Chemical ionization mass spectrometry measurements of PAN and PAA in the remote atmosphere
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Abstract
This work presents a unique data set of simultaneous measurements of the two closely
linked atmospheric trace gases peroxyacetyl nitrate (PAN) and peracetic acid (PAA) in
the remote, tropical troposphere. The measurements were obtained using a Chemical
Ionisation Mass Spectrometry (CIMS) instrument during the two Chemistry of the Atmosphere
Field Experiments (CAFE) aircraft campaigns, CAFE Brazil and CAFE Pacific,
aboard the High Altitude and Long Range Research aircraft (HALO) aircraft. The field
experiments spanned a vertical range between a few hundred meters above the ground
up to an altitude of almost 15 km.
The limit of detection (LOD) per flight for PAN varied between 20-60 parts per trillion
(ppt) during CAFE Brazil and between 10-22 ppt during CAFE Pacific. The LOD of PAA was
between 5-33 ppt and 4-11 ppt during the two campaigns. The measurement sensitivity
for PAN was derived from in-flight calibrations in dry air at altitudes above 10km to
avoid matrix effects of by-products of the photochemical calibration source in humid
air masses. A correction factor of around 0.86 was introduced to account for losses of
the detecting acetate anion (m
z =59) on formic acid. A total measurement uncertainty
of around 30% was assigned to the PAN measurements. Between the CAFE Brazil and
CAFE Pacific the CIMS calibration was changed to produce isotopically labeled 13PAN
which removed the memory effects after an in-flight calibration.
The instrument’s sensitivity to PAA was humidity corrected and calibrated in duringand
post-campaign ground experiments. Due to the lack of in-situ calibrations and inconsistencies
between different calibration methods, an uncertainty of around a factor
of 2 must be taken into consideration when using the PAA data.
The CAFE Brazil campaign was conducted above the pristine Amazonian rainforest
around the city of Manaus between December 2022 and January 2023, during
the transition between the dry and wet season. Median PAN levels of the 12 analysed
flights during CAFE Brazil were maximum in mid-tropospheric altitudes (6-10 km)
around 100 ppt. In contrast, measured PAA was generally highest at low altitudes,
with a median almost up to 500 ppt, except during flights explicitly targeting convective
outflow, which led to PAA maxima in the mid- and upper troposphere. The
sampled air masses were characterized by a low PAN-to-PAA ratio (median 0.3 at
mid-troposphere), reflecting the dominance of biogenic volatile organic compounds
(VOC)-driven hydrocarbon chemistry compared to higher-NO + NO2 (NOx) regions.
The high mid-tropospheric PAN/(PAN+NOx)-ratio of 80% highlighted the importance
of PAN as a reservoir species of NO + NO2 (NOx) in tropical Amazonia and indicated
a NOx-limited PAN formation in this region. The comparison with the global
chemical-transport model ECHAM/MESSy Atmospheric Chemistry (EMAC) revealed
that methyl glyoxal (MGLY) was the most important (approx. 29% of total model
peroxyacetyl (PA)-production) single immediate PAN and PAA precursor during CAFE
Brazil, compared to acetaldehyde (approx. 17 %) and acetone (approx. 9 %). Overall,
isoprene oxidation products were responsible for almost three quarter of the total PA
formation in the model.
The CAFE Pacific campaign was performed one year later, based in Cairns, Australia,
and covered a large area between 130-165 ◦E and 0-45 ◦S above the Australian continent
and the Southern Pacific. A special target region was the Indo-Pacific Warm Pool
region in the north-east of Cairns, where PAN and PAA mixing ratios where close to
or below the instrument’s detection limit. In general, the CAFE Pacific campaign was
characterized by low PAN and PAA levels, with medians of around 50 ppt and 100 ppt,
respectively, during 12 analysed flights in the mid-troposphere. In comparison to the
CAFE Brazil campaign, the relative contribution of the immediate biogenic PA-precursor
MGLY was lower during CAFE Pacific (approx. 16% of total model PA-production) based
on EMAC model simulations. In contrast, the higher relative contributions of acetone
(approx. 26 %) and acetaldehyde (approx. 22 %), which can have both biogenic and anthropogenic
origins, indicated the mixture of different sources of PAN and PAA during
CAFE Pacific. The presence of different air masses from different source regions was
also confirmed by tracer-tracer correlations and back-trajectory calculations, which
showed that sampled air masses originated from both maritime and continental regions.
The influence of long-range transported pollution via the jet-stream, such as
biomass burning emissions from southern Africa, on the sampled PAN and PAA mixing
ratios appeared to be relatively small, as indicated by backward trajectory analysis.
Compared to the air masses sampled during the previous CAFE Africa campaign
above the tropical Atlantic, the impact of fresh biomass burning plumes on the observed
PAN and PAA levels during CAFE Brazil and CAFE Pacific was much less significant,
based on the analysis of black carbon (BC) and satellite observations of open
fires. In contrast to air masses during CAFE Brazil and CAFE Pacific, the PAN-toPAA ratio
reached up to a factor of 6 during CAFE Africa, highlighting the seasonal and regional
variability of PAN and PAA in the troposphere.
Calculated PA formation based on the measured precursors revealed that the high
acetaldehyde levels (around 100 ppt in the mid-troposphere) measured with the gas
chromatography-mass spectrometry (GC-MS) instrument during CAFE Brazil and CAFE
Pacific are in contradiction to the observed low levels of PAN and PAA. The GC-MS acetaldehyde
measurements exceeded the EMAC simulations up to a factor of 25, underscoring
the gap between the current understanding of atmospheric sinks and sources
of acetaldehyde and the observational data basis.
In addition, steady-state calculations of PAN and PAA based on model PA-radical
concentrations resulted in much steeper gradients than in the modelled and measured
vertical profiles of both species. That indicated that, in the model, convective mixing
leads to significantly flatter vertical profiles in both regions, Amazonia and southern
Pacific, than what would be expected in a chemical steady state.
In the case of the CAFE Brazil campaign, the EMAC model generally represented observed
PAN well with a slight tendency of underestimation (by 20-50 %). In contrast,
PAA was overestimated by EMAC (by 12-70 %). The overestimation of PAA was even
larger with 300-500% during CAFE Pacific, exceeding possible measurement uncertainties
by far. These discrepancies may have several reasons in the model such as the
incomplete or inaccurate loss and production processes of the PA-radical, insufficient
parameterisation of lightning NOx and convection or underestimation of cloud scavenging
effects on organic peroxides.
The tropical aircraft measurements emphasized the need for comprehensive measurements
of air composition in the troposphere in order to improve global model predictions
with regard to PAN and PAA, notably in tropical remote areas, where sparce
measurement data is available to date. In particular, this requires precise knowledge of
the tropospheric distributions of the organic precursor species and the reactive trace
gases NOx and OH+HO2 (HOx).