Ozone and NOy in the Milan plume: The episode of June 19-21,1998

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1 Ozone and NOy in the Milan plume: The episode of June 19-21,1998 A. Thielmaim'", A.S.H. Prevot^ and J. Staehelin^ ^Institute for Atmospheric Science, Swiss Federal Institute of Technology, Zurich, Switzerland Scherrer Institute, Villigen, Switzerland EM ail: Abstract Nitrogen oxides (NOx=NO+NC>2), total reactive nitrogen NOy, ozone (Og), JNO% and volatile organic compounds (VOC) were measured for 10 weeks during spring and summer 1998 in the highly polluted Milan agglomeration. The concentrations during the episode of June 19 to 21 are analyzed. During this period ozone peaks at 160 ppb, NOy at 36 ppb. The high NOz/NOy-ratio in the afternoon indicates aged air masses reaching the station. We refer to NOz as the difference between NOy and NOx. It stands for NOx consumed during the photochemical aging of the air mass. Depending on the origin of the air mass the limitation of the ozone production shifts between VOC- and NOx-sensitivity. 1 Introduction Among the highest ozone concentrations in Europe have been measured downwind of Milan, Italy (Prevot et al. [1], Staffelbach et al. [2]). Southern Switzerland is among the regions subject to the influence of the air masses originated in the Milan agglomeration. The aim of the EUROTRAC-subproject LOOP (Limitation of Oxidant Production) is to describe in more detail the photooxidant formation in this area. The concept of indicator variables introduced by Sillman [3] provide a tool to discern between the two regimes of ozone formation. They were derived from model calculations for conditions in

2 628 Air Pollution the U.S.. VOC-sensitive chemistry is associated with indicator values of Os/NOz < 7, HCHO/NOy < 0.28, HzOz/HNO, < 0.4 and NOz > 20 ppb. Based on previous observations of the so-called Milan plume the field campaign PIPAPO (Pianura Padana Produzione di Ozono: Ozone Production in the Pobasin) was carried out from May to July We ran two stations north of Milan (Bresso at the northern border of the Milan agglomeration and Verzago, 27 km further North; cf. Figure 1). At both stations the following chemical species were measured: NO, NO%, total reactive nitrogen (NOy), Og, CO and VOC (Griiebler and Staehelin [4]). Meteorological measurements included JNO2- During the PIPAPO-campaign several ozone build-up episodes were observed where ozone reached values of up to 200 ppb (May 12 to 13; June 1 to 6; June 8 to 9 and June 19 to 21). Here we will describe the episode of June 19 to 21.. Cdtecr del Fianc Figure 1: location of the stations in the Po-basin 2 Measurement techniques Chemiluminescence detection (CLD) was used for the measurement of NO. The same type of detectors were used in conjunction with photolytical converters (PLC). These offer a highly selective reduction of NO% to NO, which is not the case for commercially available catalytic molybdenum (Mo) converters. Instead, Mo is quite suitable for the measurement of total reactive nitrogen (NOy). It

3 Air Pollution 629 catalyses the reduction of numerous NOy species to NO, which is again detected by CLD. The conversion efficiencies of two kinds of Mo-converters which were employed during PIPAPO and one gold-converter were recently tested in laboratory. These calibrations included the following NOy-components: NO2, Peroxyacetyl-nitrate (PAN), Isopropyl-nitrate (IPN), HNOs, HONO together with particulate sodium nitrate and ammonium nitrate. Possible interferences were also addressed: NH] and particulate ammonium sulfate. The interferences of all above-mentioned components on standard NOx-monitors were also measured. Preliminary results show the suitability of the Mo-converter for the measurement of NOy. Ozone is measured by UV-absorption, JNO% with radiometers. In these instruments, only sunlight in the wavelengths capable of photolysing NO] is detected. Table 1 lists the employed instruments together with the respective accuracies: Table 1: monitoring equipment parameter NO NO NO? N0% NOy NOy Os Os JNOz JNOz site Bresso Verzago Bresso Verzago Bresso Verzago Bresso Verzago Bresso Verzago instrument TECAN CLD 770 AL ppt ECOPHYSICS CLD 780 TR TECAN PLC 760 ECOPHYSICS PLC 760 Mo-converter + TE Model 42 NOx Mo-converter + CLD 780 TR Environics S300 Environics S300 Meteoconsult UV photometer Meteoconsult UV photometer precision (la at zero) 50 ppt 20 ppt same as CLD same as CLD 200 ppt 200 ppt 1 ppb 1 ppb 1E-4 s~' 1E-4 s"' 3 Meteorological Conditions from June 19 to 21,1998 The pattern of the solar radiation shows clear sky conditions throughout the considered period. This finding is also supported by METEOSAT-images. As a consequence of this stable weather conditions a heat island effect over the Alps can develop. It induces a pressure gradient between the Po-basin and the Alps and, consequently, a local wind system. The observation of this local wind system transporting polluted air from the Po-basin to the North led to the choice of our measurement sites. On June 19 and 20 the wind changes from NNW in the morning (until 7:00 AM) to WSW in the afternoon. Southern wind is dominant on the afternoon of the 21" (cf. Figure 2). The prevailing clear sky conditions are also responsible for temperatures over 25 C and a humidity of 42% in the

