Abstract. This study documents and analyses a 4-year continuous record of
aerosol optical properties measured at the Global Atmosphere Watch (GAW)
station of Chacaltaya (CHC; 5240 m a.s.l.), in Bolivia. Records of particle light
scattering and particle light absorption coefficients are used to
investigate how the high Andean Cordillera is affected by both long-range
transport and by the fast-growing agglomeration of La Paz–El Alto, located
approximately 20 km away and 1.5 km below the sampling site. The extended
multi-year record allows us to study the properties of aerosol particles for different
air mass types, during wet and dry seasons, also covering periods when the
site was affected by biomass burning in the Bolivian lowlands and the Amazon Basin. The absorption, scattering, and extinction coefficients (median annual
values of 0.74, 12.14, and 12.96 Mm−1 respectively) show a clear
seasonal variation with low values during the wet season (0.57, 7.94, and
8.68 Mm−1 respectively) and higher values during the dry season (0.80,
11.23, and 14.51 Mm−1 respectively). The record is driven by variability
at both seasonal and diurnal scales. At a diurnal scale, all records of
intensive and extensive aerosol properties show a pronounced variation
(daytime maximum, night-time minimum), as a result of the dynamic and
convective effects. The particle light absorption, scattering, and extinction
coefficients are on average 1.94, 1.49, and 1.55 times higher respectively
in the turbulent thermally driven conditions than the more stable
conditions, due to more efficient transport from the boundary layer.
Retrieved intensive optical properties are significantly different from one
season to the other, reflecting the changing aerosol emission sources of
aerosol at a larger scale. Using the wavelength dependence of aerosol particle
optical properties, we discriminated between contributions from natural (mainly
mineral dust) and anthropogenic (mainly biomass burning and urban transport
or industries) emissions according to seasons and local circulation. The
main 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
Amazon Basin. Results show a 28 % to 80 % increase in the extinction
coefficients during the biomass burning season with respect to the dry
season, which is observed in both tropospheric dynamic conditions. From this
analysis, long-term observations at CHC provide the first direct evidence
of the impact of biomass burning emissions of the Amazon Basin and urban
emissions from the La Paz area on atmospheric optical properties at a remote
site all the way to the free troposphere.
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)