Showing papers by "Ann M. Middlebrook published in 2018"

Secondary organic aerosol (SOA) yields from NO 3 radical + isoprene based on nighttime aircraft power plant plume transects

Abstract: . Nighttime reaction of nitrate radicals ( NO3 ) with biogenic volatile organic compounds (BVOC) has been proposed as a potentially important but also highly uncertain source of secondary organic aerosol (SOA). The southeastern United States has both high BVOC and nitrogen oxide ( NOx ) emissions, resulting in a large model-predicted NO3 -BVOC source of SOA. Coal-fired power plants in this region constitute substantial NOx emissions point sources into a nighttime atmosphere characterized by high regionally widespread concentrations of isoprene. In this paper, we exploit nighttime aircraft observations of these power plant plumes, in which NO3 radicals rapidly remove isoprene, to obtain field-based estimates of the secondary organic aerosol yield from NO3 + isoprene. Observed in-plume increases in nitrate aerosol are consistent with organic nitrate aerosol production from NO3 + isoprene, and these are used to determine molar SOA yields, for which the average over nine plumes is 9 % ( ±5 %). Corresponding mass yields depend on the assumed molecular formula for isoprene- NO3 -SOA, but the average over nine plumes is 27 % ( ±14 %), on average larger than those previously measured in chamber studies (12 %–14 % mass yield as Δ OA ∕ Δ VOC after oxidation of both double bonds). Yields are larger for longer plume ages. This suggests that ambient aging processes lead more effectively to condensable material than typical chamber conditions allow. We discuss potential mechanistic explanations for this difference, including longer ambient peroxy radical lifetimes and heterogeneous reactions of NO3 -isoprene gas phase products. More in-depth studies are needed to better understand the aerosol yield and oxidation mechanism of NO3 radical + isoprene, a coupled anthropogenic–biogenic source of SOA that may be regionally significant.

Airborne and ground-based observations of ammonium-nitrate-dominated aerosols in a shallow boundary layer during intense winter pollution episodes in northern Utah

Abstract: . Airborne and ground-based measurements of aerosol concentrations, chemical composition, and gas-phase precursors were obtained in three valleys in northern Utah (USA). The measurements were part of the Utah Winter Fine Particulate Study (UWFPS) that took place in January–February 2017. Total aerosol mass concentrations of PM 1 were measured from a Twin Otter aircraft, with an aerosol mass spectrometer (AMS). PM 1 concentrations ranged from less than 2 µ g m −3 during clean periods to over 100 µ g m −3 during the most polluted episodes, consistent with PM 2.5 total mass concentrations measured concurrently at ground sites. Across the entire region, increases in total aerosol mass above ∼2 µ g m −3 were associated with increases in the ammonium nitrate mass fraction, clearly indicating that the highest aerosol mass loadings in the region were predominantly attributable to an increase in ammonium nitrate. The chemical composition was regionally homogenous for total aerosol mass concentrations above 17.5 µ g m −3 , with 74±5 % (average ± standard deviation) ammonium nitrate, 18±3 % organic material, 6±3 % ammonium sulfate, and 2±2 % ammonium chloride. Vertical profiles of aerosol mass and volume in the region showed variable concentrations with height in the polluted boundary layer. Higher average mass concentrations were observed within the first few hundred meters above ground level in all three valleys during pollution episodes. Gas-phase measurements of nitric acid ( HNO3 ) and ammonia ( NH3 ) during the pollution episodes revealed that in the Cache and Utah valleys, partitioning of inorganic semi-volatiles to the aerosol phase was usually limited by the amount of gas-phase nitric acid, with NH3 being in excess. The inorganic species were compared with the ISORROPIA thermodynamic model. Total inorganic aerosol mass concentrations were calculated for various decreases in total nitrate and total ammonium. For pollution episodes, our simulations of a 50 % decrease in total nitrate lead to a 46±3 % decrease in total PM 1 mass. A simulated 50 % decrease in total ammonium leads to a 36±17 % µ g m −3 decrease in total PM 1 mass, over the entire area of the study. Despite some differences among locations, our results showed a higher sensitivity to decreasing nitric acid concentrations and the importance of ammonia at the lowest total nitrate conditions. In the Salt Lake Valley, both HNO3 and NH3 concentrations controlled aerosol formation.

