Observations of tropospheric composition
Nitrogen dioxide
OMI TCNO2 was sampled under the wintertime (November–February) NAO-high (Fig. a) and NAO-low (Fig. b) for
2005–2015. Peak TCNO2 concentrations (over
15 × 1015 molecules cm-2) in both phases are over the Po
Valley and the Benelux region. Over the UK, source-region TCNO2
ranges between 7–13 and 6–10 × 1015 molecules cm-2 in
NAO-low and NAO-high. We hypothesise that NAO-high-enhanced westerly flow transports
NO2 off the UK mainland, as seen by , who
investigated the impacts of cyclonic conditions on UK TCNO2.
Figure c supports this, highlighting significant negative
anomalies of -4 to -2 × 1015 molecules cm-2 between
the TCNO2 NAO-high composite and 11-year wintertime average.
Significant anomalies, shown in the green polygon-outlined regions, are based
on the WRT at the 95 % confidence level and where composite and
wintertime averages ± their respective random errors do
not overlap. Systematic errors will cancel when differencing the two
TCNO2 composites. NAO-low reduces westerly flow across Europe and
might be expected to aid TCNO2 accumulation, but there is actually
little change in the anomaly field (Fig. d). Only the Benelux
region (3–5 × 1015 molecules cm-2) and North Sea
(-2.0 to -1.0 × 1015 molecules cm-2) show
significant anomalies linked to NO2 accumulation and reduced
transport off the UK mainland.
Peroxyacetyl nitrate
The MIPAS PAN 200–100 hPa average volume mixing ratio, sampled under NAO-high
(NDJF) between 2002 and 2012 (Fig. a), shows peak (minimum) PAN
concentrations of 50–55 (10–20) pptv in the subtropical North Atlantic (over
Newfoundland and the Canadian Arctic). During NAO-low (Fig. b), PAN concentrations are lower over the subtropical
Atlantic but slightly larger over Newfoundland and the Canadian Arctic between
25 and 40 pptv. PAN concentrations are also larger (approximately 40 pptv) over
Iceland, southern Greenland and the Denmark Strait, leaving a spatially prominent
feature. MIPAS-derived tropopause height (see Supplement) peaks at 11 km in this
region, while it is only 9–10 km in the surrounding area (excluding the
subtropical North Atlantic). There is also an increase in pressure and
convergence of winds over this region (Fig. d), potentially
highlighting the impact of NAO-low vertical transport of PAN into the UTLS;
this is investigated further using TOMCAT (see Sect. ). In Fig. c under NAO-high, peak
significant anomalies of 5–15 pptv occur over the subtropical Atlantic and
north-eastern Arctic region (top right of the domain). There are also
significant negative anomalies (-15 to -5 pptv) over the Québec region.
Significant anomalies are based on the WRT (95 % confidence level) and where
the NAO composite and the wintertime averages plus or minus their uncertainty ranges
do not overlap. Over Iceland and Greenland (subtropical North Atlantic and
Europe), there are positive (negative) anomalies of 5–15 (-5 to -1) pptv in
NAO-low.
Figure shows the zonally averaged (90–20∘ E)
vertical profiles of MIPAS PAN under the two NAO phases. Peak PAN
concentrations at 300–250 hPa range between 100 and 130 pptv in both phases
but are larger in NAO-low between 30 and 50∘ N by 10–20 pptv. However,
northwards of 70∘ N, PAN concentrations between 225 and 100 hPa tend
to be larger under NAO-high conditions. Figure c shows the
NAO-high zonal anomalies relative to the wintertime average (hatched
anomalies are insignificant based on the WRT – 95 % confidence level).
