Total hydroxyl radical (OH) reactivity measurements were conducted at the second Station for Measuring
Ecosystem–Atmosphere Relations (SMEAR II), a boreal forest site located in Hyytiälä, Finland, from April
to July 2016.
The measured values were compared with OH reactivity calculated from a combination of data from the
routine trace gas measurements (station mast) as well as online and offline analysis with a gas chromatographer
coupled to a mass spectrometer (GC–MS) and offline liquid chromatography.
Up to 104 compounds, mostly volatile organic compounds (VOCs) and oxidized VOCs, but also inorganic compounds,
were included in the analysis, even though the data availability for each compound varied with time.
The monthly averaged experimental total OH reactivity was found to be higher in April and May (ca. 20 s
Terrestrial vegetation is responsible for about 90 % of the emissions of biogenic volatile organic
compounds (BVOCs) into the atmosphere
Total hydroxyl radical (OH) reactivity measurements can be used as a method to assess our understanding
of tropospheric chemistry
Large fractions of missing reactivity were first observed in a forest in northern Michigan
Also in the boreal forest, which represents approximately one-third of the Earth's forested surface
The two main assumptions for the missing reactivity are (1) missing primary emissions and (2) missing
oxidation products from the emissions. Several studies have been conducted to investigate these
hypotheses.
In order to investigate OH reactivity at SMEAR II in more detail, in particular its missing fraction and
the seasonal variations which are often neglected for summer intensive campaigns, a new implementation of
the CRM was developed at the Finnish Meteorological Institute
Measurements were conducted at the boreal forest site SMEAR II
The instruments were located inside a container in an opening about 115 m from the site mast,
from which meteorological data as well as ozone (
In situ measurements of the total OH reactivity (Sect.
Temperature and relative humidity (RH) are taken at 4.2 m above ground on the mast, soil properties are an average of five locations throughout the site, and radiation and precipitation data are collected at 18 m height on a nearby tower.
Orthophotograph of the SMEAR II station in Hyytiälä and its surroundings with the marked location of the station mast and the container where the measurements were performed. (Source: Land Survey of Finland Topographic Database 09/2018).
VOCs were measured with two in situ gas chromatographer–mass spectrometers (GC–MSs). The first GC–MS was used for the measurements of mono- and
sesquiterpenes, isoprene, 2-methyl-3-butenol (MBO), and C
Additional sampling took place between 27 April and 3 May in canisters and through adsorption cartridges
(24 h time resolution) to be analysed by a GC–flame ionization detector (FID) (C
The mixing layer height (MLH) was estimated from measurements with a 1.5
The OH reactivity,
Our analysis includes up to over 100 individual species from two GC–MS, GC–FID and LC–UV measurements
(see Sect.
Measurements of total OH reactivity (
The CRM is based on the monitoring of pyrrole (
Pyrrole detection is performed with a gas chromatograph (GC) equipped with a photon ionization
detector (PID) every 2 min (Synthec Spectras GC955, Synspec BV, Groningen, the Netherlands).
The sensitivity of this detector is independent of the RH of the sample (Fig.
Based on earlier tests, the sensitivity of the GC–PID does not depend on RH in the reactor (Fig.
OH is produced by the photolysis of water (
Equation
In addition, because of the dilution of the sampled air with humid nitrogen, the experimental total OH reactivity
(
Finally, the missing fraction of the total OH reactivity is obtained by comparing
As discussed in
The corrections
The correction
For instance, on 29 and 30 April total OH reactivity around 125 to 150 s
Nevertheless, reactivity calibrations were performed for the present study with a 10 ppmv
The comparison between OH reactivity expected from the standard (
The 1 h averages of total measured OH reactivity,
The calibration for
The total uncertainty for the measured total OH reactivity is derived from the following equations:
In this study we applied the model to Simulate the concentrations of Organic vapours, Sulfuric Acid and Aerosols
(SOSAA) to simulate the OH reactivity at the SMEAR II station for selected days in April, May, and July 2016.
SOSAA is a one-dimensional chemical transport model comprised of boundary layer meteorology, biogenic emission
of VOCs, gas-phase chemistry, aerosol dynamics, and gas dry deposition
The boundary layer meteorology was derived from scalar distribution
The gas-phase chemistry was created using the Kinetic PreProcessor (KPP;
Comparison between measured OH reactivity for
The model runs in the present study include the dry deposition module implemented in SOSAA by
An overview of the measured total OH reactivity together with the calculated OH reactivity from up to
104 compounds, depending on data availability, as well as selected ancillary data, such as environmental
conditions (air and surface soil temperatures as well as surface soil water content), and contributions
from different compounds and groups of compounds are presented in Fig.
The range of measured total OH reactivity values is similar to that of previous studies at the same site
in August 2008 and July–August 2010 These high total OH reactivity peaks in the spring
(with values higher than at the end of July) seem to be associated with changes in the soil water content
resulting from soil thawing. The calculated OH reactivity from measured compounds is in general lower than the measured
total OH reactivity (also for periods with a large number of compounds included in the analysis), leading to
a large fraction of missing reactivity (see Sect. Inorganic compounds (
Monthly means and standard deviations (SD) of experimental total OH reactivity
(
Keeping in mind that the experimental data have not always been acquired continuously, the total experimental
OH reactivity (
The data for July cover days that were cloudier and more humid (both air and soil) but warmer than the
period covered by the data in June leading to higher total OH reactivity.
Monthly means of ambient concentrations of locally emitted terpenoids had a weak correlation (
It should be noted, though, that the use of a correction factor based on
The calculated OH reactivity of various groups of compounds shows different diurnal patterns, which vary
with the season as well. Their normalized values are depicted in Fig.