Characterization of a catalyst-based conversion technique to measure total particulate nitrogen and organic carbon and comparison to a particle mass measurement instrument

Abstract: . The chemical composition of aerosol particles is a key aspect in determining their impact on the environment. For example, nitrogen-containing particles impact atmospheric chemistry, air quality, and ecological N deposition. Instruments that measure total reactive nitrogen ( Nr = all nitrogen compounds except for N2 and N2O ) focus on gas-phase nitrogen and very few studies directly discuss the instrument capacity to measure the mass of Nr -containing particles. Here, we investigate the mass quantification of particle-bound nitrogen using a custom Nr system that involves total conversion to nitric oxide (NO) across platinum and molybdenum catalysts followed by NO−O3 chemiluminescence detection. We evaluate the particle conversion of the Nr instrument by comparing to mass-derived concentrations of size-selected and counted ammonium sulfate ( (NH4)2SO4 ), ammonium nitrate ( NH4NO3 ), ammonium chloride ( NH4Cl ), sodium nitrate ( NaNO3 ), and ammonium oxalate ( (NH4)2C2O4 ) particles determined using instruments that measure particle number and size. These measurements demonstrate Nr -particle conversion across the Nr catalysts that is independent of particle size with 98 ± 10 % efficiency for 100–600 nm particle diameters. We also show efficient conversion of particle-phase organic carbon species to CO2 across the instrument's platinum catalyst followed by a nondispersive infrared (NDIR) CO2 detector. However, the application of this method to the atmosphere presents a challenge due to the small signal above background at high ambient levels of common gas-phase carbon compounds (e.g., CO2 ). We show the Nr system is an accurate particle mass measurement method and demonstrate its ability to calibrate particle mass measurement instrumentation using single-component, laboratory-generated, Nr -containing particles below 2.5 µ m in size. In addition we show agreement with mass measurements of an independently calibrated online particle-into-liquid sampler directly coupled to the electrospray ionization source of a quadrupole mass spectrometer (PILS–ESI/MS) sampling in the negative-ion mode. We obtain excellent correlations ( R2 = 0.99) of particle mass measured as Nr with PILS–ESI/MS measurements converted to the corresponding particle anion mass (e.g., nitrate, sulfate, and chloride). The Nr and PILS–ESI/MS are shown to agree to within ∼ 6 % for particle mass loadings of up to 120 µ g m −3 . Consideration of all the sources of error in the PILS–ESI/MS technique yields an overall uncertainty of ± 20 % for these single-component particle streams. These results demonstrate the Nr system is a reliable direct particle mass measurement technique that differs from other particle instrument calibration techniques that rely on knowledge of particle size, shape, density, and refractive index.

Wintertime ammonium nitrate aerosol pollution in urban areas: NO x and VOC control as mitigation strategies

Abstract: Wintertime ammonium nitrate aerosol pollution is a severe air quality issue affecting both developed and rapidly urbanizing regions from Europe to East Asia. In the US, it is acute in western basins subject to inversions that confine pollutants near the surface. Measurements and modeling of a wintertime pollution episode in Salt Lake City, Utah demonstrates that ammonium nitrate is closely related to photochemical ozone through a common parameter, total odd oxygen, Ox,total. We show that the traditional NOx‐VOC framework for evaluating ozone mitigation strategies also applies to ammonium nitrate. Despite being nitrate‐limited, ammonium nitrate aerosol pollution in Salt Lake City is responsive to VOC control and, counterintuitively, not initially responsive to NOx control. We demonstrate simultaneous nitrate limitation and NOx saturation and suggest this phenomenon may be general. This finding may identify an unrecognized control strategy to address a global public health issue in regions with severe winter aerosol pollution.

