Glyoxal processing by aerosol multiphase chemistry: towards a kinetic modeling framework of secondary organic aerosol formation in aqueous particles

This study presents a modeling framework based on laboratory data to describe the kinetics of glyoxal reactions that form secondary organic aerosol (SOA) in aqueous aerosol particles. Recent laboratory results on glyoxal reactions are reviewed and a consistent set of empirical reaction rate constants is derived that captures the kinetics of glyoxal hydration and subsequent reversible and irreversible reactions in aqueous inorganic and water-soluble organic aerosol seeds. Products of these processes include (a) oligomers, (b) nitrogen-containing products, (c) photochemical oxidation products with high molecular weight. These additional aqueous phase processes enhance the SOA formation rate in particles and yield two to three orders of magnitude more SOA than predicted based on reaction schemes for dilute aqueous phase (cloud) chemistry for the same conditions (liquid water content, particle size). <br><br> The application of the new module including detailed chemical processes in a box model demonstrates that both the time scale to reach aqueous phase equilibria and the choice of rate constants of irreversible reactions have a pronounced effect on the predicted atmospheric relevance of SOA formation from glyoxal. During day time, a photochemical (most likely radical-initiated) process is the major SOA formation pathway forming ∼5 μg m<sup>−3</sup> SOA over 12 h (assuming a constant glyoxal mixing ratio of 300 ppt). During night time, reactions of nitrogen-containing compounds (ammonium, amines, amino acids) contribute most to the predicted SOA mass; however, the absolute predicted SOA masses are reduced by an order of magnitude as compared to day time production. The contribution of the ammonium reaction significantly increases in moderately acidic or neutral particles (5 < pH < 7). <br><br> Glyoxal uptake into ammonium sulfate seed under dark conditions can be represented with a single reaction parameter <i>k</i><sub>effupt</sub> that does not depend on aerosol loading or water content, which indicates a possibly catalytic role of aerosol water in SOA formation. However, the reversible nature of uptake under dark conditions is not captured by <i>k</i><sub>effupt</sub>, and can be parameterized by an effective Henry's law constant including an equilibrium constant <i>K</i><sub>olig</sub> = 1000 (in ammonium sulfate solution). Such reversible glyoxal oligomerization contributes <1% to total predicted SOA masses at any time. <br><br> Sensitivity tests reveal five parameters that strongly affect the predicted SOA mass from glyoxal: (1) time scales to reach equilibrium states (as opposed to assuming instantaneous equilibrium), (2) particle pH, (3) chemical composition of the bulk aerosol, (4) particle surface composition, and (5) particle liquid water content that is mostly determined by the amount and hygroscopicity of aerosol mass and to a lesser extent by the ambient relative humidity. <br><br> Glyoxal serves as an example molecule, and the conclusions about SOA formation in aqueous particles can serve for comparative studies of other molecules that form SOA as the result of multiphase chemical processing in aerosol water. This SOA source is currently underrepresented in atmospheric models; if included it is likely to bring SOA predictions (mass and O/C ratio) into better agreement with field observations

Estimation of the mass absorption cross section of the organic carbon component of aerosols in the Mexico City Metropolitan Area (MCMA)

