Constraints on the relative roles of N2O5 heterogeneous and NO3 gas-phase chemistry in secondary aerosol production
Year Initially Funded: 2018
Principal Investigator (s): Bertram, Timothy (U. Wisconsin)
Nitrogen oxides (NOx) play a controlling role in the photochemical production of ozone (O3) at Earth’s surface. Laboratory and field measurements have demonstrated that NOx can also significantly impact the production rate of organic and inorganic aerosol mass during the daytime. More recently, the importance of nocturnal chemistry involving N2O5 and nitrate radicals (NO3) has emerged. Nocturnal reactive nitrogen chemistry impacts ozone concentrations and aerosol particle mass in at least three ways: i) efficient, terminal removal of NOx and volatile organic compounds (VOC) at night reduces O3 precursors, ii) the nocturnal oxidation of VOC by NO3 radicals is an efficient pathway for organic aerosol production, and iii) the hydrolysis of N2O5 on aerosol surfaces leads to the production of inorganic nitrate aerosol. Central to these nocturnal processes is the equilibrium between N2O5 and NO3, where the extent of NO3 chemistry can be regulated by the efficiency of N2O5 loss rates, and vice versa.
The primary scientific objective of this proposal is the direct, simultaneous measurement of NO3 gas-phase reactivity and N2O5 heterogeneous reactivity in ambient air for both urban and remote environments. Ambient measurements of NO3 and N2O5 reactivity provide unique constraints on the nighttime oxidation rates of VOCs and the extent by which NO3 reactions impede NOx loss and particulate nitrate formation via N2O5 heterogeneous reactions. To achieve this objective, we propose the following specific tasks:
1) Analysis of existing measurements of NO3 and N2O5 reactivity in ambient air at a remote site made during the Southern Oxidant and Aerosol Study (SOAS) at Look Rock, TN.
2) Conduct new measurements of NO3 and N2O5 reactivity in ambient air at an urban site with variable influence from biogenic volatile organic emissions (Madison, WI).
Measurements of NO3 gas-phase reactivity and N2O5 heterogeneous reactivity in ambient air will be determined using existing, well-characterized flow reactors equipped with a high purity and controllable N2O5/NO3 source and chemical ionization mass spectrometer for reactant and product detection. Ancillary measurements of nitric oxide, speciated volatile organic compounds, and aerosol surface area and chemical composition will be used to apportion observed loss rates and test model parameterizations.
Measurements and modeling of organic nitrate gases and aerosol: Influences on the lifetimes of NOx, ozone and aerosol
Year Initially Funded: 2018
Principal Investigator (s): Cohen, Ronald (UC Berkeley)
To address NOAA’s goal of reducing uncertainties in the role of “reactive nitrogen processes in the atmosphere as they relate to aerosol formation,” we propose research aimed at improving our understanding of how biogenic volatile organic carbon (BVOC) interacts with nitrogen oxides (NOx and NOy) to affect the spatial and temporal patterns of NOx, ozone and aerosol. In the U.S. we are moving from a regime where urban mobile sources and large power plants dominate the spatial pattern of NOx to one where agricultural sources are predominant. This is bringing about dramatic changes in the chemical regime affecting the reaction pathways that govern the interactions of NOx and BVOC. Recent research, with key contributions from our research group, has established that biogenic organic nitrate production is the dominant sink of NOx on the continents. This fact makes knowledge of the fate of biogenic organic nitrates much more important for an understanding of atmospheric chemistry. Evidence suggests that the ultimate fate of a significant fraction of RONO2 molecules includes hydrolysis in aerosol or clouds or oxidation to release NOx. At the same time a significant fraction of aerosol mass has been identified as organic nitrates. Deposition to the surface and through the stomata of plants are also likely important. We plan two lines of research to address the underlying uncertainties about these processes:
