On the sources and sinks of atmospheric VOCs: an integrated analysis of recent aircraft campaigns over North America

We apply a high-resolution chemical transport model (GEOS-Chem CTM) with updated treatment of volatile organic compounds (VOCs) and a comprehensive suite of airborne datasets over North America to (i) characterize the VOC budget and (ii) test the ability of current models to capture the distribution and reactivity of atmospheric VOCs over this region. Biogenic emissions dominate the North American VOC budget in the model, accounting for 70 % and 95 % of annually emitted VOC carbon and reactivity, respectively. Based on current inventories anthropogenic emissions have declined to the point where biogenic emissions are the dominant summertime source of VOC reactivity even in most major North American cities. Methane oxidation is a 2× larger source of nonmethane VOCs (via production of formaldehyde and methyl hydroperoxide) over North America in the model than are anthropogenic emissions. However, anthropogenic VOCs account for over half of the ambient VOC loading over the majority of the region owing to their longer aggregate lifetime. Fires can be a significant VOC source episodically but are small on average. In the planetary boundary layer (PBL), the model exhibits skill in capturing observed variability in total VOC abundance (R^2=0.36) and reactivity (R^2=0.54). The same is not true in the free troposphere (FT), where skill is low and there is a persistent low model bias (∼ 60 %), with most (27 of 34) model VOCs underestimated by more than a factor of 2. A comparison of PBL : FT concentration ratios over the southeastern US points to a misrepresentation of PBL ventilation as a contributor to these model FT biases. We also find that a relatively small number of VOCs (acetone, methanol, ethane, acetaldehyde, formaldehyde, isoprene + oxidation products, methyl hydroperoxide) drive a large fraction of total ambient VOC reactivity and associated model biases; research to improve understanding of their budgets is thus warranted. A source tracer analysis suggests a current overestimate of biogenic sources for hydroxyacetone, methyl ethyl ketone and glyoxal, an underestimate of biogenic formic acid sources, and an underestimate of peroxyacetic acid production across biogenic and anthropogenic precursors. Future work to improve model representations of vertical transport and to address the VOC biases discussed are needed to advance predictions of ozone and SOA formation

Atmospheric Reactive Trace Gas Observations in Field and Chamber Studies Using CF₃O⁻ Chemical Ionization Mass Spectrometry

This dissertation describes the development of chemical ionization mass spectrometry (CIMS) instrumentation and methods utilizing the CF3O- reagent ion for the sensitive, specific, and direct detection of many oxygenated volatile organic compounds (OVOC) and inorganic reactive trace gases in the atmosphere. These species include HNO3, HONO, HO2NO2, SO2, HCN, H2O2, CH3OOH, CH3(O)OOH, HC(O)OH, CH3C(O)OH, HC(O)CH2OH, CH3C(O)CH2OH, organic hydroperoxides (ROOH), and many additional multifunctional species (e.g., hydroxynitrates, hydroxycarbonyls, hydroxyhydroperoxides, carbonylnitrates, carbonylhydroperoxides, etc.). CF3O--tandem mass spectrometry (MSMS) is demonstrated to be useful for distinguishing and individual quantification of certain isobaric compounds, as well as solving instrumental background problems for certain species. This technology is applied in field studies conducted from aircraft and ground-based platforms and to chamber studies investigating VOC oxidation and organic aerosol formation mechanisms. Comparisons with simultaneous observations from other instrumentation for several species show good agreement with CIMS observations. CF3O--CIMS observations of HCN (a biomass burning tracer) from aircraft are used to quantify the impact of biomass burning emissions to the Mexico City region in March 2006. Biomass burning emissions are shown to contribute significantly to a number of gas and aerosol phase pollutants even in the midst of the large anthropogenic pollution emissions from Mexico City. The analysis of the photochemical aging of a fire plume over the Yucatan Peninsula (March 2006) is also reported. Observations indicate intense chemistry occurring within the fire plume evidenced by high OH levels, fast production of H2O2 and conversion of NO and NO2 (NOx) into peroxyacetylnitrate (PAN) and aerosol nitrate. This rapid chemistry is likely driven by photolysis of HONO, which is observed to be emitted in high amounts from these fires. The CIMS methods are applied to studies of VOC oxidation and organic aerosol formation conducted in chamber experiments. Specifically, new insights gained from the study of isoprene oxidation under high and low NOx conditions are reported. We quantify the formation of small carboxylic acids as well as C5-hydroxynitrates from the oxidation of isoprene under high NOx conditions. Under low NOx conditions, we show that C5-hydroxyhydroperoxides are formed in high yield. Subsequent oxidation of these hydroxyhydroperoxides is shown to occur through a unique HOx neutral mechanism that generates C5-epoxydiols, a likely precursor to organic aerosol. We utilize the high sensitivity and specificity of CF3O- -CIMS to study novel intermolecular hydrogen-shift isomerization processes in peroxy radicals formed during isoprene oxidation. We find these rates to be substantially slower than recent theoretical predictions; however, we find these isomerization rates to be fast enough that they are important in atmospheric isoprene oxidation in regions where lifetimes become long. Globally, we estimate 8-11% of isoprene peroxy radicals react through 1,6-H-shift isomerization reactions.</p

