The Milan Photooxidant Plume

 
In order to investigate the transition from NOx to VOC limited photooxidant formation in the Milan area, in summer 1998 the field experiment PIPAPO being part of the EUROTRAC project LOOP took place. Our group performed in this campaign continuous measurements of ozone, NOx, NOy and volatile hydrocarbon species (C4 to C11 compounds) including  isoprene at the two ground stations Bresso and Verzago (7 and 35 km northern to downtown Milan) from May until July 1998.

On our Map you can see all the ground stations of the campaign.

See Pictures of our PIPAPO sites in Bresso and in Verzago!

In our investigations of the Milan plume we focus the following topics:

Recent Resaerch Results

The measurements of anthropogenic hydrocarbons revealed highest mixing ratios for i-pentane and toluene of 69 ppb and 55 ppb at Bresso and 17 and 10 ppb at Verzago (Grüebler, 1999). Hydrocarbon concentrations at Bresso were generally strongly correlated, indicating the large dominance of road traffic emissions. At Verzago the correlations of the hydrocarbon concentrations were much lower reflecting the advanced photochemical age of the advected air. At Verzago high toluene peaks occurred at noon probably caused by the advection of industrial emissions from the south. Maximum concentrations of isoprene were 4.5 ppb in Bresso and 5.6 ppb in Verzago. Isoprene diurnal variation showed low afternoon concentrations and a sharp peak in the evening. They were explained with the strong dilution and photochemical degradation in the afternoon and the continuing emission after the collapse of the mixing layer in the evening and the degradation by NO3 in the early night. In Bresso anthropogenic interferences indicated anthropogenic isoprene emissions originating from road traffic leading to isoprene concentrations in the same magnitude as biogenic emissions. The averaged toluene to benezene ratio was 4.05 at Bresso which closely corresponded to the measurements of a roadside station in Como. This ratio is much higher than measurements from other sites in Europe. Most possibly, this is due to the fact, that the benzene content in Italian gasoline has been limited by law to 1% which is much lower than in other European countries.

An estimate of the contributions of the single compounds to the local ozone formation showed at both stations the highest share for isoprene, followed by m/p-xylene and C6 alkenes in Bresso and toluene and CO in Verzago. The multivariate data analysis for the determination of the most important sources basically confirmed the existence of an independent source for isoprene and one for toluene in Verzago.

In order to obtain information on the limitation of the regional photooxidant formation we analyzed the ozone concentrations as a function of NOx and ROG concentrations. In Bresso these empirical ozone isopleths clearly point to VOC-limitation: ozone increases with increasing VOC and decreases with increasing NOx. In Verzago the level of consumed nitrogen oxides (NOz=NOy-NO-NO2) limits ozone production under general conditions: ozone increases with increasing NOz and is invariant to changes in ROG. However, when the site was under the direct influence of the Milan urban plume the analysis provides evidence for photooxidant limitation by ROG. The results from Verzago were confirmed by field measurements of some "indicator variables". Indicator variables are concentration limits (NOy) or concentration ratios (e.g. O3/NOz) derived from model calculations which contain information on the sensitivity of O3 production.

An exemplary O3 plume

Figure 1: An exemplary O3 plume observed on June 19, 1998 in Verzago

Main Conclusions

  1. The large Milan photooxidant plume leads to O3 concentrations close to 200 ppb 30 km north to Milan.
  2. From our data analysis of NOz vs. ROG we found some evidence for ROG sensitivity in the Milan urban plume at Verzago, which was confirmed by measurements of the indicator variables.
  3. The ozone production is clearly NOx limited if the air is not directly advected from Milan.
  4. The indicator variables which include NOy and NOz seem to be suitable for the classification of NOx and ROG limitation regimes.
  5. The interaction between emission, photochemical degradation and mixing height probably explains the diurnal variation of the isoprene concentration.

Bibliography

For further information about these measurements please contact Axel Thielmann or Franca Grüebler.

Last update: March, 2000