Cécile Honore

Photochemical regimes in urban atmospheres: The influence of dispersion

The dependence on emissions of urban photochemistry is studied under meteorological conditions fayouting pollution episodes. We use a simple chemistry-transport model, yet realistic with respect to the main chemical and physical processes and considering the diurnal cycle of forcing parameters. For a range of NOz emissions, the diurnal equilibria of the system are searched for. It is shown that, if the photochem-istry over a large city mostly exhibits a negative sensitivity to NOx emissions, there are meteorological situations under which it becomes (positively) NOx sensitive, the dispersion time scale being a key parameter. We show that, under specific conditions, the urban atmosphere might reach two very different diurnal equilibria. We demonstrate that the sensitivity to NOz emissions evolves with time: under specific conditions, the urban atmosphere becomes NOz sensitive within two days.

Low and high NOx chemical regimes in an urban environment

Abstract In this work, a chemistry-transport box model, derived from the CHIMERE model, is used in order to examine the existence of different chemical regimes, known as the low NO x and high NO x regimes, and the existence of multiple equilibria in an urban environment. From an operationnal point of view, the existence of chemical regimes of low NO x and high NO x type and of multiple equilibria is of paramount importance for the effectiveness of the air quality policies. For the urban photochemical system under consideration, we determine the equilibria associated with primary pollutant emissions varying in a range of values typical of the parisian urban area. The influence of several variables on these regimes is also studied, through the following parameters: wind speed, upstream ozone concentration, cloudiness.

Forecasting Urban Air Quality over Cities by Statistical Adaptation of Deterministic Chemistry Transport Model Outputs

Within the European CITEAIR II project, deterministic and statistical forecasting approaches are combined to forecast an air quality index in European cities. The idea is to benefit both from chemistry-transport modelling, which provides large scale information about the evolution of photo-oxidizing pollution, and from local measurements of pollution levels. In addition to usual predictors, numerical outputs from the PREV’AIR system are used in multiple linear regression to forecast O3, NO2 and PM10 concentrations at urban stations and derive the city-level forecast index. The methodology was applied to Rotterdam, Sevilla and Prague with 2008 data set and validated against 2009 one. It can be extended to any European city involved in CITEAIR II, in particular to those having not implemented their own forecasting system yet.