Showing papers by "André S. H. Prévôt published in 2002"

Urban and rural aerosol characterization of summer smog events during the PIPAPO field campaign in Milan, Italy

Abstract: [1] A comprehensive range of aerosol parameters was measured at an urban and a rural site in the Milan, Italy metropolitan region during summer smog events in summer 1998. Measurements were performed as part of the Pianura Padana Produzione di Ozono (PIPAPO) field campaign to determine the sensitivity of O3 production to NOX and volatile organic carbon concentrations at several ground stations. Primary aerosol parameters (i.e., direct emissions) such as aerosol black carbon showed a distinct diurnal variation with maxima at about 0000 and 0800 central European summer time (CEST), in contrast to secondary aerosol parameters such as sulfate and nitrate. Aerosol number size distributions were measured under ambient conditions as well as after conditioning with volatility and hygroscopicity systems. A mode at d = 20–30 nm in the number concentration was found at 0800 CEST and exhibited high volatility at 110°C (∼80% volume lost upon heating) but no hygroscopic behavior. Based on these measurements, small particles (d < 40 nm) are thought to consist mainly of hydrophobic particulate organic matter, rather than soot or H2SO4 aerosols. Two distinct hygroscopic modes with average growth factors d/d0 ∼ 1.02 and 1.21–1.28 were found for particles with dry (relative humidity of <30%) diameters d0 = 50–200 nm. Submicrometer aerosols exhibited lower volatility at the rural than at the urban site, which is attributed to additional particulate mass produced by secondary particle formation.

Sensitivity of photooxidant production in the Milan Basin: An overview of results from a EUROTRAC-2 Limitation of Oxidant Production field experiment : LOOP ozone production in the Po Valley, Italy: The PIPAPO field experiment

Abstract: [1] The international field experiment Pianura Padana Produzione di Ozono (PIPAPO) studying the volatile organic compounds (VOC)/NO x ozone production sensitivity took place in May and June 1998 downwind of the metropolitan area of Milan. The project was embedded in the framework of the EUROTRAC-2 Limitation of Oxidant Production (LOOP) subproject. Several ground stations between the city of Milan and the Alps north of Milan combined with airborne measurements delivered a comprehensive data set suitable for observation-based analysis and validation of chemical transport models. The special section devoted to the PIPAPO project contains 10 contributions on the characterization of secondary aerosol formation, and radical precursors, and the characterization of ozone production sensitivity based on field measurements and numerical model simulations. In this paper the scientific background and the major objectives of PIPAPO are described. An overview of the field measurement program, the study site, and the meteorological conditions prevailing during the experiment are given. A general conclusion of the combined results is that under typical summer conditions in the Milan area with clear skies, low wind speeds and high temperatures, the transition from VOC- to NO x -sensitive ozone production occurs a few tens of kilometers downwind of the strongest emission sources in the city of Milan.

Sensitivity of photooxidant production in the Milan Basin: An overview of results from a EUROTRAC-2 Limitation of Oxidant Production field experiment

Abstract: [1] The international field experiment Pianura Padana Produzione di Ozono (PIPAPO) studying the volatile organic compounds (VOC)/NOx ozone production sensitivity took place in May and June 1998 downwind of the metropolitan area of Milan. The project was embedded in the framework of the EUROTRAC-2 Limitation of Oxidant Production (LOOP) subproject. Several ground stations between the city of Milan and the Alps north of Milan combined with airborne measurements delivered a comprehensive data set suitable for observation-based analysis and validation of chemical transport models. The special section devoted to the PIPAPO project contains 10 contributions on the characterization of secondary aerosol formation, and radical precursors, and the characterization of ozone production sensitivity based on field measurements and numerical model simulations. In this paper the scientific background and the major objectives of PIPAPO are described. An overview of the field measurement program, the study site, and the meteorological conditions prevailing during the experiment are given. A general conclusion of the combined results is that under typical summer conditions in the Milan area with clear skies, low wind speeds and high temperatures, the transition from VOC- to NOx-sensitive ozone production occurs a few tens of kilometers downwind of the strongest emission sources in the city of Milan.

Sensitivity of ozone production derived from field measurements in the Italian Po basin : LOOP ozone production in the Po Valley, Italy: The PIPAPO field experiment

Abstract: [1] Ozone, nitric oxides, and total reactive nitrogen, measured at two stations in the vicinity of Milan, Italy, are used to infer information on the reactive organic gas (ROG) versus NO x sensitivity of ozone production. At the rural station, ozone production per NO x consumed is less efficient when the advected air masses originate from Milan. This lower efficiency is indicative of ROG-sensitive ozone production. Furthermore, the indicator variables H 2 O 2 /NO z , HCHO/NO y , and O 3 /NO z are applied to both sets of data. The data set with lower ozone production efficiency consistently shows lower indicator values. These findings corroborate the interpretation that ozone production within the Po basin has a tendency to be NO x sensitive and that ROG-sensitive ozone production is only possible within the Milan urban plume. Important links to model calculations for the domain are highlighted.