International audienceData taken from the MCMA-2003 and the 2006 MILAGRO field campaigns are used to examine the absorption of solar radiation by the organic component of aerosols. Using irradiance data from a Multi-Filter Rotating Shadowband Radiometer (MFRSR) and an actinic flux spectroradiometer (SR), we derive aerosol single scattering albedo, ?0,?, as a function of wavelength, ?. We find that in the near-UV spectral range (250 to 400 nm) ?0,? is much lower compared to ?0,? at 500 nm indicating enhanced absorption in the near-UV range. Absorption by elemental carbon, dust, or gas cannot account for this enhanced absorption leaving the organic part of the aerosol as the only possible absorber. We use data from a surface deployed Aerodyne Aerosol Mass Spectrometer (AMS) along with the inferred ?0,? to estimate the Mass Absorption Cross section (MAC) for the organic carbon. We find that the MAC is about 10.5 m2/g at 300 nm and falls close to zero at about 500 nm; values that are roughly consistent with other estimates of organic carbon MAC. These MAC values can be considered as "radiatively correct" because when used in radiative transfer calculations the calculated irradiances/actinic fluxes match those measured at the wavelengths considered here. For an illustrative case study described here, we estimate that the light absorption by the "brown" (organic) carbonaceous aerosol can add about 40% to the light absorption of black carbon in Mexico City. This contribution will vary depending on the relative abundance of organic carbon relative to black carbon. Furthermore, our analysis indicates that organic aerosol would slow down photochemistry by selectively scavenging the light reaching the ground at those wavelengths that drive photochemical reactions. Finally, satellite retrievals of trace gases that are used to infer emissions currently assume that the MAC of organic carbon is zero. For trace gases that are retrieved using wavelengths shorter then 420 nm (i.e. SO2, HCHO, halogenoxides, NO2), the assumption of non-zero MAC values will induce an upward correction to the inferred emissions. This will be particularly relevant in polluted urban atmospheres and areas of biomass burning where organic aerosols are particularly abundant

Oxidative capacity of the Mexico City atmosphere - Part 1: A radical source perspective

A detailed analysis of OH, HO2 [HO subscript 2] and RO2 [RO subscript 2] radical sources is presented for the near field photochemical regime inside the Mexico City Metropolitan Area (MCMA). During spring of 2003 (MCMA-2003 field campaign) an extensive set of measurements was collected to quantify time-resolved ROx [RO subscript x] (sum of OH, HO2 [HO subscript 2], RO2 [RO subscript 2]) radical production rates from day- and nighttime radical sources. The Master Chemical Mechanism (MCMv3.1) was constrained by measurements of (1) concentration time-profiles of photosensitive radical precursors, i.e., nitrous acid (HONO), formaldehyde (HCHO), ozone (O3 [o subscript 3]), glyoxal (CHOCHO), and other oxygenated volatile organic compounds (OVOCs); (2) respective photolysis-frequencies (J-values); (3) concentration time-profiles of alkanes, alkenes, and aromatic VOCs (103 compound are treated) and oxidants, i.e., OH- and NO3 [NO subscript 3] radicals, O3 [O subscript 3]; and (4) NO, NO2 [NO subscript 2], meteorological and other parameters. The ROx [RO subscript x] production rate was calculated directly from these observations; the MCM was used to estimate further ROx [RO subscript x] production from unconstrained sources, and express overall ROx [RO subscript x] production as OH-equivalents (i.e., taking into account the propagation efficiencies of RO2 [RO subscript 2] and HO2 [HO subscript 2] radicals into OH radicals). Daytime radical production is found to be about 10–25 times higher than at night; it does not track the abundance of sunlight. 12-h average daytime contributions of individual sources are: Oxygenated VOC other than HCHO about 33%; HCHO and O3 [O subscript 3] photolysis each about 20%; O3/alkene [O subscript 3 / alkene] reactions and HONO photolysis each about 12%, other sources <3%. Nitryl chloride photolysis could potentially contribute ~15% additional radicals, while NO2* [NO subscript 2*] + water makes – if any – a very small contribution (~2%). The peak radical production of ~7.5 107 [10 superscript 7] molec cm−3 [cm superscript -3] s−1 [s superscript -1] is found already at 10:00 a.m., i.e., more than 2.5 h before solar noon. O3/alkene [O subscript 3 / alkene] reactions are indirectly responsible for ~33% of these radicals. Our measurements and analysis comprise a database that enables testing of the representation of radical sources and radical chain reactions in photochemical models. Since the photochemical processing of pollutants in the MCMA is radical limited, our analysis identifies the drivers for ozone and SOA formation. We conclude that reductions in VOC emissions provide an efficient opportunity to reduce peak concentrations of these secondary pollutants, because (1) about 70% of radical production is linked to VOC precursors; (2) lowering the VOC/NOx [VOC / NO subscript x] ratio has the further benefit of reducing the radical re-cycling efficiency from radical chain reactions (chemical amplification of radical sources); (3) a positive feedback is identified: lowering the rate of radical production from organic precursors also reduces that from inorganic precursors, like ozone, as pollution export from the MCMA caps the amount of ozone that accumulates at a lower rate inside the MCMA. Continued VOC reductions will in the future result in decreasing peak concentrations of ozone and SOA in the MCMA.National Science Foundation (U.S.) (Grant ATM-0528227)United States. Dept. of Energy (Grant DE-FG02-0563980)Mexico. Comisión Ambiental MetropolitanaNational Science Foundation (U.S.) (CAREER grant ATM-0847793)Alliance for Global Sustainabilit

