Influence of synoptic and local circulations on high ozone concentration episode

Poor air quality is the cause of 8,1 millions deaths annually. The Metropolitan Area of São Paulo is home to more than 20 million people and frequently presents poor air quality, creating a concerning public health issue. Its location presents some unique features: 1) despite being 700 meters above sea level, it is frequently influenced by the sea breeze circulation that comes up the coastal escarpment and the predominant wind direction in Sao Paulo, influenced by the sea breeze, is from southeast; 2) considering a southeast-northewest direction, the sea breze circulation comes from a coastal area (in the city of Cubatao) that contains an industrial complex and the largest harbor in Latin America (in the city of Santos), passes through Sao Paulo and eventually reaches another metropolitan area, centered in the city of Campinas, that also has relevant industrial activities; 3) the three metropolitan areas also form a very active economic axes that presents an intense road traffic among the 3 areas and combine approximately 25 million inhabitants. The present work analyses the influence of synoptic and mesoscale atmospheric circulations, such as sea breeze, urban heat island, cold fronts, and topographic influences, on the air quality of the Cubatão-São Paulo-Campinas region, focusing on an acute ozone pollution episode. We use the WRF model with the Single Layer Urban Canopy Model for the meteorological simulations. Air pollutant emissions were simulated using the EDGAR global dataset and MEGAN for biogenic emissions. Traffic emissions were then adjusted using local inventories. Air quality was simulated with CMAQ. The chosen episode was from 3rd to 6th October 2019, during the austral spring, when the pollutant exceeded local air quality standards. Considering NOX, sea breeze helps decrease the concentrations near the surface because of transport and dispersion due to the increased wind speed, but also because sea breeze creates an internal boundary layer. The returning branch of the sea breeze transports polluted air back to the ocean above the internal boundary layer. During pre-frontal conditions, wind is mainly from the northwest, transporting pollutants to the coast, increasing air temperature, and favoring a deeper boundary layer and vertical dispersion. These patterns also delay sea breeze propagation over the plateau and are crucial to a steep increase in ozone concentration. The front passage changes wind direction and increases its velocity, favoring transport from the coast to the continent, augmenting atmospheric instability and vertical dispersion of pollutants. A better understanding of the mechanisms that cause high ozone concentrations is key to forecasting these occurrences and choosing effective measures to prevent them.