4 630 Air Pollution afternoon. The height of the mixing layer is similar throughout the episode (900 m at noon and 1300 m at 6:00 PM). This information was derived from the balloon soundings at the airport of Milan-Linate and is considered representative for the dilution of emissions in the Milan agglomeration /19/98 6/19/98 6/20/98 6/20/98 6/21/98 6/21/98 6/22/98 0:00 12:00 0:00 12:00 0:00 12:00 0:00 date & time Figure 2: wind pattern at Verzago 4 Discussion of the diurnal patterns Northern to north-western winds in the morning hours advect relatively fresh emissions from the city of Como among other nearby sources. This finding is supported by NO concentrations of 1-2 ppb together with rising NO/NO2 ratios. This ratio attends a value of 0.2 in the morning NOx-peak and was reported previously at nearby measuring sites (Staffelbach et al. [2]). Since sunrise at 6:00 AM ozone production is ongoing. Ozone reaches a concentration of 120 ppb towards noon, mainly because of the dissolution of the residual layer. At the same time the NOz/NOy-ratio reaches a value of indicating that the monitoring site is under the influence of aged air. The age of the air mass, expressed with the NOz/NOy-ratio doesn't vary with time in the afternoon (cf. Figure 5).

5 Air Pollution 631 6/19/98 6/19/98 6/20/98 6/20/98 6/21/98 6/21/98 6/22/98 0:00 12:00 0:00 12:00 0:00 12:00 0:00 date & time Figure 3: diurnal patterns of NOx at Verzago + O3 [ppb] a NOy [ppb] 0 6/19/98 6/19/98 6/20/98 6/20/98 6/21/98 6/21/98 6/22/98 0:00 12:00 0:00 12:00 0:00 12:00 0:00 date & time Figure 4: diurnal patterns of ozone and NOy at Verzago

6 632 Air Pollution On June 19 and 20 an additional increase of 40 ppb of ozone is observed between noon and 4:00 PM (cf. Figure 4). This further increase does not take place on June 21. In spite of strong variations of O] and NOy during the afternoon and differences between the individual days, no temporal change in NOz/NOy is observed. This leads to the conclusion that the site is under the influence of air masses of similar age on all three days and that this age doesn't vary with daytime in the afternoon. NO/NO% decreases slightly to 0.11 at 2:00 PM and 0.08 at 5:00 PM. This indicates ongoing peroxide oxidation of NO and, consequently, Os-production. Again, the ratios NO/NO2 are very close to those measured by Staffelbach et al. [2]. NOy concentrations in this period of the day attain levels of 36 ppb on the 19*, 27 ppb on the 20* and 20 ppb on the 21* (cf. Figure 3). They are well correlated with the CO concentrations. This correlation still holds when the concentrations decline in the afternoon (a correlation coefficient of 89% was found for the half hour values from 1:00 PM to 7:00 PM). This implies that the decay in the NOy-concentrations in the afternoon is due to advection of different air masses either than dry deposition NOz/NOy + NO/NO /19/98 6/19/98 6/20/98 6/20/98 6/21/98 6/21/98 6/22/98 0:00 12:00 0:00 12:00 0:00 12:00 0:00 date & time Figure 5: diurnal patterns of NOy-ratios at Verzago The difference in the ozone pattern between the two first days of the episode and June 21 may be explained by either different origins of the air masses reaching the stations, varying vertical mixing or day-to-day variation of the emission strength. The first possibility may be addressed with the wind direction, the second with the analysis of the vertical profile of the potential temperature and the latter with emission inventories. Mixing layer height doesn't vary

7 Air Pollution 633 significantly (cf. section 3). The emissions on June 21 might be different, as it is a Sunday. The VOC/NOx-relation could be lower on weekends. To our understanding the wind direction is the key to the diverging pattern on the 21**. The wind at Verzago (350m) is predominantly WSW on June 19 and 20 and SSW on the 2T* (cf. Figure 2). As we mentioned earlier, the age of the air mass does not vary from day to day (afternoon values, cf. Figure 5) despite the fact that wind directions point toward different source regions. At a station on the hills (Alpe de Vicere at 900 m, close to Colma del Piano - cf. Figure 1) the difference is even more accentuated: it changes from WSW to SE on the 21*'. At the station of Brer a in the center of Milan the wind turns to eastern directions at 6:00 AM. This direction persists until 9:00 AM on the 19*, until noon on the 20* and only until 7:00 AM on the 21". Additionally, on the 21*' the wind is weaker. The easterly winds will transport fresh emissions to the west of Milan where the air masses reaching Verzago come from. Under the circumstances prevailing on June 19 and 20, the Milan plume reaches Verzago :00 12:00 0:00 12:00 0:00 12:00 0:00 date & time Figure 6: wind pattern at Brer a The question arises whether the ozone production is in a NOx- or VOC-sensitive regime. We plotted the Ox (Ox=O]+NO2) afternoon (1:00 PM to 7:00 PM) concentration against NOz (cf. Figure7).. This plot shows a similar pattern for June 19 and 20 with a slope of 3.0 ppb of ozone produced by 1 ppb NOz (corresponds to NOx consumed). Similar values were found by Prevot et al. [1] within the more polluted region of a flight track nearby (in the vicinity of Bergamo, Italy). In contrast, on June 21 the correlation is quite poor (correlation