When the total measured OH reactivity hourly average is at a minimum during the day and a maximum at night (May
to July), it follows the pattern of BVOC concentrations (and calculated OH reactivity) due to the low mixing
layer height and despite slightly lower emissions due to the lower temperatures at night
Normalized monthly averaged diurnal variations in experimental OH reactivity
While the OH reactivity daily patterns from monoterpenoids and MBO had a minimum during the day for all
months, other groups of compounds showed this reactivity pattern only for some periods. Isoprene showed
this pattern except in July, where the light-induced emissions during the day dominated.
Sesquiterpenes, other carbonyls, and
Measured total OH reactivity (
Overall, from May to July the total OH reactivity exhibits a minimum during the day and a maximum at night,
following the OH reactivity pattern for biogenic compounds (except for isoprene in July, which is present in
low concentrations in this pine forest, and has a maximum in the afternoon then). In April, the total OH
reactivity has a maximum in the afternoon, and sesquiterpenes, even though present
in low concentrations, show a similar reactivity pattern.
The comparison between the calculated and measured OH reactivity is challenging as the calculated values
are derived from a number of compounds that varies because of the availability of the measurements
(Fig.
However, this period also coincided with high reactivity peaks observed likely due
to soil thawing as mentioned previously. Only sesquiterpenes peaked at the same time as the total OH reactivity,
but their concentrations are still low, which is why we mentioned amines and non-terpene BVOCs as potential
classes of compounds contributing to the observed total OH reactivity.
As has been shown for forests dominated by isoprene emitters
Contributions of various compounds and groups of compounds to the
measured total OH reactivity (
While the trend of
Retrieving the additional reactivity from these modelled compounds that were not included in
Most of the missing reactivity could then be due to oxidation products that are
not included in the model from measured precursors such as
Amines released from soil, as mentioned previously, are a potential class of compounds that
could contribute to OH reactivity.
It is also good to keep in mind that part of the missing reactivity can be
explained by measurement uncertainties and potential overestimation due to applied correction factors.
As the data in this study have been uniformly corrected based on
A previous study by
Our results are not entirely in line with other studies that showed reductions of the missing reactivity
by constraining VOC concentrations to model their oxidation products
Finally, heterogeneous loss of OH to particles might be a contribution to missing OH
reactivity, but this process is poorly quantified
As a side note, total OH reactivity measurements were unfortunately not
available in the autumn, but
Total OH reactivity is not a simple function of a few variables. It includes many complex processes involving sources and sinks that can change dramatically depending on the environmental conditions and the time of the year. Measurement uncertainties and data availability for comparison between measured total OH reactivity values and calculated values also represent a challenge when interpreting results.
In the present study total OH reactivity measurements were performed at a
Finnish boreal forest research site (SMEAR II). The monthly averaged
experimental total OH reactivity was high in April and May (about 20 s
A suite of online and offline (O)VOCs measurements was used to calculate the
known fraction of OH reactivity to compare it to the total OH reactivity measured.
The missing fraction of the OH reactivity remained high for the measurement period.
This can be due to various reasons.
As the data availability of (O)VOCs varies, the comparison between experimental
and calculated OH reactivity is difficult but three different explanations can
lead to high missing (unexplained) OH reactivity: (1) simply the lack of
measurements, (2) not measuring oxidation products (only their precursors),
and (3) not measuring the right class of compounds.
We showed that compounds not included (or only partially included) in the analysis
due to the unavailability of measurements (e.g. due to technical problems), such as acetaldehyde, might
contribute a small but significant fraction to the total OH reactivity, in particular for low reactivity
values.
Using a one-dimensional transport model to estimate oxidation product
concentrations from measured precursor concentrations for three short periods
of 2 to 3 d in various months (with most (O)VOC data availability) it
is demonstrated that only a small fraction (up to ca. 4.5 %) of the missing
reactivity can be explained by these oxidation products.
On the one hand, this is due to the absence in the model of a degradation scheme
for detected compounds in the ambient air (e.g.
More measurements of oxidized compounds and identification of non-terpene reactive compounds from emissions from sources other than vegetation (e.g. soil) are required to better understand the reactivity and local atmospheric chemistry in the forest air in general, in particular during winter, spring, and autumn, when the forest air chemistry is not dominated by emissions from the vegetation.
All the data whose source is not explicitly specified in the text are available upon reasonable request to the corresponding author.
Numerical simulations of
Uncertainty of
Mixing ratio of measured (red line and shaded area for 50 % uncertainty) and modelled (blue line) nopinone for
the three modelled periods in April, May, and July
Averages of individual compound mixing ratios (ppt
Continued.
Continued.
APP conducted total OH reactivity measurements, offline sampling, and LC–UV analysis; performed data analysis; and led the writing of the paper. HH designed the measurement campaign, conducted GC–MS measurements and data analysis, and commented on the paper. TT assisted with GC–MS measurements and data analysis and commented on the paper. VV provided mixing layer height data and their description in the method part and commented on the paper. DC, MB, and PZ performed model runs with the help of DT and all commented on the paper.
The authors declare that they have no conflict of interest.
The authors thank Hannele Hakola for the continuous support. They also thank the staff at the SMEAR II station for their help, Petri Keronen for providing the data that we retrieved from SmartSMEAR, Jari Waldén for lending calibration standards and gas analysers, and Anne-Mari Mäkelä for the analysis of the canister samples. The authors also wish to acknowledge the CSC IT Center for Science, Finland, for computational resources.
This research has been supported by the Academy of Finland (grant no. 275608), the Academy of Finland, Biotieteiden ja Ympäristön Tutkimuksen Toimikunta (grant no. 307957), and the Academy of Finland (grant no. 272041).
This paper was edited by Yugo Kanaya and reviewed by two anonymous referees.