<p>Proton-transfer-reaction mass spectrometry (PTR-MS) has been widely used to study the emissions, distributions, and chemical evolution of volatile organic compounds (VOCs) in the atmosphere. The applications of PTR-MS have greatly promoted understanding of VOC sources and their roles in air-quality issues. In the past two decades, many new mass spectrometric techniques have been applied in PTR-MS instruments, and the performance of PTR-MS has improved significantly. This Review summarizes these developments and recent applications of PTR-MS in the atmospheric sciences. We discuss the latest instrument development and characterization work on PTR-MS instruments, including the use of time-of-flight mass analyzers and new types of ion guiding interfaces. Here we review what has been learned about the specificity of different product ion signals for important atmospheric VOCs. We present some of the recent highlights of VOC research using PTR-MS including new observations in urban air, biomass-burning plumes, forested regions, oil and natural gas production regions, agricultural facilities, the marine environment, laboratory studies, and indoor air. Finally, we will summarize some further instrument developments that are aimed at improving the sensitivity and specificity of PTR-MS and extending its use to other applications in atmospheric sciences, e.g., aerosol measurements and OH reactivity measurements.</p>

<p>tA proton-transfer-reaction mass spectrometer (PTR-MS) was used onboard the NOAA WP-3D aircraftfor atmospheric measurements over the Deepwater Horizon (DWH) oil spill in the Gulf of Mexico in2010. A mass spectrum obtained by PTR-MS downwind from the accident site revealed the presenceof a complex mixture of volatile organic compounds (VOCs). Here, we interpret this mass spectrum bycomparing it with mass spectra of crude oil, gasoline and diesel samples measured in the laboratory.Aromatics were less abundant over the spill than from crude oil samples, due to the dissolution of thesespecies in the seawater. The mass spectra obtained from aircraft measurements and crude oil samplesboth show strong signals at masses with mass-to-charge ratio (m/z) of 14 &times; n &plusmn; 1, including 43, 57, 69,71, 83, 85, 97 and 111Yamu. PTR-MS coupled with a gas chromatograph was used to identify majormass signals from crude oil samples. Cycloalkanes are important contributors to the signals of mass m/z14 &times; n &plusmn; 1, especially for masses 69, 83, 97 and 111 amu. Aromatics could be interpreted from their specificmasses without significant interference for crude oil vapors, but the interferences to benzene from higheraromatics can be significant as crude oil evaporates. The interpretation of DWH mass spectrum is notonly helpful in understanding the atmospheric emissions associated with the DWH oil spill, but also forthe interpretation of PTR-MS measurements in urban air, near natural oil seeps and oil as well as naturalgas extraction activities.</p>