Authors:	Hamryszczak, Zaneta Teresa
Title:	Hydroperoxide measurements in outdoor environments
Online publication date:	13-Nov-2023
Year of first publication:	2023
Language:	english
Abstract:	Hydroperoxides are well-acknowledged trace species in the oxidative chemistry of the atmosphere. On the one side, hydroperoxides and especially hydrogen peroxide (H2O2), serve as a reservoir of the main atmospheric oxidant, the hydroperoxyl radical (OH), and of the peroxy radicals (HO2), which are often collectively defined as HOx (HOx = OH + HO2). On the other side, hydroperoxides are known to convert sulfur dioxide (SO2) and to a minor extent nitrogen dioxide (NO2) into sulfuric acid (H2SO4) and nitric acid (HNO3) in the atmospheric aqueous phase (cloud, fog, and rain) leading to their acidification. Therefore, hydroperoxide in situ observations add to an improved understanding of self-cleansing processes in the atmosphere. This work is focused on airborne hydroperoxide measurements using a tool specifically designed to meet the requirements of dynamic, high-altitude, and long-range measurements on board the research aircraft, High-Altitude and Long-range Observatory (HALO). The Hydrogen Peroxide and Higher Organic Peroxides (HYPHOP) monitor is based on a dual-enzyme fluorescence spectroscopy technique, which enables tracking hydrogen peroxide and total organic hydroperoxides mixing ratios at a 1-Hz measurement frequency. The instrument was deployed in several airborne research projects, among which the most recent three aircraft campaigns, Chemistry of the Atmosphere: Field Experiment in Africa (CAFE-Africa), BLUESKY, and CAFE-Brazil are presented in detail regarding the hydroperoxide observations. The first section focuses on the theoretical background of the atmosphere and its main gaseous- and aqueous-phase chemistry. In the second section, the measurement technique and the corresponding analytical methods are discussed. The section characterizes the measurement method and data acquisition with special emphasis on potential measurement inconsistencies induced by dynamic flight patterns. The instrument’s precision based on the measurement results from the most recent airborne campaign, CAFE-Brazil for H2O2 and the sum of organic hydroperoxides (ROOH) were determined to be 6.4% (at 5.7 ppbv) and 3.6% (at 5.8 ppbv), respectively. The instrument’s limit of detection at a 1-Hz data acquisition frequency were 20 pptv and 19 pptv for H2O2 and ROOH, respectively. Based on the performed analyses, technical and physical challenges do not critically impact the measurement performance of the HYPHOP monitor. In situ observations of hydroperoxides based on airborne campaigns are presented in the following sections (Sect. 3. – Sect. 5.). During the BLUESKY campaign, performed in May – June 2020 mainly over central and southern Europe (35° N – 60° N; 15° W – 15° E), average mixing ratios of 0.23 (±0.18) ppbv, 0.42 (±0.25) ppbv, and 0.48 (±0.17) ppbv for H2O2 and 0.37 (±0.23) ppbv, 0.57 (±0.30) ppbv, and 0.62 (±0.36) ppbv for ROOH in the upper troposphere, the middle troposphere, and the boundary layer, respectively, were measured (Sect. 3.). In contrast to previous measurements during the HOOVER campaign (HOx Over EuRope; 2006 – 2007) and UTOPIHAN-ACT II/IIII campaign (Upper Tropospheric Ozone: Processes Involving HOx and NOx: The Impact of Aviation and Convectively Transported Pollutants in the Tropopause Region; 2002 – 2004), vertical profiles of measured H2O2 display diminished mixing ratios particularly above the boundary layer, which is most likely caused by cloud scavenging and subsequent rainout of the highly soluble trace species. The expected inverted C-shaped vertical distribution with maximum H2O2 mixing ratios at 3 – 7 km was not observed during the BLUESKY campaign. The observations are partly reproduced by the global circulation ECHAM/MESSy Atmospheric Chemistry (EMAC) model. The strong impact of the H2O2 cloud and precipitation scavenging is confirmed by a sensitivity study performed using the EMAC model. The differences arising between the H2O2 observations and simulations are most likely due to difficulties in wet scavenging simulations caused by the limited resolution of the model. Analyses of the hydroperoxide distribution during the CAFE-Africa aircraft campaign performed in August – September 2018 over the tropical Atlantic and the western coast of Africa (10° S – 50° N; 50° W – 15° E) reveal average mixing ratios of 0.18 (±0.13) ppbv, 2.19 (±1.86) ppbv and 2.25 (±1.30) ppbv for H2O2 and 0.15 (±0.07) ppbv, 0.55 (±0.25) ppbv, and 0.70 (±0.18) ppbv for ROOH in the upper troposphere, the middle troposphere, and in the boundary layer, respectively (Sect. 4.). In opposition to the