Influence of surface and subsidence thermal inversion on PM2.5 and black carbon concentration

The local meteorological factors of the cold season nocturnal particulate matter (PM) air quality problem in the western urban area of Santiago City are documented and analyzed by means of three years of multi-level temperature and turbulence measurements performed in-situ on a 30 m meteorological tower. For the 20-23 LT period in which the highest PM concentrations commonly occur, high prevalence of low wind and low vertical turbulence conditions are found. With wind speeds less than 1 m s -1 , the frequency of calms and the intensity of meandering increase, although the highest PM concentrations are associated more to meandering than to purely calm conditions. In terms of the stability of the near surface layer, while there is a general association between high PM concentrations and high stability, the most extreme PM event in the period occurred with an intermediate value of stability probably affected by the presence of mid-tropospheric clouds. Using a nocturnal vertical height scale based on the stability and the surface turbulent heat flux to compute a nocturnal ventilation factor (VF), a marked inverse relationship between VF and PM concentrations is documented. The prevalence of low winds and high meandering in this sector of Santiago is hypothesized to be related to the local topography that precludes the development of the weak but persistent drainage surface winds observed at other sites of the city. Further investigation of this very stable atmospheric boundary layer is deemed necessary for improving the numerical models used in forecasting and managing the air pollution problem of Santiago.

Section: Discussionsupporting

confidence: 63%

Section: Introductionmentioning

confidence: 99%

Surface Indices of Wind, Stability, and Turbulence at a Highly Polluted Urban Site in Santiago, Chile, and their Relationship with Nocturnal Particulate Matter Concentrations

Muñoz

Corral

2017

“…These episodes occur at night and are related to adverse meteorological conditions such as thermal inversion and low wind speed (Rutland and Garreaud, 1995;Gramsch et al, 2000) as well as the location of the episode (Gramsch et al, 2006). It is not clear whether the episodes are related to a build-up of secondary PM 2.5 components or to freshly emitted pollutants (Gramsch et al, 2014). It is also well known that Santiago has a diurnal-nocturnal, valley-mountain wind cycle and as a result, the air mass can remain an average 2-3 days trapped on the city with the consequent enhancement of the secondary PM 2.5 fraction.…”

Section: Introductionmentioning

confidence: 99%

“…1, but during episodes, the wind speed is so low, that there is no predominant direction. These meteorological conditions favor building up of contaminants (Schaefer et al, 2006;Gramsch et al, 2014) therefore increasing pollution during episodes. Another feature that can be seen in the plots is that the increase in PM 2.5 is larger at night (8 pm-5 am) than during the day.…”

Section: Pollution Behavior During Episodesmentioning

confidence: 99%

Prevalence of Freshly Generated Particles during Pollution Episodes in Santiago de Chile

Gramsch

Reyes²,

Vásquez³

et al. 2016

A winter campaign was carried out in Santiago de Chile the year 2012 in two urban sites that can be considered representative for most of the city in order to characterize formation of primary and secondary PM 1.0 during episodes. One site is located in the campus of the University of Santiago and measurements were carried out with an Aerosol Chemical Speciation Monitor and a black carbon monitor. Another site is located in a large park, about 2 km south-east of the first site, measurements of CO, NO x , SO 2 , O 3 were done in this site. A noticeable increase in most of the primary components of PM 1.0 (black carbon and organics) and primary gases (CO and NO) was observed during days in which the average PM 1.0 concentration was higher than 50 µg m -3 (episode). A small increase or no change was observed in the secondary pollutants (NH 4 , NO 3 , SO 4 and NO 2 ) at night during these episodes. Positive Matrix Factorization was used to extract four components from the ACSM data: hydrocarbon-like organic aerosol (HOA), biomass burning OA (BBOA), low volatility oxygenated OA (LV-OOA) and semi-volatile oxygenated OA (SV-OOA). The freshly generated components (HOA and BBOA) showed a clear increase at night during episodes, while the aged fraction of organic aerosol (LV-OOA and SV-OOA) showed a smaller increase or a decrease at night during episodes. Correlation of HOA and BBOA components with primary pollutants was also high, indicating that freshly created aerosols (HOA, BBOA and BC) are in large part responsible for the increase in pollution at night during episodes in Santiago de Chile.

“…Air masses following these trajectories would not be directly in contact with the polluted central Santiago region. In fact, they would be in contact with pollutants characteristics of the southern periphery of Santiago, where other studies have indicated (Seguel et al, 2013;Gramsch et al, 2014) that the air quality is not correlated with conditions in Santiago and sometimes even considered more representative of background aerosol concentrations. A detailed analysis of the individual trajectories in this cluster indicates arrival at the research site mainly between 1800 LT and 0300 LT. Recall that CN concentrations reached a maximum at the research site at 18:00 LT (Fig.…”

Section: Mesoscale Circulationsmentioning

confidence: 99%

On the Transport of Urban Pollution in an Andean Mountain Valley

Cordova

Arevalo

Marı́n

et al. 2016

Urban pollution can often impact surrounding, non-urban regions, through advection and dispersal of pollutants by the prevailing winds. Urban regions located upstream of high mountains, such as the Andes, can potentially impact the cryosphere by deposition of particles onto the surface of the snowpack and glaciers.Santiago, the capital of Chile, has more than 6 million inhabitants and regularly experiences episodes of severe pollution, particularly during the austral winter. Some studies have hypothesized that particle pollution from Santiago can reach the cryosphere downwind of the city, but the scarcity of measurements made high in the mountains prevents the validation of mesoscale models so the proof of actual impact remains elusive. A research project was designed to provide some insight into this question. The Pollution Impact on Snow in the Cordillera -Experiments and Simulations (PISCES) project was carried out in 2014 and includes both observational and modeling components. A five-week field campaign was conducted at the end of winter, at an elevated site in a mountain valley, 65 km to the southeast of the center of Santiago, to characterize some aspects of particulate pollution.During synoptic conditions that result in clear days at the site, the mesoscale mountain-valley circulation is effective in transporting pollutants upwards during the day, leading to diluted particle concentrations beyond the summits of the highest peaks. Cloudy days with reduced up-valley circulation do not show increased concentrations associated with transport. Back trajectories indicate that air masses reaching the site during the field campaign are seldom influenced by pollution from Santiago.