a factor
ARTICLE IN PRESS
Table 4
Background stations reporting increasing ozone trends at the surface, near surface (900 mb) or in the lower troposphere (850–700 mb,
betweenB1500 and 3000 m)
Location Measurement
elevation
Period of record Trend (% per yr
unless otherwise
indicated)
References
Whiteface Mountain, N.Y. Surface (1480 m) 1974–1995 +0.4570.22 Oltmans et al. (1998)
Wallops Island, Virginia Near surface 1970–1981 +0.971.2 Logan (1985)
Lower
troposphere
1970–1995 +0.0670.24 Oltmans et al. (1998)
Lassen Volcanic Nat. Park,
California
a
Surface (1756 m) 1988–2002 +0.6070.30
b
Jaffe et al. (2003)
Saturna Island, B.C. Surface (178 m) 1991–2000 + 0.9470.74 Vingarzan and Thomson
(2004)
Alert, Nunavut Surface (62 m) 1987–2001 +1.270.8 Tarasick et al. (submitted)
Resolute, Nunavut Surface (64 m) 1980–2001 +0.270.6 Tarasick et al. (submitted)
Eureka, Nunavut Surface (10 m) 1980–2001 +2.672.1 Tarasick et al. (submitted)
Point Barrow, Alaska Surface (11 m) 1974–2001 +0.0770.04
b
CMDL (2004)
Mace Head, Ireland Surface (25 m) 1987–1995 +0.19
b
Simmonds et al. (1997)
Zugspitze, Germany Surface (2962 m) 1978–1995 +1.4870.51 Oltmans et al. (1998)
Arkona, Germany Surface (42 m) 1956–1990 +1.1270.38 Lefohn et al. (1992)
+0.9870.86 Low et al. (1990)
Hohenpeissenberg, Germany Lower 1967–1995 +1.4870.22 Oltmans et al. (1998)
troposphere 1971–1988 +1.0270.60
Surface (975 m) 1976–1992 +0.970.3 Low et al. (1990)
Surface (975 m) Logan (1994)
Payerne, Switzerland Near surface 1968–1977 +0.671.5 Logan (1985)
Switzerland, various sites Surface 1991–1999 +0.4–0.9
b
Bronnimann et al. (2002)
Okinawa, Japan Near surface 1989–1997 +2.570.6 Lee et al. (1998)
Surface (76 m) 1989–1997 +2.672.0 Lee et al. (1998)
Tsukuba, Japan Lower
troposphere
1969–1995 +0.9370.26 Oltmans et al. (1998)
Sapporo, Japan Near surface 1969–1982 +1.371.7 Logan (1985)
Tateno, Japan Near surface 1969–1982 +2.3 71.0 Logan (1985)
Cape Point, South Africa Surface (260 m) 1983–1995 +0.5370.34 Oltmans et al. (1998)
Cape Grim, Australia Surface (104 m) 1982–1995 +0.1870.14 Oltmans et al. (1998)
Mauna Loa, Hawaii Surface (3397 m) 1974–2001 +0.1570.06
b
CMDL (2004)
a
Spring data.
b
Units in ppb yr
1
.
R. Vingarzan / Atmospheric Environment 38 (2004) 3431–3442 3435
which makes it difficult to characterize the southern half
of the globe.
Table 5 lists background stations reporting declining
ozone trends over the past three decades. As in Table 4,
the reader should be aware that trends spanning
different time periods may not be directly comparable.
The longest ozone dataset in the Southern Hemisphere is
for the South Pole, Antarctica. Here ozone levels
declined by approximately 15% from 1975 to 1995
(Oltmans et al., 1998) and then began to increase
thereafter (CMDL, 2004). Negative ozone trends at
Canadian locations appear to suggest a regional pattern
of declining ozone in the Canadian north. It should be
noted, however, that for the 1991–2001 period, trends
have reversed at all three sites, ranging from +0.8 to
1.8% per year (Tarasick et al., submitted).
Other studies report no trends or mixed trends. Low
and Kelly (1992) analyzed linear trends for the period
1978–1988 using surface ozone data from 20 European
stations of mixed character (remote, rural, suburban,
urban) and found that relatively few of the trends they
calculated were statistically significant. Similarly, no
evidence of trends was found for the 9 year period
(1980–1988) at Preila, Lithuania (Girgzdiene, 1991). In
North America, no obvious trend in ozone concentra-
tion was reported for urban stations in southern Ontario
and Quebec for the period 1980–1990 (Xu et al., 1996).
