Surface Inversions Research Articles

The Role of Non‐Local Effects on Surface Sensible Heat Flux Under Different Types of Thermal Structures Over the Arctic Sea‐Ice Surface

AbstractThe effects of atmospheric thermal structure on the surface energy flux are poorly understood over the Arctic sea‐ice surface. Here, we explore the mechanism of sensible heat exchange under different types of thermal structures over the Arctic sea‐ice surface by using data collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition. The quadrant analysis indicates that strong surface temperature inversions below 100 m enhance non‐local effects on the positive (upward) sensible heat flux () through entrainment of large eddies from the convective boundary layer aloft. However, strong surface inversions restrict the contributions of large eddies to the negative (downward) due to intensified surface stability. By inspecting the existing parameterization schemes, we found that the European Center for Medium‐Range Weather Forecasts Integrated Forecasting System scheme fails to predict the impacts of non‐local processes on the positive , and an adjustment term to correct the bias of parameterized is proposed.

РЕАНАЛИЗ ДЕРЕКТЕРІ БОЙЫНША ИНВЕРСИЯ ҚАБАТЫН ТАЛДАУ МҮМКІНДІКТЕРІ

The study of thermal inversion layers in the atmosphere plays an important role in understanding various atmospheric processes and solving environmental and meteorological problems. This work is devoted to evaluating the potential of thermal inversion layer analysis using ERA5 reanalysis data in the nearground layer as an alternative to the limited availability of aerological stations.The characteristics of inversions calculated using ERA5 reanalysis data and radiosonde data from the Karaganda Aerological Station for the period from 2012 to 2021 were compared. General inversions occur most often in January (15...18 times), while in spring and autumn, the average monthly number of cases is 4...10. In the summertime, surface inversions occur most frequently, with a mean monthly value of up to 25 times. The intensity of different surface inversions varies between -1 °C ... -9 °С. The results of the study showed a close similarity between the ERA5 reanalysis data and the radiosonde data. This indicates the suitability of the reanalysis data for studying thermal inversions. This study contributes to the development of knowledge in atmospheric pollution, weather forecasting, aviation meteorology, and the development of climate models by using reanalysis data to study inversion phenomena.

Influences of meteorological conditions in PM2.5 levels in Krasnoyarsk city atmosphere

The relationship between meteorological conditions and the levels of PM2.5 in Krasnoyarsk city atmosphere for the period from 2019 to 2022 were investigated. The meteorological data of the National Centers for Environmental Prediction Global Forecast System (NCEP GFS) reanalysis model was used. PM2.5 data were obtained from the ground monitoring stations. Analysis of variances (one-way and two-way ANOVA) and the Tukey Test showed statistically significant differences for temperature inversions, months in the cold period (November-February), and calm wind. In the case of high daily PM2.5 surface and elevated inversions occurred at 69% cases and strong temperature inversions at 74%. In the reverse case, in the presence of surface and elevated temperature inversions, high daily PM2.5 occurred in 53% of cases, and the presence of strong temperature inversions in 44%.

Temporal Variation of Temperature Inversions and their Effect on Fine Particulate Matter (PM2.5) Concentrations in a Vietnamese Metropolis

This study aims to investigate the temporal variation of temperature inversions, including inversion frequency and inversion strength, in Ho Chi Minh City (HCM), a metropolis of Vietnam. The effect of the temperature inversions on PM2.5 concentrations in the dry and rainy seasons of HCM City was also identified. The results revealed that the surface inversions, which are temperature inversions below 300 m, were more frequent at 12Z (7 PM local time) in the rainy season. In addition, the stronger inversions were found in the dry season, when the higher PM2.5 concentrations were also observed, suggesting an influence of the temperature inversions on an increase in PM2.5 concentrations. This suggestion was also supported by the significantly positive correlation between PM2.5 concentrations and inversion frequency in the dry season. However, the temperature inversions were believed to have a minor effect on the PM2.5 concentrations in the rainy season since no significant correlation between the PM2.5 concentrations and inversion frequency or strength was found for this season. The findings of this study contribute to understanding of the temperature inversions and their effect on PM2.5 concentrations in a metropolis of Vietnam, a developing country in Southeast Asia. Based on these results, solutions for reducing the PM2.5 pollution in the study area can be obtained.

