A52A-01 INVITED
Top-down estimates of tropospheric OH: past and future
Since the start of the ALE (Atmospheric Lifetime Experiment) program, measurements of methyl chloroform (1,1,1 trichloro ethane, or MCF) have been used to estimate tropospheric OH concentrations. These top-down estimates of OH are obtained by minimizing differences between measured and modeled MCF concentrations in an inverse modelling framework. Historic controversies about the inverse methodology will be reviewed. These controversies include the discussion on (i) whether or not initial MCF concentrations should be optimised (ii) how well emission estimates of MCF can be trusted, and (iii) the role of model errors. These details become important if, apart from a long-term average OH concentration, also information about the year-to-year variation of OH is obtained in the inverse procedure. Due to the Montreal protocol and its amendments, recent emissions of MCF have become very small. Hence, the atmospheric MCF concentrations strongly reflect the oxidizing action of OH. From the decaying burden of MCF in the atmosphere it should in the case of negligible emissions be possible to deduce year-to-year variations in OH. Recent progress in this field will be reviewed and alternatives for the vanishing OH-calibration gas MCF will be discussed.
A52A-02 INVITED
Hydroxyl Radical Determination From Methyl Chloroform: Current Utility and Future Viability
The hydroxyl free radical (OH) is the major oxidizing chemical in the atmosphere destroying about 3.7 Pg of trace gases each year. Global observations of methyl chloroform (CH3CCl3), available since 1978 from the ALE/GAGE/AGAGE network, have been widely used to indirectly estimate OH concentrations on hemispheric to global spatial scales and annual time scales using optimal estimation inverse methods and chemical transport models. As reviewed in the IPCC 2007 assessment, these inverse studies suggest that global OH levels grew between 1979 and 1989, then declined between 1989 and 1998, and then recovered again so that in 2003 they are comparable to those in 1979. The circa 1998 OH minimum coincides with major global wildfires and an intense El Nino at this time. The inferred OH levels also exhibit significant inter-annual variations, but only conclusions about their phasing, and not their amplitude, are robust. These decadal and inter-annual OH estimates remain even after accounting for additional recent lingering anthropogenic CH3CCl3 emissions, and the proposal that the polar oceans stored methyl chloroform during its pre-peak (1992) years and began re-emitting it in subsequent years. We will update previous OH estimates using AGAGE CH3CCl3 measurements through 2007. Looking to the future, the continued use of CH3CCl3 for these purposes could potentially become very limited because its concentrations are rapidly decreasing and because any lingering future anthropogenic, wildfire and oceanic emissions would increase the errors in the OH estimates. We will present an extensive analysis of southern hemispheric methyl chloroform measurements that indicates that the proposed substantial oceanic re-emission did not occur. We will also review the significant evidence for rapidly declining anthropogenic emissions and small wildfire emissions. We will finally show the results of a numerical inverse modelling study that calculates the errors in future OH estimates using CH3CCl3 under a range of assumptions regarding instrumental precision, atmospheric variability, and lingering emissions of this gas. We conclude that, with achievable increases in instrumental sensitivity and precision, CH3CCl3 could remain viable for OH estimations for another decade with an accuracy comparable to that achieved today.
A52A-03 INVITED
Interannual Variability in Atmospheric Hydroxyl as Inferred From Measurements of CH3CCl3, CH4, and Other Trace Gases
Interannual variations in the abundance of the atmospheric hydroxyl radical (OH) have been inferred from observations of certain long-lived trace gases. Accurate estimates of OH variability are possible provided emission magnitudes of a trace gas are precisely known or insignificant and provided variations in global mean trace gas mixing ratios are well characterized. Analyses conducted previously of CH4 and CH3CCl3 observations prior to 1998 suggest very different conclusions regarding the stability of OH from year to year: while the CH3CCl3 data imply relatively large interannual OH changes (8%), CH4 data suggest only a small interannual variability in OH (1-2%). Since 1998, however, we will show that the observations of both CH4 and CH3CCl3 suggest relatively small interannual variations in the global atmospheric OH burden. Although the underlying causes for the discrepancy inferred for OH before 1998 are not well known, we argue here that the consistency inferred for OH variability in recent years is likely the result of an enhanced ability of CH3CCl3 measurements to discern OH variability since 1998, a period when CH3CCl3 emissions and atmospheric gradients were substantially diminished. Furthermore, during this latter period we have noted a similar phase to changes in the atmospheric growth rates of multiple trace gases including CH4, CH3CCl3, CH3Cl, and C2Cl4. While some of these similarities arise from variations in a common source ( e.g., biomass burning), for those not produced in substantial quantities from burning (CH3CCl3 and C2Cl4) a coherent picture regarding OH variability and phasing has emerged.
A52A-04 INVITED
Do tropospheric OH measurements agree with models?
In situ measurements of OH from aircraft provide insight into the global mean OH that is calculated by models and tracer lifetimes. Although it is difficult to compare in situ aircraft OH measurements with the OH calculated in global models, it is not impossible. One method is to compare measured OH to OH calculated by a box model that is constrained by simultaneous in situ measurements of everything that affects OH. The chemical mechanisms used in box models and chemical transport models can then be compared. Another is to compare the measured OH to OH obtained by "flying" the aircraft through the box model. A recent recalibration of our in situ OH measurements has brought measured OH from three field studies into general agreement with OH calculated with a box model. The agreement is best in clean air and degrades as air quality degrades. Agreement between measurements and chemical transport models (CTM) is generally less good, as would be expected, since errors in the CTM transport and emissions also affect calculated OH. These comparisons and their implications for global OH will be discussed.