In the United States, Lefohn and Shadwick (1991)
reported either no trends or increasing trends for the
period 1979–1988 in some forested and agricultural
areas in the eastern part of the country.
It should be noted that part of the recent uncertainty
about ozone trends may be due to a growing number of
studies reporting declining trends in ozone concentra-
tions at urban sites or sites downwind of urban centers
at locations in North America and Europe (Cox, 1998;
Lin et al., 2000, 2001;Fiore et al., 1998;Holland et al.,
1999;Wolff, 2001;Gardner and Dorling, 2000). This
trend is strongest for concentrations at the high end of
the distribution, while increasing trends are often seen
for values at the mid to low end of the distribution
(Lin et al., 2000;Bronnimann et al., 2002). Declines at
the high end of the distribution appear to be associated
with declining emissions in ozone precursors, as opposed
to background concentrations. In addition, some back-
ground sites have reported declining trends for ozone
measured on days when air masses were from polluted
areas (Simmonds et al., 1997).
7. Possible reasons for observed trends
Chemical transport models such as GEOS-CHEM
(Harvard University, 2004) and NASA GISS (NASA,
2004) have been used to investigate the observed ozone
trends in the Northern Hemisphere. Using sensitivity
studies, a number of factors have been investigated to
explain the rising trends in background ozone over the
past few decades.
Changes in emissions of ozone precursors are
commonly believed to have affected background ozone
levels. According to model results presented by Fusco
and Logan (2003), increased surface emissions of NO
x
from fossil-fuel combustion have had the largest effect
on ozone in the lower troposphere since 1970. These
increased emissions are estimated to be responsible for
more than a 10% increase in year round ozone over
Canada, Europe and Japan and as much as a 20%
increase over Europe and Japan in the summer. In
contrast to NO
x
emissions, increases in hydrocarbon
emissions from fossil-fuel combustion have been more
modest. Although long-term changes in hydrocarbon
emissions have affected the strength of regional high
ozone episodes, model results indicate that they have not
contributed significantly to mean tropospheric ozone
trends (Lefohn et al., 1998;Fiore et al., 2002).
The global rise in methane levels, which occurred
primarily from the late 1970s to the late 1980s
(Finlayson-Pitts and Pitts, 2000), may also have
contributed to the observed increase in tropospheric
ozone. Model results estimate that this rise in methane
levels is responsible for roughly one-fifth of the
ARTICLE IN PRESS
Table 5
Background stations reporting declining ozone trends at the surface or near surface (900 mb)
Location Measurement
elevation
Period of
record
Trend (% per yr unless
otherwise indicated)
References
Goose Bay, Labrador Surface (44 m) 1980–2001 0.770.4 Tarasick et al. (submitted)
Churchill, Manitoba Surface (35 m) 1980–2001 0.670.4 Tarasick et al. (submitted)
Edmonton, Alberta Surface (766 m) 1980–2001 1.470.7 Tarasick et al. (submitted)
Izana, Canary Is. Surface (2360 m) 1987–1995 0.2270.43 Oltmans et al. (1998)
Kagoshima, Japan Near surface 1969–1982 0.7071.5 Logan (1985)
American Samoa Surface (82 m) 1976–2001 0.0270.04
a
CMDL (2004)
South Pole, Antarctica Surface (2835) 1975–2001 0.0170.03
a
CMDL (2004)
Aspendale, Australia Near surface 1965–1982 0.270.8 Logan (1985)
a
Units in ppb yr
1
.
R. Vingarzan / Atmospheric Environment 38 (2004) 3431–34423436
anthropogenically induced increase in tropospheric
ozone at northern midlatitudes and to a global ozone
increase of 3–4% over the past quarter century (Fusco
and Logan, 2003).
The influence of stratospheric–tropospheric ozone
exchange has also been investigated as a possible cause
of changes in the background ozone level. Modeling
studies presented by Fusco and Logan (2003) indicate
that stratospheric ozone sources are estimated to exert a
small but significant influence on ozone levels in the
lower troposphere during the winter and spring at
northern midlatitudes, where approximately 10% of
ozone is estimated to be of stratospheric origin. During
the summer the stratospheric ozone influence is believed
to be negligible. It is estimated that due to ozone
depletion in the lowermost stratosphere during the
winter and spring, the stratospheric flux of ozone into
the troposphere may have declined by as much as 30%
from the early 1970s to the mid-1990s.