A Study of Air Pollution Episode Over Delhi National Capital Region, India During October 29, 2016- November 03, 2016

Northern India has an increase in surface air pressure during winter; this causes the atmosphere to clearout, resulting in the production of significant surface inversions. High-pressure zones are also calm andwind direction changes only when a western disturbance enters the area. The wind is substantially strongerwhen there is a low-pressure system. As a result, the pollutants in northern India are regulated by a sequenceof high- and low-pressure weather situations. October 29, 2016, was when the wind direction unexpectedlyshifted from NW to NE and then to N. Wind speeds dropped from 1-6 km/h to near-calm during this time,which indicated the passing of a high-pressure zone, based on the readings of the pressure gauges. Pollutionfrom northern states was brought into the national capital region by a combination of low wind speed andthe wind’s direction. NCR was clogged with high levels of particulate matter during Diwali celebrations,which were exacerbated by heavy traffic, factory production, and firecrackers. The grave situation persisteduntil the wind shifted directions and increased its velocity. On the monostatic acoustic radar installed atREMTech, Shamli, detected the persistence of a substantially raised layer over the region, implying thatvertical mixing of pollutants was severely harmed or limited. As a result of this state, pollutants accumulated,resulting in a severe bout of air pollution. The findings of this study details an incident of such air pollution happened over Delhi NCR.

A Model-Based Climatology of Low-Level Jets in the Weddell Sea Region of the Antarctic

Low-level jets (LLJs) are climatological features in polar regions. It is well known that katabatic winds over the slopes of the Antarctic ice sheet are associated with strong LLJs. Barrier winds occurring, e.g., along the Antarctic Peninsula may also show LLJ structures. A few observational studies show that LLJs occur over sea ice regions. We present a model-based climatology of the wind field, of low-level inversions and of LLJs in the Weddell Sea region of the Antarctic for the period 2002–2016. The sensitivity of the LLJ detection on the selection of the wind speed maximum is investigated. The common criterion of an anomaly of at least 2 m/s is extended to a relative criterion of wind speed decrease above and below the LLJ. The frequencies of LLJs are sensitive to the choice of the relative criterion, i.e., if the value for the relative decrease exceeds 15%. The LLJs are evaluated with respect to the frequency distributions of height, speed, directional shear and stability for different regions. LLJs are most frequent in the katabatic wind regime over the ice sheet and in barrier wind regions. During winter, katabatic LLJs occur with frequencies of more than 70% in many areas. Katabatic LLJs show a narrow range of heights (mostly below 200 m) and speeds (typically 10–20 m/s), while LLJs over the sea ice cover a broad range of speeds and heights. LLJs are associated with surface inversions or low-level lifted inversions. LLJs in the katabatic wind and barrier wind regions can last several days during winter. The duration of LLJs is sensitive to the LLJ definition criteria. We propose to use only the absolute criterion for model studies.

Nocturnal atmospheric conditions and their impact on air pollutant concentrations in the city of Stuttgart

AbstractMeteorological and air pollutant measurements were conducted in the area of Stuttgart during winter and summer seasons. Stuttgart is situated in moderate mountainous terrain in southwestern Germany. We focus on the connection between atmospheric conditions and air pollutants in the urban nocturnal boundary layer. This is done by relating the bulk Richardson number (Rib), turbulence intensity, cloudiness, and winds, as well as NOx and O3 data. Turbulence intensity is inversely related to Rib, with the lower values occurring at Rib >0.33. The coefficient of determination for the exponential regression is only moderate, which partly can be attributed to sporadic turbulence in the transition from dynamically unstable to stable flows. Dynamically unstable flows (Rib <0.33) occur frequently in winter, as a result of the presence of low‐level clouds and strong winds, supporting low buoyant suppression and strong shear generation of turbulence. Dynamically stable flows (Rib >1.25) are found preferably under clear skies in summer with the build‐up of strong surface inversions, so that buoyant suppression is strong and shear generation of turbulence is weak. The nocturnal NOx concentrations are positively correlated with Rib. The correlation is weak, which is mainly related to the large variability of air pollutant concentrations in a range around Rib = 0.33. In this range, many low‐level jets are present that can cause sporadic turbulent coupling between the atmosphere and the surface. Reduced mixing under dynamically stable flows causes NOx values about 3 times higher than under dynamically unstable flows. The overall lowest NOx concentrations occur during winter when low clouds and strong winds are present.

