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  • Item type: Item , DissertationAccess status: Open Access ,
    Chemical ionization mass spectrometry measurements of PAN and PAA in the remote atmosphere
    (2025) Nelson, Anna Carolina; Lelieveld, Jos; Curtius, Joachim
    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).
  • Item type: Item , DissertationAccess status: Open Access ,
  • Item type: Item , DissertationAccess status: Open Access ,