Black carbon emission and transport mechanisms to the free troposphere at the La Paz/El Alto (Bolivia) metropolitan area based on the Day of Census (2012)

Abstract

Urban development, growing industrialization, and increasing demand for mobility have led to elevated levels of<br> air pollution in many large cities in Latin America, where air quality standards and WHO guidelines are frequently<br> exceeded. The conurbation of the metropolitan area of La Paz/El Alto is one of the fastest growing urban<br> settlements in South America with the particularity of being located in a very complex terrain at a high altitude.<br> As many large cities or metropolitan areas, the metropolitan area of La Paz/El Alto and the Altiplano region are<br> facing air quality deterioration. Long-term measurement data of the equivalent black carbon (eBC) mass concentrations<br> and particle number size distributions (PNSD) from the Global Atmosphere Watch Observatory<br> Chacaltaya (CHC; 5240m a.s.l., above sea level) indicated a systematic transport of particle matter from the<br> metropolitan area of La Paz/El Alto to this high altitude station and subsequently to the lower free troposphere.<br> To better understand the sources and the transport mechanisms, we conducted eBC and PNSDs measurements<br> during an intensive campaign at two locations in the urban area of La Paz/El Alto from September to November<br> 2012. While the airport of El Alto site (4040m a.s.l.) can be seen as representative of the urban and Altiplano<br> background, the road site located in Central La Paz (3590m a.s.l.) is representative for heavy traffic-dominated<br> conditions. Peaks of eBC mass concentrations up to 5 μgm−3 were observed at the El Alto background site in the<br> early morning and evening, while minimum values were detected in the early afternoon, mainly due to thermal<br> convection and change of the planetary boundary layer height. The traffic-related eBC mass concentrations at<br> the road site reached maximum values of 10–20 μgm−3. A complete traffic ban on the specific Bolivian Day of<br> Census (November 21, 2012) led to a decrease of eBC below 1 μgm−3 at the road site for the entire day.<br> Compared to the day before and after, particle number concentrations decreased by a factor between 5 and 25<br> over the particle size range from 10 to 800 nm, while the submicrometer particle mass concentration dropped by<br> approximately 80%. These results indicate that traffic is the dominating source of BC and particulate air pollution<br> in the metropolitan area of La Paz/El Alto. In general, the diurnal cycle of eBC mass concentration at the<br> Chacaltaya observatory is anti-correlated to the observations at the El Alto background site. This pattern indicates<br> that the traffic-related particulate matter, including BC, is transported to higher altitudes with the developing<br> of the boundary layer during daytime. The metropolitan area of La Paz/El Alto seems to be a significant<br> source for BC of the regional lower free troposphere. From there, BC can be transported over long distances and<br> exert impact on climate and composition of remote southern hemisphere.