Investigation of the effect of anthropogenic influences on the change in the geoecological conditions of urbanized territories

The types and degree of anthropogenic impacts have an effect on the ecological state of the environment. There are two main sources of pollution on the territory of cities: industry and motor transport. Currently, with a significant reduction in industrial production, road transport is one of the main sources of environmental pollution. The largest amount (more than 200) of air pollutants is provided by gasoline carburetor engines: carbon monoxide, nitrogen oxide, unburned hydrocarbons. When 1 liter of gasoline is burned, 200 - 400 mg of lead is released, thus, one passenger car emits up to 1 kg of lead into the city atmosphere per year. Diesel engines pollute the atmosphere with soot, sulfur compounds, benzo pyrene. According to American geochemists, in cities more often than in rural areas, we observe cloudy weather, more rain and less snow. Atmospheric dust in cities is 100 times more, exhaust gases, 5-25 times more than in agricultural landscapes. All this affects the health of the population. The favorable factor is that the main streets with heavy traffic are located on the watersheds, in the upper part of the city, where conditions are better for dispersing impurities. However, on streets descending into depressions, where air stagnation and surface inversions are more likely, and where uphill vehicle emissions increase, air pollution is upraised.

Micrometeorological Aspects of Spraying within a Surface Inversion

AbstractPesticide applications during surface inversions can lead to spray drift causing severe damage up to several kilometers off-target. Current regulations in Australia prohibit spray application of certain agricultural chemicals when hazardous surface inversions exist. This severely limits spray opportunities. Surface inversions can be classified as weakly or strongly stable. In the weakly stable case, moderate to strong turbulent mixing is not supportive of long-distance concentrated drift. In the very stable case, weak turbulent mixing can support the transport of high concentrations of fine material over long distances. A review of the literature and our analyses indicate that if the turbulence, as measured by the standard deviation of the vertical wind speed σw, is greater than about 0.2 m s−1 then turbulence-driven mixing and dispersion is moderate to strong and conversely if σw is less than about 0.2 m s−1 then turbulence-driven mixing and dispersion is weaker (an order of magnitude). The concept of maximum downward heat flux as a natural division between the regimes is applied within Monin–Obukhov stability theory, and it is shown that the observed mean σw of 0.2 m s−1 aligns with the ridge line of maximum heat flux in stable conditions. The level of turbulence in the weakly stable regime is comparable to the turbulence typically observed in near-neutral conditions that are recommended under current guidelines as suitable for spraying and is therefore seen as an acceptable prerequisite to avoid nondispersive spraying conditions.

Evaluation of the Rapid Refresh Numerical Weather Prediction Model over Arctic Alaska

AbstractDespite a need for accurate weather forecasts for societal and economic interests in the U.S. Arctic, thorough evaluations of operational numerical weather prediction in the region have been limited. In particular, the Rapid Refresh Model (RAP), which plays a key role in short-term forecasting and decision-making, has seen very limited assessment in northern Alaska, with most evaluation efforts focused on lower latitudes. In the present study, we verify forecasts from version 4 of the RAP against radiosonde, surface meteorological, and radiative flux observations from two Arctic sites on the northern Alaskan coastline, with a focus on boundary layer thermodynamic and dynamic biases, model representation of surface inversions, and cloud characteristics. We find persistent seasonal thermodynamic biases near the surface that vary with wind direction, and may be related to the RAP’s handling of sea ice and ocean interactions. These biases seem to have diminished in the latest version of the RAP (version 5), which includes refined handling of sea ice, among other improvements. In addition, we find that despite capturing boundary layer temperature profiles well overall, the RAP struggles to consistently represent strong, shallow surface inversions. Further, while the RAP seems to forecast the presence of clouds accurately in most cases, there are errors in the simulated characteristics of these clouds, which we hypothesize may be related to the RAP’s treatment of mixed-phase clouds.

