Biomass burning and urban emission impacts in the Andes Cordillera region based on in situ measurements from the Chacaltaya observatory, Bolivia (5240 m a.s.l.)

Abstract

Abstract. This study documents and analyses a 4-year continuous record of\naerosol optical properties measured at the Global Atmosphere Watch (GAW)\nstation of Chacaltaya (CHC; 5240\u2009m\u2009a.s.l.), in Bolivia. Records of particle light\nscattering and particle light absorption coefficients are used to\ninvestigate how the high Andean Cordillera is affected by both long-range\ntransport and by the fast-growing agglomeration of La Paz–El Alto, located\napproximately 20\u2009km away and 1.5\u2009km below the sampling site. The extended\nmulti-year record allows us to study the properties of aerosol particles for different\nair mass types, during wet and dry seasons, also covering periods when the\nsite was affected by biomass burning in the Bolivian lowlands and the Amazon Basin. The absorption, scattering, and extinction coefficients (median annual\nvalues of 0.74, 12.14, and 12.96\u2009Mm −1 respectively) show a clear\nseasonal variation with low values during the wet season (0.57, 7.94, and\n8.68\u2009Mm −1 respectively) and higher values during the dry season (0.80,\n11.23, and 14.51\u2009Mm −1 respectively). The record is driven by variability\nat both seasonal and diurnal scales. At a diurnal scale, all records of\nintensive and extensive aerosol properties show a pronounced variation\n(daytime maximum, night-time minimum), as a result of the dynamic and\nconvective effects. The particle light absorption, scattering, and extinction\ncoefficients are on average 1.94, 1.49, and 1.55 times higher respectively\nin the turbulent thermally driven conditions than the more stable\nconditions, due to more efficient transport from the boundary layer.\nRetrieved intensive optical properties are significantly different from one\nseason to the other, reflecting the changing aerosol emission sources of\naerosol at a larger scale. Using the wavelength dependence of aerosol particle\noptical properties, we discriminated between contributions from natural (mainly\nmineral dust) and anthropogenic (mainly biomass burning and urban transport\nor industries) emissions according to seasons and local circulation. The\nmain sources influencing measurements at CHC are from the urban area of La Paz–El Alto in the Altiplano and from regional biomass burning in the\nAmazon Basin. Results show a 28\u2009% to 80\u2009% increase in the extinction\ncoefficients during the biomass burning season with respect to the dry\nseason, which is observed in both tropospheric dynamic conditions. From this\nanalysis, long-term observations at CHC provide the first direct evidence\nof the impact of biomass burning emissions of the Amazon Basin and urban\nemissions from the La Paz area on atmospheric optical properties at a remote\nsite all the way to the free troposphere.