Ice and snow in the Central Andes contain significant amounts of light-absorbing particles such as black carbon. The consequent accelerated melting of the cryosphere is not only a threat from a ...climate perspective but also for water resources and snow-dependent species and activities, worsened by the mega-drought affecting the region since the last decade. Given its proximity to the Andes, emissions from the Metropolitan Area of Santiago, Chile, are believed to be among the main contributors to deposition on glaciers. However, no evidence backs such an assertion, especially given the usually subsident and stable conditions in wintertime, when the snowpack is at its maximum extent. Based on high-resolution chemistry-transport modeling with WRF-CHIMERE, the present work shows that, for the month of July 2015, up to 40% of black carbon dry deposition on snow or ice covered areas in the Central Andes downwind from the Metropolitan area can be attributed to emissions from Santiago. Through the analysis of aerosol tracers we determine (i) that the areas of the Metropolitan Area where emissions matter most when it comes to export towards glaciers are located in Eastern Santiago near the foothills of the Andes, (ii) the crucial role of the network of Andean valleys that channels pollutants up to remote locations near glaciers, following gentle slopes. A direct corollary is that severe urban pollution, and deposition of impurities on the Andes, are anti-correlated phenomena. Finally, a two-variable meteorological index is developed that accounts for the dynamics of aerosol export towards the Andes, based on the zonal wind speed over the urban area, and the vertical diffusion coefficient in the valleys close to ice and snow covered terrain. Numerous large urban areas are found along the Andes so that the processes studied here can shed light on similar investigations for other glaciers-dependent Andean regions.
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•Modeling shows that BC emissions from Santiago account for 40% of deposition on central Andean glaciers.•Channeling through valleys and vertical export deep into the canyons is the main pathway.•Urban pollution and glaciers darkening by soot are anti-correlated phenomena.•This work provides insight for systematic studies regarding urban pollution along the Andes.
This study delves into the photochemical atmospheric changes reported globally during the pandemic by analyzing the change in emissions from mobile sources and the contribution of local meteorology ...to ozone (O3) and particle formation in Bogotá (Colombia), Santiago (Chile), and São Paulo (Brazil). The impact of mobility reductions (50%–80%) produced by the early coronavirus-imposed lockdown was assessed through high-resolution vehicular emission inventories, surface measurements, aerosol optical depth and size, and satellite observations of tropospheric nitrogen dioxide (NO2) columns. A generalized additive model (GAM) technique was also used to separate the local meteorology and urban patterns from other drivers relevant for O3 and NO2 formation. Volatile organic compounds, nitrogen oxides (NOx), and fine particulate matter (PM2.5) decreased significantly due to motorized trip reductions. In situ nitrogen oxide median surface mixing ratios declined by 70%, 67%, and 67% in Bogotá, Santiago, and São Paulo, respectively. NO2 column medians from satellite observations decreased by 40%, 35%, and 47%, respectively, which was consistent with the changes in mobility and surface mixing ratio reductions of 34%, 25%, and 34%. However, the ambient NO2 to NOx ratio increased, denoting a shift of the O3 formation regime that led to a 51%, 36%, and 30% increase in the median O3 surface mixing ratios in the 3 respective cities. O3 showed high sensitivity to slight temperature changes during the pandemic lockdown period analyzed. However, the GAM results indicate that O3 increases were mainly caused by emission changes. The lockdown led to an increase in the median of the maximum daily 8-h average O3 of between 56% and 90% in these cities.