1) We propose to flesh out a comprehensive chemical mechanism for BVOC oxidation for use in WRF-CHEM and WRF-CMAQ and to use this mechanism in a series of model calculations and analyses to assess the role of organic nitrates in the chemistry of the atmosphere. The mechanism will conserve carbon and nitrogen and include production and oxidation of many individual anthropogenic and biogenic nitrates. It will have detailed representation of isoprene and monoterpene (more than one) chemistry. For all of these molecules, the mechanism will represent both gas and aerosol processes and deposition to surfaces and stomata. Our research will include optimization of chemical mechanisms, comparison to field observations of concentrations of total and individual gas and aerosol RONO2, comparison to field observations of correlations of RONO2 with other major co-products such as aerosol mass, or H2CO. One focus of the work will be comparison of calculations to in situ and space-based observations of nitrogen oxides and aerosol in wet and dry environments to assess the effects of clouds and humidity on the chemistry. Another focus will be an assessment of the range of uncertainty introduced by incomplete knowledge of boundary layer mixing during day and night. The role of nighttime chemistry is strongly mediated by boundary layer dynamics, both because of remnant effects of composition in the residual layer and because the extent of mixing of emissions of monoterpenes into the thin nocturnal layer near the surface determine the volume over which reactions take place.
2) The extent to which organic nitrates are removed by stomatal uptake vs. simple deposition to all surfaces is a largely unexplored aspect of RONO2 chemistry. We propose a series of laboratory experiments to observe the deposition of some example organic nitrates at the leaf level. Using tools we have developed for the study of leaf level NO2 emission and uptake, we will expose a suite of plants to specific organic nitrates and measure the role of surface and stomatal uptake on their deposition rates. These results will guide development of modeling to represent the two distinct deposition processes affecting RONO2 concentrations.
The aging of aerosol nitrate and implications for the global nitrogen cycle
Year Initially Funded: 2018
Principal Investigator (s): Kroll, Jesse H. (MIT); Murphy, Jennifer (PI, U. Toronto)
Co-PI (s): Heald, Colette L. (MIT)
The atmospheric chemistry of nitrogen oxides (NOx), biogenic volatile organic compounds (BVOCs), and secondary organic aerosol (SOA) are closely linked: NOx has a governing role on the mechanism of BVOC oxidation and SOA formation, and SOA in turn can sequester NOx in the particle phase in the form of nitrates. Particulate nitrate (which includes both organic and inorganic nitrate) can make up an important fraction of reactive nitrogen in the atmosphere, but the fate of nitrogen in the particle phase is poorly understood. Nitrate particles have traditionally been considered to be a sink of reactive oxidized nitrogen (NOy) via wet removal, though recent studies suggest that particulate nitrate can photolyze, emitting NOx and HONO to the gas phase. This so-called “renoxification” can have important implications for atmospheric chemistry away from anthropogenic sources, but little is known about the detailed chemistry of the process, or its dependence on key parameters such as relative humidity, wavelength, etc. Similarly, the fate of particulate organic nitrates is poorly understood – the further “aging” (photolysis or heterogeneous oxidation) of organic nitrates within SOA may be an important component of the overall BVOC oxidation mechanism and the atmospheric cycling of reactive nitrogen, but there exist few constraints on either the rates or products of such aging reactions.
Here we propose a joint laboratory-modeling study to explore the importance of aging of aerosol nitrate to the atmospheric lifecycles of both BVOC-derived SOA and NOy. Laboratory studies will focus on the kinetics and products of aging reactions, which will be examined by generating nitrate-containing particles in a large environmental chamber, sampling them into a flow reactor to simulate several days of photolytic/oxidative aging, and measuring reactants and products (in the gas and particle phases) with a suite of state-of-the-art analytical instruments. The focus of these experiments will be on particulate organic nitrates generated from BVOC oxidation (by OH in the presence of NO, or by NO3), though the photolysis of inorganic nitrates, a simpler chemical system, will also be explored. Such laboratory measurements will provide new constraints on the chemistry of particulate nitrate, which will then be incorporated into a global chemical-transport model (GEOS-Chem). Results from this model will be tested against field measurements of NOy species, most importantly particulate nitrates (both organic and inorganic), and will provide insight into the impact of this nitrate aging chemistry (e.g., renoxification) on global particulate nitrate loadings and, more generally, the chemistry of the troposphere.