A consequence of declining stratospheric ozone levels
is an increase in UV radiation reaching the lower
troposphere. This effect would have a potential impact
on both photochemical production and loss of ozone.
Model results indicate that increases in UV radiation
due to stratospheric ozone depletion do not appear to
have significantly reduced tropospheric ozone, except at
midlatitudes in the Southern Hemisphere following the
breakup of the ozone hole (Fusco and Logan, 2003).
Intercontinental transport appears to be an important
factor that may explain observed ozone trends. Recent
studies have indicated that trans-Pacific transport of
Asian pollution affects North America (Jacob et al.,
1999;Jaffe et al., 1999;Wilkening et al., 2000;Yienger
et al., 2000), trans-Atlantic transport of North American
pollution affects Europe (Derwent et al., 1998;Ryall
et al., 1998;Stohl and Trickl, 1999) and trans-Eurasian
transport of European pollution affects Asia (Liu et al.,
2002). Studies focusing on trans-Pacific transport have
quantified an Asian pollution influence of about 3–10
ppb on background ozone levels in the western United
States (Jacob et al., 1999;Yienger et al., 2000;Fiore
et al., 2002). This effect is most pronounced during the
spring, when storm and frontal activity in Asia is most
prevalent and westerly transport of Asian air across the
North Pacific is strongest (Merrill, 1989;Savoie et al.,
1989). Further modeling studies indicate that the
influence of Asian and European emissions is greatest
(up to 14 ppb) under moderately polluted conditions
(50–70 ppb O
3
), when subsidence from the free tropo-
sphere associated with convective events combines with
subsequent ozone production in the boundary layer
(Fiore et al., 2002). The background influence is less
during acute high ozone episodes associated with
regional stagnation.
If trends in background ozone levels are influenced by
intercontinental transport, it is expected that they would
follow trends in emissions of ozone precursors from
source areas. The last two decades have seen increasing
NO
x
emissions from East Asia in the range of 4–6% per
year (Akimoto and Narita, 1994;Streets et al., 2001).
The largest increase in NO
x
emissions has been in China
where the steady increase until the late 1990s (Streets
et al., 2001) has contributed to poor air quality in many
regions of the country (Chameides et al., 1999). Recent
statistics on energy use in China suggest a slight decline
in emissions since 1996 as a result of reductions in coal
use and declines in the Asian economy (Carmichael et al.,
2002). It is difficult to predict if this decline will persist
or if there will be a return to increasing trends due to the
rapid pace of industrialization. Nevertheless, if emis-
sions from developing countries follow their projected
increasing trends (Prather and Ehhalt, 2001), the Asian
contribution to background levels will likely rise.
8. Future background ozone levels
Studies using coupled climate-tropospheric chemistry
models indicate that surface ozone concentrations are
expected to rise significantly throughout the 21st century
(IPCC, 2001). This rise is expected to occur primarily as
a result of the projected rise in ozone precursor
emissions over the next century, with about half the
rise being due to the rise in methane emissions and half
due to the rise in NO
x
emissions (IPCC, 2001). Of the
range of global emission scenarios studied under the
most recent IPCC assessment (Nakicenovic, 2000), all
but one projected increases in global tropospheric ozone
during the 21st century.
