Impact of NOx and OH on secondary organic aerosol formation from beta-pinene photooxidation

Sarrafzadeh, M., Wildt, J., Pullinen, I., Springer, M., Kleist, E., Tillmann, R., Schmitt, S. H., Wu, C., Mentel, T. F., Zhao, D., Hastie, D. R., and Kiendler-Scharr

Published September 2016 in Atmos. Chem. Phys., 16, 11237-11248, doi:10.5194/acp-16-11237-2016.

Abstract. In this study, the NO_x dependence of secondary organic aerosol (SOA) formation from photooxidation of the biogenic volatile organic compound (BVOC) β-pinene was comprehensively investigated in the Jülich Plant Atmosphere Chamber. Consistent with the results of previous NO_x studies we found increases of SOA yields with increasing [NO_x] at low-NO_x conditions ([NO_x]₀  <  30 ppb, [BVOC]₀ ∕ [NO_x]₀  >  10 ppbC ppb⁻¹). Furthermore, increasing [NO_x] at high-NO_x conditions ([NO_x]₀  >  30 ppb, [BVOC]₀ ∕ [NO_x]₀  ∼  10 to ∼  2.6 ppbC ppb⁻¹) suppressed the SOA yield. The increase of SOA yield at low-NO_x conditions was attributed to an increase of OH concentration, most probably by OH recycling in NO + HO₂  →  NO₂ + OH reaction. Separate measurements without NO_x addition but with different OH primary production rates confirmed the OH dependence of SOA yields. After removing the effect of OH concentration on SOA mass growth by keeping the OH concentration constant, SOA yields only decreased with increasing [NO_x]. Measuring the NO_x dependence of SOA yields at lower [NO] ∕ [NO₂] ratio showed less pronounced increase in both OH concentration and SOA yield. This result was consistent with our assumption of OH recycling by NO and to SOA yields being dependent on OH concentrations. Our results furthermore indicated that NO_x dependencies vary for different NO_x compositions. A substantial fraction of the NO_x-induced decrease of SOA yields at high-NO_x conditions was caused by NO_x-induced suppression of new particle formation (NPF), which subsequently limits the particle surface where low volatiles condense. This was shown by probing the NO_x dependence of SOA formation in the presence of seed particles. After eliminating the effect of NO_x-induced suppression of NPF and NO_x-induced changes of OH concentrations, the remaining effect of NO_x on the SOA yield from β-pinene photooxidation was moderate. Compared to β-pinene, the SOA formation from α-pinene photooxidation was only suppressed by increasing NO_x. However, basic mechanisms of the NO_x impacts were the same as that of β-pinene.

Stable carbon isotope ratios of ambient aromatic volatile organic compounds

Anna Kornilova, Lin Huang, Marina Saccon, and Jochen Rudolph

Published September 21, 2016 in Atmos. Chem. Phys., 16, 11755-11722.

Abstract. Measurements of mixing ratios and stable carbon isotope ratios of aromatic volatile organic compounds (VOC) in the atmosphere were made in Toronto (Canada) in 2009 and 2010. Consistent with the kinetic isotope effect for reactions of aromatic VOC with the OH radical the observed stable carbon isotope ratios are on average significantly heavier than the isotope ratios of their emissions. The change of carbon isotope ratio between emission and observation is used to determine the extent of photochemical processing (photochemical age, ∫ [OH]dt) of the different VOC. It is found that ∫ [OH]dt of different VOC depends strongly on the VOC reactivity. This demonstrates that for this set of observations the assumption of a uniform ∫ [OH]dt for VOC with different reactivity is not justified and that the observed values for ∫ [OH]dt are the result of mixing of VOC from air masses with different values for ∫ [OH]dt. Based on comparison between carbon isotope ratios and VOC concentration ratios it is also found that the varying influence of sources with different VOC emission ratios has a larger impact on VOC concentration ratios than photochemical processing. It is concluded that for this data set the use of VOC concentration ratios to determine ∫ [OH]dt would result in values for ∫ [OH]dt inconsistent with carbon isotope ratios and that the concept of a uniform ∫ [OH]dt for an air mass has to be replaced by the concept of individual values of an average ∫ [OH]dt for VOC with different reactivity.

Laboratory Studies of Carbon Kinetic Isotope Effects on the Production Mechanism of Particulate Phenolic Compounds Formed by Toluene Photooxidation: A Tool to Constrain Reaction Pathways

Satoshi Irei, Jochen Rudolph, Lin Huang, Janeen Auld, Fabrice Collin and Donald Hastie

The Journal of Physical Chemistry A, 2015, 119 (1), pp. 5-13, DOI: 10.1021/jp5104609.
Abstract:

In this study, we examined compound-specific stable carbon isotope ratios for phenolic compounds in secondary organic aerosol (SOA) formed by photooxidation of isotope-label-free toluene. SOA generated by photooxidation of toluene using a continuous-flow reactor and an 8 m(3) indoor smog chamber was collected on filters, which were extracted with acetonitrile for compound-specific analysis. Eight phenolic compounds were identified in the extracts using a gas chromatograph coupled with a mass spectrometer, and their compound-specific stable carbon isotope ratios were determined using a gas chromatograph coupled with a combustion furnace followed by an isotope ratio mass spectrometer. The majority of products, including methylnitrophenols and methylnitrocatechols, were isotopically depleted by 5-6‰ compared to the initial isotope ratio of toluene, whereas the isotope ratio for 4-nitrophenol remained identical to that of toluene. On the basis of the reaction mechanisms proposed in previous reports, stable carbon isotope ratios of these products were calculated. By comparing the observed isotope ratios with the predicted isotope ratios, we explored possible production pathways for the particulate phenolic compounds.