Despite the decline in partially (PM
10
) and fully (PM
2.5
) inhalable particles observed in recent decades, Santiago in Chile shows high levels of particle and ozone pollution. Attainment plans ...have emphasized measures aimed at curbing primary and, to some extent, secondary particles, but little attention has been paid to photochemical pollution. Nevertheless, ozone hourly mixing ratios in Eastern Santiago regularly exceed 110 ppbv in summer, and in winter maximum mixing ratios often reach 90 ppbv. Moreover, the sum of ozone and nitrogen dioxide shows an increasing trend of more than 3.5 ppbv per decade at 5 out of 8 stations. This trend is driven by increasing NO
2
, possibly associated with increasing motorization but also with changes in photochemistry. To estimate the fraction of secondary particles in PM
2.5
and due to the lack of long-term speciation data for particles, we use carbon monoxide as a proxy of primary particles and ozone daily maxima as a proxy for secondary particle formation. We find a growing fraction of secondary particles due to an increase in the oxidizing capacity of Santiago's atmosphere. This stresses the need for new curbing measures to tackle photochemical pollution. This is particularly needed in the context of a changing climate.
Subregional inversion of North African dust sources Escribano, Jerónimo; Boucher, Olivier; Chevallier, Frédéric ...
Journal of geophysical research. Atmospheres,
27 July 2016, Letnik:
121, Številka:
14
Journal Article
Recenzirano
Odprti dostop
The emission of mineral dust aerosols in arid and semiarid regions is a complex process whose representation in atmospheric models remains crude, due to insufficient knowledge about the aerosol ...lifting process itself, the lack of global data on soil characteristics, and the impossibility for the models to resolve the fine‐scale variability in the wind field that drives some of the dust events. As a result, there are large uncertainties in the total emission flux of mineral dust, its natural variability at various timescales, and the possible contribution from anthropogenic land use changes. This work aims for estimating dust emissions and reduces their uncertainty over the Sahara desert and the Arabian Peninsula—the largest dust source region of the globe. We use a data assimilation approach to constrain dust emission fluxes at a monthly resolution for 18 subregions. The Moderate Resolution Imaging Spectroradiometer satellite‐derived aerosol optical depth is assimilated in a regional configuration of a general circulation model coupled to an aerosol model. We describe this data assimilation system and apply it for 1 year, resulting in a total mineral dust emissions flux estimate of 2900 Tg yr−1 over the Sahara desert and the Arabian Peninsula for the year 2006. The analysis field of aerosol optical depth shows an improved fit relative to independent Aerosol Robotic Network measurements as compared to the model prior field.
Key Points
Satellite AOD retrievals are assimilated for a 1 year period over North Africa
Strengths and weaknesses of the source inversion system are discussed
Retrievals of fine and coarse AOD over land and ocean with reduced biases would further improve emission estimates
The effective radiative forcings (including rapid adjustments) and feedbacks associated with an instantaneous quadrupling of the preindustrial CO2 concentration and a counterbalancing reduction of ...the solar constant are investigated in the context of the Geoengineering Model Intercomparison Project (GeoMIP). The forcing and feedback parameters of the net energy flux, as well as its different components at the top‐of‐atmosphere (TOA) and surface, were examined in 10 Earth System Models to better understand the impact of solar radiation management on the energy budget. In spite of their very different nature, the feedback parameter and its components at the TOA and surface are almost identical for the two forcing mechanisms, not only in the global mean but also in their geographical distributions. This conclusion holds for each of the individual models despite intermodel differences in how feedbacks affect the energy budget. This indicates that the climate sensitivity parameter is independent of the forcing (when measured as an effective radiative forcing). We also show the existence of a large contribution of the cloudy‐sky component to the shortwave effective radiative forcing at the TOA suggesting rapid cloud adjustments to a change in solar irradiance. In addition, the models present significant diversity in the spatial distribution of the shortwave feedback parameter in cloudy regions, indicating persistent uncertainties in cloud feedback mechanisms.
