Mobile NO2 Measurements for Air Pollution Mapping and Personal Exposure Studies Using an ICAD (Iterative Cavity-Enhanced DOAS) Instrument

The spatial and temporal variability of nitrogen dioxide (NO2) concentrations in cities, along with the resulting personal exposure of pedestrians, cyclists and car passengers, remains poorly characterized due to the reliance of official monitoring on a small number of fixed stations and passive samplers that provide only limited spatial coverage, and in the case of passive samplers, only long averaging periods. This study aims to demonstrate the use of a mobile iterative cavity-enhanced differential optical absorption spectroscopy (DOAS) NO2 instrument for (i) city-wide spatial mapping and (ii) personal-exposure assessment across transport modes, then to (iii) develop and validate a method to extrapolate annual mean NO2 concentrations from short-term mobile measurements and (iv) evaluate activated-charcoal cabin air filters as a practical exposure reduction measure for car occupants. NO2 concentrations were measured in several German cities between 2015 and 2019 using a mobile iterative cavity-enhanced DOAS (ICAD) instrument (accuracy better than 1.7μmathvariantsans-serifg/m3 at 2 s integration time) mounted on a bicycle trailer or inside a car. Short-term measurements were combined with concurrent data from nearby reference stations to extrapolate local annual means. The approach was validated using long-term station data from Berlin and independently checked against passive sampler data in Frankfurt and stationary ICAD measurements in Walldorf. In-cabin exposure and cabin air filter performance were assessed by alternating sampling between the vehicle interior and the ambient air. For a single continuous 32 h measurement, 80% of the extrapolated annual means deviate from the measured annual mean by between −30 and +40% (10–90th percentile) (up to −43 to +74% for 1 h). This reduces to about −7 to + 12 % when 32 one-hour measurement periods are distributed across the year instead. Mobile bicycle mapping in Walldorf and in Mannheim/Ludwigshafen showed pronounced spatial and temporal variability, with extrapolated annual means ranging from about 16μmathvariantsans-serifg/m3 in residential areas/parks to 44μmathvariantsans-serifg/m3 on main roads without a separated cycle path. In-cabin NO2 concentrations reached above 1000μmathvariantsans-serifg/m3 in tunnels with a standard pollen filter, while activated charcoal cabin air filters reduced in-cabin concentrations by 92–96% (new filters) and 66–78% (filters with 30,000 km of use). Mobile ICAD measurements combined with the proposed extrapolation approach provide a cost-effective screening tool for identifying NO2 hotspots and characterizing personal exposure patterns that are frequently underestimated by fixed monitoring networks.