Northwards of 50∘ N, significant positive anomalies (5–15 pptv) exist
between 300 and 125 hPa. MIPAS-derived tropopause height under NAO-high (see Supplement)
is typically higher than in NAO-low over the North Atlantic and Europe by 1–2 km. The higher tropopause signifies enhanced vertical transport, which in
this case is the propagation of polluted air masses (i.e. large PAN content)
from further down in the troposphere into the UTLS. Southwards of
50∘ N, positive anomalies occur between 200 and 100 hPa, while negative
anomalies are found between 300 and 250 hPa. Under NAO-low conditions (Fig. d), there are significant positive anomalies (5–15 pptv) at
300–275 hPa between 30 and 90∘ N, and they reach up to 125 hPa at
60–80∘ N coinciding with peak PAN concentrations over Iceland in
Fig. b. Significant negative anomalies (-5 to -1 pptv) exist
at 225–50 hPa, which coincides with the negative anomalies in Fig. d over the subtropical Atlantic. Between 30 and 50∘ N,
there is an altitude anomaly dipole reversal with NAO-high showing
significant positive (negative) anomalies at 200–100 hPa (300–250 hPa) and
NAO-low highlighting significant positive (negative) anomalies at 300–275 hPa
(225–50 hPa). These patterns are linked to changes to regional circulation
patterns under the different NAO phases and will be explored further using
TOMCAT in Sect. .
Ozone
Figure shows TES vertical profiles averaged over four regions
(Zones 1–4) covering the North Atlantic between 2005 and 2011, which were sampled
during significant wintertime (November–February) NAO events. These four
domains are selected because TES has infrequent spatial sampling
meaning spatial ozone distributions are often
noisy or unclear. In Zone 1 (UK), TES ozone sampled under NAO-high (red line) is
significantly larger (90 % – squares and 95 % – diamonds) than in NAO-low
(blue line) by 3–4 ppbv throughout the region of peak sensitivity (900–650 hPa – yellow box). Similar patterns exist in surface ozone measurements from
the UK Automatic Urban and Rural Network (AURN, ). Under
NAO-high, surface ozone concentrations were significantly higher than in
NAO-low by 5–10 µg m-3 across the UK (see Supplement). The opposite is true
for AURN surface NO2 where concentrations across the UK are lower by
5–10 µg m-3 in NAO-high. This supports the hypothesis that NAO-high
increases (decreases) ozone concentrations over western Europe (western
Atlantic) through enhanced westerly transport of ozone and dispersion of
other species involved in its removal (e.g. NO) over Europe. Zone 2
(Newfoundland) has the opposite signal whereby the NAO-low ozone profile is
significantly (95 %) larger than the NAO-high profile by 2–4 ppbv. Again,
this is potentially linked to enhanced westerly ozone transport across the
Atlantic towards Europe during NAO-high. In Zone 3 (North Atlantic), there
are insignificant differences at approximately 900 hPa. However, ozone is
significantly greater under NAO-low between 875 and 350 hPa. There are
insignificant differences in Zone 4 ozone up to 600 hPa, but NAO-high ozone
is significantly larger above this altitude.
TOMCAT surface NO2 (ppbv) averaged between 2006 and 2015
sampled under the wintertime (NDJF) NAO. Panel (a) NAO positive phase, (b) NAO
negative phase, (c) shows the NAO positive phase anomaly relative to the
wintertime average and (d) shows the negative NAO phase anomaly relative to
the wintertime average. Wind vectors show the horizontal 10m winds and the
red, green and blue contours represent 980, 1000 and 1020 hPa. Green
polygon-outlined regions in (c) and (d) highlight significant differences at the 95 %
confidence level based on the WRT.
TOMCAT NO2 (ppbv) cross section at 0∘ E averaged
between 2006 and 2015 sampled under the wintertime (NDJF) NAO. (a) NAO
positive phase, (b) NAO negative phase, (c) shows the NAO positive phase
anomaly relative to the wintertime average and (d) shows the negative NAO
phase anomaly relative to the wintertime average. Green dashed lines
represents the dynamical tropopause. Wind vectors represent the cross section
(0∘ E) meridional and vertical (scaled by 104) winds.
