Measuring the OH reactivity
In 1993, William H. Brune conceived for the first time the concept of total OH reactivity as the direct measure of OH loss rate. A few years later the first measurements of total OH reactivity were performed independently by two research groups in the laboratory based on LIDAR (Calpini et al., 1999) and in ambient air based on laser induced fluorescence for detecting OH (Kovacs and Brune, 2001).
Currently, the total OH reactivity can be measured directly using three diﬀerent methods:
(i) Total OH Loss Rate Measurement (TOLRM) (Kovacs and Brune, 2001; Mao et al., 2009; Hansen et al., 2014); Pump and probe technique (Calpini et al., 1999; Sadanaga et al., 2004; Yoshino et al., 2006; Ingham et al., 2009; Lou et al., 2010) and Comparative Reactivity Method (CRM) (Sinha et al., 2008; N¨olscher et al., 2012a; Dolgorouky et al., 2012; Kim et al., 2011; Kumar and Sinha, 2014).
Total OH Loss Rate Measurement was first developed by Kovacs and Brune (2001). It consists of a flow tube used to sample ambient air at flow rates in the order of 50-400 sL min−1, wherein a large amount of OH is added through a movable injector. OH concentra-tion is quantified at diﬀerent reaction times using a FAGE apparatus (Fluorescence Assay by Gas Expansion, see Faloona et al., (2004) and Dusanter et al., (2009)) at the exit of the flow tube by moving the injector. A decay curve for OH is obtained due to a change in distance between the OH source and the OH detector.
The pump and probe technique was first pioneered by Calpini et al., (1999) and Jeanneret et al., (2001) and then adapted with some diﬀerences by other groups (Sadanaga et al., 2004, Yoshino et al., 2006; Ingham et al., 2009 and Lou et al., 2010). The instrument consists of three main parts: a flow tube to sample ambient air, a pulsed laser to generate OH in the sampling reactor, and a FAGE apparatus to quantify OH. The sampling flow is set around 10-20 sL min−1 and assuming laminar flow the sample has about 1 s residence time for reaction with OH. The hydroxyl radical OH is generated inside the reactor from ozone photolysis. The decay of OH is measured considering the summed laser pulses using the FAGE instrument.
The Comparative Reactivity Method was more recently developed (Sinha et al., 2008) and then adopted by several research groups in the last decade (Kim et al., 2011; Dolgorouky et al., 2012; Michoud et al., 2015). The popularity of this method comes from the use of analytical techniques such as gas chromatography and mass spectrometry, already in use by many research groups working in the field of the atmospheric sciences. Indeed, in CRM a small glass flow reactor is coupled to a detector such a Proton Transfer Reaction Mass Spectrometer (PTR-MS) or a Gas Chromatography Photo-Ionization Detector (GC-PID).
Total OH reactivity: a change in philosophy
The chosen detector monitors the concentration of a reference molecule, whose reactivity with OH is well known, throughout diﬀerent experimental stages. The reference molecule, so far always pyrrole, is first diluted in clean air, then reacts with OH radicals generated inside the reactor, then competes for the OH radicals when ambient air is sampled. The competition between pyrrole and reactants in ambient air for OH is acquired in real time, while data processing produces raw data of reactivity with a time resolution of about 10 minutes. Since the CRM is the technique adopted for my PhD project, an exhaustive description of the method can be found in the experimental chapter of this thesis.
OH reactivity in the world
Measurements of OH reactivity were performed so far at four diﬀerent scales: (i) in branch enclosures/plant cuvettes (Kim et al, 2011; N¨olscher et al., 2013); (ii) in flow tubes and environmental chambers (Nakashima et al., 2012; N¨olscher et al., 2014); (iii) ground-based ambient measurements (e.g. Ren et al., 2003; Lee et al., 2010; Edwards et al., 2013) and (iv) airborne measurements (Mao et al., 2009).
Ground-based measures were conducted in diﬀerent environments from diﬀerent parts of the world. Fig. 1.7 illustrates the diﬀerent sites in the world where the OH reactivity was measured. Green, gray and blue frames represent the type of environment where the measures were performed: i.e. forests, urban and coastal areas, respectively. Bold red font is used to highlight the sites where a maximum OH reactivity>50 s−1 was observed. Finally the star indicates the measurements that were conducted with the Comparative Reactivity Method.
