Ground Level Ozone Formation Near a Traffic Intersection: Lisbon “Rotunda De Entrecampos” Case Study

Abstract

Ground-level ozone in cities is increasing mainly due to traffic exhaust aftertreatment devices, i.e., tailpipe catalytic converters. The chemical reaction of O3 formation indicates radiation and nitrogen oxides as main players. Thus, we investigate correlations between O3, global radiation, nitrogen oxides, temperature, and precipitation in several periods of the year (2017) near a traffic roundabout in Lisbon city (coordinates 38°44’55’’ lat, −9°08’56’’ long). The weekend effect, school break versus school period, day and night, and seasonal effect were explored. Low-cost sensors (LCS) of O3, NOx, and temperature were tested to see if they can be used to get historical data on other cities and locations. The main innovation is the calibration of the sensor directly with real data (uncontrolled environment). Raw data were compared and led us to conclude that MQ-131 has a better performance than the MICS-4514 sensor. The results indicate that the diurnal cycle of ozone concentration has a mid-day peak around 1–2 pm and a lower nighttime concentration below 5 ppb Weekends and school break period (251 days a year) have the highest values of Ozone, this is due to lower NOx emissions and thus lower levels of ozone destruction reaction (NOx-titration reaction). August is a hotspot month with a maximum concentration of 71 ppb.

Urban Ground-Level O3 Trends: Lessons from Portuguese Cities, 2010–2018

“…Our 10-year datasets of pollution and weather will allow us to observe daily, monthly and yearly trends, day-night cycles, weekday (WD) versus weekend (WE) trends, seasonality, and NO 2 , O 3 , NO and GR relationships. This will be useful to draw our own conclusions for Lisbon, Porto and Coimbra pollution trends and compare our values with known and consistent worldwide observed data regarding the weekend effect [4], seasonality and yearly trends.…”

“…Following the same logic, the observed increase in CO 2eq emissions would have the opposite effect, less solar energy available (light scattering) for the photochemical reaction which could potentially reduce O 3 . The effects of global radiation (GR) on ground-level O 3 through the photochemical reaction have already been established in another work by the author [4]. Other studies have also established GR-O 3 correlation, which ranges from 0.4 to 0.6 (R 2 ) [23,24].…”

“…Changes in surface O 3 are influenced by meteorological conditions and anthropogenic precursors, hence, changes due to NO x precursor mitigation may be masked by the varying meteorology. In an author's previous study there is evidence that O 3 is not NO x sensitive [4] in Portugal for the year 2017.…”

“…Ground-level O 3 is a result of NO x and VOC emissions from mobile and stationary combustion sources in the presence of radiation. These interactions of VOCs and NO x (NO + NO 2 ), are now suspected to play a key role on a counter-intuitive phenomenon called the "Weekend Effect", in which ozone concentrations are higher during the weekend, a time where it would be expected that lower NO 2 production would form less O 3 [4]. This effect was identified in other cities around the world, including Lisbon [4], but the metric to quantify this phenomenon is not uniform as it will be discussed later.…”

“…These interactions of VOCs and NO x (NO + NO 2 ), are now suspected to play a key role on a counter-intuitive phenomenon called the "Weekend Effect", in which ozone concentrations are higher during the weekend, a time where it would be expected that lower NO 2 production would form less O 3 [4]. This effect was identified in other cities around the world, including Lisbon [4], but the metric to quantify this phenomenon is not uniform as it will be discussed later. Furthermore, the weekend effect has been theorized to have a correlation with the weekly pattern of traffic flow [5], showing an inverse relationship between NO 2 and O 3 .…”