Abstract
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.
Highlights
Natural and anthropogenic aerosol particle emissions significantly influence the global and regional climate by absorbing and scattering solar radiation (Charlson et al, 1992; Boucher et al, 2013; Kuniyal and Guleria, 2018)
The extended multiyear record allows us to study the properties of aerosol particles for different air mass types, during wet and dry seasons, covering periods when the site was affected by biomass burning in the Bolivian lowlands and the Amazon Basin
Using the wavelength dependence of aerosol particle optical properties, we discriminated between contributions from natural and anthropogenic emissions according to seasons and local circulation
Summary
Natural and anthropogenic aerosol particle emissions significantly influence the global and regional climate by absorbing and scattering solar radiation (Charlson et al, 1992; Boucher et al, 2013; Kuniyal and Guleria, 2018). Hamburger et al (2013) present long-term (3-year) ground-based measurements at Pico Espejo (4765 m a.s.l.), Venezuela They show the influences of the local Venezuelan savannah and of the Amazon Basin biomass burning emissions, mainly during the dry period and in the whole tropospheric layer. This information has to be treated with caution since source influences are expected to be homogeneous and have been reported from several regions. Back-trajectory analysis and optical wavelength dependencies are presented to identify the impacts of local and regional aerosol sources
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