Anthropogenic influence on the oxidation of biogenic volatile organic compounds: implications for formation of secondary organic aerosols
Year Initially Funded: 2018
Principal Investigator (s): Mao, Jingqiu (U. Alaska – Fairbanks)
Co-PI (s): Ng, Nga Lee (co-PI, Georgia Tech); Naik, Vaishali (co-I, NOAA/GFDL), Horowitz (co-I, NOAA/GFDL)
Oxidation of biogenic volatile organic compounds (BVOCs) leads to the formation of secondary organic aerosols (SOA). The production of SOA from BVOCs is greatly modulated by anthropogenic NOx emissions, but the role of SOA in climate system and how anthropogenic activity plays a role is largely unknown. Southeast US is heavily influenced by both anthropogenic and biogenic emissions. In particular, anthropogenic NOx emissions have been substantially reduced in the past decade, providing an unprecedented opportunity to evaluate the anthropogenic influence on the formation of secondary organic aerosol on a decadal scale. We aim to advance the science in this area by addressing the following questions:
How do SOA and its precursors (isoprene epoxydiol (IEPOX), organic nitrates and glyoxal) respond to NOx emission changes in Southeast US?
What are the possible drivers for this declining trend of organic aerosol over Southeast US? Can we use the trend of organic aerosol in Southeast US to quantify the anthropogenic influence on biogenic SOA?
Can we extend estimates of this anthropogenic influence from decadal scale to the century scale, to better quantify the radiative forcing from biogenic SOA from the preindustrial period to the present day? What are the uncertainties associated with this estimate?
Laboratory Chamber Studies on Organic Nitrate and Secondary Organic Aerosol (SOA) Formation from Oxidation of Monoterpenes
Year Initially Funded: 2018
Principal Investigator (s): Ng, Nga Lee (Georgia Tech)
A key mechanism that couples anthropogenic emissions with biogenic emissions is the effect of NOx on organic nitrates (ON) and biogenic secondary organic aerosols (SOA) formation. The overall goal is to establish a fundamental and quantitative understanding of the formation mechanisms, yields, gas-particle partitioning, and fates of ON and SOA from monoterpenes under all oxidation pathways (ozonolysis, photooxidation, and nitrate radical oxidation). Specific objectives are 1) to investigate formation mechanisms and quantify ON yields from ozonolysis, OH, and NO3 oxidation of monoterpenes, 2) to quantify the rates and extents of hydrolysis of particle-phase ON generated from different oxidation pathways, 3) to systematically evaluate the photochemical fates of gas-phase and particle-phase ON, and 4) to obtain closure of reactive oxidized nitrogen species in chamber experiments to achieve a quantitative understanding of the roles of ON species in NOx cycling. A series of laboratory chamber experiments will be conducted to investigate ON and SOA formation from α-pinene and β-pinene oxidations. We will systematically investigate the effects of various factors on ON formation and fates: relative humidity, seed type and particle acidity, hydrolysis, and photochemical aging (hν reaction). The changes in gas and aerosol composition, aerosol chemical and physical properties will be continuously monitored by a suite of analytical instruments surrounding the chamber facility. The proposed study will provide fundamental, comprehensive, and quantitative insights into monoterpene organic nitrogen chemistry. We expect the results from this study to significantly transform our understanding of the roles of ON in NOx cycling, ozone and SOA formation. Results from this study will not only contribute to the interpretation of ambient observations, but will also provide the much-needed fundamental data for improved parameterizations of ON and SOA formation from BVOC oxidations in models, facilitating more accurate predictions of the control of ON over NOx cycling, ozone, and OA as NOx emissions continue to change in the future.