IPCC ozone projections for the 21st century are
illustrated in Fig. 1 along with historic and current
surface ozone concentrations reported in the various
studies referenced in this paper. Projections of future
ozone levels are based on average results of 14 coupled
models participating in the OxComp Workshop (IPCC,
2003) using five emission scenarios (A1FI, A2, A2p,
B2p, IS92a), each assuming different economic condi-
tions, population growth curves and access to non-fossil
fuel technologies (IPCC, 2001;Nakicenovic, 2000). It
should be noted that these scenarios do not take into
account atmospheric changes associated with an overall
warming of the climate. These can include global-scale
changes in background ozone due to temperature and
water vapor as well as meteorological changes that alter
the prevalence or intensity of smog episodes. Such
changes could be very important but were not assessed
in the IPCC third assessment report due to lack of
published research. Although there is significant un-
certainty in the projected concentrations, as indicated
by the vertical error bars, all projections show a rise
in surface ozone over the 21st century. Note that
projected concentrations for the 21st century exceed
ARTICLE IN PRESS
R. Vingarzan / Atmospheric Environment 38 (2004) 3431–3442 3437
internationally accepted environmental criteria, ranging
around 40–50 ppb to protect human health, crops and
vegetation (WHO, 1987). Given that these values
represent background conditions, any additional ozone
production associated with smog episodes would make it
very difficult to achieve a clean air standard of o80 ppb
over most-populated regions (IPCC, 2001). Fig. 1 also
illustrates the broad range of modern day ozone
concentrations. In the lower part of the range are
relatively un-impacted sites such as American Samoa,
ARTICLE IN PRESS
1840 1860 1880 1900 1920 1940 1960 1980 2000 2020 2040 2060 2080 2100 2120
O3 (ppb)
0
10
20
30
40
50
60
70
80
90
100
Fig. 1. Historical, current and projected background surface ozone annual concentrations. a—Athens, Greece (Varotsos and Cartalis,
1991). b—Europe (Bojkov, 1986)–avg of daily maxima. c—Montsouris, France (Volz and Kley, 1988). d—Arosa, Switzerland,
(Staehelin et al., 1994). e—Arosa, Switzerland, (Staehelin et al., 1994). f—Pt. Barrow, Alaska (CMDL, 2004). g—Virgin Islands
National Park, US Virgin Islands (CASTNet, 2004). h—American Samoa (CMDL, 2004). i—South Pole, Antarctica (CMDL, 2004).
j—Arrival Heights, Antarctica (CMDL, 2004). k—Ny Alesund, Svalbard, Norway (CMDL, 2004). l—Mauna Loa, Hawaii (CMDL,
2004). m—Mount Rainier National Park, Washington (CASTNet, 2004). n—Denali National Park, Alaska (CASTNet, 2004). o—
Glacier National Park, Montana (CASTNet, 2004). p—Lassen National Park, California (CASTNet, 2004). q—Rocky Mountain
National Park, Colorado (CASTNet, 2004). r—Theodore Roosevelt National Park, North Dakota (CASTNet, 2004). s—Yellowstone
National Park, Wyoming (CASTNet, 2004). t—Kejimkujik, Nova Scotia (CAPMoN, 2003). u—Montmorency, Quebec (CAPMoN,
2003). v—Algoma, Ontario (CAPMoN, 2003). w—Chalk River, Ontario, (CAPMoN, 2003). x—Egbert, Ontario (CAPMoN, 2003).
y—Experimental Lakes Area (ELA), Ontario, (CAPMoN, 2003). z—Bratt’s Lake, Saskatchewan (CAPMoN, 2003). $—Esther,
Alberta (CAPMoN, 2003). &—Saturna, British Columbia (CAPMoN, 2003). i—Range of surface O
3
projections for the 2040 (IPCC-
DDC, 2004). ii—Range of surface O
3
projections for the 2060 (IPCC-DDC, 2004). iii—Range of surface O
3
projections for the 2080
(IPCC-DDC, 2004). iv—Range of surface O
3
projections for 2100 (IPCC-DDC, 2004).
R. Vingarzan / Atmospheric Environment 38 (2004) 3431–34423438
US Virgin Islands and Mt. Rainier National Park. Most
low-elevation stations fall in the middle range, with
annual average values between 25–35 ppb. At the upper
part of the range are the high elevation, free tropo-
spheric sites: Mauna Loa, Hawaii, Rocky Mountain
National Park, Yellowstone National Park and modern
day Arosa, Switzerland.
9. Summary and conclusions
The annual cycle of ozone at background sites in the
Northern Hemisphere is characterized by a spring
maximum peaking during the month of May. Sites
which are affected to some extent by local ozone
production exhibit a broad summer maximum. There
is no overarching concensus as to the origin of the spring
maximum, as evidence supports both enhanced photo-
chemistry in the free troposphere and stratospheric–
tropospheric exchange.
Although historical ozone measurements are only
semi-quantitative, they give a rough indication of how
current ozone levels compare to those over a century
ago. Based on this comparison, present day ozone levels
appear to have approximately doubled, with the greatest
increase having occurred since the 1950s. Modern day
annual average background ozone concentrations range
between approximately 20 and 45 ppb with variability
being a function of geographic location, elevation and
extent of anthropogenic influence. Annual average
ozone concentrations at Canadian background stations
currently fall between 23 and 34 ppb, a range similar to
low elevation background sites in the US and around the
world.