Sensitivity of ozone production derived from field measurements in the Italian Po basin

Abstract: [1] Ozone, nitric oxides, and total reactive nitrogen, measured at two stations in the vicinity of Milan, Italy, are used to infer information on the reactive organic gas (ROG) versus NOx sensitivity of ozone production. At the rural station, ozone production per NOx consumed is less efficient when the advected air masses originate from Milan. This lower efficiency is indicative of ROG-sensitive ozone production. Furthermore, the indicator variables H2O2/NOz, HCHO/NOy, and O3/NOz are applied to both sets of data. The data set with lower ozone production efficiency consistently shows lower indicator values. These findings corroborate the interpretation that ozone production within the Po basin has a tendency to be NOx sensitive and that ROG-sensitive ozone production is only possible within the Milan urban plume. Important links to model calculations for the domain are highlighted.

Characterization of the photooxidant formation in the metropolitan area of Milan from aircraft measurements : LOOP ozone production in the Po Valley, Italy: The PIPAPO field experiment

Abstract: [1] Measurements of O 3 , NO 2 , HCHO, H 2 O 2 , and hydrocarbons were performed on MetAir's airborne sampling platform in the area of Milan within the Italian Po Basin. The objective was to characterize the O 3 formation process and its dependence on ambient levels of NO x and volatile organic compounds (VOCs). In the metropolitan area and downwind a regional plume developed of the order of 50 km wide and atop of this an urban plume 10-15 km wide. In the Po Basin, ozone mixing ratios were around 80-130 ppb, while in the regional plume, ozone levels were 15-30 ppb higher. In addition, the urban plume adds another 10-30 ppb. Maximum afternoon ozone mixing ratios reached 195 ppb. Downwind of Milan, ozone strongly increased (0.6-1 ppb km -1 ), whereas peroxide usually decreased. An observation based budget analysis of the photooxidant formation yielded chemical ozone production rates between 5 and 15 ppb h -1 , occasionally exceeding 20 ppb h -1 . The chemical peroxide production rate was negligible in the urban and regional plume and small in the adjacent areas. Ozone production rates derived from an observation-driven steady state model were mostly lower than calculated from the budget approach, while peroxide production rates were negligible. Missing VOCs due to incomplete sampling might be responsible for the discrepancies between the model and measurements. The analysis indicates VOC-sensitive ozone formation within and downwind of the metropolitan area of Milan. This would still be the result of the model calculation in the case of doubled VOC levels.

Climatology of ozone transport from the free troposphere into the boundary layer south of the Alps during North Foehn

Abstract: [1] In the Swiss Alpine foothills south of the Alps, on the northern edge of the Po Basin of Italy, 10 years of ozone data were analyzed to deduce the climatological influence of North Foehn on the boundary layer concentrations of ozone. It was found that northerly winds are connected with low relative humidity and small differences in potential temperature between the Jungfraujoch (3600 m above sea level (asl)) and Mendrisio (350 m asl) and therefore are primarily caused by North Foehn. This confirms that North Foehn transports air masses from high altitudes above the Alpine crest down into the lower boundary layer. On days with North Foehn the yearly cycles of ozone are very similar at Jungfraujoch and Mendrisio, around 40 ppb in winter and 60 ppb in summer. Compared to the whole data set in Mendrisio, these values are, at comparable temperatures, high in winter and very low in summer, when ozone concentrations often exceed 120 ppb during daytime. In contrast, during nighttime the North Foehn causes an enhancement of the ozone concentration near the ground during the whole year. The frequencies of the North Foehn events are 4–6% from May to October and 6–10% from November to April. Thus North Foehn is an important process for the ozone concentrations in the Po Basin.

Characterization of the photooxidant formation in the metropolitan area of Milan from aircraft measurements

Abstract: [1] Measurements of O3, NO2, HCHO, H2O2, and hydrocarbons were performed on MetAir's airborne sampling platform in the area of Milan within the Italian Po Basin. The objective was to characterize the O3 formation process and its dependence on ambient levels of NOx and volatile organic compounds (VOCs). In the metropolitan area and downwind a regional plume developed of the order of 50 km wide and atop of this an urban plume 10–15 km wide. In the Po Basin, ozone mixing ratios were around 80–130 ppb, while in the regional plume, ozone levels were 15–30 ppb higher. In addition, the urban plume adds another 10–30 ppb. Maximum afternoon ozone mixing ratios reached 195 ppb. Downwind of Milan, ozone strongly increased (0.6–1 ppb km−1), whereas peroxide usually decreased. An observation based budget analysis of the photooxidant formation yielded chemical ozone production rates between 5 and 15 ppb h−1, occasionally exceeding 20 ppb h−1. The chemical peroxide production rate was negligible in the urban and regional plume and small in the adjacent areas. Ozone production rates derived from an observation-driven steady state model were mostly lower than calculated from the budget approach, while peroxide production rates were negligible. Missing VOCs due to incomplete sampling might be responsible for the discrepancies between the model and measurements. The analysis indicates VOC-sensitive ozone formation within and downwind of the metropolitan area of Milan. This would still be the result of the model calculation in the case of doubled VOC levels.