Key Points
Analysis of 10 ESMs focusing on energy balance at the TOA and surface
Climate feedback's distribution is almost identical for both forcing mechanisms
ERF at the TOA suggest a rapid cloud adjustment to change in solar irradiance
Black carbon transport from the Santiago Metropolitan Area, Chile, up to the adjacent Andes Cordillera and its glaciers is of major concern. Its deposition accelerates the melting of the snowpack, ...which could lead to stress on water supply in addition to climate feedback. A proposed pathway for this transport is the channelling through the network of canyons that connect the urban basin to the elevated summits, as suggested by modelling studies, although no observations have validated this hypothesis so far. In this work, atmospheric measurements from a dedicated field campaign conducted in winter 2015, under severe urban pollution conditions, in Santiago and the Maipo canyon, southeast of Santiago, are analysed. Wind (speed and direction) and particulate matter concentrations measured at the surface and along vertical profiles, demonstrate intrusions of thick layers (up to 600 m above ground) of urban black carbon deep into the canyon on several occasions. Transport of PM down-valley occurs mostly through shallow layers at the surface except in connection with deep valley intrusions, when a secondary layer in altitude with return flow (down-valley) at night is observed. The transported particulate matter is mostly from the vicinity of the entrance to the canyon and uncorrelated to concentrations observed in downtown Santiago. Reanalyses data show that for 10% of the wintertime days, deep intrusions into the Maipo canyon are prevented by easterly winds advecting air pollutants away from the Andes. Also, in 23% of the cases, intrusions proceed towards a secondary north-eastward branch of the Maipo canyon, leaving 67% of the cases with favourable conditions for deep penetrations into the main Maipo canyon. Reanalyses show that the wind directions associated to the 33% anomalous cases are related to thick cloud cover and/or the development of coastal lows.
•Transport of urban BC in deep, dense layers is found far into an Andean canyon.•A return flow in an elevated secondary layer is observed with valley intrusions.•Air pollutants transported into the canyon are from the vicinity of its entrance.•Climatologically, intrusions into the canyon occur 90% of the wintertime days.•Cloud cover prevents air masses flow towards the Andes 10% of winter days.
The city of Santiago experiences extreme pollution events during winter due to particulate matter and the associated health impact depends on the exposure to this pollutant, particularly to PM2.5. We ...present and apply a method that estimates the exposure of users of the public transport system of Santiago by combining smart card mobility data with measured surface concentrations from the monitoring network of Santiago and simulated concentrations by the CHIMERE model. The method was applied between July 20th and 24th of 2015 to 105,588 users corresponding to 12% of the frequent users of the public transport system and approximately 2% of the total population of Santiago. During those five days, estimated exposure based on measured concentrations varied between 44 and 75 μg/m3 while exposure based on simulated concentrations varied between 45 and 89 μg/m3. Furthermore, including socioeconomic conditions suggests an inverse relationship between exposure and income when measured concentrations are used, i.e. the lower the income the higher the exposure, whereas no such relationship is observed when using simulated concentrations. Although only exposure to PM2.5 was considered in this study, the method can also be applied to estimate exposure to other urban pollutant such as ozone.
•Exposure to PM2.5 based on observations varied between 44 and 75 μg/m3.•Exposure based on simulated concentrations varied between 45 and 89 μg/m3.•Observations suggest inverse relationship between exposure and income.•No relation between exposure and income based on simulations.•Relation between exposure and income is introduced with use of I/O ratios.
Santiago (33.5°S, 70.5°W), the capital of Chile, is frequently affected by extreme air pollution events during wintertime deteriorating air quality (AQ) and thus affecting the health of its ...population. Intense residential heating and on-road transport emissions combined with poor circulation and vertical mixing are the main factors responsible for these events. A modelling system composed of a chemistry-transport model (CHIMERE) and a meteorological model (WRF) was implemented to assess the AQ impacts of residential and transportation sources in the Santiago basin. A two-week period of July 2015 with various days with poor AQ was simulated focusing on the impact on AQ with respect to fully inhalable particles (PM2.5) and nitrogen oxides (NOX). Three emission scenarios, within the range of targeted reductions of the decontamination plan of Santiago, were tested; namely 50% reduction of residential emission, 50% reduction of transport emissions and the combination of both. An additional scenario decreasing transport emissions in 10% was carried out to examine whether a linear dependence of surface concentrations on changes in emissions exists.