Oxidative capacity of the Mexico City atmosphere - Part 2: A ROx radical cycling perspective

A box model using measurements from the Mexico City Metropolitan Area study in the spring of 2003 (MCMA-2003) is presented to study oxidative capacity (our ability to predict OH radicals) and ROx (ROx=OH+HO2+RO2+RO) radical cycling in a polluted (i.e., very high NOx=NO+NO2) atmosphere. Model simulations were performed using the Master Chemical Mechanism (MCMv3.1) constrained with 10 min averaged measurements of major radical sources (i.e., HCHO, HONO, O3, CHOCHO, etc.), radical sink precursors (i.e., NO, NO2, SO2, CO, and 102 volatile organic compounds (VOC)), meteorological parameters (temperature, pressure, water vapor concentration, dilution), and photolysis frequencies. Modeled HOx (=OH+HO2) concentrations compare favorably with measured concentrations for most of the day; however, the model under-predicts the concentrations of radicals in the early morning. This "missing reactivity" is highest during peak photochemical activity, and is least visible in a direct comparison of HOx radical concentrations. We conclude that the most likely scenario to reconcile model predictions with observations is the existence of a currently unidentified additional source for RO2 radicals, in combination with an additional sink for HO2 radicals that does not form OH. The true uncertainty due to "missing reactivity" is apparent in parameters like chain length. We present a first attempt to calculate chain length rigorously i.e., we define two parameters that account for atmospheric complexity, and are based on (1) radical initiation, n(OH), and (2) radical termination, ω. We find very high values of n(OH) in the early morning are incompatible with our current understanding of ROx termination routes. We also observe missing reactivity in the rate of ozone production (P(O3)). For example, the integral amount of ozone produced could be under-predicted by a factor of two. We argue that this uncertainty is partly accounted for in lumped chemical codes that are optimized to predict ozone concentrations; however, these codes do not reflect the true uncertainty in oxidative capacity that is relevant to other aspects of air quality management, such as the formation of secondary organic aerosol (SOA). Our analysis highlights that apart from uncertainties in emissions, and meteorology, there is an additional major uncertainty in chemical mechanisms that affects our ability to predict ozone and SOA formation with confidence.National Science Foundation (U.S.) (ATM-0528227)United States. Dept. of Energy (Grant DE-FG02-0563980)Alliance for Global SustainabilityHenry & Camille Dreyfus FoundationAlexander von Humboldt-StiftungNational Science Foundation (U.S.) (CAREER award ATM-0847793

Characterizing ozone production in the Mexico City Metropolitan Area: a case study using a chemical transport model

International audienceAn episodic simulation is conducted to characterize ozone (O3) photochemical production and investigate its sensitivity to emission changes of ozone precursors in the Mexico City Metropolitan Area (MCMA) using the Comprehensive Air Quality Model with extensions (CAMx). High Ox (O3+NO2) photochemical production rates of 10?80 ppb/h are predicted due to the high reactivity of volatile organic compounds (VOCs) in which alkanes, alkenes, and aromatics exert comparable contributions. The predicted ozone production efficiency is between 4?10 O3 molecules per NOx molecule oxidized, and increases with VOC-to-NO2 reactivity ratio. Process apportionment analyses indicate significant outflow of pollutants such as O3 and peroxyacetyl nitrate (PAN) from the urban area to the surrounding regional environment. PAN is not in chemical-thermal equilibrium during the photochemically active periods. Sensitivity studies of O3 production suggest that O3 formation in the MCMA urban region with less chemical aging (NOz/NOy3 production and its sensitivities to precursors suggest that midday O3 formation during this episode is VOC sensitive in the urban region on the basis of the current emissions inventory. More episodic studies are needed to construct a comprehensive and representative picture of the O3 production characteristics and its response to emission controls