Isotope ratio studies of atmospheric organic compounds

Dr. Jochen Rudolph is co-author of Isotope ratio studies of atmospheric organic compounds: Principles, methods, applications and potential published in the International Journal of Mass Spectrometry, March 2014.

Authors: Iulia Gensch, Astrid Kiendler-Scharr and Jochen Rudolph

Abstract: In the atmosphere, both gas and particle phase organic trace compounds (OTC) have multiple effects on air quality and climate. Gaps exist in a fundamental understanding of the sources and sinks of organics and thus, knowledge needed to steer regulatory purposes is far from complete. Isotopes provide specific “fingerprints” in OTC. These fingerprints result from the isotopic composition at emission, as well as from chemical and physical processes in the atmosphere. Compound specific isotope ratio mass spectrometry (IRMS) in atmospheric OTC is therefore a promising tool to improve our understanding of sources and the atmospheric fate of OTC. Due to analytical challenges originating from the small sample amounts and a huge variety of physical and chemical properties of OTC present in the atmosphere, such measurements are not routinely performed. We present an overview of basic concepts as well as instrumental and measurement procedures used for compound specific IRMS in atmospheric OTC. Concepts for the interpretation of ambient observations are reviewed together with available literature data on source specific and ambientδ13C values of gas and particle phase OTC. Full deployment of the IRMS potential in future atmospheric studies will depend on the availability of laboratory kinetic data. Further method developments, such as increasing sensitivity and accuracy, as well as techniques for simultaneous isotope ratio measurement of multiple atoms are expected to further extend the potential use of isotope ratios for studies of atmospheric OTC.

Isotope ratio studies of atmospheric organic compounds: Principles, methods, applications and potential

Authors: Iulia Gensch, Astrid Kiendler-Scharr and Jochen Rudolph

Method for the determination of concentration and stable carbon isotope ratios of atmospheric phenols

Marina Saccon, Ph.D candidate, is co-author of Method for the determination of concentration and stable carbon isotope ratios of atmospheric phenols published in the Atmospheric Measurement Techniques Journal, an interactive open access journal of the European Geosciences Union, released on November 5 2013.

Marina Saccon presenting thesis

Authors: M. Saccon, R. Busca, C. Facca, L. Huang, S. Irei, A. Kornilova, D. Lane and J. Rudolph

Abstract: A method for the determination of the stable carbon isotopic composition of atmospheric nitrophenols in the gas and particulate phases is presented. It has been proposed to use the combination of concentration and isotope ratio measurements of precursor and product to test the applicability of results of laboratory studies to the atmosphere. Nitrophenols are suspected to be secondary products formed specifically from the photooxidation of volatile organic compounds. XAD-4TM resin was used as an adsorbent on quartz filters to sample ambient phenols using conventional high volume air samplers at York University in Toronto, Canada. Filters were extracted in acetonitrile, with a HPLC (high-performance liquid chromatography) clean-up step and a solid phase extraction step prior to derivatization with BSTFA (bis(trimethylsilyl) trifluoroacetamide). Concentration measurements were done with gas chromatography– mass spectrometry and gas chromatography–isotope ratio mass spectrometry was used for isotope ratio analysis. The technique presented allows for atmospheric compound-specific isotopic composition measurements for five semi-volatile phenols with an estimated accuracy of 0.3–0.5‰ at atmospheric concentrations exceeding 0.1 ngm−3 while the detection limits for concentration measurements are in the pgm−3 range. Isotopic fractionation throughout the entire extraction procedure and analysis was proven to be below the precision of the isotope ratio measurements. The method was tested by conducting ambient measurements from September to December 2011.

Radiative absorption enhancements due to the mixing state of atmospheric black carbon

Ibraheem Nuaaman, Phd, candidate, is co-author on a science paper, published in Science, Aug, 31, 2012, Vol. 337, no. 6098 pp. 1078-1081 - DOI: 10.1126/science. 1223447.

F3_medium

Abstract: Atmospheric black carbon (BC) warms Earth’s climate, and its reduction has been targeted for near-term climate change mitigation. Models that include forcing by BC assume internal mixing with non-BC aerosol components that enhance BC absorption, often by a factor of ~2; such model estimates have yet to be clearly validated through atmospheric observations. Here, direct in situ measurements of BC absorption enhancements (E_abs) and mixing state are reported for two California regions. The observed E_abs is small—6% on average at 532 nm—and increases weakly with photochemical aging. The E_abs is less than predicted from observationally constrained theoretical calculations, suggesting that many climate models may overestimate warming by BC. These ambient observations stand in contrast to laboratory measurements that show substantial E_abs for BC are possible.