Characterization of a catalyst-based total nitrogen and carbon conversion technique to calibrate particle mass measurement instrumentation

Abstract: The chemical composition of aerosol particles is a key aspect in determining their impact on the environment. For example, nitrogen (N)-containing particles impact atmospheric chemistry, air quality, and ecological N-deposition. Instruments that measure total reactive nitrogen (N r = all nitrogen compounds except for N 2 and N 2 O) focus on gas-phase nitrogen and very few studies directly discuss the instrument capacity to measure the mass of N r –containing particles. Here, we investigate the mass quantification of particle-bound nitrogen using a custom N r system that involves total conversion to nitric oxide (NO) across platinum and molybdenum catalysts followed by NO-O 3 chemiluminescence detection. We evaluate the particle conversion of the N r instrument by comparing to mass derived concentrations of size-selected and counted ammonium sulfate ((NH 4 ) 2 SO 4 ), ammonium nitrate (NH 4 NO 3 ), ammonium chloride (NH 4 Cl), sodium nitrate (NaNO 3 ), and ammonium oxalate ((NH 4 ) 2 C 2 O 4 ) particles determined using instruments that measure particle number and size. These measurements demonstrate N r -particle conversion across the N r catalysts that is independent of particle size with 98 ± 10 % efficiency for 100–600 nm particle diameters. We also show conversion of particle-phase organic carbon species to CO 2 across the instrument’s platinum catalyst followed by a non-dispersive infrared (NDIR) CO 2 detector. We show the N r system is an accurate particle mass measurement method and demonstrate its ability to calibrate particle mass measurement instrumentation using single component, laboratory generated, N r -containing particles below 2.5 µm in size. In addition we show agreement with mass measurements of an independently calibrated on-line particle-into-liquid sampler directly coupled to the electrospray ionization source of a quadrupole mass spectrometer (PILS-ESI/MS) sampling in the negative ion mode. We obtain excellent correlations (R 2 = 0.99) of particle mass measured as N r with PILS-ESI/MS measurements converted to the corresponding particle anion mass (e.g. nitrate, sulfate, and chloride). The N r and PILS-ESI/MS are shown to agree to within ~ 6 % for particle mass loadings up to 120 µg m −3 . Consideration of all the sources of error in the PILS-ESI/MS technique yields an overall uncertainty of ±20 % for these single component particle streams. These results demonstrate the N r system is a reliable direct particle mass measurement technique that differs from other particle instrument calibration techniques that rely on knowledge of particle size, shape, density, and refractive index.

Limited impact of sulfate-driven chemistry on black carbon aerosol aging in power plant plumes

Abstract: The aging of refractory black carbon (rBC) aerosol by sulfate-driven chemistry has been constrained in coal-fired power-plant plumes using the NOAA WP-3D research aircraft during the Southern Nexus (SENEX) study, which took place in the Southeastern US in June and July of 2013. A Single Particle Soot Photometer (SP2) determined the microphysical properties of rBC-containing particles including single-particle rBC mass and the presence and amount of internally-mixed non-rBC material, hereafter referred to as “coatings”. Most power-plant influenced air was associated with very slightly increased amounts of non-refractory material, likely sulfate internally mixed with the rBC, however this increase was statistically insignificant even after semi-Lagrangian exposure for up to 5 h. On average, the increase in coating thickness was 2 ± 4 nm for particles containing 3–5 fg rBC. Similarly, the number fraction of rBC-containing particles that could be identified as internally mixed was increased by plume chemistry by only 1.3 ± 1.3%. These direct measurements of microphysical aging of rBC-containing aerosol by power plant emissions constrain the enhancement of sulfate chemistry on both rBC’s column-integrated absorption optical depth, and rBC-containing aerosol’s ability to act as cloud condensation nuclei. Appling Mie and k -Kohler theories to the SP2 observations, permits the resulting effect on rBC ambient light-absorption to be capped at the 2–6% level.