Total OH reactivity in pristine and polluted environments: Investigating atmospheric chemistry in the Anthropocene
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Abstract
Since the start of industrialization and increasingly since the 1950s, anthropogenic
activities have altered the Earth’s atmospheric composition significantly with consequences
for climate, weather, and the health of both humans and ecosystems. Reactive
trace gases are, on the one hand, part of the anthropogenic emissions that fuel
air pollution and impact climate, and on the other hand, are impacted by the manmade
changes in environmental conditions in a feedback loop. A way to quantify
the total atmospheric load of reactive trace gases is the measurement of total OH
reactivity, i.e. the loss rate of the most important tropospheric oxidant, the hydroxyl
(OH) radical.
In this doctoral project, total OH reactivity measurements were used to investigate
two points in the feedback loop of human activity and atmospheric reactants: Firstly,
the indirect impact of anthropogenic climate change and deforestation, which will
lead to an increasing frequency of drought and heat events in the Amazon rainforest,
is thought to influence biogenic trace gas emissions. This was investigated using
total OH reactivity observations during an extreme El Niño event. Secondly, direct
human impact above the seaways around the Arabian Peninsula, detectable by anthropogenically
emitted trace gases from ships and oil/gas production, was studied
with total OH reactivity observations and a regional ozone formation assessment.
The method-oriented part of this doctoral project was based on the need for robust,
accurate long-term observations of total OH reactivity for understanding atmospheric
photochemistry in the Anthropocene epoch.
During the drought and heat conditions of the extreme 2015/16 El Niño event, the
diel cycle of total OH reactivity in the Amazon rainforest exhibited a striking difference
to "normal" diel behavior. After the usual early afternoon OH reactivity maximum,
a second, higher peak was observed during the sunset hours. A possible explanation
for the increased sunset reactivity was found in stronger turbulent transport
inside and above the canopy related with the changed meteorological conditions,
combined with a stress-related release of monoterpenes and other (unmeasured)
BVOCs by vegetation.
Total OH reactivity measured around the Arabian Peninsula was comparable to highly
populated urban areas, due to a combination of shipping emissions and petrochemical
pollution. The extreme regional ozone concentrations could be explained by a
favorable mixture of NOx and VOCs coupled with intense solar irradiation, causing
rapid photochemical reactions.
A new Comparative Reactivity Method (CRM) instrument for long-term autonomous
measurements of total OH reactivity was successfully characterized and tested in
Helsinki. Interferences were quantified and compared to a model of the CRM reactor chemistry. The total OH reactivity observed in winter in Helsinki was with an overall
median of 7.6 s^−1 at the lower end of worldwide urban observations.
In the first comprehensive intercomparison of OH reactivity measurements, the CRM
method was compared to all other available instrument types by simultaneous measurements
at an atmospheric simulation chamber. Results showed that the CRM
device is suited for a range of atmospheric mixtures. However, significant deviations
were seen under terpene-dominated and high NO conditions. A sensitivity towards
ozone, which also impacted the size of the NO2 interference, was newly discovered
for CRM. Photolysis inside the reactor and the HO2 concurrently produced with OH
were identified as major sources of interferences and uncertainties. With the aim of
improving the method, laboratory studies were conducted in the aftermath of the
intercomparison, focusing on reducing photolysis and increasing CRM sensitivity.