Outdoor and indoor aerosol size, number, mass and compositional dynamics at an urban background site during warm season: Fid-Bau Portal

Outdoor and indoor aerosol size, number, mass and compositional dynamics at an urban background site during warm season

Talbot, N. / Kubelova, L. / Makes, O. / Cusack, M. / Ondracek, J. / Vodička, P. / Schwarz, J. / Zdimal, V.

Abstract This paper describes the use of a unique valve switching system that allowed for high temporal resolution indoor and outdoor data to be collected concurrently from online C-ToF-AMS, SMPS and OC/EC, and offline BLPI measurements. The results reveal near real-time dynamic aerosol behaviour along a migration path from an outdoor to indoor environment. An outdoor reduction in NR-PM1 mass concentration occurred daily from AM (06:00–12:00) to PM (12:00–18:00). SO4 (26%–37%) [AM/PM] increased proportionally during afternoons at the expense of NO3 (18%–7%). The influences of mixing height, temperature and solar radiation were considered against the mean mass concentration loss for each species. Losses were then calculated according to species via a basic input/output model. NO3 lost the most mass during afternoon periods, which we attribute to the accelerated dissociation of NH4NO3 through increasing temperature and decreasing relative humidity. Indoor/outdoor (I/O) ratios varied from 0.46 for <40 nm to 0.65 for >100 nm. These ratios were calculated using average SMPS PNC measurements over the full campaign and corroborated using a novel technique of calculating I/O penetration ratios through the indoor migration of particles during a new particle formation event. This ratio was then used to observe changes in indoor composition relative to those outdoors. Indoor sampling was carried out in an undisturbed room with no known sources. Indoor concentrations were found to be proportional to those outdoors, with organic matter [2.7 μg/m³] and SO4 [1.7 μg/m³] being the most prominent species. These results are indicative of fairly rapid aerosol penetration, a source-free indoor environment and small afternoon I/O temperature gradients. Fine fraction NO3 was observed indoors in both real-time AMS PM1 and off-line BLPI measurements. Greater mass concentration losses were observed from filter measurements, highlighting an important time dependency factor when investigating semi-volatiles. Coarse mode NO3 was observed by impactor measurements, ascribing value to observing the full particle mass size distribution for understanding aerosol origin.

Highlights • A unique automated switching system allowing for concurrent I/O sampling described. • On-line C-ToF-AMS, SMPS and OC/EC reveal dynamic behaviour of outdoor aerosols migrating indoors. • Diurnal fluctuations in chemical composition outdoors investigated through a mass balance model. • I/O ratio derived from the indoor penetration of outdoor originating new particles. • Mass concentration losses of NH4NO3 from online AMS compared to offline BLPI measurements.