- Email: [email protected]
Urban NO2 and NO pollution in relation to the North Atlantic Oscillation NAO
Atmospheric Environment 45 (2011) 883e888
Contents lists available at ScienceDirect
Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Urban NO2 and NO pollution in relation to the North Atlantic Oscillation NAO M. Grundström a, H.W. Linderholm b, J. Klingberg a, H. Pleijel a, * a b
University of Gothenburg, Plant and Environmental Sciences, P.O. Box 461, 405 30 Göteborg, Sweden University of Gothenburg, Department of Earth Sciences, P.O. Box 460, 405 30 Göteborg, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2010 Received in revised form 10 November 2010 Accepted 12 November 2010
The North Atlantic Oscillation (NAO), a measure of the strength of the zonal wind across the North Atlantic Ocean, strongly influences weather conditions in NW Europe, e.g. temperature, precipitation and wind, especially during winter. It was hypothesised that elevated concentrations of nitrogen oxides in Gothenburg would be enhanced during negative NAO index (NAOI) conditions, representing more anticyclonic weather situations and thus leading to limited air mixing in the urban atmosphere, than situations with NAOI > 0. Hourly wintertime (DecembereFebruary) concentrations (1997e2006) of NO2, NO, air pressure, temperature and wind direction from an urban rooftop (30 m above street level) in the centre of the City of Gothenburg were analysed in relation to NAOI. Air pressure, the average concentration of nitrogen oxides (NOx ¼ NO2 þ NO), as well as the fraction of hourly NO2 and NO concentrations exceeding 90 mg m3 and the fraction of daily NO concentrations exceeding 60 mg m3, were significantly and negatively related to NAOI. Air temperature was positively correlated with NAOI. Southerly and westerly winds were more common in months with positive NAOI, while easterly and northerly winds were overrepresented in months with negative NAOI. High pollution concentrations dominantly occurred in situations with northerly and easterly wind directions. High NO2 and NO concentrations were associated with negative NAOI, especially in the morning when the traffic rush coincided with restricted air mixing. Over the tenyear period there were trends for more negative NAOI and increased time fractions with hourly NO2 concentrations exceeding 90 mg m3. The conclusion of this study is that a climate shift towards higher or lower NAOI has the potential to significantly influence urban air pollution in North-West Europe, and thus the possibility to reach air quality standards, even if emissions remain constant. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: North Atlantic Oscillation NO2 NOx Urban pollution Climate Gothenburg
1. Introduction High air pollution concentrations in urban areas represent a serious environmental problem (Curtis et al., 2006). Air quality standards (AQS) for high priority air pollutants, including nitrogen dioxide (NO2), particulate matter and ozone (O3), have been introduced nationally in many countries and by the European Union. The possibility of meeting the AQSs depend not only on the emission rates of pollutants but also on the meteorological factors that govern air mixing and dispersion of air pollutants (Flocas et al., 2009). NO2 is often considered to be the more toxic of the nitrogen oxides (NOx ¼ NO þ NO2), but NO and NOx are sometimes used as proxies for traffic related air pollution in general. Emissions of NOx take place mainly in the form of NO, although the fraction of primary NO2 in vehicle exhausts has been increasing since the end
* Corresponding author. Tel.: þ46 31 786 2532; fax: þ46 31 786 2560. E-mail address: [email protected] (H. Pleijel). 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2010.11.023
of the 1990s in European cities (Carslaw et al., 2007), including Gothenburg (Pleijel et al., 2009). In reaction with ozone (O3) NO is oxidised to NO2 (Brimblecombe, 1996), and consequently a substantial fraction (w15% up to almost 100%) of NOx is present in the form of NO2 in the urban air of e.g. Gothenburg (Pleijel et al., 2009). While the ratio between NO and NO2 varies with the presence of ozone and the photolysis of NO2 (Brimblecombe, 1996), NOx remains uninfluenced by oxidation with ozone and photolysis. Typically, NOx pollution in urban areas is dominated by local emissions. Recently, the scientific discussion of the influence of climate change and climate patterns on air pollution exposure has intensified. Since there are important examples of the strong influence of climate patterns on the occurrence of air pollutants (Flocas et al., 2009), especially particulate matter and ozone (Leibensberger et al., 2008; Jacob and Winner, 2009; Demuzere et al., 2009), the influence of climate change and climate patterns on the possibility to reach AQSs has become an important issue. In the case of urban NO2 pollution, Delaney and Dowding (1998) provided evidence that synoptic circulation patterns strongly affected the occurrence of
884
M. Grundström et al. / Atmospheric Environment 45 (2011) 883e888
high NO2 episodes in Dublin, Ireland. In that study, most of the high pollution days were associated with high pressure cells east of Ireland, but in one case the high NO2 levels were most likely associated with advection of a polluted air mass from continental Europe and Britain. The North Atlantic Oscillation (NAO) is a large-scale meridional oscillation in atmospheric mass, with centres of action near Iceland and over the subtropical Atlantic (Visbeck et al., 2001). It is one of the leading climate modes in the North Atlantic region (Hurrell et al., 2003), and influences climate variability, e.g. temperature, precipitation and storminess, from eastern United States into western Eurasia, from the Arctic to the subtropical Atlantic, especially during boreal winter (Hurrell and Deser, 2009). When the pressure gradient between the Icelandic low and the subtropical high pressure centre during winter is stronger than normal (positive NAO), the westerly winds are stronger, bringing maritime air masses into Northern Europe where climate becomes wetter and milder, e.g. in Sweden (Chen and Hellström, 1999). At times of a weak pressure gradient (negative NAO), weaker westerly winds result in a domination of high pressure conditions over Northern Europe, yielding cold and dry winters. Consequently, when the NAO index (NAOI) is negative, temperature inversions, associated with restricted air mixing and high NOx concentrations, are likely to occur more frequently than during the low pressure conditions (e.g. Flocas et al., 2009) associated with a positive NAO. The City of Gothenburg, the centre of a conurbation with w900,000 inhabitants on the west coast of southern Sweden, is not extremely polluted due to its moderate size and location near the coast with rather strong winds ventilating the urban air most of the time. However, during anticyclonic weather conditions in winter, strong temperature inversions occur, leading to periods of strongly elevated pollution concentrations, including nitrogen oxides (Janhäll et al., 2006). The aim of this investigation was to study the relationship between the degree of urban pollution, as represented by nitrogen oxides, and the large-scale atmospheric circulation, represented by a NAO index (NAOI), during the winter. The hypothesis was that nitrogen oxides concentrations correlate negatively to the NAOI, since a low NAOI is expected to represent a higher probability of stagnant, anticyclonic weather conditions. 