Grantee Research Project Results

Instrument development/characterization. A novel analytical technique for the online, volatility-resolved measurement of the loadings and chemical composition of LVOCs was developed. Briefly, the instrument measures gas-phase organic species in bulk using high-resolution electron impact mass spectrometry, whose utility for the ensemble measurement of atmospheric organics has been demonstrated by Aerodyne’s Aerosol Mass Spectrometer (AMS). The technique utilizes cryogenic collection (preconcentration) of sampled gas-phase organics, followed by temperature-programmed desorption into the mass spectrometer according to their volatility (vapor pressure). Collection temperatures are held just above the dewpoint of the air being sample; this prevents major interferences from water in the mass spectrometer, and moreover ensures that only low-volatility species (and no VOCs) are measured. High-resolution mass spectrometry allows for the determination of both the amount and the ensemble chemical properties (e.g., elemental ratios) of the organics introduced, and is amenable to the use of factor analysis techniques (e.g., positive matrix factorization) in the interpretation of results.

 

The technique was demonstrated and characterized via an ongoing series of calibration experiments, to quantify both the sensitivity of the instrument (via determination of the ion-signal-to-mass conversion) and the ability of the instrument to estimate the volatilities of the LVOCs (via determination of the temperature-to-vapor-pressure conversion). Two different calibration systems, in which organics are delivered in known concentrations either by a syringe pump or a capillary, were developed and used for such experiments, allowing for a wide range of organic species and concentrations to be accessed. Sensitivity studies were carried out by introducing several concentrations of a standard compound (typically hexadecane, C₁₆H₃₄) into the instrument; signal was found to be linear with concentration, with high (~picogram) sensitivity of the technique. The volatility calibrations were carried out by introducing a mixture of n-alkanes (with even carbon numbers ranging from 10 to 24); the peak desorption temperatures of each compound were found to decrease linearly with the logarithm of the compound’s saturation vapor pressure, allowing for a simple, straightforward approximation of the volatility distribution of the LVOCs sampled.

 

Diesel engine experiments. Two sets of diesel engine characterization experiments were carried out at MIT’s Sloan Automotive Lab. The LVOC instrument was used to characterize emissions from a medium-duty diesel engine (Cummins ISB 300, 5.9L), as well as a “fuel burner” for generating accelerated emissions over a short period of time. In addition, a wide range of other instruments were used in parallel, including a soot particle aerosol mass spectrometer (SP-AMS, for refractory and nonrefractory components of the particles), an aerosol chemical speciation monitor (for nonrefractory components of the aerosol only), a scanning mobility particle sizer (SMPS, for particle mobility diameter), and a multi-angle absorption photometer (MAAP, for soot loadings). Additionally, particles were collected on filters for ICP-MS analysis, and major gas-phase combustion products were measured using an FTIR and a combustion analyzer. For the diesel exhaust experiments, measurements were made over a wide range of operating parameters. This included engine speed, engine load, and use of an uncatalyzed diesel particulate filter (DPF). The focus was on not just the steady-state emissions but also the transients that arise from changes in conditions: cold starts, increases in load, and DPF regeneration.

 

Changes to engine speed (with the load held constant) were found to have only a minor effect on LVOC emissions; instead, LVOC emissions were most sensitive to engine power. In general the highest LVOC emissions are highest at idle and at the lowest loads. This is expected, since combustion is least efficient at low engine power settings (and is consistent with the well-known behavior of VOC emissions). The opposite trend is seen for particle mass loading, due to the increased emissions of black carbon (soot) at higher powers. LVOC emissions appeared to be sensitive to transients – for example, step-increases in engine power led to temporary spikes in emissions. Such transients have implications for real-world emissions, suggesting that simple emission factors (emissions per amount of fuel consumed) might not always capture the true emissions by an on-road vehicle, and highlight the utility of the semicontinuous technique used. Very little change to LVOC emissions was observed downstream of an uncatalyzed DPF, pointing to the central importance of catalytic oxidation for removing organic vapors from engine exhaust. In addition, the SP-AMS measurements provided evidence for metals and other trace elements within the exhaust particles; such elements have since been shown to be useful markers for diesel emissions during on-road mobile-source measurements.

 

Aircraft emissions measurements. We deployed the LVOC instrument to Palmdale, CA, as part of the second Alternative Aviation Fuels Experiment (AAFEX-2), in collaboration with Aerodyne Research, Inc. The goal of such measurements was to make the first chemistry- and volatility-resolved measurements of LVOCs from aircraft, at a higher time resolution than has been possible previously. Our sampling trailer was located 143 m downwind of a DC-8 aircraft, for the measurement of LVOC emissions as a function of aircraft engine power. In addition, the two engines sampled were burning different fuels, one with standard JP-8 jet fuel and the other with synthetic (Fischer-Tropsch) fuel; however by the time they reached our inlet, both plumes had merged and mixed emissions from the two engines were captured in most of our measurements.

 

The plume intercepted our trailer a number of times, including for a complete engine power “sweep,” allowing for detailed measurements of the LVOC profile (loadings, chemical composition, and volatility) of the jet engine exhaust as a function of engine power. As in the diesel engine case, LVOC emissions, normalized to measured CO2 enhancement, are strongly dependent on engine power, with the emissions decreasing as power increases (as a result of the increased combustion efficiencies). The LVOCs also shift to lower vapor pressures (high desorption temperatures) at higher powers, indicating a switch from volatile LVOC species (e.g., evaporated fuels) to nonvolatile ones (e.g., pyrolysis products and soot precursors). At the highest powers, LVOCs represent a large (and even dominant) fraction of total emissions. Analysis of the volatility-resolved mass spectra provided additional information about the nature of these LVOCs. Positive matrix factorization (PMF) of the time- (temperature-) dependent mass spectra, allowed for the separation of LVOC species into three key classes: aliphatic (corresponding mostly to unburned fuel), aromatic (both from unburned fuel and pyrolysis), and oxygenated organics (possibly from the high-temperature oxidation of lubricant oil and/or primary combustion products). When the plume from only one engine (burning Fischer-Tropsch fuel) was intercepted, the aromatic component was considerably smaller and lower in volatility, consistent with the negligible aromatic content of that fuel.