TOMCAT surface PAN (pptv) averaged between 2006 and 2015 sampled
under the wintertime (NDJF) NAO. Panel (a) NAO positive phase, (b) NAO
negative phase, (c) shows the NAO positive phase anomaly relative to the
wintertime average and (d) shows the negative NAO phase anomaly relative to
the wintertime average. Wind vectors show the horizontal 10 m winds and the
red, green and blue contours represent 980, 1000 and 1020 hPa surface
pressure. Green polygon-outlined regions in (c) and (d) highlight significant
differences at the 95 % confidence level based on the WRT.
TOMCAT PAN (pptv) averaged between 200 to 100 hPa for 2006–2015
sampled under the wintertime (NDJF) NAO. Panel (a) NAO positive phase, (b) NAO
negative phase, (c) shows the NAO positive phase anomaly relative to the
wintertime average and (d) shows the negative NAO phase anomaly relative to
the wintertime average. Wind vectors show the horizontal 200–100 hPa winds.
Green polygon-outlined regions in (c) and (d) highlight significant differences at
the 95 % confidence level based on the WRT.
TOMCAT zonally averaged (90∘ W to 20∘ E) PAN (pptv)
between 2006 and 2015 sampled under the wintertime (NDJF) NAO. Panel (a) NAO
positive phase, (b) NAO negative phase, (c) shows the NAO positive phase
anomaly relative to the wintertime average and (d) shows the negative NAO
phase anomaly relative to the wintertime average. Green dashed lines
represents the dynamical tropopause. Wind vectors represent the zonally
averaged (90∘ W to 20∘ E) meridional and vertical (scaled by
104) winds.
TOMCAT surface ozone (ppbv) averaged between 2006 and 2015 sampled
under the wintertime (NDJF) NAO. Panel (a) NAO positive phase, (b) NAO
negative phase, (c) shows the NAO positive phase anomaly relative to the
wintertime average and (d) shows the negative NAO phase anomaly relative to
the wintertime average. Wind vectors show the horizontal 10 m winds and the
red, green and blue contours represent 980, 1000 and 1020 hPa surface
pressure. Green polygon-outlined regions in (c) and (d) highlight significant
differences at the 95 % confidence level based on the WRT.
TOMCAT ozone (ppbv) averaged between 200 and 100 hPa for 2006–2015
sampled under the wintertime (NDJF) NAO. Panel (a) NAO positive phase, (b) NAO
negative phase, (c) represents the NAO positive phase anomaly relative to the
wintertime average and (d) represents the negative NAO phase anomaly relative
to the wintertime average. Wind vectors show the horizontal 200–100 hPa
winds. Green polygon-outlined regions in (c) and (d) highlight significant
differences at the 95 % confidence level based on the WRT.
TOMCAT ozone (ppbv) cross section at 0∘ E averaged between
2006 and 2015 sampled under the wintertime (NDJF) NAO. Panel (a) NAO positive
phase, (b) NAO negative phase, (c) shows the NAO positive phase anomaly
relative to the wintertime average and (d) shows the negative NAO phase
anomaly relative to the wintertime average. Green dashed lines represents
the dynamical tropopause. Wind vectors represent the cross section
(0∘ E) meridional and vertical (scaled by 104) winds.
TOMCAT ozone (ppbv) cross section at 56.25∘ W averaged
between 2006 and 2015 sampled under the wintertime (NDJF) NAO. Panel (a) NAO
positive phase, (b) NAO negative phase, (c) shows the NAO positive phase
anomaly relative to the wintertime average and (d) shows the negative NAO
phase anomaly relative to the wintertime average. Green dashed lines
represents the dynamical tropopause. Wind vectors represent the cross section
(56.25∘ W) meridional and vertical (scaled by 104) winds.
Model results
TOMCAT has been evaluated in multiple studies (e.g.
) for NO2, PAN and ozone, which are
discussed in detail in the Supplement. We also have evaluated TOMCAT
surface and tropospheric ozone against a range of observations covering western
Europe and the North Atlantic. In all cases, TOMCAT can suitably
represent these chemical tracers and their responses to the NAO circulation
patterns (see Supplement).
