Diurnal variability, photochemical production and loss processes of hydrogen peroxide in the boundary layer over Europe

Abstract

Abstract. Hydrogen peroxide (H2O2) plays a significant role in the oxidizing capacity of the atmosphere. It is an efficient oxidant in the liquid phase and serves as a temporary reservoir for the hydroxyl radical (OH), the most important oxidizing agent in the gas phase. Due to its high solubility, removal of H2O2 due to wet and dry deposition is efficient, being a sink of HOx (OH+HO2) radicals. In the continental boundary layer, the H2O2 budget is controlled by photochemistry, transport and deposition processes. Here we use in situ observations of H2O2 and account for chemical source and removal mechanisms to study the interplay between these processes. The data were obtained during five ground-based field campaigns across Europe from 2008 to 2014 and bring together observations in a boreal forest, two mountainous sites in Germany, and coastal sites in Spain and Cyprus. Most campaigns took place in the summer, while the measurements in the south-west of Spain took place in early winter. Diel variations in H2O2 are strongly site-dependent and indicate a significant altitude dependence. While boundary-layer mixing ratios of H2O2 at low-level sites show classical diel cycles with the lowest values in the early morning and maxima around local noon, diel profiles are reversed on mountainous sites due to transport from the nocturnal residual layer and the free troposphere. The concentration of hydrogen peroxide is largely governed by its main precursor, the hydroperoxy radical (HO2), and shows significant anti-correlation with nitrogen oxides (NOx) that remove HO2. A budget calculation indicates that in all campaigns, the noontime photochemical production rate through the self-reaction of HO2 radicals was much larger than photochemical loss due to reaction with OH and photolysis, and that dry deposition is the dominant loss mechanism. Estimated dry deposition velocities varied between approximately 1 and 6 cm s−1, with relatively high values observed during the day in forested regions, indicating enhanced uptake of H2O2 by vegetation. In order to reproduce the change in H2O2 mixing ratios between sunrise and midday, a variable contribution from transport (10 %–100 %) is required to balance net photochemical production and deposition loss. Transport is most likely related to entrainment from the residual layer above the nocturnal boundary layer during the growth of the boundary layer in the morning.

Measurement report: Photochemical production and loss rates of formaldehyde and ozone across Europe

“…Please note that all times are in UTC (coordinated universal time). The time difference between 12:00 UTC and local noon are UTC + 128 min for Cyprus, UTC + 44 min for Germany and UTC + 96 min for Finland (Fischer et al, 2019).…”

“…(5), a transport term T(HCHO) such as advection or entrainment (can be positive or negative) and a term for primary emissions (Fischer et al, 2019). The reaction of O( 3 P) with molecular oxygen (O 2 ) subsequently yields O 3 (Jacob, 1999).…”

“…with an uncertainty of 20 %. BLH measurements were not performed and instead adopted from Fischer et al (2019) which are 1500 m for daytime (j(NO 2 ) > 10 −3 s −1 ) and 200 m for nighttime (j(NO 2 ) < 10 −3 s −1 ). We determined the deposition velocity 11).…”

Modelling the impact of gas-phase pyruvic acid on acetaldehyde and peroxy radical formation in the boreal forest

“…Note that the calculated emission rates are subject to substantial uncertainties arising from the measurement of pyruvic acid, MT, OH, O3, NO3, hMXL and Jpyr. In particular, the results are very sensitive to the estimated deposition velocity (vdep) of pyruvic acid which is an estimate based on the deposition velocity of H2O2 which itself has an uncertainty of ~ 90 % (Fischer et al, 2019). Further, our calculations are based on the assumption that the sources for pyruvic acid and MT emissions are evenly distributed and measurements made at ~ 8.5 m above the ground are representative of the entire boundary layer (i.e.…”

“…The Max Planck Institute data used for the IBAIRN analysis is archived with Zenodo at https://doi.org/10.5281/zenodo.3254828 (Crowley and Fischer, 2019). HUMPPA-COPEC-2010 data can be obtained on request (via John N. Crowley) from the owners.…”