John Ortega

Secondary Organic Aerosol Formation and Organic Nitrate Yield from NO3 Oxidation of Biogenic Hydrocarbons

The secondary organic aerosol (SOA) mass yields from NO3 oxidation of a series of biogenic volatile organic compounds (BVOCs), consisting of five monoterpenes and one sesquiterpene (α-pinene, β-pinene, Δ-3-carene, limonene, sabinene, and β-caryophyllene), were investigated in a series of continuous flow experiments in a 10 m3 indoor Teflon chamber. By making in situ measurements of the nitrate radical and employing a kinetics box model, we generate time-dependent yield curves as a function of reacted BVOC. SOA yields varied dramatically among the different BVOCs, from zero for α-pinene to 38–65% for Δ-3-carene and 86% for β-caryophyllene at mass loading of 10 μg m–3, suggesting that model mechanisms that treat all NO3 + monoterpene reactions equally will lead to errors in predicted SOA depending on each location’s mix of BVOC emissions. In most cases, organonitrate is a dominant component of the aerosol produced, but in the case of α-pinene, little organonitrate and no aerosol is formed.

Organosulfates as Tracers for Secondary Organic Aerosol (SOA) Formation from 2-Methyl-3-Buten-2-ol (MBO) in the Atmosphere

2-Methyl-3-buten-2-ol (MBO) is an important biogenic volatile organic compound (BVOC) emitted by pine trees and a potential precursor of atmospheric secondary organic aerosol (SOA) in forested regions. In the present study, hydroxyl radical (OH)-initiated oxidation of MBO was examined in smog chambers under varied initial nitric oxide (NO) and aerosol acidity levels. Results indicate measurable SOA from MBO under low-NO conditions. Moreover, increasing aerosol acidity was found to enhance MBO SOA. Chemical characterization of laboratory-generated MBO SOA reveals that an organosulfate species (C5H12O6S, MW 200) formed and was substantially enhanced with elevated aerosol acidity. Ambient fine aerosol (PM2.5) samples collected from the BEARPEX campaign during 2007 and 2009, as well as from the BEACHON-RoMBAS campaign during 2011, were also analyzed. The MBO-derived organosulfate characterized from laboratory-generated aerosol was observed in PM2.5 collected from these campaigns, demonstrating that it is a molecular tracer for MBO-initiated SOA in the atmosphere. Furthermore, mass concentrations of the MBO-derived organosulfate are well correlated with MBO mixing ratio, temperature, and acidity in the field campaigns. Importantly, this compound accounted for an average of 0.25% and as high as 1% of the total organic aerosol mass during BEARPEX 2009. An epoxide intermediate generated under low-NO conditions is tentatively proposed to produce MBO SOA.

An annual cycle of size‐resolved aerosol hygroscopicity at a forested site in Colorado

Received 12 September 2011; revised 11 January 2012; accepted 22 January 2012; published 16 March 2012. [1] The ability of particles composed wholly or partially of biogenic secondary organic compounds to serve as cloud condensation nuclei (CCN) is a key characteristic that helps to define their roles in linking biogeochemical and water cycles. In this paper, we describe size-resolved (14–350 nm) CCN measurements from the Manitou Experimental Forest in Colorado, where particle compositions were expected to have a large biogenic component. These measurements were conducted for 1 year as part of the Bio-hydro-atmosphere Interactions of Energy, Aerosols, Carbon, H2O, Organics, and Nitrogen program and determined the aerosol hygroscopicity parameter, k, at five water supersaturations between � 0.14% and � 0.97%. The average k value over the entire study and all supersaturations was kavg = 0.16 � 0.08. Kappa values decreased slightly with increasing supersaturation, suggesting a change in aerosol composition with dry diameter. Furthermore, some seasonal variability was observed with increased CCN concentrations and activated particle number fraction, but slightly decreased hygroscopicity, during the summer. Small particle events, which may indicate new particle formation, were observed throughout the study period, especially in the summer, leading to increases in CCN concentration, followed by a gradual increase in the aerosol mode size. The condensing material appeared to be predominantly composed of organic compounds and ledto a small decrease inkat the larger activation diameters during and immediately after those events.

Secondary Organic Aerosol Formation via 2-Methyl-3-buten-2-ol Photooxidation: Evidence of Acid-Catalyzed Reactive Uptake of Epoxides

Secondary organic aerosol (SOA) formation from 2-methyl-3-buten-2-ol (MBO) photooxidation has recently been observed in both field and laboratory studies. Similar to the level of isoprene, the level of MBO-derived SOA increases with elevated aerosol acidity in the absence of nitric oxide; therefore, an epoxide intermediate, (3,3-dimethyloxiran-2-yl)methanol (MBO epoxide), was synthesized and tentatively proposed to explain this enhancement. In this study, the potential of the synthetic MBO epoxide to form SOA via reactive uptake was systematically examined. SOA was observed only in the presence of acidic aerosol. Major SOA constituents, 2,3-dihydroxyisopentanol and MBO-derived organosulfate isomers, were chemically characterized in both laboratory-generated SOA and in ambient fine aerosol collected from the BEACHON-RoMBAS field campaign during the summer of 2011, where MBO emissions are substantial. Our results support the idea that epoxides are potential products of MBO photooxidation leading to the formation of atmospheric SOA and suggest that reactive uptake of epoxides may explain acid enhancement of SOA observed from other biogenic hydrocarbons.