2. Materials and methods 2.1. Measurement site and instrumentation Air quality and meteorological monitoring was performed on top of a building, 30 m above street level, surrounded by buildings of similar or lower height. This site is located in the commercial district of Gothenburg city centre (‘Femman’; 5742.5220 , 1158.2360 ), adjacent to the central terminal for trains and buses and approximately 300 m away from a busy traffic route (E45). The location of the monitoring station was selected to represent the polluted urban background. Hourly monitoring of NO and NO2 (Tecan CLD 700 AL chemiluminescence instrument), wind direction (Gill ultrasonic anemometer), temperature (Campbell Rotronic MP101 thermometer/hygrometer) and air pressure (Vaisala PA11A electronic barometer) were carried out. Data for the ten-year period 1997e2006 were used. The pollutants NO and NO2 and meteorology were analysed for the winter months (January, February and December) of each year. Winter season data (Fig. 7a and b) refer to December one year and January and February in the consecutive year. Data coverage was essentially complete, apart from wind direction for which 1 month of data was missing. Traffic emission data covering every second year from 2000 to 2006 was retrieved from the Environmental Administration of the City of
Gothenburg, which holds a data base (using the EnviMan system, www.opsis.se) for local air pollution emissions including traffic, based on detailed emission inventory. 2.2. NAOI The NAOI data used in this study was obtained from the Climate Research Unit, University of East Anglia; (http://www.cru.uea.ac. uk/wtimo/datapages/naoi.htm). The NAOI was calculated on a monthly basis from the difference between the normalised sea level pressure over Gibraltar and the normalised sea level pressure over Southwest Iceland (e.g. Jones et al., 1997). Monthly index values were used throughout the study with the exception of Fig. 7, where winter season (DecembereFebruary average) data are used. The monthly index values varied between 2.25 and 5.26. 2.3. Calculations and evaluation of air quality standards The hourly values of each pollutant were converted from mole fraction (expressed in parts per billion, ppb, equivalent to nmol mol1) units to mg m3 units using the ideal gas law. Monthly averages of air pressure, air temperature and air pollutants (NO, NO2, and NOx) were calculated from hourly values. Linear regression, using the Pearson product moment correlation coefficient, was used to assess the association between the NAOI and monthly averages of NOx concentration, temperature and air pressure. The AQSs for NO2 valid in Sweden (EU and national standards) are presented in Table 1. Since only winter conditions were included in the present study, the annual AQS was not considered. Moreover, hourly concentrations above 200 mg m3 were rare (0.10% of the total dataset). Therefore the evaluation of AQSs focused on the 60 mg m3 daily and 90 mg m3 hourly standards. They are evaluated on a yearly basis, but since they are expressed as time fractions (the fraction of time that the thresholds are allowed to be exceeded) they can be considered also on a monthly or winter season basis. The 60 mg m3 daily and 90 mg m3 hourly thresholds were also applied to the nonregulated gas NO to reflect the total situation for nitrogen oxides. These data for NO and NO2 for each year during the study period are presented in Table 2. The frequency of concentrations (NO2 and NO) above the daily 60 and hourly 90 mg m3 thresholds were compared for months with NAOI > 0 and NAOI < 0 to reflect the differences between the two phases of the NAO. The Pearson’s chi-squared test in the software R was used to test for statistical significance between positive and negative NAOI with respect to the frequency of concentrations exceeding the threshold values. The null hypothesis was that there was no difference between the two NAOI groups regarding the fraction of hours exceeding the thresholds. An extended analysis was carried out for the diurnal variation of the hourly NO2 and NO concentrations exceeding 90 mg m3 during negative and positive NAOI with the same null hypothesis. Further, the distribution of wind directions (four wind sectors centred around S, W, N and E, respectively) during positive and negative NAOI was investigated and the significance of differences was tested using the Pearson’s chi-squared test with the null hypothesis that wind direction did not depend on NAOI being
Table 1 Air quality standards for NO2 in Sweden. Concentration threshold
Time resolution
Limit not to be exceeded
40 mg m3 60 mg m3 90 mg m3 200 mg m3
year day (24 h) hour hour
average 7 days yr1 175 h yr1 17 h yr1
M. Grundström et al. / Atmospheric Environment 45 (2011) 883e888
885
Table 2 Number of days (d) with NO2 and NO greater than 60 mg m3 and hours (h) with NO2 and NO greater than 90 mg m3 during January, February and December 1997e2006. JFD
d NO2 > 60
h NO2 > 90
d NO > 60
h NO > 90
1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
3 4 4 3 6 1 9 3 4 7
18 45 24 31 58 19 64 55 37 74
11 9 10 6 13 5 10 6 6 11
157 103 144 88 172 70 161 113 96 166
larger or smaller than 0. In addition, the occurrence of hourly NO2 and NO concentrations exceeding 90 mg m3 within the four wind sectors was investigated. Differences between the four wind directions with respect to the number of hours with concentrations exceeding 90 mg m3 was tested using the Pearson’s chi-squared test with the null hypothesis that concentrations of NO and NO2 occur with the same frequency in all wind directions. 3. Results 3.1. Relationships of air pressure and temperature with the NAOI
Fig. 2. Linear regression between monthly mean air temperature and monthly mean NAOI in Gothenburg for January, February and December 1997e2006.
photolysis and oxidation with ozone, it should reflect the pollution level in fairly direct relation to emission. 3.3. Relationships of concentration threshold exceedance for NO2 and NO with the NAOI
Fig. 3 shows the linear regression between the monthly average concentrations of NOx and the NAOI. There was a statistically significant negative correlation between NOx concentration and NAOI (p ¼ 0.0093). Since the NOx concentration is not influenced by
The exceedance of the daily threshold of 60 mg m3 and the hourly threshold of 90 mg m3 for NO2 and NO was negatively associated with the NAOI (Fig. 4). The daily NO2 average of 60 mg m3 (allowed to be exceeded 2% of the time on a yearly basis) was exceeded 5.5% of the time in months with NAOI < 0, while during NAOI > 0 it was exceeded 4.4% of the time. In this case the difference between negative and positive NAOI was not statistically significant, which is likely to be due to the relatively small number of daily NO2 averages above 60 mg m3 during the study period (Table 2). The hourly 90 mg m3 standard for NO2, also allowed to be exceeded 2% of the time (98-percentile), was exceeded 2.3% of the time during negative NAOI but less than 1.7% of the time for positive NAOI. The difference between the fraction of hours with NO2 concentrations exceeding 90 mg m3 between months with NAOI > 0 and NAOI < 0 was strongly significant (p < 0.0001). Studying months with a strongly negative NAOI (NAOI < 2), the fraction of days exceeding 60 mg m3 was found to be 8.2% for NO2, compared to 1.9% during months with strongly positive NAOI (NAOI > 2.5). Similarly, the fraction of hours with NO2
Fig. 1. Linear regression between monthly mean air pressure and monthly mean NAOI in Gothenburg for January, February and December 1997e2006.
Fig. 3. Linear regression between monthly mean concentration of NOx (NO þ NO2) and monthly mean NAOI in Gothenburg for January, February and December 1997e2006.
