Atmospheric chemical source signatures from proton-transfer reaction mass spectrometry aid in the interpretation of ambient aerosol distributions

Trace gases and aerosols play a fundamental role for climate and air quality, yet predicting their ambient distributions remains challenging, particularly in urban environments. Primary and secondary sources of trace gases and aerosols in the urban atmosphere are diverse and dominated by combustion sources (e.g. traffic or residential energy use), direct emissions (e.g. solvents), or secondary formation processes (e.g. gas to particle conversion). Novel instrumentation is needed to better constrain the fate of these species. With the advance of chemical ionisation mass spectrometry it is now possible to quantify a range of important atmospheric species. Here we use selected volatile organic compound (VOC) and organic aerosol (OA) observations obtained by PTR-TOF-MS and CHARON PTR-TOF-MS technology to decipher chemical source signatures in the urban area of Innsbruck. Chemically speciated gas phase and aerosol measurements were analyzed using positive matrix factorization, suggesting an externally mixed aerosol population with characteristic source contributions from domestic biomass burning, traffic, secondary sources from atmospheric oxidation, gas-fired furnaces and possibly from aircraft operations. We identified specific ions in the mass spectra associated with biomass burning, (e.g. levoglucosan (C6H10O5.H+ and fragment ions), furan (C4H4O.H+), methyl furans (C5H6O.H+), benzene (C6H6.H+) and polycyclic aromatic hydrocarbons (C10H8.H+)) that were elevated after the passage of a cold front leading to increased domestic biomass burning activities. Overall, the influence of biomass burning on PM1 (particulate matter <1 mu m) is heavily dependent on meteorology as wood operated fireplaces are predominantly used as additional heating in the residential sector. Our observations show that the cumulative number density of fine particulate (CFP) matter (defined as 190-1357 nm) was dominated by secondary (e.g. photochemically aged) sources with contributions from primary sources such as traffic, local primary sources and biomass burning, whilethe traffic sector contributes notably to ultrafine particulate matter (UFP) (<90 nm). In addition, a second source of ultrafine particulate matter was identified in the west sector, which includes likely emissions from a nearby airport and/or a gas-fired district heating plant.