Oxidation mechanism and organic aerosol formation from the α-pinene and pinonaldehyde reaction with NO3 radicals under near-ambient conditions
Year Initially Funded: 2018
Principal Investigator (s): Nguyen, Tran B. (UC Davis)
Introduction of the problem: The reaction of α-pinene with the nitrate (NO3) radical exerts significant control over the formation of secondary organic aerosol (SOA) and organic nitrates in the Southeast US, and other regions with joint biogenic and anthropogenic influence. Yet, there are large uncertainties in both the organic nitrate yield (10-30%) and the SOA yield (0-30%) for this reaction in the literature, and an incomplete understanding of the role of the organic nitrates as NOx reservoirs in ambient aerosols. These large uncertainties undermine accuracy in modeling SOA and reactive nitrogen from BVOC+NO3 sources in nature.
Rationale for proposed work: Discrepancies in yields and knowledge gaps in the reaction mechanism of α-pinene + NO3 may be due to the large range of chemical and physical conditions under which this reaction has been studied in past chamber studies, including those where the reaction is shifted toward higher volatility products instead of SOA. This work will thoroughly characterize the reaction of α-pinene+NO3 under near-ambient conditions by finely tuning reaction parameters, and provide a systematic understanding of (1) how surface area and liquid water content of inorganic seeds affect SOA yields in the chamber; (2) how the reaction responds to changing chemistry regimes, specifically to NO3 and HO2 levels that approach those representative of the atmosphere; and (3) how the second-generation chemistry of pinonaldehyde+NO3 contribute to α-pinene’s ability to produce SOA and organic nitrates.
Brief summary of work: A series of atmospheric chamber experiments are planned to measure the yield and molecular composition of SOA and organic nitrates in the α-pinene+NO3 and pinonaldehyde+NO3 reactions, under a variety of conditions that mimic the reaction branching observed in the Southeast US nighttime (e.g., mainly RO2+HO2 and RO2+NO). A range of NO3 production rates (PNO3), relative humidity, and particle surface area will be investigated for each reaction subset. In addition to SOA yields, the oxidized VOC products (including gaseous organic nitrates) will be measured in-situ with a CF3O- CIMS, the SOA molecular composition (including aerosol organic nitrates) will be characterized with high-resolution nanospray mass spectrometry, along with other planned measurements.
Relevance to the AC4 Program: This work directly addresses two out of three priorities in the AC4 Program, Priority 1 and Priority 2, i.e., (a) promote a better understanding of BVOC+NO3 oxidation mechanisms; (b) provide better constraints on the yields of SOA and gaseous products from BVOC+NO3; and (c) provide accurate measurements of aerosol organic nitrate production and loss to understand their role in atmospheric NOx removal. Relevance to NOAA’s climate goals This research is expected to help reduce uncertainties in atmospheric model simulations of SOA and reactive nitrogen in certain regional environments. Our focus on the dependence of the reactions on PNO3 rates has relevance to understanding a changing future atmosphere, which is a benefit to society in general. As emission controls further reduce NOx in the United States, we will see lower NO2 and O3 concentrations (and thus, lower PNO3).
Development of a Formaldehyde Product from the Ozone Mapping Profiler Suite Nadir Mapper (OMPS-N) on Suomi-NPP and JPSS-1
Year Initially Funded: 2018
Principal Investigator (s): Nowlan, Caroline (Smithsonian Astrophysical Observatory)
Co-PI (s): González Abad, Gonzalo (Smithsonian Astrophysical Observatory)
Formaldehyde is one of the most abundant non-methane volatile organic compounds (NMVOCs) in the troposphere. Over land, high levels of formaldehyde can result from the oxidation of NMVOCs from biogenic, anthropogenic and pyrogenic activities, as well as from direct emission by industrial activity and fires. When present in sufficient amounts, formaldehyde can be detected using nadir sounding satellite instruments viewing backscattered light in the ultraviolet. Its short lifetime of a few hours means that formaldehyde can be used as a proxy of NMVOCs, and as a top-down constraint on the isoprene emissions that contribute to the formation of secondary organic aerosols in certain chemical regimes.