Rapid cycling of reactive nitrogen in the marine boundary layer

Nitrogen oxides are essential for the formation of secondary atmospheric aerosols and of atmospheric oxidants such as ozone and the hydroxyl radical, which controls the self-cleansing capacity of the atmosphere. Nitric acid, a major oxidation product of nitrogen oxides, has traditionally been considered to be a permanent sink of nitrogen oxides. However, model studies predict higher ratios of nitric acid to nitrogen oxides in the troposphere than are observed. A ‘renoxification’ process that recycles nitric acid into nitrogen oxides has been proposed to reconcile observations with model studies, but the mechanisms responsible for this process remain uncertain. Here we present data from an aircraft measurement campaign over the North Atlantic Ocean and find evidence for rapid recycling of nitric acid to nitrous acid and nitrogen oxides in the clean marine boundary layer via particulate nitrate photolysis. Laboratory experiments further demonstrate the photolysis of particulate nitrate collected on filters at a rate more than two orders of magnitude greater than that of gaseous nitric acid, with nitrous acid as the main product. Box model calculations based on the Master Chemical Mechanism suggest that particulate nitrate photolysis mainly sustains the observed levels of nitrous acid and nitrogen oxides at midday under typical marine boundary layer conditions. Given that oceans account for more than 70 per cent of Earth’s surface, we propose that particulate nitrate photolysis could be a substantial tropospheric nitrogen oxide source. Recycling of nitrogen oxides in remote oceanic regions with minimal direct nitrogen oxide emissions could increase the formation of tropospheric oxidants and secondary atmospheric aerosols on a global scale.

Characterization and Nonparametric Regression of Rural and Urban Coarse Particulate Matter Mass Concentrations in Northeastern Colorado

The Colorado Coarse Rural Urban Sources and Health study (CCRUSH) is an ongoing study of the relationship between coarse particulate mass concentrations (PM10–2.5, particulate matter with diameter between 2.5 and 10 μm) and selected health effects. For two urban monitoring sites in Denver, CO, and two comparatively rural sites in Greeley, CO, hourly mass concentrations of PM10–2.5 and fine particulate matter (PM2.5, diameter less than 2.5 μm) have been measured by using dichotomous tapered element oscillating microbalances (TEOMs) with Filter Dynamics Measurement Systems (FDMS). This paper presents air quality results from just over a year of PM2.5 and PM10–2.5 measurements. Average PM2.5 concentrations ranged from 7.7 to 9.2 μg m−3 across the four sites with higher concentrations in Denver than Greeley. Average PM10–2.5 concentrations ranged from 9.0 to 15.5 μg m−3 with the highest values at the site in northeast Denver. Temporal variability in PM10–2.5 was higher than that in PM2.5 concentrations at all four sites. The two Greeley sites displayed moderate spatial correlation for PM2.5 and high correlation for PM10–2.5, whereas the two Denver sites showed lower spatial correlation for both PM sizes. PM10–2.5 concentrations in Denver were highest with winds from the direction of the citys urban core. PM10–2.5 concentrations in Greeley were moderately elevated with winds from the southwest to the northwest, coming from Denver and other large Front Range communities. Wind speed regressions for PM10–2.5 at the Denver sites primarily exhibited resuspension effects, while PM10–2.5 concentrations in Greeley showed relatively complex wind speed dependence. Copyright 2012 American Association for Aerosol Research

New Particle Formation and Growth in an Isoprene-Dominated Ozark Forest: From Sub-5 nm to CCN-Active Sizes

Particle Investigations at a Northern Ozarks Tower: NOx, Oxidant, Isoprene Research (PINOT NOIR) were conducted in a Missouri forest dominated by isoprene emissions from May to October 2012. This study presents results of new particle formation (NPF) and the growth of new particles to cloud condensation nuclei (CCN)-active sizes (∼100 nm) observed during this field campaign. The measured sub-5 nm particles were up to ∼20,000 cm−3 during a typical NPF event. Nucleation rates J1 were relatively high (11.0 ± 10.6 cm−3 s−1), and one order of magnitude higher than formation rates of 5 nm particles (J5). Sub-5 nm particle formation events were observed during 64% of measurement days, with a high preference in biogenic volatile organic compounds (BVOCs)- and SO2-poor northwesterly (90%) air masses than in BVOCs-rich southerly air masses (13%). About 80% of sub-5 nm particle events led to the further growth. While high temperatures and high aerosol loadings in the southerly air masses were not favorable for nucleation, high BVOCs in the southerly air masses facilitated the growth of new particles to CCN-active sizes. In overall, 0.4–9.4% of the sub-5 nm particles grew to CCN-active sizes within each single NPF event. During a regional NPF event period that took place consecutively over several days, concentrations of CCN size particles increased by a factor of 4.7 in average. This enhanced production of CCN particles from new particles was commonly observed during all 13 regional NPF events during the campaign period. Copyright 2014 American Association for Aerosol Research

A fast-scanning DMA train for precision quantification of early nanoparticle growth

Understanding the mechanisms of secondary organic aerosol formation is of critical importance for assessing aerosol radiative forcing and improving air quality control. A key quantity describing the gas-to-particle conversion is the rate at which nanoparticles of certain size are formed per unit volume and time. The quantification of nanoparticle formation rates requires precise knowledge of growth rates. Here we present a newly developed DMA train capable of providing definitive growth rates from size distribution scan frequencies of 1 Hz. This system has proven to yield statistically significant data even at ultra-low post-classification concentrations of 0.1 cm−3. A chamber study of α-pinene ozonolysis under close-to-ambient conditions demonstrates the need for particle size distribution measurements with increased time resolution.