Although there is good evidence for an increase in the
global background level of ozone over the past century,
there is less certainty regarding trends in the past few
decades. Some of the reasons for this uncertainty stem
from the relatively small number of background
stations, difficulties in identifying stations that are
representative of background conditions and difficulties
with comparisons of data among stations due to
differences in choice of study period and reported
statistic. In spite of these limitations, there is some
indication that background ozone levels over the
midlatitudes of the Northern Hemisphere have contin-
ued to rise over the past three decades and that this rise
has been in the range of approximately 0.5–2% per year.
Rising trends have not been uniform, however, as the
relatively steep trends of the 1970s and 1980s have given
way to more modest trends throughout 1990s. The
slower rate of increase, or in some cases lack of an
increase, over the past decade is believed to reflect recent
declines in ozone precursor emissions in North America
and Europe. Ozone trend data for the Southern Hemi-
sphere is sparse, and no overall pattern of ozone trends
can be determined at this time.
Part of the recent uncertainty in determining hemi-
spheric or regional ozone trends has been due to the
seemingly inconsistent reports regarding ozone trends. A
number of sites affected by urban pollution have been
reporting declining trends, which are generally in the
upper part of the ozone distribution. At the same time
other studies are reporting increasing trends, which are
often in the mid to lower part of the distribution. This
apparent discrepancy reflects the influence of local
pollution, which is generally seen during the summer
months, and is thus reflected in trends in maximum
values, compared to that of background ozone, which is
present year round and is thus reflected in median or
below median values.
Modeling studies have been used to investigate the
reasons for the observed ozone trends. Model results
indicate that increases in NO
x
emissions since the 1970s
account for a 10–20% increase in background ozone
over certain areas of the globe. Rising methane levels
from industry and agriculture are believed to have
increased global ozone levels by 3–4%. Countering this,
are estimates of declines in the ozone flux from the
stratosphere to the troposphere, resulting from strato-
spheric ozone depletion. Among the most compelling
results are those related to intercontinental transport of
ozone. Recent global chemical transport model studies
indicate that Asian pollution contributes about 3–10
ppb to background ozone levels in the western United
States during the spring. A continued rise in anthro-
pogenic emissions from Asia is expected to increase the
background level even further.
The projected rise in global emissions over the 21st
century is expected to have a profound effect on surface
ozone levels around the world. Using five of the less
conservative IPCC emission scenarios, the average
global surface ozone concentration is expected to be in
the range of 35–48 ppb by 2040, 38–71 ppb by 2060, 41–
87 by 2080 and 42–84 ppb by 2100. Such increases would
exceed internationally accepted environmental criteria
and have negative implications on human health, crops
and vegetation.
In conclusion, much work remains to be done in the
characterization of regional, hemispheric and global
ozone trends. The continued collection of data at
background stations is of critical importance in this
work. An increase in the number of monitoring stations,
especially in Asia and the Southern Hemisphere, would
greatly assist in filling in some of the current gaps. A
global network would facilitate characterization of
background ozone in a manner that is not possible
today. Such a network would be invaluable in assessing
impacts of intercontinental transport and emissions
changes on background levels. As the gap between air
quality standards and objectives and background levels
ARTICLE IN PRESS
R. Vingarzan / Atmospheric Environment 38 (2004) 3431–3442 3439
narrows, it becomes increasingly important to have a
good understanding of the anthropogenic enhancement
to the background. This enhancement over background
can then be used as a target for future emission controls.
Acknowledgements
The author would like to thank Bruce Thomson and
Bill Taylor for their valuable input to the manuscript.
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ARTICLE IN PRESS
R. Vingarzan / Atmospheric Environment 38 (2004) 3431–34423442
... Now, available reports showed that the ground level O 3 concentrations have increased in the urban areas due to increase the emission of NO 2 and hydrocarbons. The earliest O 3 measurements began in mid-1800s when more than 300 stations recorded O 3 concentrations in different parts of Europe and USA (Vingarzan, 2004). However, the continuity of O 3 monitoring was maintained only at a few stations and hence long term data are limited (Vingarzan, 2004). ...