The Ozone Mapping and Profiler Suite nadir-viewing instruments (OMPS-N) on the Suomi National Polar-orbiting Partnership (NPP) and Joint Polar Satellite System (JPSS)-1 satellites operate in the ultraviolet, with a focus on measurements of ozone. To date, there has been no formal directive from either NASA or NOAA to produce a formaldehyde product from OMPS. Formaldehyde retrievals from OMPS-N have, however, been demonstrated by two research groups, including our own at the Smithsonian Astrophysical Observatory, which has previously produced four years of global formaldehyde retrievals from OMPS-N on Suomi NPP.
We propose to generate satellite retrieval products of formaldehyde from OMPS-N on the Suomi NPP and JPSS-1 satellites by:
Developing consistent retrievals of formaldehyde with quantitative uncertainties from the two OMPS-N instruments on board Suomi NPP and JPSS-1, using well-established retrieval algorithms developed at the Smithsonian Astrophysical Observatory;
Validating our retrievals using airborne measurement data from field campaigns; and
Creating and disseminating the OMPS-N formaldehyde products.
The role of in-canopy processes on biogenic VOC oxidation and formation of organic nitrogen aerosols
Year Initially Funded: 2018
Principal Investigator (s): Steiner, Allison (U. Michigan)
The balance between biogenic volatile organic compound (BVOC), nitrogen oxides (NOx), and hydrogen oxides (HOx) is key to determining photochemical regimes and aerosol formation. Field campaigns in forested regions have improved our understanding of the role of BVOC emissions on gas-phase tropospheric chemistry (e.g., isoprene oxidation under a range of NOx conditions), yet large uncertainties remain about the role of organic nitrates in aerosol formation. Typically, regional and global scale models represent the forest canopy and resulting BVOC emissions as a single lower boundary condition. However, the three-dimensional nature of the forest canopy has been shown to be important for BVOC emissions and atmospheric chemistry. Because BVOC are emitted from vegetation and react quickly in the atmosphere, the potential for oxidation and aerosol formation within the forest canopy remains an open question. Other physical processes, such as deposition, are also important for the loss of aerosol precursors and particulate matter. The large concentration gradients of reactive species within forest canopies coupled with large temperature gradients that influence partitioning highlight the need for evaluating these processes at a finer vertical scale than can be included in most regional models.
The proposed work use a 1-D canopy-chemistry model (FORCAsT, [Ashworth et al., 2015]) with existing field observations at two forests sites to address two scientific questions:
What are the role of physical and chemical in-canopy processes on BVOC oxidation and the formation of organic nitrate aerosol?
How do in-canopy processes inform emissions and chemistry of organic nitrates at the regional and global scales?
Work tasks will include adding new partitioning parameterizations for organic nitrates, evaluating these changes with existing field observations, and analyzing the chemical and physical processes that drive the formation of organic nitrates within and above a forest canopy. At the project conclusion, we will evaluate key processes to include in regional and global scale atmospheric chemistry models to accurately represent the formation of biogenically derived organic nitrates.
Constraining NOx-BVOC Oxidation Chemistry and Feedbacks Using Oxygen Stable Isotopes
Year Initially Funded: 2018
Principal Investigator (s): Walters, Wendell (Brown U.)
Co-PI (s): Hastings, Meredith (Brown U.); Ng, Nga Lee (Georgia Tech.)