Atmospheric Acetaldehyde: Importance of Air-Sea Exchange and a Missing Source in the Remote Troposphere

We report airborne measurements of acetaldehyde (CH₃CHO) during the first and second deployments of the National Aeronautics and Space Administration (NASA) Atmospheric Tomography Mission (ATom). The budget of CH₃CHO is examined using the Community Atmospheric Model with chemistry (CAM‐chem), with a newly‐developed online air‐sea exchange module. The upper limit of the global ocean net emission of CH₃CHO is estimated to be 34 Tg a⁻¹ (42 Tg a⁻¹ if considering bubble‐mediated transfer), and the ocean impacts on tropospheric CH₃CHO are mostly confined to the marine boundary layer. Our analysis suggests that there is an unaccounted CH₃CHO source in the remote troposphere and that organic aerosols can only provide a fraction of this missing source. We propose that peroxyacetic acid (PAA) is an ideal indicator of the rapid CH₃CHO production in the remote troposphere. The higher‐than‐expected CH₃CHO measurements represent a missing sink of hydroxyl radicals (and halogen radical) in current chemistry‐climate models

Chemical ionization tandem mass spectrometer for the in situ measurement of methyl hydrogen peroxide

A new approach for measuring gas-phase methyl hydrogen peroxide [(MHP) CH_3OOH] utilizing chemical ionization mass spectrometry is presented. Tandem mass spectrometry is used to avoid mass interferences that hindered previous attempts to measure atmospheric CH_3OOH with CF_3O− clustering chemistry. CH_3OOH has been successfully measured in situ using this technique during both airborne and ground-based campaigns. The accuracy and precision for the MHP measurement are a function of water vapor mixing ratio. Typical precision at 500 pptv MHP and 100 ppmv H_2O is ±80 pptv (2 sigma) for a 1 s integration period. The accuracy at 100 ppmv H_2O is estimated to be better than ±40%. Chemical ionization tandem mass spectrometry shows considerable promise for the determination of in situ atmospheric trace gas mixing ratios where isobaric compounds or mass interferences impede accurate measurements

Isoprene Peroxy Radical Dynamics

Approximately 500 Tg of 2-methyl-1,3-butadiene (isoprene) is emitted by deciduous trees each year. Isoprene oxidation in the atmosphere is initiated primarily by addition of hydroxyl radicals (OH) to C_4 or C_1 in a ratio 0.57 ± 0.03 (1σ) to produce two sets of distinct allylic radicals. Oxygen (O_2) adds to these allylic radicals either δ (Z or E depending on whether the allylic radical is cis or trans) or β to the OH group forming six distinct peroxy radical isomers. Due to the enhanced stability of the allylic radical, however, these peroxy radicals lose O_2 in competition with bimolecular reactions. In addition, the Z-δ hydroxy peroxy radical isomers undergo unimolecular 1,6 H-shift isomerization. Here, we use isomer-resolved measurements of the reaction products of the peroxy radicals to diagnose this complex chemistry. We find that the ratio of δ to β hydroxy peroxy radicals depends on their bimolecular lifetime (τ_(bimolecular)). At τ_(bimolecular) ≈ 0.1 s, a transition occurs from a kinetically to a largely thermodynamically controlled distribution at 297 K. Thus, in nature, where τ_(bimolecular) > 10 s, the distribution of isoprene hydroxy peroxy radicals will be controlled primarily by the difference in the relative stability of the peroxy radical isomers. In this regime, β hydroxy peroxy radical isomers comprise ∼95% of the radical pool, a much higher fraction than in the nascent (kinetic) distribution. Intramolecular 1,6 H-shift isomerization of the Z-δ hydroxy peroxy radical isomers produced from OH addition to C_4 is estimated to be ∼4 s^(–1) at 297 K. While the Z-δ isomer is initially produced in low yield, it is continually reformed via decomposition of the β hydroxy peroxy radicals. As a result, unimolecular chemistry from this isomer contributes about half of the atmospheric fate of the entire pool of peroxy radicals formed via addition of OH at C_4 for typical atmospheric conditions (τ_(bimolecular) = 100 s and T = 25 C). In contrast, unimolecular chemistry following OH addition at C_1 is slower and less important

On the sources and sinks of atmospheric VOCs: an integrated analysis of recent aircraft campaigns over North America

