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Abstract. Organic nitrate (RONO2) formation in the atmosphere represents a sink of NOx(NOx = NO + NO2) and termination of the NOx/HOx(HOx = HO2 + OH) ozone formation and radical propagation cycles, can act as a NOx reservoirtransporting reactive nitrogen, and contributes to secondary organic aerosol formation. While some fraction of RONO2 is thought to reside in the particle phase, particle-phase organic nitrates (pRONO2) are infrequently measured and thus poorly understood. There is anincreasing prevalence of aerosol mass spectrometer (AMS) instruments, which have shown promise for determining the quantitative total organic nitratefunctional group contribution to aerosols. A simple approach that relies on the relative intensities of NO+ and NO2+ ions inthe AMS spectrum, the calibrated NOx+ ratio for NH4NO3, and the inferred ratio for pRONO2 hasbeen proposed as a way to apportion the total nitrate signal to NH4NO3 and pRONO2. This method is increasingly beingapplied to field and laboratory data. However, the methods applied have been largely inconsistent and poorly characterized, and, therefore, adetailed evaluation is timely. Here, we compile an extensive survey of NOx+ ratios measured for variouspRONO2 compounds and mixtures from multiple AMS instruments, groups, and laboratory and field measurements. All data and analysispresented here are for use with the standard AMS vaporizer. We show that, in the absence of pRONO2 standards, thepRONO2 NOx+ ratio can be estimated using a ratio referenced to the calibrated NH4NO3 ratio, aso-called “Ratio-of-Ratios” method (RoR = 2.75 ± 0.41). We systematically explore the basis for quantifyingpRONO2 (and NH4NO3) with the RoR method using ground and aircraft field measurements conducted over a largerange of conditions. The method is compared to another AMS method (positive matrix factorization, PMF) and other pRONO2 andrelated (e.g., total gas + particle RONO2) measurements, generally showing good agreement/correlation. A broad survey of ground andaircraft AMS measurements shows a pervasive trend of higher fractional contribution of pRONO2 to total nitrate with lower totalnitrate concentrations, which generally corresponds to shifts from urban-influenced to rural/remote regions. Compared to ground campaigns,observations from all aircraft campaigns showed substantially lower pRONO2 contributions at midranges of total nitrate(0.01–0.1 up to 2–5 µg m−3), suggesting that the balance of effects controlling NH4NO3 and pRONO2formation and lifetimes – such as higher humidity, lower temperatures, greater dilution, different sources, higher particle acidity, andpRONO2 hydrolysis (possibly accelerated by particle acidity) – favors lower pRONO2 contributions for thoseenvironments and altitudes sampled. 

Abstract. We present a rapid method for apportioning the sources of atmospheric organic aerosol composition measured by gas chromatography–mass spectrometry methods. Here, we specifically apply this new analysis method to data acquired on a thermal desorption aerosol gas chromatograph (TAG) system. Gas chromatograms are divided by retention time into evenly spaced bins, within which the mass spectra are summed. A previous chromatogram binning method was introduced for the purpose of chromatogram structure deconvolution (e.g., major compound classes) (Zhang et al., 2014). Here we extend the method development for the specific purpose of determining aerosol samples' sources. Chromatogram bins are arranged into an input data matrix for positive matrix factorization (PMF), where the sample number is the row dimension and the mass-spectra-resolved eluting time intervals (bins) are the column dimension. Then two-dimensional PMF can effectively do three-dimensional factorization on the three-dimensional TAG mass spectra data. The retention time shift of the chromatogram is corrected by applying the median values of the different peaks' shifts. Bin width affects chemical resolution but does not affect PMF retrieval of the sources' time variations for low-factor solutions. A bin width smaller than the maximum retention shift among all samples requires retention time shift correction. A six-factor PMF comparison among aerosol mass spectrometry (AMS), TAG binning, and conventional TAG compound integration methods shows that the TAG binning method performs similarly to the integration method. However, the new binning method incorporates the entirety of the data set and requires significantly less pre-processing of the data than conventional single compound identification and integration. In addition, while a fraction of the most oxygenated aerosol does not elute through an underivatized TAG analysis, the TAG binning method does have the ability to achieve molecular level resolution on other bulk aerosol components commonly observed by the AMS. 

Abstract. Airborne and ground-based measurements of aerosol concentrations, chemicalcomposition, and gas-phase precursors were obtained in three valleys innorthern Utah (USA). The measurements were part of the Utah Winter FineParticulate Study (UWFPS) that took place in January–February 2017. Totalaerosol mass concentrations of PM₁ were measured from a Twin Otteraircraft, with an aerosol mass spectrometer (AMS). PM₁ concentrationsranged from less than 2µgm⁻³ during clean periods to over100µgm⁻³ during the most polluted episodes, consistent withPM_2.5 total mass concentrations measured concurrently at groundsites. Across the entire region, increases in total aerosol mass above∼2µgm⁻³ were associated with increases in theammonium nitrate mass fraction, clearly indicating that the highest aerosolmass loadings in the region were predominantly attributable to an increase inammonium nitrate. The chemical composition was regionally homogenous fortotal aerosol mass concentrations above 17.5µgm⁻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 regionshowed variable concentrations with height in the polluted boundary layer.Higher average mass concentrations were observed within the first few hundredmeters above ground level in all three valleys during pollution episodes. Gas-phase measurements of nitric acid (HNO₃) and ammonia (NH₃) duringthe pollution episodes revealed that in the Cache and Utah valleys, partitioningof inorganic semi-volatiles to the aerosol phase was usually limited by theamount of gas-phase nitric acid, with NH₃ being in excess. The inorganicspecies were compared with the ISORROPIA thermodynamic model. Total inorganicaerosol mass concentrations were calculated for various decreases in totalnitrate and total ammonium. For pollution episodes, our simulations of a50% decrease in total nitrate lead to a 46±3% decrease in totalPM₁ mass. A simulated 50% decrease in total ammonium leads to a36±17%µgm⁻³ decrease in total PM₁ mass, over the entirearea of the study. Despite some differences among locations, ourresults showed a higher sensitivity to decreasing nitric acid concentrationsand the importance of ammonia at the lowest total nitrate conditions. In theSalt Lake Valley, both HNO₃ and NH₃ concentrations controlledaerosol formation.