Evaluation of Six Atmospheric Reanalyses over Arctic Sea Ice from Winter to Early Summer

AbstractThis study evaluates the performance of six atmospheric reanalyses (ERA-Interim, ERA5, JRA-55, CFSv2, MERRA-2, and ASRv2) over Arctic sea ice from winter to early summer. The reanalyses are evaluated using observations from the Norwegian Young Sea Ice campaign (N-ICE2015), a 5-month ice drift in pack ice north of Svalbard. N-ICE2015 observations include surface meteorology, vertical profiles from radiosondes, as well as radiative and turbulent heat fluxes. The reanalyses simulate surface analysis variables well throughout the campaign, but have difficulties with most forecast variables. Wintertime (January–March) correlation coefficients between the reanalyses and observations are above 0.90 for the surface pressure, 2-m temperature, total column water vapor, and downward longwave flux. However, all reanalyses have a positive wintertime 2-m temperature bias, ranging from 1° to 4°C, and negative (i.e., upward) net longwave bias of 3–19 W m−2. These biases are associated with poorly represented surface inversions and are largest during cold-stable periods. Notably, the recent ERA5 and ASRv2 datasets have some of the largest temperature and net longwave biases, respectively. During spring (April–May), reanalyses fail to simulate observed persistent cloud layers. Therefore they overestimate the net shortwave flux (5–79 W m−2) and underestimate the net longwave flux (8–38 W m−2). Promisingly, ERA5 provides the best estimates of downward radiative fluxes in spring and summer, suggesting improved forecasting of Arctic cloud cover. All reanalyses exhibit large negative (upward) residual heat flux biases during winter, and positive (downward) biases during summer. Turbulent heat fluxes over sea ice are simulated poorly in all seasons.

Temperature inversion and mixing height: critical indicators for air pollution in hot arid climate

Boundary layer temperature inversion and mixing heights are important parameters in understanding the atmospheric dispersion of air pollution. Surface and elevated inversions were analyzed within the planetary boundary layer over Kuwait, a coastal urban city situated in a typical hot, arid climate. Temperature inversions constitute a natural hazard by trapping smog emitted from pollution sources. Temperature inversions were measured by the Meteorological Temperature Profiler version (MTP-5H) using a scanning microwave radiometer over the course of a complete year (2013). Remote continuous measurements (4 min interval) of temperature profiles were collected (total readings = 130,986). Daytime and nocturnal temperature inversion variations were presented. The effects of five atmospheric parameters, namely ambient temperature, relative humidity, wind speed, solar radiation, and precipitation on the surface and elevated inversion, were presented. In general, there was a significant correlation between surface and elevated inversions with four meteorological parameters (p value ≤ .01). Results show a weak correlation between the inversions analyzed by MTP-5H and the MH modeled by the WRF.

The Intraseasonal and Interannual Variability of Arctic Temperature and Specific Humidity Inversions

Temperature and humidity inversions are common in the Arctic’s lower troposphere, and are a crucial component of the Arctic’s climate system. In this study, we quantify the intraseasonal oscillation of Arctic temperature and specific humidity inversions and investigate its interannual variability using data from the Surface Heat Balance of the Arctic (SHEBA) experiment from October 1997 to September 1998 and the European Centre for Medium-Range Forecasts (ECMWF) Reanalysis (ERA)-interim for the 1979–2017 period. In January 1998, there were two noticeable elevated inversions and one surface inversion. The transitions between elevated and surface-based inversions were associated with the intraseasonal variability of the temperature and humidity differences between 850 and 950 hPa. The self-organizing map (SOM) technique is utilized to obtain the main modes of surface and elevated temperature and humidity inversions on intraseasonal time scales. Low (high) pressure and more (less) cloud cover are related to elevated (surface) temperature and humidity inversions. The frequency of strong (weak) elevated inversions over the eastern hemisphere has decreased (increased) in the past three decades. The wintertime Arctic Oscillation (AO) and Arctic Dipole (AD) during their positive phases have a significant effect on the occurrence of surface and elevated inversions for two Nodes only.

Modeling Extreme Warm‐Air Advection in the Arctic: The Role of Microphysical Treatment of Cloud Droplet Concentration