Diﬀerent sites have been investigated so far: observations are reported for urban, forests, rural and remote environments. Forests of diﬀerent climatic areas were analysed: boreal, temperate, mixed and tropical forests. Most of the measurements reported in literature were conducted, for logistical reasons, in northern hemisphere as can be seen in Fig. 1.7. However, measurements at tropical sites in Suriname and Borneo are also reported in literature (Sinha et al., 2008 and Edwards et al., 2013, respectively) and soon will be available the results of the first measures of OH reactivity in the pristine Amazonian rainforest.
Figure 1.7 permits to conclude:
• In only 15 years since the development of the first measurements of OH reactivity, a great eﬀort for measuring this parameter and improving our understanding on atmospheric processes at diﬀerent sides of the world has been made.
• Many interesting areas of the world are still unexplored: no measures of OH reactivity existed before my thesis project in the Mediterranean basin, no measures are reported in pristine areas of Asia and Africa, in the Siberian boreal forest, in densely populated urban areas as New Delhi, and in clean remote environments as the poles.
• The magnitude of OH reactivity does not depend on the method used to measure it. Highest values of OH reactivity were reported from measurements where the three existing methods were applied (it is the case for the studies conducted in Mexico city and Paris, using respectively the TOLRM and CRM methods).
• Urban and forest sites are the environments better characterized. However, there is more variability in type of emission in forests than in urban sites, which means that we could explore more and more forested areas and still miss much information. Be-sides, unknowns on oxidation processes exist at diﬀerent levels, generating more and more complex systems, e.g. primary reactants emitted from diﬀerent plant species, secondary species formed from the primary biogenic precursors, interaction between biogenic precursors and anthropogenic components for the ecosystem influenced by pollution.
• Highest values of OH reactivity were reported in megacities and tropical forests (red highlighted sites). Despite being marked as a coastal area, maximum of OH reactivity at El Arenosillo (Spain) were observed when the site was influenced by continental pollution plumes from big cities as Madrid and Sevilla. Maximum values
of OH reactivity observed in megacities are alarming: 200 s−1 in Mexico City during spring 2003 (Shirley et al., 2006); 130 s−1 in Paris during winter 2010 (Dolgorouky et al., 2012); 100 s−1 in Tokyo during autumn 2004, in New York City during winter 2004 and in Beijing during summer 2006 (Yoshino et al., 2006; Ren at el., 2006; Lu et al., 2013). In tropical forests local drivers as higher ambient temperature and solar radiation trigger the emissions of BVOCs, resulting in a more intense photochemistry, therefore higher OH reactivity compared to temperate environment.
• The magnitude of OH reactivity depends mainly on the type of environment but also on the season of measurements. Observations conducted in forested sites report a clear seasonal dependence, with the OH reactivity being larger in during spring and summer. Observations conducted in urban sites, as Tokyo, during all seasons
provide an example: wintertime maximum OH reactivity was slightly below 50 s−1, while during the other seasons was between 80-100 s−1.
Figure 1.7: OH reactivity measurements world map. Arrows indicate the sites where measurements of OH reactivity using the three diﬀerent experimental methods were conducted. Green, gray and blue frames refer to forested, urban and coastal sites. Red font indicates the sites where a maximum value of OH reactivity>50 s−1 was reported. The star indicates the measurements carried out with the comparative reactivity method.
Total OH reactivity: a change in philosophy
The missing OH reactivity
The missing OH reactivity is simply the fraction of reactivity which is not explained by the simultaneous measurements of concentration of gaseous atmospheric constituents. In other words, it corresponds to the unmeasured compounds in ambient air. These com-pounds can be unmeasured for three main reasons:
(i) they are not measured because of the lack of an appropriate technique for measuring them;
(ii) they are measured but their contribute to the OH reactivity is not known due to tech-nical issues for identifying the species or to the not known kinetics of reaction;
(iii) they are not measured because they are not known to exist.
It is especially this last hypothesis that has contributed considerably to pose the interest on the OH reactivity parameter. It is this last hypothesis to make us confident that there are more species in air than those measured or estimated to be (Goldstein and Galbally, 2007).
It is diﬃcult to compare studies of OH reactivity using diﬀerent methods, deployed by diﬀerent operators while the suite of complementary measures available in the gas phase is diﬀerent. Therefore comparisons among missing reactivity provided by diﬀerent works have to be taken with caution. However, OH reactivity in forests have shown so far the largest gaps between OH reactivity measured and calculated. Those values are larger than in urban environments, for instance.