... The earliest O 3 measurements began in mid-1800s when more than 300 stations recorded O 3 concentrations in different parts of Europe and USA (Vingarzan, 2004). However, the continuity of O 3 monitoring was maintained only at a few stations and hence long term data are limited (Vingarzan, 2004). But the higher levels of O 3 concentrations are reported from rural and remote areas which are located 100 or 1000 kilometre away from the original source of emission (Prather et al. ,2003). ...
... At Varanasi, India, O 3 concentration varied between 10.3 and 15.4 ppb during winter season, and 9.7 -58.5 ppb in summer (Agarwal, 2005 (CASTNet, 2004). Canadian Air and Precipitation Network has recorded annual median O 3 concentrations ranging between 23 to 34 ppb at Canadian background sites, while annual maxima ranged from 63 to 108 ppb (Vingarzan, 2004). ...
... Following the industrial revolution-apart from increasing levels of CO 2 -there has been a dramatic increase in the tropospheric load of atmospheric pollutants, including the oxidant species nitric oxides (NO x ) and ozone (Olivier et al., 1998;Hauglustaine and Brasseur, 2001). Ozone, for example, has increased from approximately 10 ppbV or less in preindustrial times (Hauglustaine and Brasseur, 2001) to current averages in North America of 20-45 ppbV (Vingarzan, 2004), with spikes as high as 120 ppbV during summertime ozone events (Fiore et al., 2002;Vingarzan, 2004). Both, NO x and ozone are highly reactive oxidants that can react with the carboncarbon double bonds commonly found in volatiles emitted from food sources (Atkinson and Arey, 2003;Baker et al., 2004) and pheromones (Arndt, 1995). ...
... Following the industrial revolution-apart from increasing levels of CO 2 -there has been a dramatic increase in the tropospheric load of atmospheric pollutants, including the oxidant species nitric oxides (NO x ) and ozone (Olivier et al., 1998;Hauglustaine and Brasseur, 2001). Ozone, for example, has increased from approximately 10 ppbV or less in preindustrial times (Hauglustaine and Brasseur, 2001) to current averages in North America of 20-45 ppbV (Vingarzan, 2004), with spikes as high as 120 ppbV during summertime ozone events (Fiore et al., 2002;Vingarzan, 2004). Both, NO x and ozone are highly reactive oxidants that can react with the carboncarbon double bonds commonly found in volatiles emitted from food sources (Atkinson and Arey, 2003;Baker et al., 2004) and pheromones (Arndt, 1995). ...
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The planet is presently undergoing dramatic changes caused by human activities. We are living in the era of the Anthropocene, where our activities directly affect all living organisms on Earth. Insects constitute a major part of the world’s biodiversity and currently, we see dwindling insect biomass but also outbreaks of certain populations. Most insects rely on chemical communication to locate food, mates, and suitable oviposition sites, but also to avoid enemies and detrimental microbes. Emissions of, e.g., CO 2 , NO x , and ozone can all affect the chemical communication channel, as can a rising temperature. Here, we present a review of the present state of the art in the context of anthropogenic impact on insect chemical communication. We concentrate on present knowledge regarding fruit flies, mosquitoes, moths, and bark beetles, as well as presenting our views on future developments and needs in this emerging field of research. We include insights from chemical, physiological, ethological, and ecological directions and we briefly present a new international research project, the Max Planck Centre for Next Generation Insect Chemical Ecology (nGICE), launched to further increase our understanding of the impact of human activities on insect olfaction and chemical communication.
... The rise in surface ozone concentration contributes to increase in global warming roughly by 10%, though the value is quite uncertain (Tropospheric Ozone Research (TOR) -2 final report, 2003; Saini et al., 2017). The ground and free tropospheric ozone concentrations at different locations all over the globe have shown both increasing and decreasing trends (Cooper et al., 2014;Kurokawa et al., 2009;Oltmans et al., 2008;Jaffe and Ray, 2007;Vingarzan, 2004). The yearly average surface ozone concentrations over Germany have doubled from 1950s till the end of 20 th century (Parrishet al., 2012). ...