AbstractAs the Arctic climate is changing fast, with increasing areas of open water in summer, there is a growing interest in the processes related to the marginal ice zones. Recent studies have indicated that such a critical process may be the advection of warm and moist air from the south. In this study, the performance of the Weather Research and Forecasting (WRF) model is evaluated during an extreme warm advection episode over melting sea ice that occurred near the Arctic ice edge in summer 2014. The model gives a reasonably good representation of the atmospheric conditions and the Arctic boundary layer, characterized by very strong surface inversions and the frequent presence of low‐level jets. However, the representation of the highly variable cloud conditions, from optically thick to optically thin, dissipating clouds, is sensitive to the choice of cloud droplet treatment in WRF. Simulations with relatively high cloud droplet number concentrations (Ndrop ≥ 100 cm−3) are more successful in representing the optically thick cloud state, whereas to reproduce optically thin and tenuous clouds Ndrop should be <50 cm−3. The WRF‐Chem model, with a realistic treatment of the cloud‐aerosol interactions, allows for large variations in Ndrop and hence can reproduce the cloud water properties reasonably well for most of the simulation time. This contributes to an improved representation of the cloud longwave radiative effect, compared to the simulations where a less adaptive treatment of Ndrop is applied.

Arctic Summer Airmass Transformation, Surface Inversions, and the Surface Energy Budget

During the Arctic Clouds in Summer Experiment (ACSE) in summer 2014 a weeklong period of warm-air advection over melting sea ice, with the formation of a strong surface temperature inversion and dense fog, was observed. Based on an analysis of the surface energy budget, we formulated the hypothesis that, because of the airmass transformation, additional surface heating occurs during warm-air intrusions in a zone near the ice edge. To test this hypothesis, we explore all cases with surface inversions occurring during ACSE and then characterize the inversions in detail. We find that they always occur with advection from the south and are associated with subsidence. Analyzing only inversion cases over sea ice, we find two categories: one with increasing moisture in the inversion and one with constant or decreasing moisture with height. During surface inversions with increasing moisture with height, an extra 10–25 W m−2of surface heating was observed, compared to cases without surface inversions; the surface turbulent heat flux was the largest single term. Cases with less moisture in the inversion were often cloud free and the extra solar radiation plus the turbulent surface heat flux caused by the inversion was roughly balanced by the loss of net longwave radiation.

Surface Temperature Inversions and Risk of Off-Target Herbicide Damage in the Soybean- and Cotton-Growing Regions of the US

Core Ideas Temperature inversions form most nights between 15 May and 30 June. Temperature inversions commonly form before sundown under winds greater than 3 mph. Frequency of temperature inversions in 2017 were similar to those in 2012 through 2016. The approval of the dicamba-tolerant trait in soybean and cotton as well as new dicamba spray formulations to control weeds in cotton and soybean fields has resulted in broad usage of dicamba-based herbicides. As dicamba-based herbicide usage increased, reports of non-target crop damage dramatically increased. Temperature inversions have been implicated in this off-target damage. Short-term climatologies of surface inversions at six locations across the soybean- and cotton-growing regions of the US were developed for the period 15 May through 30 June between 2012 and 2017 using National Climate Reference Network measurements. Inversions occurred most nights, occasionally collapsing and then reforming in the same night. Inversions often formed before or around sunset as the surface temperature cooled more rapidly than the overlying air. Inversion development was similar for most of the soybean-growing region with the greatest change in temperature difference between the air and the ground occurring at sundown. Higher wind speeds corresponded to decreased inversion intensity and shortened the duration of the inversion. Differences in inversion conditions in 2017 versus those of 2013–2016 were not consistent; consequently, the increased occurrence of off-target damage by dicamba in 2017 cannot be explained by an increase in the number or intensity of inversions or differences in winds during inversions. The presence of inversions when they are not expected (before sunset, after sunrise and when winds exceed 3 mph) increases the risk of unexpected off-target crop damage. Since inversions are commonly a local phenomenon, predicting inversion occurrence requires understanding the conditions at specific locations.

An Aircraft-Based Study of Strong Gap Flows in Nares Strait, Greenland

AbstractGap flows and the stable boundary layer were studied in northwest Greenland during the aircraft-based Investigation of Katabatic Winds and Polynyas during Summer (IKAPOS) experiment in June 2010. The measurements were performed using the research aircraft POLAR 5 of Alfred Wegener Institute (AWI; Bremerhaven). Besides navigational and basic meteorological instrumentation, the aircraft was equipped with radiation and surface temperature sensors and a turbulence measurement system. In the area of Smith Sound at the southern end of the Nares Strait, a stable, but fully turbulent, boundary layer with strong winds of up to 22 m s−1 was found during conditions of synoptically induced northerly winds through the Nares Strait. Strong surface inversions were present in the lowest 100–200 m. As a consequence of channeling effects, a well-pronounced low-level jet system was documented for each of four flights. The wind maximum is located at 20–50-km distance from the exit of Smith Sound. The 3D boundary layer structure past this gap is studied in detail. The channeling process is consistent with gap flow theory. The flow through the gap and over the surrounding mountains leads to the lowering of isotropic surfaces and the acceleration of the flow. The orographically channeled flow through Smith Sound plays a key role for the formation of the North Water polynya being the largest ice-producing polynya in the Arctic.

