Spatial and size distributions of ultrafine particles in the port and city of Rotterdam, Netherlands

We present mobile measurements of ultrafine particulate matter pollution in the city and port of Rotterdam, collected in Fall 2022 and Spring 2023 as part of the Ruisdael Observatory's Urban-Atmosphere Interactions Intensive Trace-gas and Aerosol Measurements Campaign (RITA2022). During this campaign, an instrumented mobile monitoring vehicle was deployed to measure for >80 hours driving around the urban core and harbor of Rotterdam, covering as many streets as possible.  The full spatial map of measured UFP concentrations is compared to an emissions inventory developed by TNO to identify hotspots and compare the accuracy of relative strengths of different sources, by comparing emissions patterns to concentration patterns.  We find relatively higher measured concentrations on freeways and relatively lower at airports, oil refineries and industry, compared to emissions inventory fractions. We additionally conduct bicycle mobile measurements along a transect from port into urban core, following the wind direction, to assess UFP growth and losses due to coagulation scavenging. We find elevated concentrations of the smallest particles in the harbor and growth in diameter moving east along the wind direction into the urban core.  We compare observed diameter changes to theoretical coagulation rates to conclude that UFPs are lost substantially to coagulation as they are transported inland. Finally, we compare UFP concentrations to fine particle concentrations (PM2.5) and find an anticorrelation, consistent with UFP lifetimes being limited by loss to larger particles.