A52A-05
High-Resolution Mass Spectrometric Analysis of Oligomers Formed in Ozonation of Selected Monoterpenes
Monoterpenes constitute a significant source of the secondary organic aerosols (SOA) because of their abundant emissions from plants and high reactivity with ozone. It has been estimated that more than 50% of the total organic aerosols in specific regions are produced from monoterpene precursors. Although recent studies indicate that a significant part of secondary organic aerosols formed as a result of ozonation of monoterpenes consist of oligomeric products with high molecular weight (MW) detailed mechanism of oligomer formation is currently poorly understood. Knowledge of the molecular structure of the high MW organic products is essential for understanding of climate related properties of SOA such as hygroscopicity, CCN activity, light scattering and absorption. This work focuses on the identification of the monomeric and oligomeric chemical species present in SOA particles produced from the ozone-induced oxidation á-Pinene and d-Limonene. We take advantage of the rapidly developing tools of high-resolution mass spectrometry (HR-MS) that have the potential to analyze the aerosol particle composition without chromatographic separation techniques. High-resolution mass spectra reveal a large number of both monomeric and oligomeric products of oxidation. The combination of high resolving power (m/Δm = 60,000) and Kendrick mass defect analysis makes it possible to unambiguously determine the elemental composition for hundreds of individual compounds in SOA samples. It allows us to identify monomeric building blocks for all major oligomeric products. Positive and negative modes of HR-MS analysis provide complementary information on the composition of SOA, because less oxidized products are better observed in the positive mode while highly oxidized products tare more readily detected in the negative mode. Additional experiments using derivatization of SOA components with isotopically labeled methanol were conducted to identify compounds with aldehyde groups. An extended reaction mechanism for the formation of the monomeric and oligomeric components is proposed and will be discussed in the presentation.
A52A-06
An investigation for unexpected high yield of peroxides from isoprene through aqueous phase ozonolysis
It has recently become evident that isoprene, the atmosphere's most abundant non-methane hydrocarbon, and its oxidation products can considerably result in formation of secondary organic aerosols (SOA) through the acid- catalyzed aqueous phase reaction with hydrogen peroxide. However, the peroxide source in the atmospheric aqueous process is unclear. The present study revealed a potentially important route to the formation of aqueous peroxides, including hydrogen peroxide and hydroxylmethyl hydroperoxide, from the aqueous phase ozonolysis of isoprene. In this study, the atmospheric aqueous phase ozonolysis of isoprene at different pHs and temperatures were studied with the method of laboratory simulation. The major products, including peroxides and carbonyl compounds, were well-characterized, with a measured carbon balance approaching 100%, and the detailed reaction mechanisms were proposed. Most strikingly, peroxides have been found in the aqueous phase ozonolysis of isoprene with unexpected high yields. Considering the huge amount of isoprene in the atmosphere, we suggest that the aqueous phase ozonolysis of isoprene and its first-generation oxidation products may contribute a considerable and even the main source of oxidants to the atmospheric aqueous phase. This means that isoprene and its oxidation products can be transformed into SOA by peroxides provided from their aqueous phase ozonolysis reactions, even if there is no other peroxide source.
A52A-07
The Fate of Carbon From Isoprene Over West Africa
During the summer of 2006, a comprehensive suite of trace gases and aerosol properties were measured aboard the BAe-146 U.K. research aircraft during the African Monsoon Multidisciplinary Analysis campaign. Good spatial agreement was found between isoprene concentrations measured using a Proton Transfer Reaction Mass Spectrometer and those predicted based on the underlying vegetation. Within minutes to hours following emission, the isoprene is oxidized to form several possible products, many of which were observed from the aircraft, including methyl vinyl ketone, methacrolein, formaldehyde, organic peroxides and carbon monoxide. We use the ratios of isoprene and its oxidation products and a simple box model to calculate a regional average for the flux of carbon derived from isoprene into the atmosphere. Despite these large emissions of isoprene in the region, the average loading of organic aerosol observed by the Aerodyne Aerosol Mass Spectrometer (AMS) was very low (< 1 μg/m3). These observations are in conflict with chamber measurements, which predict a significant yield of secondary organic aerosol (SOA) from isoprene, and observations over the United States and Europe, which show large amounts of biogenically-derived SOA. We explore two other possibilities that isoprene-derived SOA was formed, but 1) partitioned to larger mode aerosol (PM1.0) not measurable using the AMS, or 2) that the lifetime of SOA in the lower troposphere over West Africa is extremely short (less than a few hours).
A52A-08
Development of an explicit mechanistic model for isoprene oxidation, assessed by atmospheric chamber experiments
Isoprene is the single largest non-methane volatile organic carbon source to the atmosphere. Its flux from terrestrial ecosystems is sensitive to temperature, water, and light levels and thus its emissions may be significantly altered by climate change. The oxidation of isoprene, typically initiated by reaction with the hydroxyl radical, broadly influences atmospheric composition. Several recent studies suggesting that oxidation products of isoprene yield significant secondary organic aerosol have fueled renewed interest in its oxidation chemistry. We describe the development and testing of an explicit mechanism of the photooxidation of isoprene initiated by reaction with the hydroxyl radical. We evaluate our mechanism with observations obtained in the Caltech environmental chamber of the time dependent evolution of numerous volatile organic compounds with a novel CIMS technique. Many of these compounds have not previously been observed. Experiments include oxidation in atmospheres with both initially low and high NOx concentrations, providing a comprehensive test of the mechanism. Special emphasis is placed on unraveling the production and loss mechanisms of organic nitrates and acids. Finally, we test several hypotheses on the chemistry leading o aerosol formation.