Nitrogen dioxide
In NAO-high and NAO-low (Fig. a and b), where TOMCAT has been
sampled under the NAO phases in Fig. a, the model TCNO2
over western Europe ranges between 3 and 9 × 1015 molecules cm-2
and 6 to over 10 × 1015 molecules cm-2. Over the UK, NAO-high-enhanced westerly flow transports
NO2 off the mainland (Fig. c) with significant negative
anomalies of -2.0 to -0.5 × 1015 molecules cm-2 relative to
the wintertime average. OMI TCNO2 has a similar NAO-high UK signal
(c), but it is less spatially extensive and does not cover as
much of continental Europe. In NAO-low, OMI (Fig. d) only
shows accumulation of TCNO2 over the Benelux region, while TOMCAT
(positive anomalies over 1.5 × 1015 molecules cm-2)
accumulates TCNO2 over all of continental Europe (Fig. d). Potential reasons for model-satellite NAO-low anomaly
differences (Fig. d and d) included the following. (1) As OMI
has peak retrieval sensitivity in the middle–upper troposphere it potentially
underestimates the full TCNO2 under NAO-low conditions when the more
stable conditions trap NO2 in the boundary layer. (2) The model NO2
lifetime in the NAO-low composite is potentially longer than the satellite
equivalent as it represents all-sky conditions, while the satellite composite
represents clear-sky conditions only (i.e. more photochemical loss of
NO2).
At the surface, TOMCAT surface NO2 ranges between 0–6 and
2–8 ppbv in NAO-high and NAO-low. TOMCAT does have a
systematic surface NO2 low bias against surface observations (see
Supplement), but this systematic offset is removed when considering anomalies
(Fig. c and d) relative to the wintertime average. TOMCAT
surface anomalies typically have similar spatial patterns to the TOMCAT
TCNO2, but they are less spatially extensive. Under the NAO-high,
there are significant negative (positive) anomalies of -0.5 (0.2) ppbv over
the UK (North Sea), highlighting the westerly transport of NO2 off
the UK mainland. Under NAO-low, significant positive anomalies (0.0 to 1.0 ppbv) highlight the accumulation of NO2 from reduced westerly flow
across the UK. This is consistent with the AURN results presented in the Supplement.
The model also shows a significant anomaly dipole over Scandinavia which
reverses between phases. This, in combination with the reduced spatial impact
on surface NO2 compared with the tropospheric pattern, implies that
processes above the surface also influence the response of the tropospheric
NO2 distribution to the NAO.
Figure shows the TOMCAT NO2 meridional vertical
cross section at 0∘ E. Between 35 and 60∘ N TOMCAT simulates
NO2 concentrations above 1.0 ppbv from 1000 hPa to 900 (850) hPa in
NAO-high (NAO-low). Negative anomalies (under -0.05 ppbv) relative to the
wintertime average from 1000 to 900 hPa at 50∘ N (Fig. c), show the enhanced NAO-high westerly flow transporting
NO2 throughout the boundary layer away from UK source regions. This
NO2 is transported into the North Sea, yielding positive anomalies of 0.02 ppbv northwards of 55∘ N with vertical ascent into the
mid-troposphere (approximately 600–700 hPa at 60–70∘ N). Under
NAO-low (Fig. d), there are positive (above 0.05 pptv)
anomalies between 35 and 65∘ N as the weakened meridional winds have a
southerly flow with ascent (descent) at 65 (40)∘ N. This highlights
reduced NO2 transport from the climatological westerlies aiding
accumulation in the lower troposphere (1000–700 hPa). Therefore, processes
throughout the lower troposphere over the UK are important in governing the
tropospheric column burden during the two NAO phases.
Peroxyacetyl nitrate
At the surface, although PAN has lower concentrations
than NO2 in source regions, it has a longer lifetime resulting in more
significant responses to the seasonal average under the different NAO phases.