8 634 Air Pollution coefficient of 30% against 88% for all values of the 19* and 20*). This is not astonishing regarding the stable ozone concentration. Additionally we addressed the question of the sensitivity using the concept of indicator variables introduced by Sillman [3]. The indicators Og/NOz and NOy were calculated from our measurements. In Figure 8 VOC-sensitivity corresponds to the upper left quadrant, NOx-sensitivity to the lower right. The half-hour afternoon values of the episode in question show that ourjneasurements are consistent with the model results. There are no contradictory pairs of indicators. In a more recent study, Sillman et al. [5] suggest the use of Og/NOz = 8-10 to discern between the two regimes. This would mean that also the value of NOy should be adapted to 18-14, respectively. On June 19 and less pronounced on June 20 a gradual shift from VOC- to NOx-sensitivity is observed. June 20 is a day with prevailing VOC-sensitivity. On June, 21 all pairs of data indicate NOx-sensitivity June 19th, 1998 n June 20th, 1998 A June 21st, NOz [ppb] Figure 7: Ox-NOz correlation at Verzago (1:00 PM<t<7:OOPM)

9 Air Pollution p a * June 19th, 1998 n June 20th, 1998 A June 21st, O3/NOz Figure 8: NOy-Og/NOz correlation at Verzago (1:00 PM<t<7:OOPM) 5 Conclusions Under the meteorological circumstances prevailing on June 19 and 20, the Milan plume reaches Verzago. As a consequence an additional increase of 40 ppb of ozone is observed between noon and 4:00 PM. This further increase does not take place on June 21. Verzago is under the influence of air masses of similar age on all three days of the episode in the afternoon. NOy-concentrations in the afternoon decay due to advection of different air masses either than dry deposition. On June 19 and less pronounced on June 20 a gradual shift from VOC- to NOx-sensitivity is observed. June 20 is a day with prevailing VOCsensitivity. On June, 21 all pairs of data indicate NOx-sensitivity. Further analyses need to focus on days when peroxide and formaldehyde measurements are available. In this way all indicators proposed by Sillman could be inter compared. Acknowledgements The authors would like to thank Mr. T. Staffelbach from the Swiss Federal Research Station for Agroecology and Agriculture and Mr. G. Lanzani of the Provincial Administration of Como for their data from the stations of Verzago and Alpe de Vicere, respectively. Furthermore, we would like to express our

10 636 Air Pollution gratitude to the Komission fur Technologie und Innovation and the Swiss Environmental Protection Agency BUWAL for their financial support. References 1. Prevot, A.S.H., Staehelin, J., Kok, G.L., Schillawski, R.D., Neininger, B., Staffelbach, T., Neftel, A., Wernli, H. and Dommen,J., The Milan photooxidant plume, Journal of Geophysical Research, Vol. 102 D19, pp 23'375-23'388, Staffelbach, T., Neftel, A., Blatter, A., Gut, A., Fahrni, A., Staehelin, J., Prevot, A.S.H., Hering, A., Lehning, M., Neininger, B., Baumle, M., Kok, G.L., Dommen, J., Hutterli, M. and Anklin, M., Photochemical oxidant formation over southern Switzerland 1. Results from summer 1994, Journal of Geophysical Research, Vol. 102D19, pp *362, Sillman, S., The use of NOy, H^Oz and HNO] as indicators for ozone- NOx-hydrocarbon sensitivity in urban locations, Journal of Geophysical Research, 100, pp , Gruebler, F. and Staehelin, J., Temporal and spatial variation of volatile hydrocarbon compounds in the Milan area, this issue, 1999 Sillman, S., He, D., Pippin, M. R., Daum, P.H., Imre, D.G., Kleinman, L.I., Lee, J.H. and Weinstein-Lloyd, J., Model correlations for ozone, reactive nitrogen, and peroxides for Nashville in comparison with measurements: Implications for O3-NOx-hydrocarbon chemistry, Journal of Geophysical Research, Vol. 103 D17, pp 22*629-22*644, 1998.

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