Isomer-Resolved Real-Time Quantification and Dynamics of Indoor Carbonyl Compounds via NO+ Chemical Ionization in PTR-MS.

Volatile carbonyl compounds are ubiquitous indoors and outdoors, yet their real-time measurements are challenging. Conventional proton-transfer-reaction mass spectrometry, i.e., chemical ionization using H3O+ (H3O+-CIMS), cannot distinguish aldehyde and ketone isomers, complicating accurate identification and source apportionment. To address this, we developed and validated a NO+-CIMS method for isomer-resolved, real-time carbonyl detection. Laboratory characterization revealed stable sensitivities for aldehydes and a 20-40% increase in ketone sensitivities when the relative humidity (RH) rose from 20% to 80%. Field measurements in office, corridor, and outdoor settings demonstrated the reliability of NO+-CIMS, with total carbonyl signals comparable to H3O+-CIMS data (r > 0.87, deviations within ± 50%). The average indoor carbonyl mixing ratios ranged from 0.074 to 25 ppb, dominated by acetaldehyde and acetone. Contrasting behaviors between compound classes were observed; aldehydes and acetone exhibited strong diurnal variation linked to human activities, while ketones remained stable, indicating constant background sources. Furthermore, H3O+-CIMS quantification can be biased by a factor of 0.19 to 5.0 when a signal sensitivity factor is applied to mixed carbonyl signals, which may even distort the temporal trends. These results highlight the advantages of NO+-CIMS in resolving isomeric carbonyls and capturing real-time dynamics, offering an advanced approach for application in diverse environments.