Under NAO-high (Fig. a), TOMCAT surface PAN peaks between
200 and 220 pptv over the western Atlantic. Over Europe, PAN ranges between
150 and 170 pptv as, like NO2, enhanced westerly flow transports PAN away
from western European source regions, replacing it with cleaner subtropical
North Atlantic air (100–150 pptv). Through reduced transport, NAO-low
conditions aid pollutant accumulation over continental Europe with PAN
concentrations of 190 to over 300 pptv. The NAO-high TOMCAT PAN anomalies
(Fig. c) relative to the wintertime average highlight
reduced concentrations of -50 to -20 pptv over continental Europe, while in
the western North Atlantic there are no significant anomalies. This infers
similar transport processes to the wintertime average, resulting in minimal
PAN changes, yet NAO-low (Fig. d) weakens or reverses the
winds, yielding significant negative anomalies of -20 to -10 pptv. Therefore,
westerly flow, similar under NAO-high and average wintertime conditions,
aids the long-range transport of PAN from North America. As NAO-low
interrupts this transport pathway, there is a significant decrease in
background PAN.
We now investigate whether TOMCAT reproduces the MIPAS UTLS PAN patterns
under the NAO phases, despite the slightly different time periods. Previous
studies (e.g. ) have shown that TOMCAT PAN
compares reasonably well with aircraft observations, but there is a
systematic difference between TOMCAT and KIT MIPAS PAN (see Supplement). Therefore,
we primarily focus on the anomalies relative to the wintertime average
under the NAO phases, as this systemic difference is removed. TOMCAT PAN
200–100 hPa average peaks (over 50 pptv) in NAO-high over the western
subtropical North Atlantic (Fig. a). The south-westerly flow
(approximately 30 m s-1) at this altitude transports PAN across the
Atlantic,
reaching 35 pptv over Iberia. However, at approximately 0∘ E, a
southerly shift in the winds over the Mediterranean leads to lower
continental Europe PAN concentrations (20–30 pptv). Northwards of
70∘ N, the flow (20–30 m s-1 ) accumulates PAN over the Arctic region
(20–24 pptv). Similar spatial patterns are seen in MIPAS with peak PAN
concentrations over the western subtropical Atlantic, minimum PAN over
Canada and Hudson Bay and elevated PAN in the Arctic region. However, MIPAS PAN
absolute concentrations are systematically higher than TOMCAT (see Supplement).
Vertical transport will also have an important impact on NAO-high, as
signified by the higher MIPAS-derived tropopause height (see Supplement), with
propagation of polluted air masses from the lower troposphere into the
UTLS. In NAO-low, peak PAN (over 40 ppbv) occurs in the subtropical Atlantic
where the winds are predominately zonal (westerly), yielding lower PAN
concentrations (15–25 pptv) over the mid-North Atlantic. Continental Europe
PAN concentrations decrease (10–20 pptv), as north-westerly flow
transports cleaner Arctic air masses into the region. PAN accumulation over
Iceland and southern Greenland (25 ppbv) correlates with the large UTLS
pressure increase shown in Fig. d. Figure d
highlights the significant enhancement of PAN over Iceland and southern Greenland
with positive anomalies relative to the wintertime average of 5–10 pptv.
Again, the MIPAS-derived tropopause height in NAO-low peaks in this region
(approximately 11 km – see Supplement) suggesting sufficiently strong vertical
transport of tropospheric air masses. In NAO-low, as the strong westerly flow
in NAO-high (Fig. b) has shifted equatorwards, there are
significant negative anomalies under -15 pptv across the North Atlantic,
which match the MIPAS equivalent in Fig. d. There are some
similarities between the TOMCAT (Fig. c) and MIPAS (Fig. c) NAO-high PAN anomalies with increased PAN in the eastern
Arctic. However, TOMCAT simulates positive anomalies (0–5 pptv) over the
North Atlantic between 35 and 45∘ N, while MIPAS has significant positive
anomalies of 10 pptv. TOMCAT also simulates significant negative anomalies
over the UK and the eastern North Atlantic, which are not observed by MIPAS.
Therefore, the model results only allow for limited assessment of the NAO
influence of UTLS PAN in NAO-high over these regions.