The chemical coupling between nitrogen oxides (NOx = NO + NO2) and biogenic volatile organic compounds (BVOCs) have important consequences for air quality and climate, playing a key role in the global budget for reactive nitrogen, ozone (O3), hydrogen oxide radicals (OH + RO2 + HO2) and secondary organic aerosols (SOA). Despite its significance for the climate system and human health, there are numerous uncertainties related to the chemical coupling of NOx and BVOC chemistry. These include a lack of understanding of the feedback of NOx and BVOC interactions on local/regional oxidation chemistry, the mechanisms responsible for SOA formation and NOx loss, and expected changes in NOx and BVOC oxidation chemistry in the future as anthropogenic NOx emissions are expected to decrease. Here we plan to conduct controlled laboratory chamber studies to further our understanding of the chemical coupling between NOx and BVOC utilizing oxygen stable isotopes (Δ17O and δ18O) of NO2, organic nitrate (both gas and particle phase), and inorganic nitrate. This will aid in evaluating the oxidation coupling between NOx and important BVOC molecules (isoprene, α- pinene, and β-pinene), and their role in organic nitrate formation pathways and NOx loss under varying conditions. Our specific objectives include to: (1) examine various organic nitrate formation pathways (OH, NO3, and O3) and their impact on NOx loss processes and oxidant budgets, (2) evaluate changes in the coupling of NOx and BVOC oxidation chemistry and the role this plays in reactive nitrogen chemistry cycling, and (3) probe the fate of peroxy radicals in the nighttime atmosphere and its role in NOx oxidation and organic nitrate formation. The combination of an established experimental chamber setup (GA Tech), suite of sensitive collection and measurement techniques, and isotope measurement facility (Brown Univ) put us in an excellent position to examine many of the major uncertainties in NOx-BVOC chemistry.
Quantitative Investigation of Mechanisms of Oxidation and SOA Formation from Terpenes for Different Oxidants and NOx Regimes
Year Initially Funded: 2018
Principal Investigator (s): Ziemann, Paul J. (U. Colorado)
Co-PI (s): de Gouw, Joost (U. Colorado); Jimenez, Jose (U. Colorado)
Secondary organic aerosol (SOA) formed from the oxidation of biogenic hydrocarbons is one of the largest sources of organic aerosol in Earth’s atmosphere, and a significant component of total aerosol. While the carbon in biogenic SOA is derived from naturally emitted volatile organic compounds (VOCs), it has become clear that anthropogenic pollutants like NOx and SO2 play important roles in biogenic SOA formation, and thus affect both air quality and climate. While progress has been made elucidating mechanisms of biogenic SOA formation in the presence of anthropogenic pollutants, the net effect of anthropogenic emissions on biogenic SOA loadings in the atmosphere are still highly uncertain. In this proposed project, titled “Quantitative Investigation of Mechanisms of Oxidation and SOA Formation from Terpenes for Different Oxidants and NOx Regimes”, we will conduct experimental and modeling studies in order to achieve an improved understanding and quantitative description of the mechanisms of atmospheric oxidation of monoterpenes, with an emphasis on the effect of oxidation regime and NOx on reaction pathways, gas- and particle-phase products, and SOA formation. Such information is necessary to understand the role of NOx in the evolution of SOA composition due to future changes in NOx and BVOC emissions and climate. Experiments will be conducted in the CU-Boulder chamber facility using a suite of state-of-the-art online and offline instruments and methods and will focus on reactions of OH and NO3 radicals, the major daytime and nighttime oxidants, respectively, with three of the most atmospherically abundant monoterpenes: α-pinene, β-pinene, and Δ-3-carene. Results will be used to evaluate and improve monoterpene oxidation and SOA formation mechanisms in the explicit GECKO-A model and in simplified versions suitable for use in chemical transport models. The investigators on this proposal were major participants in the SENEX, SOAS, SAS, SEAC4RS and GoAmazon field campaigns, and because the chamber experiments will be conducted with some of the same instruments used in the field the results will be immediately useful to the interpretation of field data.