The system was validated against surface and vertically resolved meteorological measurements. The model reproduces the daily surface concentration variability from the AQ monitoring network of Santiago. However, the model not fully captures the emissions variations inferred from the observations which may be due to missing sources such as resuspension of dust.
Results show that, during the period studied, although both residential and transportation sources contribute to observed AQ levels in Santiago, reducing transport emissions is more effective in terms of reducing the number of days with pollution events than decreasing residential combustion. This difference in impact is largely due to the spatial distribution of the emission sources. While most of the residential combustion is emitted in the outskirts of the city, most of the transport emissions occur within the city, where most of the stations from AQ monitoring network of Santiago are located. As can be expected, the largest improvement of AQ in Santiago is achieved by the combined reduction of emissions in both sectors. Sensitivity analysis with 10% reduction in transport emissions reveals a linear behavior between emissions and concentrations for NOX and approximate linear behavior for PM2.5. The absence of secondary aerosols formation and dust resuspension in the current simulation could explain this deviation from linearity for fine particles. Nevertheless, it suggests that the results can be used for mitigation policies with emissions reductions below the 50% used in this study.
•WRF-CHIMERE modelling system simulates dispersion patterns of particulate matter and gaseous pollutants in Santiago.•CHIMERE model reproduces the daily variability of PM2.5 and NOX concentrations in Santiago.•Impact of reduction in residential and/or transport emissions on PM2.5 concentration is explored.•Mitigating transport emissions is more effective than decreasing residential emissions in reducing the number of events.
Abstract
The population of Santiago, Chile, experiences air pollution above global health guidelines that is attributable in part to large anthropogenic emissions. This is compounded by geographic ...features and meteorological conditions that are prone to pollution accumulation as well as secondary pollution production. In recent years, there have been improvements in air quality; however, the future of air pollution in Santiago remains unclear due to its growing population and increased vehicle use. Mitigation efforts can be supported by characterizing sources of air pollution and estimating how changes in emissions could affect air quality in future years. In this study, we conduct simulations using a chemical transport model (GEOS‐Chem) and perform adjoint calculations to characterize the relationship between health impacts associated with exposure to PM
2.5
, O
3
, and NO
2
and anthropogenic emissions. We incorporate model updates in a new nested domain simulation over Central South America including local and regional anthropogenic emissions inventories for Chile. We estimate that 2,490 (1,360, 4,060) PM
2.5
‐ and O
3
‐related premature deaths and 5,350 (1,320, 11,330) NO
2
‐related new pediatric asthma cases were associated with pollution exposure in Santiago in 2015 and that a majority of these health impacts were attributable to anthropogenic emissions. We identify emissions from transportation, energy generation, and residential combustion as the leading contributors to these health impacts. Additionally, we estimate that Chile's commitment to attain carbon neutrality by 2050 could result in benefits in Santiago of 3,230 (1,240, 7,160) avoided deaths and 2,590 (640, 5,500) avoided pediatric asthma cases in 2050 compared to business‐as‐usual emissions.
Plain Language Summary
The population of Santiago, Chile is exposed to unhealthy levels of air pollution that are above global health guidelines. Emissions originating from human activities contribute to this poor air quality. In recent years, air quality has improved; however, the future of air pollution in Santiago remains unclear due to a growing population and increased vehicle use. In this study we use a chemical transport model framework to identify how emissions affect air pollution in Santiago. We estimate that 2,490 premature deaths and 5,350 new asthma cases were associated with pollution in Santiago in 2015 and found that much of these health impacts were associated with emissions from transportation, power‐plants, and residential combustion. Additionally, we estimate that Chile's commitment to the Paris Climate Agreement by 2050 could avoid 3,230 deaths and 2,590 new asthma cases in 2050.
Key Points
We estimate that Chile's commitment to attain carbon neutrality could avoid 3,230 premature deaths and 2,590 pediatric asthma cases in 2050
We identify transportation, energy and residential emissions as the leading contributors to Santiago's air pollution related health impacts
We incorporate model updates in a new nested domain over South America for GEOS‐Chem and find comparable performance to other simulations
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