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Surface ozone (O3) data at Pune (1998-2014) and Delhi (1998-2013) are studied to examine their temporal characteristics. Study also examines role of meteorology and atmospheric boundary layer height (ABLH) in modulating surface O3 at these sites. Using diurnal variability of surface O3, rate of change of surface O3, [d(O3)/dt] is estimated to infer the nature of surface O3 formation/destruction mechanisms. Analysis of data reveals that at both locations, surface O3 concentrations during daytime are significantly high as compared to those during nighttime. Seasonally, at Pune averaged daytime surface O3 concentrations are high during pre-monsoon and low in monsoon while those during winter and post-monsoon are found to be significantly higher than those in monsoon but half as compared to those in pre-monsoon. At Delhi, averaged daytime surface O3 concentration is minimum in winter and maximum in pre-monsoon with monsoon and post-monsoon values being about 0.79-0.82 times with respect to premonsoon O3 concentrations. High natural/anthropogenic pollutant concentration, abundance of ozone precursor gases, high temperature and high rate of photo-oxidation of precursor gases due to solar flux are the causal factors for increased surface O3 concentrations in pre-monsoon season. Reduced solar flux decreases photo-dissociation of ozone precursor gases resulting in low O3 concentration during winter season. Occurrence of low surface O3 during early morning hours in monsoon, post-monsoon and winter seasons is because of low ABLH and low stratosphere-troposphere exchange (STE). [d(O3)/dt] values during morning/evening at Pune and Delhi are indicative of asymmetric and symmetric nature of ozone formation/destruction mechanisms.
... The ozone concentration (O 3 ) may increase by 0.5-2% per year (Vingarzan, 2004), and the mean monthly concentration will range from 40 ppb to 70 bbp (Sitch et al., 2007) which may influence plant defense responses to simultaneous herbivore attack and O 3 exposure. The attack of Phyllobius pyri L. on hybrid Aspens, exposed to elevated O 3 increased the emission of α-pinene and β-pinene while the damage caused by Tetranychus urticae Koch. on Lima bean, grown at 60 and 120 ppb O 3 enhanced the emission of MeSA (Pinto et al., 2007). ...
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Volatile organic compounds emitted by plants (pVOCs) protect themselves from abiotic and biotic stresses. Plants are under constant threats from biotic stress especially herbivores which facilitate plants to emit herbivore-induced plant volatiles (HIPVs), an inducible defense in plants against herbivores by communicating to herbivores’ natural enemies and neighboring plants. HIPVs are reported to act as feeding and/or oviposition deterrents to herbivores, belonging to four major groups including terpene/terpenoids, benzenoids and phenylpropanoids, the volatile fatty acid derivatives, and the volatile aminoacid derivatives. However, the plant volatile profiles induced by herbivores have been reported to be altered by silicon (Si)-fertilization, priming of plants with chemical elicitors and climate change which can induce plants to produce and emit novel plant volatiles that are not expressed by plants in response to damage by herbivory. Transgenic crops and/or sentinel crops with enhanced pVOCs emission profiles have been proposed to improve plant resistance, health and yields. Currently several technological advances in devices including field-portable electronic devices (e-Nose) and hand-held smartphone-based biosensors for in situ early detection of pVOCs profiles which are now available and supported by artificial intelligence (AI), to increase detection accuracy of these systems. The present review highlights the multi-dimension approaches emerging in pVOCs research for early and rapid detection of specific VOCs indicators as markers with associated plant biotic stressors.
... Tropospheric ozone (O 3 ) is a gaseous secondary air pollutant that is detrimental to human and vegetation health (Ainsworth et al., 2012;Fowler et al., 2009;Karnosky et al., 2007). Surface O 3 trends have varied regionally over the recent decades, with reductions in Europe and North America and increases in many regions in Asia due to changes in anthropogenic emissions from industrial and agricultural processes (Cooper et al., 2014;Tarasick et al., 2019;Vingarzan, 2004). One of the major removal pathways of tropospheric O 3 is dry deposition onto the land surface, accounting for ∼ 25 % of total tropospheric O 3 removal (Wild, 2007). ...
