Megacity Levels of Black Carbon in the Port of Valparaiso, Chile: The Role of Synoptic and Mesoscale Circulations

Measurements of equivalent black carbon (eBC) have been made during three seasons (Winter, Spring and Summer) in Valparaiso, a coastal city that is located in the subtropics and is the largest commercial port in Chile. In addition to the ships in the harbor and the trucks that service the shipping industry, the primary public transport in the city is the bus system that uses diesel as its primary fuel source. Adding to the emissions of black and brown carbon (BC, BrC) from mobile sources is an oil refinery approximately 30 km to the north and in the winter many residences use wood burning as a primary source of heating. In winter the wind speeds are low, the boundary layer is shallow and there are frequent nighttime thermal inversions. The meteorology, coupled with the topography of very steep hillsides surrounding the bay, leads to episodes when the maximum eBC, measured with the filter-based Particle Soot Aerosol Photometer (PSAP, Bond et al., 1999) and the photoacoustic PAX technique (Retama et al., 2015), often exceeds 10 µg m-3 . As shown in the figure, the median and 75% quantiles of eBC mass concentration are lowest in the summer and highest in the winter, reflecting the stronger winds and deeper boundary layer in the summer and the weaker winds, shallow boundary layer and deep thermal inversions in the winter. An analysis of the mesoscale features of the seasonal meteorology shows that the relative role of local emissions, i.e. from ships, trucks, buses and private vehicles, compared to the emissions from heavy industry like the nearby oil refinery, is dependent on the strength of the land-sea breeze circulation and larger scale passage of high and low pressure systems. The former leads to stagnant conditions that trap the local emissions in the city while the latter brings higher winds and precipitation that remove the pollutants from the urban area. Local authorities do not currently forecast high pollution events in Valparaiso that put the population at risk, especially the elderly and those with respiratory conditions. However, with the results from the measurement campaign and an understanding of how these high levels of BC are modulated by the weather, there is an opportunity to develop forecasting tools that can accurately predict when measures should be taken to either mitigate the high levels of pollution or alert the population of the potential risk. References Bond, T. C. and Bergstrom, R. W. (2006) Light absorption by carbonaceous particles: An investigative review, Aerosol Sci. Technol., 40, 27–67, 2006. Retama, A., Baumgardner, D., Raga, G., McMeeking, G. and Walker, J.W. (2015) Seasonal Trends in Black Carbon Properties and Co-Pollutants in Mexico City, Atmos. Phys. Chem. Disc., 15, 12539–12582, 2015