Characterizing ozone production and response under different meteorological conditions in Mexico City

International audienceTropospheric photochemistry, particularly the formation of ozone (O3), depends not only on pollutant emissions, but also on meteorological conditions. In this study a 3-D chemical transport model CAMx was employed to investigate the O3 formation and its response to emission reductions under three distinctively different meteorological conditions ("Cold Surge", "O3-North" and "O3-South") in the Mexico City Metropolitan Area during the MCMA-2003 field measurement campaign. The O3 formation characteristics and sensitivity to emissions changes were found to be weakly dependent on the meteorological conditions. The evolution of O3 formation and sensitivity were also examined along the photochemical plume transport pathway. The midday O3 production was found to undergo a rapid increase in a narrow range of chemical aging, while plumes in the downwind were characterized with low and constant O3 production, and plumes along their transport pathway were featured by a combination of the two. The O3 formation was more VOC sensitive near the source area, but as the plume became chemically aged, O3 formation became progressively VOC insensitive and more NOx sensitive

Impacts of HONO sources on the photochemistry in Mexico City during the MCMA-2006/MILAGO Campaign

The contribution of HONO sources to the photochemistry in Mexico City is investigated during the MCMA-2006/MILAGO Campaign using the WRF-CHEM model. Besides the homogeneous reaction of NO with OH, four additional HONO sources are considered in the WRF-CHEM model: secondary HONO formation from NO2 heterogeneous reaction with semivolatile organics, NO2 reaction with freshly emitted soot, NO2 heterogeneous reaction on aerosol and ground surfaces. The WRF-CHEM model with the five HONO sources performs reasonably well in tracking the observed diurnal variation of HONO concentrations. The HONO sources included are found to significantly improve the HOx (OH+HO2) simulations during daytime and the partition of NO/NO2 in the morning. The HONO sources also accelerate the accumulation of O3 concentrations in the morning by about 2 h and subsequently result in a noticeable enhancement of O3 concentrations over the course of the day with a midday average of about 6 ppb. Furthermore, these HONO sources play a very important role in the formation of secondary aerosols in the morning. They substantially enhance the secondary organic aerosol concentrations by a factor of 2 on average in the morning, although they contribute less during the rest of the day. The simulated particle-phase nitrate and ammonium are also substantially enhanced in the morning when the four HONO sources are included, in good agreement with the measurements. The impact of the HONO sources on the sulfate aerosols is negligible because of the inefficient conversion of H2SO4 from SO2 reacting with OH.National Science Foundation (U.S.) (Atmospheric Chemistry Program (ATM-0528227))Molina Center for Energy and the Environmen

Measurements of OH and HO2 concentrations during the MCMA-2006 field campaign - Part 1: Deployment of the Indiana University laser-induced fluorescence

Measurements of tropospheric hydroxyl (OH) and hydroperoxy (HO2) radicals were made during the MCMA (Mexico City Metropolitan Area) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project during March 2006. These radicals were measured using a laser-induced fluorescence instrument developed at Indiana University. This new instrument takes advantage of the Fluorescence Assay by Gas Expansion technique (FAGE) together with direct excitation and detection of OH at 308 nm. HO2 is indirectly measured as OH by titration with NO inside the fluorescence cell. At this stage of development, IU-FAGE is capable of detecting 3.9×105 molecule/cm3 of both OH and HO2, with a signal to noise ratio of 1, an averaged laser power of 10-mW and an averaging time of 5-min. The calibration accuracies (1 omega) are ±17% for OH and ±18% for HO2 using the water-vapor photolysis/O2 actinometry calibration technique. OH and HO2 concentrations were successfully measured at an urban site in Mexico City, with observed concentrations comparable to those measured in other polluted environments. Enhanced levels of OH and HO2 radicals were observed on several days between 09:30–11:00 a.m. and suggest an intense photochemistry during morning hours that may be due to elevated sources of HOx (OH+HO2) and a fast cycling between the radicals under the high NOx (NO+NO2) conditions of the MCMA.National Science Foundation (U.S.) (ATM-9984152)National Science Foundation (U.S.) (0612738)Henry & Camille Dreyfus Foundatio