Significant linear regression relationships with the monthly NAOI were obtained for monthly air pressure (Fig. 1) and monthly air temperature (Fig. 2). As expected, air pressure, being the variable on which the NAOI is based, posed a negative correlation (p ¼ 0.0045) with the NAOI, while air temperature showed a positive correlation with NAOI (p ¼ 0.0067). Worth noting is that the figure also shows that monthly index values of the NAO were most common within the range between 2.0 and 2.5, where 24 of the total 30 data points were found. More extreme negative or positive NAOI values were relatively uncommon during the study period. 3.2. Relationship between the average concentration of NOx and the NAOI
886
M. Grundström et al. / Atmospheric Environment 45 (2011) 883e888
Fig. 4. Comparison of the fraction of days (d) exceeding 60 mg m3 and the fraction of hours (h) exceeding 90 mg m3, regarding NO2 and NO, for months with NAOI < 0 and NAOI > 0. Statistical significances (**p < 0.01; ***p < 0.001) refer to the chi-square test used to compare the two NAOI intervals with respect to the frequency of concentrations above the concentrations thresholds.
concentrations >90 mg m3 was 3.5% for months with NAOI < 2 and 1.2% in months with NAOI > 2.5. Thus, the difference in occurrence of high air pollution events becomes more apparent comparing strongly negative and positive NAOI situations than comparing all negative with all positive NAOI situations. Analysis of NO in relation to NAOI revealed similar but clearer patterns and larger fractions of time exceeding the thresholds concentrations compared to NO2. The fraction of days exceeding 60 mg m3 (12.5% at NAOI < 0 and 7.6% at NAO > 0) and the fraction of hours exceeding 90 mg m3 (7.4% at NAOI < 0 and 4.8% at NAOI > 0) for NO were both statistically significant (p < 0.001 and p < 0.01, respectively). The differences between negative and positive NAOI were larger than for NO2. The larger degrees of exceedance of thresholds for NO reflects the predominance of NO in the vehicle exhausts which, in addition, is not being oxidised to NO2 to any large degree in many high pollution situations. Considering months with NAOI < 2, the fraction of days exceeding 60 mg m3 was found to be 13.1% for NO, compared to 4.2% during months with NAOI > 2.5. The fraction of hours with NO concentrations >90 mg m3 was 8.7% for months with NAOI < 2 and 3.2% in months with NAOI > 2.5. Thus, as for NO2, the difference in air pollution is more pronounced comparing strongly negative and positive NAOI situations than comparing all months with NOAI < 0 with all months with NAOI > 0. In Fig. 5a and b, the diurnal variation of the fraction of hours with NO2 and NO concentrations larger than 90 mg m3 for months with negative and positive NAOI is shown. The key observation to be made from Fig. 5a is that the main difference between negative and positive NAOI with respect to NO2 pollution is the much higher concentrations during the morning hours 06:00e09:00 when high emissions from the traffic peak often coincide with a stable boundary layer after night-time surface cooling. The chi-square test revealed a significant difference between months with NAOI < 0 and NAOI > 0 for hours 06:00e07:00. A similar, but clearer, pattern was obtained by considering NO (Fig. 5b). The NO concentrations were higher for all hours, apart from the 4th, during negative NAO. The differences were significant mainly during morning and afternoon hours when traffic generally peaks. It is obvious that pollution from morning traffic plays an important role in explaining
Fig. 5. Diurnal variation in the fraction of hourly NO2 (a) and NO (b) concentrations exceeding 90 mg m3 for months with NAOI < 0 and NAOI > 0. Statistical significances ((*)p < 0.10; *p < 0.05; **p < 0.01; ***p < 0.001) refer to the chi-square test testing the difference in frequency of concentrations above 90 mg m3 between the two NAOI intervals.
the larger probabilities to exceed the hourly threshold during negative NAOI shown in Fig. 4, especially for NO2. 3.4. Relationships between NO2 and NO concentrations >90 mg m3 and wind direction Fig. 6a shows the observed distribution of wind directions in months with negative or positive NAOI. Northerly and easterly winds were more common during negative NAOI, while westerlies and southerlies were more common in positive NAOI. The NAOI effects on wind direction were strongly significant. The generally most common wind direction was from south. As evident from Fig. 6b, elevated (>90 mg m3) NO2 and NO occurred much more commonly in northerly and partly easterly winds than in southerly and westerly winds. Since these wind directions were more frequent in the negative NAO phase, this association highlights the influence of negative NAOI on air pollution levels. 3.5. Trends of the NAOI and NO2 concentrations >90 mg m3 during the study period In Fig. 7a and b the trends of the NAOI and the fraction of hours with NO2 concentrations >90 mg m3, respectively, over the nine
M. Grundström et al. / Atmospheric Environment 45 (2011) 883e888
887
Fig. 7. Trends for (a) the time fraction of NO2 concentrations exceeding 90 mg m3, and (b) NAOI, for nine DecembereFebruary winter seasons, the first starting with December 1997 and the last ending with February 2006. Fig. 6. a) The distribution of wind directions: north, east, south and west for months with NAOI < 0 and NAOI > 0, and b) the number of hourly NO2 and NO concentrations >90 mg m3 in situations with wind directions from north, east, south and west during the study period. Statistical significances (***p < 0.001) refer to a) the chi-square test of the difference in frequency of the different wind directions between the two NAOI intervals, and b) the occurrence of NO2 or NO concentrations > 90 mg m3 in the four wind directions.