Ozone and volatile organic compounds in the metropolitan area of Lima-Callao, Peru

This study analyzes ozone formation in the metropolitan area of Lima-Callao as a function of meteorological patterns and the concentrations of nitrogen oxides and reactive organic gases. The study area is located on the west coast of South America (12°S) in an upwelling region that is markedly affected by the Southeast Pacific anticyclone. The vertical stability and diurnal evolution of the mixing layer were analyzed from radiosondes launched daily during 1992–2014 and from two intensive campaigns in 2009. Vertical profiles show that during June–November, the subsidence inversion base ranges from 0.6 to 0.9 km above sea level (asl). In contrast, during December–May, subsidence inversion dissipates, leading to weak surface inversions from 0.1 to 0.6 km asl. At the surface level, compliance with the ozone standard of 51 parts per billion by volume (ppbv) is explained by the marine boundary layer effect and by strong inhibition of ozone formation due to titration with nitric oxide. Day-of-the-week variations in ozone and nitrogen oxides suggest a VOC-limited ozone-formation regime in the atmosphere of Lima. Furthermore, the pattern of C6–C12 species indicates that gasoline-powered vehicles are the main source of volatile organic compounds (VOCs), whereas the species with the greatest ozone-forming potential corresponded to the sum of the isomers m- and p-xylene. Mean benzene concentrations exceeded the standard of 0.63 ppbv, reaching 1.2 ppbv east of Lima. Nevertheless, the cancer risk associated with the inhalation of benzene was deemed acceptable, according to USEPA and WHO criteria.

Projected change in characteristics of near surface temperature inversions for southeast Australia

Air pollution has significant impacts on human health. Temperature inversions, especially near surface temperature inversions, can amplify air pollution by preventing convective movements and trapping pollutants close to the ground, thus decreasing air quality and increasing health issues. This effect of temperature inversions implies that trends in their frequency, strength and duration can have important implications for air quality. In this study, we evaluate the ability of three reanalysis-driven high-resolution regional climate model (RCM) simulations to represent near surface inversions at 9 sounding sites in southeast Australia. Then we use outputs of 12 historical and future RCM simulations (each with three time periods: 1990–2009, 2020–2039, and 2060–2079) from the NSW/ACT (New South Wales/Australian Capital Territory) Regional Climate Modelling (NARCliM) project to investigate changes in near surface temperature inversions. The results show that there is a substantial increase in the strength of near surface temperature inversions over southeast Australia which suggests that future inversions may intensify poor air quality events. Near surface inversions and their future changes have clear seasonal and diurnal variations. The largest differences between simulations are associated with the driving GCMs, suggesting that the large-scale circulation plays a dominant role in near surface inversion strengths.

Particulate Matter Exposure of Rural Interior Communities as Observed by the First Tribal Air Quality Network in the Yukon Flat

A tribal-owned network of aerosol monitors and meteorological stations was installed at Ts’aahudaaneekk’onh Denh (Beaver), Gwichyaa Zheh (Fort Yukon), Jalgiitsik (Chalkyitsik), and Danzhit Khanlaii (Circle) in the Yukon Flats, Alaska. Surface inversions occurred under calm wind conditions due to radiative cooling. In May, local emissions governed air quality with worst conditions related to road and river dust. As the warm season progressed, worst air quality was due to transport of pollutants from upwind wildfires. During situations without smoke or when smoke existed at layers above the surface inversion, concentrations of particulate matter of less than 2.5 micrometer in diameter or less (PM2.5) were explainable by the local emissions; 24-h means remained below 25 μg·m-3. Absorption of solar radiation in the smoke layer and upward scattering enhanced stability and fostered the persistence of the surface inversions. During smoke episodes without the presence of a surface inversion, daily mean concentrations exceeded 35 μg·m-3 often for several consecutive days, at all sites. Then concentrations temporally reached levels considered unhealthy.