Importance of reactive halogens in the tropical marine atmosphere: A regional modelling study using WRF-Chem

This study investigates the impact of halogens on atmospheric chemistry in the tropical troposphere and explores the sensitivity of this to uncertainties in the fluxes of halogens to the atmosphere and the chemical processing. To do this the regional chemistry transport model WRF-Chem has been extended, for the first time, to include halogen chemistry (bromine, chlorine and iodine chemistry), including heterogeneous recycling reactions involving sea-salt aerosol and other particles, reactions of Br with volatile organic compounds (VOCs), along with oceanic emissions of halocarbons, VOCs and inorganic iodine. The study focuses on the tropical East Pacific using field observations from the TORERO campaign (January-February 2012) to evaluate the model performance. Including all the new processes, the model does a reasonable job reproducing the observed mixing ratios of BrO and IO, albeit with some discrepancies, some of which can be attributed to difficulties in the model’s ability to reproduce the observed halocarbons. This is somewhat expected given the large uncertainties in the air-sea fluxes of the halocarbons in a region where there are few observations of seawater concentrations. We see a considerable impact on the Bry partitioning when heterogeneous chemistry is included, with a greater proportion of the Bry in active forms such as BrO, HOBr and dihalogens. Including debromination of sea-salt increases BrO slightly throughout the free troposphere, but in the tropical marine boundary layer, where the sea-salt particles are plentiful and relatively acidic, debromination leads to overestimation of the observed BrO. However, it should be noted that the modelled BrO was extremely sensitive to the inclusion of reactions between Br and the VOCs, which convert Br to HBr, a far less reactive form of Bry. Excluding these reactions leads to modelled BrO mixing ratios greater than observed. The reactions between Br and aldehydes were found to be particularly important, despite the model underestimating the amount of aldehydes observed in the atmosphere. There are only small changes to Iy partitioning and IO when the heterogeneous reactions, primarly on sea-salt, are included. Our model results show that the tropospheric Ox loss due to halogens is 31%. This loss is mostly due to I (16%) and Br (14%) and it is in good agreement with other estimates from state-of-the-art atmospheric chemistry models

Measurements of Volatile Organic Compounds Using Proton Transfer Reaction - Mass Spectrometry during the MILAGRO 2006 Campaign

Volatile organic compounds (VOCs) were measured by proton transfer reaction – mass spectrometry (PTR-MS) on a rooftop in the urban mixed residential and industrial area North Northeast of downtown Mexico City as part of the Megacity Initiative – Local and Global Research Observations (MILAGRO) 2006 field campaign. Thirty eight individual masses were monitored during the campaign and many species were quantified including methanol, acetaldehyde, toluene, the sum of C2 benzenes, the sum of C3 benzenes, acetone, isoprene, benzene, and ethyl acetate. The VOC measurements were analyzed to gain a better understanding of the type of VOCs present in the MCMA, their diurnal patterns, and their origins. Diurnal profiles of weekday and weekend/holiday aromatic VOC concentrations showed the influence of vehicular traffic during the morning rush hours and during the afternoon hours. Plumes including elevated toluene as high as 216 parts per billion (ppb) and ethyl acetate as high as 183 ppb were frequently observed during the late night and early morning hours, indicating the possibility of significant industrial sources of the two compounds in the region. Wind fields during those peak episodes revealed no specific direction for the majority of the toluene plumes but the ethyl acetate plumes arrived at the site when winds were from the Southwest or West. The PTR-MS measurements combined with other VOC measuring techniques at the field site as well as VOC measurements conducted in other areas of the Mexico City Metropolitan Area (MCMA) will help to develop a better understanding of the spatial pattern of VOCs and its variability in the MCMA. Atmos. Chem. Phys., 9, 467–481, 2009.Robert A. Welch Foundation (Grant A-1417)Texas A & M University. Center for Atmospheric Chemistry and EnvironmentNational Science Foundation (U.S.) (ATM-0528227