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Measurements of the vertical profile of ozone concentration using balloon-borne ECC ozonesondes have been made weekly since 1980 at several sites in Canada (Edmonton, Goose Bay, Churchill and Resolute), since 1987 at Alert and since 1992 at Eureka. Previous analyses of ozone trends over Canada have shown strong negative trends in tropospheric ozone. Here, with data up to the end of 2001, we find that while for the 1980-2001 period the overall linear trends are primarily negative, both in tropospheric and stratospheric ozone, when the data for 1991-2001 only are considered, the trends are positive, even in the lower stratosphere. When the time series are compared with previously reported trends (to 1993), it is evident that ozone has rebounded at all levels below about 63 hPa. These differences do not appear to be related to changes in tropopause height, as the average height of the tropopause (as measured over the ozonesonde stations) has not changed over either the 22-year or the 11-year period. Nevertheless, comparison with another dynamical indicator, the wintertime frequency of occurrence of laminae in the ozone profile, suggests that this rebound may be partly a result of small changes in the atmospheric circulation, rather than a recovery of the ozone layer from halocarbon-induced depletion. The long-term trends in average tropospheric ozone concentrations over Canada are similar to corresponding lower stratospheric trends, and tropospheric ozone levels show significant correlation with lower stratospheric ozone amounts.
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Measurements of ozone throughout the troposphere clearly show an annual cycle. Over the last couple of decades it has become apparent that the measured annual cycle of ozone in certain locations shows a distinct maximum during spring and the magnitude of the maximum seems to have increased. There has been much debate as to the origins of this phenomenon. There is broad agreement that much of the ozone found in the troposphere is of photochemical origin. In contrast, there is still no over-arching consensus as to the mechanisms that lead to the formation of the spring ozone maximum. Part of the problem would seem to lie in the interpretation of measurements and the interactions of processes occurring on differing scales from the local to the global scale. This paper reviews both the experimental evidence concerning the origin of the spring ozone maximum and the supporting modelling studies. The roles of stratospheric-tropospheric exchange and photochemistry in the appearance of the spring ozone maximum are discussed; the evidence for various mechanisms for accumulation of ozone and its precursors are considered. The paper concludes with a summary of the state of the knowledge with respect to the spring ozone maximum and some possible areas for future consideration. The spring ozone phenomenon may well be a proxy for the continuing changes to the atmospheric composition owing to man's activities. Understanding the appearance of the spring ozone maximum and the mechanisms that lead to its formation therefore remains an issue fundamental to tropospheric chemistry.
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Ozone is important as an oxidant, a precursor for highly reactive radicals, and a significant absorber of ultraviolet and infrared radiation. Research is, therefore, conducted with the long-term goal to develop an atmospheric transport chemistry model which simulates the current climatology of ozone and is able to predict its responses to perturbations, both natural and anthropogenic. The present paper has the objective to determine the role which large-scale atmospheric transport plays in the behavior of tropospheric ozone, identify potential roles for tropospheric chemistry, and gain a more global picture of tropospheric ozone. The role of atmospheric transport is explored by comparing available observations with the results of a Geophysical Fluid Dynamics Laboratory (GFDL) general circulation/transport model which specifically excludes tropospheric chemistry.
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Variations in the 18-year (1971-1988) Hohenpeissenberg (Federal Republic of Germany) and the 33-year (1956-1988) Arkona (German Democratic Republic) surface ozone data sets are analyzed. Over the full period of the records (as made available to us), both stations, which are about 800 km apart, show a positive long-term trend of about 1.0% per year (strongest in winter at Hohenpeissenberg and in spring at Arkona). There are, however, marked differences in the fluctuations over various subperiods at the two locations. For example, the Hohenpeissenberg data show a larger increase in mean surface ozone concentration in the 1970s (about 2.1% per year, strongest in winter) compared with that in the 1980s (about 0.5% per year, strongest in summer). The Arkona data show a significant linear increase from 1956 to 1979 (about 2.4% per year, strongest in winter in the 1960s and in summer in the 1970s) but a linear decrease in the 1980s (about -2.4% per year, strongest in winter) and over the period 1971-1988 (about -1.2% per year, strongest in autumn and weakest in spring). The decrease is caused by the considerably lower concentrations in the 1980-1985 period when concentrations declined at about -10% per year. Over the same period 1971-1988, the seasonal cycle at Hohenpeissenberg exhibits a summer maximum in July (with a broad peak), while that at Arkona displays a spring maximum in May (with a sharp peak). The causes of these differences are likely to be complex: a combination of photochemistry (which depends on the distribution of precursors, particularly NOx), differences in surface deposition, local departures in the atmospheric circulation and, possibly, data quality. The pre-1976 Hohenpeissenberg data and the pre-1972 Arkona data (before filters were used to remove SO2) and the post-1982 Arkona data (after a new measuring instrument was installed) may need to be further scrutinized to ensure consistency in data quality.