We apply a high-resolution chemical transport model (GEOS-Chem CTM) with updated treatment of volatile organic compounds (VOCs) and a comprehensive suite of airborne datasets over North America to (i) characterize the VOC budget and (ii) test the ability of current models to capture the distribution and reactivity of atmospheric VOCs over this region. Biogenic emissions dominate the North American VOC budget in the model, accounting for 70 % and 95 % of annually emitted VOC carbon and reactivity, respectively. Based on current inventories anthropogenic emissions have declined to the point where biogenic emissions are the dominant summertime source of VOC reactivity even in most major North American cities. Methane oxidation is a 2× larger source of nonmethane VOCs (via production of formaldehyde and methyl hydroperoxide) over North America in the model than are anthropogenic emissions. However, anthropogenic VOCs account for over half of the ambient VOC loading over the majority of the region owing to their longer aggregate lifetime. Fires can be a significant VOC source episodically but are small on average. In the planetary boundary layer (PBL), the model exhibits skill in capturing observed variability in total VOC abundance (R^2=0.36) and reactivity (R^2=0.54). The same is not true in the free troposphere (FT), where skill is low and there is a persistent low model bias (∼ 60 %), with most (27 of 34) model VOCs underestimated by more than a factor of 2. A comparison of PBL : FT concentration ratios over the southeastern US points to a misrepresentation of PBL ventilation as a contributor to these model FT biases. We also find that a relatively small number of VOCs (acetone, methanol, ethane, acetaldehyde, formaldehyde, isoprene + oxidation products, methyl hydroperoxide) drive a large fraction of total ambient VOC reactivity and associated model biases; research to improve understanding of their budgets is thus warranted. A source tracer analysis suggests a current overestimate of biogenic sources for hydroxyacetone, methyl ethyl ketone and glyoxal, an underestimate of biogenic formic acid sources, and an underestimate of peroxyacetic acid production across biogenic and anthropogenic precursors. Future work to improve model representations of vertical transport and to address the VOC biases discussed are needed to advance predictions of ozone and SOA formation

Atmospheric Acetaldehyde: Importance of Air-Sea Exchange and a Missing Source in the Remote Troposphere

We report airborne measurements of acetaldehyde (CH₃CHO) during the first and second deployments of the National Aeronautics and Space Administration (NASA) Atmospheric Tomography Mission (ATom). The budget of CH₃CHO is examined using the Community Atmospheric Model with chemistry (CAM‐chem), with a newly‐developed online air‐sea exchange module. The upper limit of the global ocean net emission of CH₃CHO is estimated to be 34 Tg a⁻¹ (42 Tg a⁻¹ if considering bubble‐mediated transfer), and the ocean impacts on tropospheric CH₃CHO are mostly confined to the marine boundary layer. Our analysis suggests that there is an unaccounted CH₃CHO source in the remote troposphere and that organic aerosols can only provide a fraction of this missing source. We propose that peroxyacetic acid (PAA) is an ideal indicator of the rapid CH₃CHO production in the remote troposphere. The higher‐than‐expected CH₃CHO measurements represent a missing sink of hydroxyl radicals (and halogen radical) in current chemistry‐climate models

Airborne measurements of western U.S. wildfire emissions: Comparison with prescribed burning and air quality implications

Wildfires emit significant amounts of pollutants that degrade air quality. Plumes from three wildfires in the western U.S. were measured from aircraft during the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC^4RS) and the Biomass Burning Observation Project (BBOP), both in summer 2013. This study reports an extensive set of emission factors (EFs) for over 80 gases and 5 components of submicron particulate matter (PM_1) from these temperate wildfires. These include rarely, or never before, measured oxygenated volatile organic compounds and multifunctional organic nitrates. The observed EFs are compared with previous measurements of temperate wildfires, boreal forest fires, and temperate prescribed fires. The wildfires emitted high amounts of PM_1 (with organic aerosol (OA) dominating the mass) with an average EF that is more than 2 times the EFs for prescribed fires. The measured EFs were used to estimate the annual wildfire emissions of carbon monoxide, nitrogen oxides, total nonmethane organic compounds, and PM_1 from 11 western U.S. states. The estimated gas emissions are generally comparable with the 2011 National Emissions Inventory (NEI). However, our PM_1 emission estimate (1530 ± 570 Gg yr^(−1)) is over 3 times that of the NEI PM_(2.5) estimate and is also higher than the PM_(2.5) emitted from all other sources in these states in the NEI. This study indicates that the source of OA from biomass burning in the western states is significantly underestimated. In addition, our results indicate that prescribed burning may be an effective method to reduce fine particle emissions

Stereoselectivity in Atmospheric Autoxidation

We show that the diastereomers of hydroxy peroxy radicals formed from OH and O_2 addition to C2 and C3, respectively, of crotonaldehyde (CH_3CHCHCHO) undergo gas-phase unimolecular aldehydic hydrogen shift (H-shift) chemistry with rate coefficients that differ by an order of magnitude. The stereospecificity observed here for crotonaldehyde is general and will lead to a significant diastereomeric-specific chemistry in the atmosphere. This enhancement of specific stereoisomers by stereoselective gas-phase reactions could have widespread implications given the ubiquity of chirality in nature. The H-shift rate coefficients calculated using multiconformer transition state theory (MC-TST) agree with those determined experimentally using stereoisomer-specific gas-chromatography chemical ionization mass spectroscopy (GC–CIMS) measurements