complete winter seasons (DecembereFebruary) during the study period are shown. The NAOI showed a negative trend, while the fraction of hours with NO2 concentrations >90 mg m3 showed a positive trend, both reaching only marginal significance (p ¼ 0.062, p ¼ 0.058). The winter 2000/2001 deviated by having a high average NAOI and very low NO2 concentrations, which is probably simply a random effect. Emission inventory of NOx from traffic covering every second year from 2000 to 2006 showed a weak declining trend of 1.4% per year, which would be expected to result in somewhat fewer exceedances of AQSs, in the absence of climatic effects, but in fact did not. It seems likely that the tendency towards lower NAOI values during the study period can explain this. 4. Discussion In the present investigation significant associations between the local weather, nitrogen oxides and the NAOI during winter were observed. As expected, months with negative NAOI were associated with low average temperatures and high air pressure compared to high NAOI situations. Furthermore, high pollution events, represented by the exceedance of daily NO2 and NO concentrations higher than 60 mg m3 and hourly NO2 and NO concentrations
higher than 90 mg m3, were observed more frequently in the negative phase of the NAO. This is in line with the observations made by Delaney and Dowding (1998) that the synoptic weather situation associated with high NO2 levels in Dublin was mostly characterised by high pressure cells east of Ireland. There was a considerable difference between the negative and positive phases of the NAO and with respect to the distribution of wind directions in Gothenburg. Since high pollution events were much more common in northerly and easterly wind, this is an important aspect of the climatic influence on local air pollution by the NAO. When studying relationships of air pollutants with wind direction in a city it has to be kept in mind that different emission sources (traffic and other) are not completely evenly distributed within the urban landscape, which can influence pollution levels in different wind sectors at a certain site. We have no reason to believe that this strongly affected the results of the present study. A potential source of error in the present study is that the atmospheric circulation during the individual winter months DecembereFebruary are treated as equal, while they are in reality subject to seasonal dynamics. However, high pollution events occurred in any of these months and focussing the study to the three most pronounced winter months should limit the importance of seasonal variation. To perform the analysis separately for each month of the year would have left too few observations for analysis and to limit the study to winter season averages would have removed much of the month-to-month variation in NAOI, meteorology and air pollution. It was obvious in the present study that the probability of exceeding the AQSs for NO2 was higher in the negative phase of the NAO, especially in months with NAOI < 2. Thus, any shift in the
888
M. Grundström et al. / Atmospheric Environment 45 (2011) 883e888
frequency of high or low NAOI has a strong potential to affect the possibility to reach AQSs, even if the emissions remain constant. Fig. 5 revealed that the main difference between NAOI < 0 and NAOI > 0 in terms of the fraction of hours when NO2 and NO concentrations of 90 mg m3 is exceeded is most pronounced in the morning hours when peak traffic coincides with restricted air mixing, especially for NO2. This observation calls for further investigation of the relationship between urban air pollution at rooftop monitoring stations, as in the present case, and the situation at street level, reflecting the complexities in the ventilation of street canyons (Louka et al., 2000). Possibly, the contrast between situations with low and high NAOI in terms of air pollution levels in the morning traffic rush hours is considerably stronger in the street canyons than at neighbouring rooftops. The NAOI showed a downward trend while exceedances of the AQS for 90 mg m3 NO2 showed an upward trend during the study period. Although these trends do not completely mirror of each other, a coupling is obvious, especially considering the decreasing traffic emissions since year 2000, which would be expected to lower the number of exceedances of AQSs. Caution is however necessary before excluding the influence from emissions. Since high pollution events, and thus exceedance of AQSs, can be associated with low temperatures, the detailed emissions pattern in these situations does not necessarily follow the general emissions trend. It should also be noted that the fact that winter NAOI tended to decline during the study period cannot be extrapolated to a longterm trend. This study showed a potential for climate patterns such as the NAO to influence air pollution levels as indicated by Flocas et al. (2009). The effect of a future climate change on air pollution is hard to assess with high confidence today. Analysing the NAO response to increasing concentrations of CO2 from 18 global coupled general circulation models, Stephenson et al. (2006) found that most models simulated a slightly increasing trend in the NAO with increasing levels of atmospheric CO2. If this turns out to be correct, fewer wintertime episodes of AQS exceedance for NO2 related to the NAO can be expected in the future if the emissions rates remain unchanged. In any case, any shift in the climatic pattern represented by the NAO has the potential to significantly affect air quality in cities of e.g. North-West Europe. 5. Conclusions The most important conclusions from the present investigation were: Concentrations of nitrogen oxides as well as air pressure were negatively related to the NAOI, while air temperature was positively related to the NAOI, in the City of Gothenburg during the winter months for the period 1997e2006. The relationship between the fraction of hourly NO2 and NO concentrations exceeding 90 mg m3 and the NAOI was most
pronounced in the morning rush hours and in situations with wind directions from north and east. Acknowledgements Thanks are due to the Environment Administration of the City of Gothenburg for providing air pollution and meteorological data from the monitoring site Femman, and for traffic emission data. This research presented in this paper is a contribution to the Swedish strategic research area ModElling the Regional and Global Earth system, MERGE (http://www.miljovetenskap.lu.se/merge) and cofunded by the Gothenburg Atmospheric Science Centre (GAC). References Brimblecombe, P., 1996. Air Composition and Chemistry, second ed.. Cambridge University Press, Cambridge. Carslaw, D.C., Beevers, S.D., Bell, M.C., 2007. Risks of exceeding the hourly EU limit value for nitrogen dioxide resulting from increased road transport emissions of primary nitrogen dioxide. Atmospheric Environment 41, 2073e2082. Chen, D., Hellström, C., 1999. The influence of the North Atlantic Oscillation on the regional temperature variability in Sweden: spatial and temporal variations. Tellus 51A, 505e516. Curtis, L., Rea, W., Smith-Willis, P., Fenyves, E., Pan, Y., 2006. Adverse health effects of outdoor air pollutants. Environment International 32, 815e830. Delaney, C., Dowding, P., 1998. The relationship between extreme nitrogen oxide (NOx) concentrations in Dublin’s atmosphere and meteorological conditions. Environmental Monitoring and Assessment 52, 159e172. Demuzere, M., Trigo, R.M., Vila-Guerau de Arellano, J., van Lipzig, N.P.M., 2009. The impact of weather and atmospheric circulation on O3 and PM10 levels at a rural mid-latitude site. Atmospheric Chemistry and Physics 9, 2695e2714. Flocas, H., Kelessis, A., Helmis, C., Petrakakis, M., Zoumakis, M., Pappas, K., 2009. Synoptic and local scale atmospheric circulatiation associated with air pollution episodes in an urban Mediterranean area. Theoretical and Applied Climatology 95, 265e277. Hurrell, J., Deser, C., 2009. North Atlantic climate variability: the role of the North Atlantic oscillation. Journal of Marine Systems 78, 28e41. Hurrell, J.W., Kushnir, Y., Ottersen, G., Visbeck, M., 2003. An overview of the north Atlantic oscillation. The North Atlantic Oscillation: climatic significance and environmental impact. Geophysical Monogrraphs 134, 1e35. Jacob, D.J., Winner, D.A., 2009. Effect of climate change on air quality. Atmospheric Environment 43, 51e63. Janhäll, S., Olofsson, K.F.G., Andersson, P.U., Pettersson, J., Hallquist, M., 2006. Evolution of the urban aerosol during winter temperature inversion episodes. Atmospheric Environment 40, 5355e5366. Jones, P.D., Jonsson, T., Wheeler, D., 1997. Extension to the North Atlantic Oscillation using early instrumental pressure observations from Gibraltar and south-west Iceland. International Journal of Climatology 17, 1433e1450. Leibensberger, E.M., Mickley, L.J., Jacob, D.J., 2008. Sensitivity of US air quality to mid-latitude cyclone frequency and implications of 1980e2006 climate change. Atmospheric Chemistry and Physics 8, 7075e7086. Louka, P., Belcher, B.E., Harrison, R.G., 2000. Coupling between air flow in street and the well-developed boundary layer aloft. Atmospheric Environment 34, 2613e2621. Pleijel, H., Klingberg, J., Bäck, E., 2009. Characteristics of NO2 pollution in the city of Gothenburg, south-west Sweden e relation to NOx and O3 levels, photochemistry and monitoring location. Water Air and Soil Pollution Focus 9, 15e25. Stephenson, D.B., Pavan, V., Collins, M., Junge, M.M., Quadrelli, R., 2006. North Atlantic Oscillation response to transient greenhouse gas forcing and the impact on European winter climate: a CMIP2 multi-model assessment. Climate Dynamics 27, 401e420. Visbeck, M.H., Hurrell, J.W., Polvani, L., Cullen, H.M., 2001. The North Atlantic Oscillation: past, present, and future. PNAS 98, 12876e12877.