expected latitudinal dependency in the distribution of hydroperoxides, H2O2 observations do not display any clear trend from the tropics towards the subtropics. Locally increased H2O2 mixing ratios of up to 1 ppbv were detected in the Intertropical Convergence Zone (ITCZ), in proximity to the tropical storm Florence over the Atlantic Ocean, and over the west African coast. Observation-based photostationary steady-state (PSS) calculations produce up to a factor of 2 lower H2O2 mixing ratios in the ITCZ and in the north of the sampled region relative to in situ observations. In contrast, in the south of the sampled area, PSS calculations tend to overestimate the H2O2 levels by up to a factor of 3. Analogously, simulations performed by the EMAC model tend to overestimate the H2O2 mixing ratios in the southern part of the sampled region. Based on PSS calculations and EMAC simulations a latitudinal trend towards the subtropics with maximum H2O2 mixing ratios in the tropics was expected. However, according to in situ observations, the spatial distribution of H2O2 displays nearly no latitudinal dependency originating from photochemical processes in the tropical upper troposphere. The observations suggest an influence of convective transport processes in tropical and subtropical regions. Convective processes in the ITCZ and the enhanced presence of clouds in the south of the ITCZ are most likely the main causes for the deviations between the observations and both, PSS calculations and EMAC simulations. The most recent airborne hydroperoxide measurements were performed during the CAFE-Brazil campaign in December 2022 – January 2023 mainly over the Amazon Basin (12 °S–4 °N; 70–38 °W; Sect. 5.). Average H2O2 mixing ratios of 0.74 (±0.25) ppbv, 0.45 (±0.26) ppbv, and 0.12 (±0.09) ppbv were measured in the boundary layer, the middle and upper troposphere. Accordingly, average ROOH mixing ratios of 0.89 (±0.31) ppbv, 0.62 (±0.34) ppbv, and 0.22 (±0.12) ppbv were detected in the sampled region. The highest average hydroperoxide mixing ratios were detected at altitudes of approximately 2 km, according to expectations. In the boundary layer, decreasing hydroperoxide mixing ratios reflect the effect of deposition processes on the trace species. The levels decrease towards the upper troposphere due to diminished availability of the hydroperoxide precursor, HO2. However, at 10 – 13 km in the upper troposphere, the hydroperoxide levels rise to approximately 20% of the measured maxima above the boundary layer, most likely due to convective transport. Generally, ROOH mixing ratios are approximately by a factor of 1.3 – 1.5 higher relative to the H2O2 observations throughout the entire tropospheric column. Specifically, in the upper troposphere (above 8 km) ROOH mixing ratios seem to be up to a factor of 5 higher than H2O2 levels. The increased ROOH mixing ratios are most likely due to sufficient availability of the hydroperoxide precursor, HO2, and vegetation emission-based production. Hydrogen peroxide is expected to be removed by deposition via precipitation and vegetation uptake in the lower troposphere. In the upper troposphere, H2O2 is assumed to be removed via temporal or permanent processes in the lower part of the convective clouds. The ROOH mixing ratios seem not to be affected by wet deposition removal. In the perspective of previous measurements in the South-American region, i. e. GABRIEL campaign (Guyanas Atmosphere-Biosphere exchange and Radicals Intensive Experiment with the Learjet; 2005), the measured hydroperoxide levels are significantly lower (up to a factor of 5). The high deviations between the campaign results are most likely due to contrasting meteorological conditions and the resulting significantly higher cloud scavenging and precipitation during CAFE-Brazil. EMAC simulations of the trace species tend to overestimate the mixing ratios of hydroperoxides, especially in the boundary layer and in the tropospheric regions affected by clouds. Most likely, the differences between the simulations and in situ observations are caused by difficulties in deposition simulations due to the limitations in the model’s resolution.
DDC:	500 Naturwissenschaften 500 Natural sciences and mathematics 540 Chemie 540 Chemistry and allied sciences
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 09 Chemie, Pharmazie u. Geowissensch.
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-9649
URN:	urn:nbn:de:hebis:77-openscience-25bafe01-a9c0-4fe9-a6eb-835dc77a51ed1
Version:	Original work
Publication type:	Dissertation
License:	CC BY
Information on rights of use:	https://creativecommons.org/licenses/by/4.0/
Extent:	158, xxviii Seiten ; Illustrationen, Diagramme
Appears in collections:	JGU-Publikationen