Figure shows the zonal average (90∘ W–20∘ E)
meridional-vertical TOMCAT PAN distribution under both NAO phases. Between
1000 and 600 hPa, PAN concentrations are above 200 pptv apart from in the
region 30–40∘ N. From 400 to 200 hPa, there is a sharp PAN decrease to
less than 30 pptv. The vertical PAN profile between 30 and 40∘ N differs
from other latitude bands, with PAN peaking at 150–200 pptv from 1000 to 700 hPa
and then from 60–100 pptv up to 200 hPa. Between 200 and 100 hPa, PAN concentrations
(30–60 pptv) are larger than other latitude bands linked to the higher
tropopause (also observed by MIPAS – see Supplement). The significant decreases in
surface PAN over Europe from NAO-high enhanced westerly flow (Fig. c) occur throughout the troposphere, with negative zonal
anomalies of -10 to -3 pptv (Fig. c). Above the tropopause
(dashed green line), strong vertical (winds are scaled by 104 for
clarity) meridional transport accumulates PAN (positive anomalies over 10 pptv) in the Arctic UTLS. Under NAO-low (Fig. d), there are
positive (5–10 pptv) and negative (-10 to -5 pptv) anomalies between 30–50
and 60–90∘ N throughout the troposphere. The surface patterns (Fig. d), where reduced transport aids PAN accumulation over Europe,
appear to account for this zonal tropospheric pattern. Between
30 and 50∘ N, there is limited meridional flow aiding PAN accumulation
over Europe in NAO-low. The vertical flow contributes to positive anomalies
(3–5 pptv) propagating into the UTLS, which is consistent with the PAN accumulation shown
in Fig. d between 50 and 70∘ N.
Ozone
TOMCAT surface ozone under NAO-high (Fig. a) peaks at
approximately 28–30 ppbv over the subtropical and western North Atlantic
co-located with the enhanced westerlies. Over continental Europe, ozone
concentrations are significantly larger (1–2 ppbv – Fig. c)
than the wintertime average, ranging between 16 and 25 ppbv. This matches a
similar pattern in the observations: AURN surface ozone was significantly
higher over the UK under NAO-high than NAO-low (see Supplement),
and TES lower-tropospheric ozone (Zone 1, Fig. ) was larger under NAO-high.
In Zone 2, TES lower-tropospheric ozone was higher under NAO-low, which
correlates with the surface TOMCAT pattern. found similar
patterns with significant positive (negative) correlations over Europe
(western North Atlantic) between surface ozone and the NAOI in DJF. Under
NAO-low conditions, TOMCAT ozone concentrations are consistent across the
North Atlantic (28–30 ppbv) as the weakened or reversed westerlies limit the
transport of ozone-enriched Atlantic into Europe yielding lower
concentrations of 13–20 ppbv. Over the western North Atlantic (Europe), ozone
concentrations (Fig. d) have increased (decreased) with
significant positive (negative) anomalies of 2–3 ppbv (-3 to -1 pptv). Again,
presented similar results which also match TES and AURN
ozone observations (see Supplement). TOMCAT tropospheric column ozone (not shown
here) also showed similar anomalies.
At the surface, the PAN and NO2 spatial anomalies are anti-correlated
with ozone, so UTLS ozone (Fig. ) was investigated to see if
this relationship was consistent at higher altitudes. TOMCAT 200–100 hPa
average ozone, sampled under NAO-high, ranges from 800 to 1000 ppbv northwards of 60∘ N but decreases towards the subtropical North
Atlantic with minimum concentrations of 150–200 ppbv. A similar pattern
occurs under NAO-low except for the ozone-reduced air mass (500–700 pptv),
stretching from approximately 45 to 65∘ N along 15–45∘ W. Higher
ozone concentrations (800–1000 ppbv) also propagate further south in NAO-low
on either side of the Atlantic, surrounding the reduced ozone limb. The UTLS
ozone anomalies (Fig. c and d) are also anti-correlated with
the PAN. Whereas PAN has positive anomalies across the Atlantic basin in
NAO-high, there are significant negative ozone anomalies (under -100 ppbv).