Contents lists available at ScienceDirect
Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Urban NO2 and NO pollution in relation to the North Atlantic Oscillation NAO M. Grundström a, H.W. Linderholm b, J. Klingberg a, H. Pleijel a, * a b
University of Gothenburg, Plant and Environmental Sciences, P.O. Box 461, 405 30 Göteborg, Sweden University of Gothenburg, Department of Earth Sciences, P.O. Box 460, 405 30 Göteborg, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2010 Received in revised form 10 November 2010 Accepted 12 November 2010
The North Atlantic Oscillation (NAO), a measure of the strength of the zonal wind across the North Atlantic Ocean, strongly influences weather conditions in NW Europe, e.g. temperature, precipitation and wind, especially during winter. It was hypothesised that elevated concentrations of nitrogen oxides in Gothenburg would be enhanced during negative NAO index (NAOI) conditions, representing more anticyclonic weather situations and thus leading to limited air mixing in the urban atmosphere, than situations with NAOI > 0. Hourly wintertime (DecembereFebruary) concentrations (1997e2006) of NO2, NO, air pressure, temperature and wind direction from an urban rooftop (30 m above street level) in the centre of the City of Gothenburg were analysed in relation to NAOI. Air pressure, the average concentration of nitrogen oxides (NOx ¼ NO2 þ NO), as well as the fraction of hourly NO2 and NO concentrations exceeding 90 mg m3 and the fraction of daily NO concentrations exceeding 60 mg m3, were significantly and negatively related to NAOI. Air temperature was positively correlated with NAOI. Southerly and westerly winds were more common in months with positive NAOI, while easterly and northerly winds were overrepresented in months with negative NAOI. High pollution concentrations dominantly occurred in situations with northerly and easterly wind directions. High NO2 and NO concentrations were associated with negative NAOI, especially in the morning when the traffic rush coincided with restricted air mixing. Over the tenyear period there were trends for more negative NAOI and increased time fractions with hourly NO2 concentrations exceeding 90 mg m3. The conclusion of this study is that a climate shift towards higher or lower NAOI has the potential to significantly influence urban air pollution in North-West Europe, and thus the possibility to reach air quality standards, even if emissions remain constant. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: North Atlantic Oscillation NO2 NOx Urban pollution Climate Gothenburg
1. Introduction High air pollution concentrations in urban areas represent a serious environmental problem (Curtis et al., 2006). Air quality standards (AQS) for high priority air pollutants, including nitrogen dioxide (NO2), particulate matter and ozone (O3), have been introduced nationally in many countries and by the European Union. The possibility of meeting the AQSs depend not only on the emission rates of pollutants but also on the meteorological factors that govern air mixing and dispersion of air pollutants (Flocas et al., 2009). NO2 is often considered to be the more toxic of the nitrogen oxides (NOx ¼ NO þ NO2), but NO and NOx are sometimes used as proxies for traffic related air pollution in general. Emissions of NOx take place mainly in the form of NO, although the fraction of primary NO2 in vehicle exhausts has been increasing since the end
* Corresponding author. Tel.: þ46 31 786 2532; fax: þ46 31 786 2560. E-mail address: [email protected] (H. Pleijel). 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2010.11.023
of the 1990s in European cities (Carslaw et al., 2007), including Gothenburg (Pleijel et al., 2009). In reaction with ozone (O3) NO is oxidised to NO2 (Brimblecombe, 1996), and consequently a substantial fraction (w15% up to almost 100%) of NOx is present in the form of NO2 in the urban air of e.g. Gothenburg (Pleijel et al., 2009). While the ratio between NO and NO2 varies with the presence of ozone and the photolysis of NO2 (Brimblecombe, 1996), NOx remains uninfluenced by oxidation with ozone and photolysis. Typically, NOx pollution in urban areas is dominated by local emissions. Recently, the scientific discussion of the influence of climate change and climate patterns on air pollution exposure has intensified. Since there are important examples of the strong influence of climate patterns on the occurrence of air pollutants (Flocas et al., 2009), especially particulate matter and ozone (Leibensberger et al., 2008; Jacob and Winner, 2009; Demuzere et al., 2009), the influence of climate change and climate patterns on the possibility to reach AQSs has become an important issue. In the case of urban NO2 pollution, Delaney and Dowding (1998) provided evidence that synoptic circulation patterns strongly affected the occurrence of
884
M. Grundström et al. / Atmospheric Environment 45 (2011) 883e888
high NO2 episodes in Dublin, Ireland. In that study, most of the high pollution days were associated with high pressure cells east of Ireland, but in one case the high NO2 levels were most likely associated with advection of a polluted air mass from continental Europe and Britain. The North Atlantic Oscillation (NAO) is a large-scale meridional oscillation in atmospheric mass, with centres of action near Iceland and over the subtropical Atlantic (Visbeck et al., 2001). It is one of the leading climate modes in the North Atlantic region (Hurrell et al., 2003), and influences climate variability, e.g. temperature, precipitation and storminess, from eastern United States into western Eurasia, from the Arctic to the subtropical Atlantic, especially during boreal winter (Hurrell and Deser, 2009). When the pressure gradient between the Icelandic low and the subtropical high pressure centre during winter is stronger than normal (positive NAO), the westerly winds are stronger, bringing maritime air masses into Northern Europe where climate becomes wetter and milder, e.g. in Sweden (Chen and Hellström, 1999). At times of a weak pressure gradient (negative NAO), weaker westerly winds result in a domination of high pressure conditions over Northern Europe, yielding cold and dry winters. Consequently, when the NAO index (NAOI) is negative, temperature inversions, associated with restricted air mixing and high NOx concentrations, are likely to occur more frequently than during the low pressure conditions (e.g. Flocas et al., 2009) associated with a positive NAO. The City of Gothenburg, the centre of a conurbation with w900,000 inhabitants on the west coast of southern Sweden, is not extremely polluted due to its moderate size and location near the coast with rather strong winds ventilating the urban air most of the time. However, during anticyclonic weather conditions in winter, strong temperature inversions occur, leading to periods of strongly elevated pollution concentrations, including nitrogen oxides (Janhäll et al., 2006). The aim of this investigation was to study the relationship between the degree of urban pollution, as represented by nitrogen oxides, and the large-scale atmospheric circulation, represented by a NAO index (NAOI), during the winter. The hypothesis was that nitrogen oxides concentrations correlate negatively to the NAOI, since a low NAOI is expected to represent a higher probability of stagnant, anticyclonic weather conditions. 