Table of contents :
1 Introduction to total OH reactivity
1.1 Theoretical background on tropospheric chemical processes and reactive constituents
1.1.1 Atmospheric relevance of reactive gases
1.1.2 The hydroxyl radical and other atmospheric oxidants
1.1.3 Nitrogen Oxides
1.1.4 Volatile Organic Compounds (VOCs)
1.2 Total OH reactivity: a change in philosophy
1.2.1 OH reactivity relevance
1.2.2 Measuring the OH reactivity
1.2.3 OH reactivity in the world
1.2.4 The missing OH reactivity
1.3 The Mediterranean basin
1.3.1 General aspects
1.3.2 Natural and anthropogenic local emissions
1.3.3 A hotspot for climate change
1.4 Thesis objectives
2.1 Proton Transfer Reaction-Mass Spectrometer (PTR-MS)
2.1.1 Applications in atmospheric sciences
2.1.2 Instrumental operation
2.1.3 Compounds sensitivity and volume mixing ratio
2.2 Comparative Reactivity Method for OH reactivity studies
2.2.1 General principle
2.2.2 Derivation of the basic equation for CRM
2.2.3 The reactor
2.2.4 The detector
2.2.5 Method calibration
2.2.7 Data processing
2.2.8 Limit of detection and measurement uncertainties
2.3.1 Optimization of the Comparative Reactivity Method at LSCE
2.3.2 CRM-LSCE performance
2.3.3 CRM-LSCE: field deployment
3 Intercomparison of two comparative reactivity method instruments in the Mediterranean basin during summer 2013
3.2.1 The comparative reactivity method
3.2.2 Data processing
3.2.3 Comparative Reactivity Method set up
3.2.4 Description of the field site and experiments
3.3 Results and discussion
3.3.1 C1 acquired with the conventional and scavenger approaches
3.3.2 Assessment of the correction for humidity differences between C2 and C3
3.3.3 Assessment of the correction for the kinetics regime
3.3.4 Correction for dilution
3.3.5 Measurement uncertainty
3.3.6 Intercomparison of OH reactivity results
3.4 Summary and conclusions
4 OH reactivity and concentrations of Biogenic Volatile Organic Com-pounds in a Mediterranean forest of downy oak trees
4.2.1 Description of the field site
4.2.2 Ambient air sampling
4.2.3 Comparative Reactivity Method and instrument performance
4.2.4 Complementary measurements at the field site
188.8.131.52 Proton Transfer Reaction-Mass Spectrometer
184.108.40.206 Gas chromatography-flame ionization detector
220.127.116.11 Formaldehyde analyzer
18.104.22.168 NOx analyzer
22.214.171.124 GC-MS offline analysis
126.96.36.199 O3, CH4, CO
188.8.131.52 Meteorological parameters
4.3.1 Trace gases profiles and atmospheric regime
4.3.2 Total OH reactivity
4.3.3 Measured and calculated OH reactivity
4.3.4 Nighttime missing reactivity
4.3.5 OH reactivity at other biogenic sites
4.4 Summary and conclusion
5 Total OH reactivity at a receptor coastal site in the Mediterranean basin during summer 2013
5.2 Field site
5.3.1 Comparative Reactivity Method
5.3.2 Ancillary measurements at the field site
184.108.40.206 Proton Transfer Reaction-Mass Spectrometry
220.127.116.11 Online Chromatography
18.104.22.168 Offline Chromatography
22.214.171.124 Hantzsch method for the analysis of formaldehyde
126.96.36.199 Chemiluminescence for the analysis of NOx
188.8.131.52 Wavelength-scanned cavity ring down spectrometry (WSCRDS)
5.3.3 Box model for mixing ratios and OH reactivity evaluation
5.4.1 Air masses regime
5.4.2 Total measured OH reactivity
5.4.3 Calculated OH reactivity and importance of biogenic VOCs at the measuring site
184.108.40.206 Long-term variability
220.127.116.11 Contributions from different classes of compounds
18.104.22.168 Impact of biogenic VOCs
5.4.4 Comparison between measured and calculated reactivity
5.4.5 Clues on the missing OH reactivity: a mix of primary emission and secondary production
22.214.171.124 Unmeasured terpenes
126.96.36.199 estimated reactivity of the unmeasured terpenes
188.8.131.52 unmeasured secondary products
5.4.6 Modeled OH reactivity
184.108.40.206 Model inputs
220.127.116.11 Model results and sensitivity
18.104.22.168 Contributions to the modelled OH reactivity
6 Conclusion and future research