Unexpected vertical profiles over complex terrain due to the incomplete formulation of transport processes in the SAIMM/UAM-V air quality model

Abstract: The air quality model (AQM) package SAIMM/UAM-V Systems Applications International Mesoscale Model/Urban Airshed Model with Variable grid was applied to a domain with complex topography including Switzerland. The output pressure at a fixed height above sea level generated by one of the SAIMM post-processor showed unexpected variations of about 40 hPa over the domain. We found that these fluctuations were caused by an incorrect definition of the pressure reference height. After the change of this definition, realistic pressure values were obtained. Using the correct pressure field as input, we simulated the 3-dimensional mixing ratios of pollutants with UAM-V for the summer smog period of July 28–29, 1993. The values tended to increase with height above the surface and with surface elevation above sea level. The Swiss topography was mirrored in the mixing ratio fields. By using CO as a quasi-inert tracer, it became evident that transport phenomena such as advection and diffusion did not consider expansions or compressions due to pressure and temperature variations. After the conversion of the concentrations to a common reference pressure and temperature before the calculation of transport, these strange topographic features in the mixing ratios vanished completely. The CO mixing ratio was underestimated by 10–15 ppb (7–10%) in the lowest layer over the Swiss Plateau due the omission of compression or expansion. For O 3 and NO 2 , the differences were 0–5 ppb (0–10%) and 0–0.2 ppb (0–15%), respectively.

Applicability of Indicator-Based Approach to Assess Ozone Sensitivities: A Model Study in Switzerland

Abstract: The threshold values of indicator species and ratios delineating the transition between NOx and VOC sensitivity of ozone formation are assumed to be universal by various investigators. However, our previous studies suggested that these values may vary according to the locations and conditions. In this study, threshold values derived from various model simulations in Switzerland by the UAM Model are examined using a new approach. Possible definitions for the distinction of NOx and VOC sensitive 03 production regimes are given. The dependence of the threshold values of indicators and indicator ratios such as NOy, 03/NOz, HCHO/NOy, and H202/HN03 on the definition of NOx and VOC sensitivity is discussed. Then the variation of threshold values under two different meteorological and emission conditions is examined to check how the model responds to changes in environmental conditions. Both perturbed cases - i.e. low emissions and less-stagnant meteorology - lead to similar shifts in threshold values towards more NOx sensitive chemistry. 03/NOz and HCHO/NOy are predicted to be unsatisfactory as indicators. Although H202/HN03 provides a good separation of NOx and VOC sensitive regimes, threshold values for this indicator ratio are affected by emission and meteorological perturbations studied in this work.

Urban and rural aerosol characterization of summer smog events during the PIPAPO field campaign in Milan, Italy : LOOP ozone production in the Po Valley, Italy: The PIPAPO field experiment

Abstract: [1] A comprehensive range of aerosol parameters was measured at an urban and a rural site in the Milan, Italy metropolitan region during summer smog events in summer 1998. Measurements were performed as part of the Pianura Padana Produzione di Ozono (PIPAPO) field campaign to determine the sensitivity of O 3 production to NO X and volatile organic carbon concentrations at several ground stations. Primary aerosol parameters (i.e., direct emissions) such as aerosol black carbon showed a distinct diurnal variation with maxima at about 0000 and 0800 central European summer time (CEST), in contrast to secondary aerosol parameters such as sulfate and nitrate. Aerosol number size distributions were measured under ambient conditions as well as after conditioning with volatility and hygroscopicity systems. A mode at d = 20-30 nm in the number concentration was found at 0800 CEST and exhibited high volatility at 110°C (∼80% volume lost upon heating) but no hygroscopic behavior. Based on these measurements, small particles (d < 40 nm) are thought to consist mainly of hydrophobic particulate organic matter, rather than soot or H 2 SO 4 aerosols. Two distinct hygroscopic modes with average growth factors d/d 0 ∼ 1.02 and 1.21-1.28 were found for particles with dry (relative humidity of <30%) diameters do = 50-200 nm. Submicrometer aerosols exhibited lower volatility at the rural than at the urban site, which is attributed to additional particulate mass produced by secondary particle formation.

Effects Of Some Model Parameters On The Indicator Values For Ozone Production Sensitivity

Abstract: It is very important for ozone control strategies to know whether ozone production is sensitive to NOX or VOC emissions to avoid unwished effects due to the nonlinearity of the photochemistry, Indicator species derived from measurements can be useful to assess the sensitivity. However, indicator values can vary under certain circumstances. In an earlier model study, we showed that the thresholds of indicators are affected by the wind fields and emissions. In this work, we studied the variability of indicator thresholds further by model simulations. A 3-day episode in July 1993 in Switzerland was modelled using the meteorological model SAIMM and the photochemical model CAMX, The studied indicators are NOY, HCHO/NOY, 03/NOZ and H202/HN03. The effects of some factors such as the definition of sensitivity, boundary concentrations and degree of emission reductions are discussed.