2. Materials and methods 2.1. Measurement site and instrumentation Air quality and meteorological monitoring was performed on top of a building, 30 m above street level, surrounded by buildings of similar or lower height. This site is located in the commercial district of Gothenburg city centre (‘Femman’; 5742.5220 , 1158.2360 ), adjacent to the central terminal for trains and buses and approximately 300 m away from a busy traffic route (E45). The location of the monitoring station was selected to represent the polluted urban background. Hourly monitoring of NO and NO2 (Tecan CLD 700 AL chemiluminescence instrument), wind direction (Gill ultrasonic anemometer), temperature (Campbell Rotronic MP101 thermometer/hygrometer) and air pressure (Vaisala PA11A electronic barometer) were carried out. Data for the ten-year period 1997e2006 were used. The pollutants NO and NO2 and meteorology were analysed for the winter months (January, February and December) of each year. Winter season data (Fig. 7a and b) refer to December one year and January and February in the consecutive year. Data coverage was essentially complete, apart from wind direction for which 1 month of data was missing. Traffic emission data covering every second year from 2000 to 2006 was retrieved from the Environmental Administration of the City of
Gothenburg, which holds a data base (using the EnviMan system, www.opsis.se) for local air pollution emissions including traffic, based on detailed emission inventory. 2.2. NAOI The NAOI data used in this study was obtained from the Climate Research Unit, University of East Anglia; (http://www.cru.uea.ac. uk/wtimo/datapages/naoi.htm). The NAOI was calculated on a monthly basis from the difference between the normalised sea level pressure over Gibraltar and the normalised sea level pressure over Southwest Iceland (e.g. Jones et al., 1997). Monthly index values were used throughout the study with the exception of Fig. 7, where winter season (DecembereFebruary average) data are used. The monthly index values varied between 2.25 and 5.26. 2.3. Calculations and evaluation of air quality standards The hourly values of each pollutant were converted from mole fraction (expressed in parts per billion, ppb, equivalent to nmol mol1) units to mg m3 units using the ideal gas law. Monthly averages of air pressure, air temperature and air pollutants (NO, NO2, and NOx) were calculated from hourly values. Linear regression, using the Pearson product moment correlation coefficient, was used to assess the association between the NAOI and monthly averages of NOx concentration, temperature and air pressure. The AQSs for NO2 valid in Sweden (EU and national standards) are presented in Table 1. Since only winter conditions were included in the present study, the annual AQS was not considered. Moreover, hourly concentrations above 200 mg m3 were rare (0.10% of the total dataset). Therefore the evaluation of AQSs focused on the 60 mg m3 daily and 90 mg m3 hourly standards. They are evaluated on a yearly basis, but since they are expressed as time fractions (the fraction of time that the thresholds are allowed to be exceeded) they can be considered also on a monthly or winter season basis. The 60 mg m3 daily and 90 mg m3 hourly thresholds were also applied to the nonregulated gas NO to reflect the total situation for nitrogen oxides. These data for NO and NO2 for each year during the study period are presented in Table 2. The frequency of concentrations (NO2 and NO) above the daily 60 and hourly 90 mg m3 thresholds were compared for months with NAOI > 0 and NAOI < 0 to reflect the differences between the two phases of the NAO. The Pearson’s chi-squared test in the software R was used to test for statistical significance between positive and negative NAOI with respect to the frequency of concentrations exceeding the threshold values. The null hypothesis was that there was no difference between the two NAOI groups regarding the fraction of hours exceeding the thresholds. An extended analysis was carried out for the diurnal variation of the hourly NO2 and NO concentrations exceeding 90 mg m3 during negative and positive NAOI with the same null hypothesis. Further, the distribution of wind directions (four wind sectors centred around S, W, N and E, respectively) during positive and negative NAOI was investigated and the significance of differences was tested using the Pearson’s chi-squared test with the null hypothesis that wind direction did not depend on NAOI being
Table 1 Air quality standards for NO2 in Sweden. Concentration threshold
Time resolution
Limit not to be exceeded
40 mg m3 60 mg m3 90 mg m3 200 mg m3
year day (24 h) hour hour
average 7 days yr1 175 h yr1 17 h yr1
M. Grundström et al. / Atmospheric Environment 45 (2011) 883e888
885
Table 2 Number of days (d) with NO2 and NO greater than 60 mg m3 and hours (h) with NO2 and NO greater than 90 mg m3 during January, February and December 1997e2006. JFD
d NO2 > 60
h NO2 > 90
d NO > 60
h NO > 90
1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
3 4 4 3 6 1 9 3 4 7
18 45 24 31 58 19 64 55 37 74
11 9 10 6 13 5 10 6 6 11
157 103 144 88 172 70 161 113 96 166
larger or smaller than 0. In addition, the occurrence of hourly NO2 and NO concentrations exceeding 90 mg m3 within the four wind sectors was investigated. Differences between the four wind directions with respect to the number of hours with concentrations exceeding 90 mg m3 was tested using the Pearson’s chi-squared test with the null hypothesis that concentrations of NO and NO2 occur with the same frequency in all wind directions. 3. Results 3.1. Relationships of air pressure and temperature with the NAOI
Fig. 2. Linear regression between monthly mean air temperature and monthly mean NAOI in Gothenburg for January, February and December 1997e2006.
photolysis and oxidation with ozone, it should reflect the pollution level in fairly direct relation to emission. 3.3. Relationships of concentration threshold exceedance for NO2 and NO with the NAOI
Fig. 3 shows the linear regression between the monthly average concentrations of NOx and the NAOI. There was a statistically significant negative correlation between NOx concentration and NAOI (p ¼ 0.0093). Since the NOx concentration is not influenced by
The exceedance of the daily threshold of 60 mg m3 and the hourly threshold of 90 mg m3 for NO2 and NO was negatively associated with the NAOI (Fig. 4). The daily NO2 average of 60 mg m3 (allowed to be exceeded 2% of the time on a yearly basis) was exceeded 5.5% of the time in months with NAOI < 0, while during NAOI > 0 it was exceeded 4.4% of the time. In this case the difference between negative and positive NAOI was not statistically significant, which is likely to be due to the relatively small number of daily NO2 averages above 60 mg m3 during the study period (Table 2). The hourly 90 mg m3 standard for NO2, also allowed to be exceeded 2% of the time (98-percentile), was exceeded 2.3% of the time during negative NAOI but less than 1.7% of the time for positive NAOI. The difference between the fraction of hours with NO2 concentrations exceeding 90 mg m3 between months with NAOI > 0 and NAOI < 0 was strongly significant (p < 0.0001). Studying months with a strongly negative NAOI (NAOI < 2), the fraction of days exceeding 60 mg m3 was found to be 8.2% for NO2, compared to 1.9% during months with strongly positive NAOI (NAOI > 2.5). Similarly, the fraction of hours with NO2
Fig. 1. Linear regression between monthly mean air pressure and monthly mean NAOI in Gothenburg for January, February and December 1997e2006.
Fig. 3. Linear regression between monthly mean concentration of NOx (NO þ NO2) and monthly mean NAOI in Gothenburg for January, February and December 1997e2006.
