Improved method for characterising temporal variability in urban air quality part I: Traffic emissions in central Poland

Abstract

Four years of observations in Central Poland are used to assess the ability of a recently-improved radon-based stability classification technique to investigate temporal variability in urban air quality (specifically NOx and CO). After isolating periods of greatest synoptic non-stationarity, observations were assigned (in whole 24-h blocks) to one of four atmospheric mixing categories. Wind speed more than doubled from the most stable to the most well mixed nocturnal conditions in non-winter months, reducing nocturnal radon accumulation by a factor of five. A strong seasonality in NOx and CO concentrations was observed, contributed to by changing atmospheric mixing depths (day and night) and additional cold-season combustion sources. Peak daily NO concentrations typically increased by a factor of 2–4 from the most well-mixed to most stable conditions, with smaller changes for NO2 and CO (factor of 1.5–2). For non-winter months urban air quality was typically best in the afternoons, when mixing was deepest, but also improved in the 3–4 h after midnight due to advection of cleaner air from outside the city. Specific air quality influences were attributed to parts of Łódź and localised sources in the region, but the topographic setting of the monitoring site also played an important role. Emissions from nearby point sources were often brought to the site in katabatic flows when gradient winds were light. Observed daytime local CO contributions in summer were almost entirely attributable to vehicle emissions, whereas an additional afternoon source of around 48 mg m−2 h−1 in winter was attributed to combustion for heating (likely more at night). Winter local afternoon CO contributions attributable to local domestic combustion were estimated to be up to a factor of three less than the CO being advected to the study region from upwind European source regions.