This anti-correlation is also prominent under NAO-low, where significant
TOMCAT ozone anomalies (50–200 ppbv) exist over the mid-North Atlantic and
Europe but are significantly negative for PAN. As shown in Figs. and , tropospheric positive PAN anomalies
propagate into the UTLS over Iceland and southern Greenland, but the ozone
anomalies are significantly negative (-150 to -50 ppbv). Potential reasons
for the PAN-ozone anti-correlation include the air mass origin or the PAN
(NOx)-ozone chemistry. The thermal decomposition of PAN forms the
peroxyacetyl radical and NO2, which is an UTLS ozone sink (i.e.
conversion of NO2 to NO and then reaction with ozone), while a
tropospheric ozone source is in the presence of volatile organic compounds
. However, lower UTLS temperatures (i.e. around 250 K)
yield a PAN lifetime of several months and are a less likely
factor in the PAN NOx-ozone anomaly anti-correlations.
show that UTLS ozone-HOx chemistry is a more significant sink pathway
for ozone; however, there is no clear correlation between the NAO HO2
and ozone anomalies. Methane, a good air mass tracer due to its approximate
9-year lifetime (e.g. ) and anthropogenic source, was
sampled under the NAO phases (not shown) and highlighted similar anomaly
patterns to PAN, again anti-correlated with ozone. Therefore, PAN and methane
(ozone) act as signatures for the transport of polluted (clean) air masses in
the troposphere for the different NAO phases.
Figure shows the TOMCAT ozone cross section at 0∘ E,
similar to NO2 in Fig. . Under both NAO phases,
lower-tropospheric (UTLS – above 300 hPa) ozone ranges between 25 and 35 (above
100) ppbv. Meridionally, there is a decreasing poleward lower-tropospheric
ozone gradient, while in the UTLS peak (minimum) concentrations are at the
pole (30∘ N). The anomalies, as discussed above, are anti-correlated
with NO2. In NAO-high, there are small positive (negative) anomalies
over the UK (North Sea) consistent with ozone-enriched air transported into
the UK from the North Atlantic and ozone loss downwind due to source-region
NOx transport, which propagates up to approximately 600 hPa. The positive
anomalies in the UTLS at 60∘ N are consistent with ozone accumulation
seen in Fig. . Under NAO-low conditions, the negative
anomalies (approximately -3 ppbv) between 45–65∘ N and 1000–700 hPa
are linked to UK NOx accumulation (Fig. d). Atmospheric
downwelling leads to UTLS ozone (positive anomalies over 3 ppbv) propagation
into the mid-troposphere at 40–50∘ N. At high latitudes, small
positive anomalies throughout the troposphere, which are anti-correlated with
PAN, show clean-air transport around the UTLS Icelandic–southern Greenland
anticyclone in which PAN accumulates.
A second TOMCAT ozone cross section at 56.25∘ W (Fig. ) has similar absolute ozone concentrations to the
0∘ E cross sections, but the anomalies highlight important
differences. In NAO-high, both cross sections have similar lower-tropospheric
ozone anomalies except at 50∘ N (Figs. c and
c) with positive anomalies (approximately 1 ppbv) over the UK
region and near 0 ppbv over the western North Atlantic. Under NAO-low
conditions, there are positive anomalies (Fig. d, 1–3 ppbv)
between 1000–600 hPa and 50–70∘ N, but the eastern cross section
(Fig. d) highlights negative anomalies in this region (-3 to
-1 pptv). While there is downwelling of stratospheric ozone in the eastern
cross section into the middle–upper troposphere during NAO-low, the western
cross section has an upwelling of ozone-reduced air into the UTLS with negative
anomalies of less than -5 ppbv. Overall, in the lower troposphere, the TOMCAT
cross section anomalies support the signals in the TES data. Over the UK
(Zone 1, Fig. , and eastern cross section, Fig. ), lower-tropospheric ozone is larger (lower) than the
wintertime average under NAO-high (NAO-low), while the opposite occurs in the
western North Atlantic (Zone 2 and western cross section, Fig. ).