Significant linear regression relationships with the monthly NAOI were obtained for monthly air pressure (Fig. 1) and monthly air temperature (Fig. 2). As expected, air pressure, being the variable on which the NAOI is based, posed a negative correlation (p ¼ 0.0045) with the NAOI, while air temperature showed a positive correlation with NAOI (p ¼ 0.0067). Worth noting is that the figure also shows that monthly index values of the NAO were most common within the range between 2.0 and 2.5, where 24 of the total 30 data points were found. More extreme negative or positive NAOI values were relatively uncommon during the study period. 3.2. Relationship between the average concentration of NOx and the NAOI
886
M. Grundström et al. / Atmospheric Environment 45 (2011) 883e888
Fig. 4. Comparison of the fraction of days (d) exceeding 60 mg m3 and the fraction of hours (h) exceeding 90 mg m3, regarding NO2 and NO, for months with NAOI < 0 and NAOI > 0. Statistical significances (**p < 0.01; ***p < 0.001) refer to the chi-square test used to compare the two NAOI intervals with respect to the frequency of concentrations above the concentrations thresholds.
concentrations >90 mg m3 was 3.5% for months with NAOI < 2 and 1.2% in months with NAOI > 2.5. Thus, the difference in occurrence of high air pollution events becomes more apparent comparing strongly negative and positive NAOI situations than comparing all negative with all positive NAOI situations. Analysis of NO in relation to NAOI revealed similar but clearer patterns and larger fractions of time exceeding the thresholds concentrations compared to NO2. The fraction of days exceeding 60 mg m3 (12.5% at NAOI < 0 and 7.6% at NAO > 0) and the fraction of hours exceeding 90 mg m3 (7.4% at NAOI < 0 and 4.8% at NAOI > 0) for NO were both statistically significant (p < 0.001 and p < 0.01, respectively). The differences between negative and positive NAOI were larger than for NO2. The larger degrees of exceedance of thresholds for NO reflects the predominance of NO in the vehicle exhausts which, in addition, is not being oxidised to NO2 to any large degree in many high pollution situations. Considering months with NAOI < 2, the fraction of days exceeding 60 mg m3 was found to be 13.1% for NO, compared to 4.2% during months with NAOI > 2.5. The fraction of hours with NO concentrations >90 mg m3 was 8.7% for months with NAOI < 2 and 3.2% in months with NAOI > 2.5. Thus, as for NO2, the difference in air pollution is more pronounced comparing strongly negative and positive NAOI situations than comparing all months with NOAI < 0 with all months with NAOI > 0. In Fig. 5a and b, the diurnal variation of the fraction of hours with NO2 and NO concentrations larger than 90 mg m3 for months with negative and positive NAOI is shown. The key observation to be made from Fig. 5a is that the main difference between negative and positive NAOI with respect to NO2 pollution is the much higher concentrations during the morning hours 06:00e09:00 when high emissions from the traffic peak often coincide with a stable boundary layer after night-time surface cooling. The chi-square test revealed a significant difference between months with NAOI < 0 and NAOI > 0 for hours 06:00e07:00. A similar, but clearer, pattern was obtained by considering NO (Fig. 5b). The NO concentrations were higher for all hours, apart from the 4th, during negative NAO. The differences were significant mainly during morning and afternoon hours when traffic generally peaks. It is obvious that pollution from morning traffic plays an important role in explaining
Fig. 5. Diurnal variation in the fraction of hourly NO2 (a) and NO (b) concentrations exceeding 90 mg m3 for months with NAOI < 0 and NAOI > 0. Statistical significances ((*)p < 0.10; *p < 0.05; **p < 0.01; ***p < 0.001) refer to the chi-square test testing the difference in frequency of concentrations above 90 mg m3 between the two NAOI intervals.
the larger probabilities to exceed the hourly threshold during negative NAOI shown in Fig. 4, especially for NO2. 3.4. Relationships between NO2 and NO concentrations >90 mg m3 and wind direction Fig. 6a shows the observed distribution of wind directions in months with negative or positive NAOI. Northerly and easterly winds were more common during negative NAOI, while westerlies and southerlies were more common in positive NAOI. The NAOI effects on wind direction were strongly significant. The generally most common wind direction was from south. As evident from Fig. 6b, elevated (>90 mg m3) NO2 and NO occurred much more commonly in northerly and partly easterly winds than in southerly and westerly winds. Since these wind directions were more frequent in the negative NAO phase, this association highlights the influence of negative NAOI on air pollution levels. 3.5. Trends of the NAOI and NO2 concentrations >90 mg m3 during the study period In Fig. 7a and b the trends of the NAOI and the fraction of hours with NO2 concentrations >90 mg m3, respectively, over the nine
M. Grundström et al. / Atmospheric Environment 45 (2011) 883e888
887
Fig. 7. Trends for (a) the time fraction of NO2 concentrations exceeding 90 mg m3, and (b) NAOI, for nine DecembereFebruary winter seasons, the first starting with December 1997 and the last ending with February 2006. Fig. 6. a) The distribution of wind directions: north, east, south and west for months with NAOI < 0 and NAOI > 0, and b) the number of hourly NO2 and NO concentrations >90 mg m3 in situations with wind directions from north, east, south and west during the study period. Statistical significances (***p < 0.001) refer to a) the chi-square test of the difference in frequency of the different wind directions between the two NAOI intervals, and b) the occurrence of NO2 or NO concentrations > 90 mg m3 in the four wind directions.
complete winter seasons (DecembereFebruary) during the study period are shown. The NAOI showed a negative trend, while the fraction of hours with NO2 concentrations >90 mg m3 showed a positive trend, both reaching only marginal significance (p ¼ 0.062, p ¼ 0.058). The winter 2000/2001 deviated by having a high average NAOI and very low NO2 concentrations, which is probably simply a random effect. Emission inventory of NOx from traffic covering every second year from 2000 to 2006 showed a weak declining trend of 1.4% per year, which would be expected to result in somewhat fewer exceedances of AQSs, in the absence of climatic effects, but in fact did not. It seems likely that the tendency towards lower NAOI values during the study period can explain this. 4. Discussion In the present investigation significant associations between the local weather, nitrogen oxides and the NAOI during winter were observed. As expected, months with negative NAOI were associated with low average temperatures and high air pressure compared to high NAOI situations. Furthermore, high pollution events, represented by the exceedance of daily NO2 and NO concentrations higher than 60 mg m3 and hourly NO2 and NO concentrations
higher than 90 mg m3, were observed more frequently in the negative phase of the NAO. This is in line with the observations made by Delaney and Dowding (1998) that the synoptic weather situation associated with high NO2 levels in Dublin was mostly characterised by high pressure cells east of Ireland. There was a considerable difference between the negative and positive phases of the NAO and with respect to the distribution of wind directions in Gothenburg. Since high pollution events were much more common in northerly and easterly wind, this is an important aspect of the climatic influence on local air pollution by the NAO. When studying relationships of air pollutants with wind direction in a city it has to be kept in mind that different emission sources (traffic and other) are not completely evenly distributed within the urban landscape, which can influence pollution levels in different wind sectors at a certain site. We have no reason to believe that this strongly affected the results of the present study. A potential source of error in the present study is that the atmospheric circulation during the individual winter months DecembereFebruary are treated as equal, while they are in reality subject to seasonal dynamics. However, high pollution events occurred in any of these months and focussing the study to the three most pronounced winter months should limit the importance of seasonal variation. To perform the analysis separately for each month of the year would have left too few observations for analysis and to limit the study to winter season averages would have removed much of the month-to-month variation in NAOI, meteorology and air pollution. It was obvious in the present study that the probability of exceeding the AQSs for NO2 was higher in the negative phase of the NAO, especially in months with NAOI < 2. Thus, any shift in the
888
M. Grundström et al. / Atmospheric Environment 45 (2011) 883e888
frequency of high or low NAOI has a strong potential to affect the possibility to reach AQSs, even if the emissions remain constant. Fig. 5 revealed that the main difference between NAOI < 0 and NAOI > 0 in terms of the fraction of hours when NO2 and NO concentrations of 90 mg m3 is exceeded is most pronounced in the morning hours when peak traffic coincides with restricted air mixing, especially for NO2. This observation calls for further investigation of the relationship between urban air pollution at rooftop monitoring stations, as in the present case, and the situation at street level, reflecting the complexities in the ventilation of street canyons (Louka et al., 2000). Possibly, the contrast between situations with low and high NAOI in terms of air pollution levels in the morning traffic rush hours is considerably stronger in the street canyons than at neighbouring rooftops. The NAOI showed a downward trend while exceedances of the AQS for 90 mg m3 NO2 showed an upward trend during the study period. Although these trends do not completely mirror of each other, a coupling is obvious, especially considering the decreasing traffic emissions since year 2000, which would be expected to lower the number of exceedances of AQSs. Caution is however necessary before excluding the influence from emissions. Since high pollution events, and thus exceedance of AQSs, can be associated with low temperatures, the detailed emissions pattern in these situations does not necessarily follow the general emissions trend. It should also be noted that the fact that winter NAOI tended to decline during the study period cannot be extrapolated to a longterm trend. This study showed a potential for climate patterns such as the NAO to influence air pollution levels as indicated by Flocas et al. (2009). The effect of a future climate change on air pollution is hard to assess with high confidence today. Analysing the NAO response to increasing concentrations of CO2 from 18 global coupled general circulation models, Stephenson et al. (2006) found that most models simulated a slightly increasing trend in the NAO with increasing levels of atmospheric CO2. If this turns out to be correct, fewer wintertime episodes of AQS exceedance for NO2 related to the NAO can be expected in the future if the emissions rates remain unchanged. In any case, any shift in the climatic pattern represented by the NAO has the potential to significantly affect air quality in cities of e.g. North-West Europe. 5. Conclusions The most important conclusions from the present investigation were: Concentrations of nitrogen oxides as well as air pressure were negatively related to the NAOI, while air temperature was positively related to the NAOI, in the City of Gothenburg during the winter months for the period 1997e2006. The relationship between the fraction of hourly NO2 and NO concentrations exceeding 90 mg m3 and the NAOI was most
pronounced in the morning rush hours and in situations with wind directions from north and east. Acknowledgements Thanks are due to the Environment Administration of the City of Gothenburg for providing air pollution and meteorological data from the monitoring site Femman, and for traffic emission data. This research presented in this paper is a contribution to the Swedish strategic research area ModElling the Regional and Global Earth system, MERGE (http://www.miljovetenskap.lu.se/merge) and cofunded by the Gothenburg Atmospheric Science Centre (GAC). References Brimblecombe, P., 1996. Air Composition and Chemistry, second ed.. Cambridge University Press, Cambridge. Carslaw, D.C., Beevers, S.D., Bell, M.C., 2007. Risks of exceeding the hourly EU limit value for nitrogen dioxide resulting from increased road transport emissions of primary nitrogen dioxide. Atmospheric Environment 41, 2073e2082. Chen, D., Hellström, C., 1999. The influence of the North Atlantic Oscillation on the regional temperature variability in Sweden: spatial and temporal variations. Tellus 51A, 505e516. Curtis, L., Rea, W., Smith-Willis, P., Fenyves, E., Pan, Y., 2006. Adverse health effects of outdoor air pollutants. Environment International 32, 815e830. Delaney, C., Dowding, P., 1998. The relationship between extreme nitrogen oxide (NOx) concentrations in Dublin’s atmosphere and meteorological conditions. Environmental Monitoring and Assessment 52, 159e172. Demuzere, M., Trigo, R.M., Vila-Guerau de Arellano, J., van Lipzig, N.P.M., 2009. The impact of weather and atmospheric circulation on O3 and PM10 levels at a rural mid-latitude site. Atmospheric Chemistry and Physics 9, 2695e2714. Flocas, H., Kelessis, A., Helmis, C., Petrakakis, M., Zoumakis, M., Pappas, K., 2009. Synoptic and local scale atmospheric circulatiation associated with air pollution episodes in an urban Mediterranean area. Theoretical and Applied Climatology 95, 265e277. Hurrell, J., Deser, C., 2009. North Atlantic climate variability: the role of the North Atlantic oscillation. Journal of Marine Systems 78, 28e41. Hurrell, J.W., Kushnir, Y., Ottersen, G., Visbeck, M., 2003. An overview of the north Atlantic oscillation. The North Atlantic Oscillation: climatic significance and environmental impact. Geophysical Monogrraphs 134, 1e35. Jacob, D.J., Winner, D.A., 2009. Effect of climate change on air quality. Atmospheric Environment 43, 51e63. Janhäll, S., Olofsson, K.F.G., Andersson, P.U., Pettersson, J., Hallquist, M., 2006. Evolution of the urban aerosol during winter temperature inversion episodes. Atmospheric Environment 40, 5355e5366. Jones, P.D., Jonsson, T., Wheeler, D., 1997. Extension to the North Atlantic Oscillation using early instrumental pressure observations from Gibraltar and south-west Iceland. International Journal of Climatology 17, 1433e1450. Leibensberger, E.M., Mickley, L.J., Jacob, D.J., 2008. Sensitivity of US air quality to mid-latitude cyclone frequency and implications of 1980e2006 climate change. Atmospheric Chemistry and Physics 8, 7075e7086. Louka, P., Belcher, B.E., Harrison, R.G., 2000. Coupling between air flow in street and the well-developed boundary layer aloft. Atmospheric Environment 34, 2613e2621. Pleijel, H., Klingberg, J., Bäck, E., 2009. Characteristics of NO2 pollution in the city of Gothenburg, south-west Sweden e relation to NOx and O3 levels, photochemistry and monitoring location. Water Air and Soil Pollution Focus 9, 15e25. Stephenson, D.B., Pavan, V., Collins, M., Junge, M.M., Quadrelli, R., 2006. North Atlantic Oscillation response to transient greenhouse gas forcing and the impact on European winter climate: a CMIP2 multi-model assessment. Climate Dynamics 27, 401e420. Visbeck, M.H., Hurrell, J.W., Polvani, L., Cullen, H.M., 2001. The North Atlantic Oscillation: past, present, and future. PNAS 98, 12876e12877.