Abstract
Abstract. Trends in the CO andC2H6 partial columns ~0–15 km) have been estimated from four European ground-based solar FTIR (Fourier Transform InfraRed) stations for the 1996–2006 time period. The CO trends from the four stations Jungfraujoch, Zugspitze, Harestua and Kiruna have been estimated to −0.45 ± 0.16% yr−1, −1.00 ± 0.24% yr−1, −0.62 ± 0.19 % yr−1 and −0.61 ± 0.16% yr−1, respectively. The corresponding trends for C2H6 are −1.51 ± 0.23% yr−1, −2.11 ± 0.30% yr−1, −1.09 ± 0.25% yr−1 and −1.14 ± 0.18% yr−1. All trends are presented with their 2-σ confidence intervals. To find possible reasons for the CO trends, the global-scale EMEP MSC-W chemical transport model has been used in a series of sensitivity scenarios. It is shown that the trends are consistent with the combination of a 20% decrease in the anthropogenic CO emissions seen in Europe and North America during the 1996–2006 period and a 20% increase in the anthropogenic CO emissions in East Asia, during the same time period. The possible impacts of CH4 and biogenic volatile organic compounds (BVOCs) are also considered. The European and global-scale EMEP models have been evaluated against the measured CO and C2H6 partial columns from Jungfraujoch, Zugspitze, Bremen, Harestua, Kiruna and Ny-Ålesund. The European model reproduces, on average the measurements at the different sites fairly well and within 10–22% deviation for CO and 14–31% deviation for C2H6. Their seasonal amplitude is captured within 6–35% and 9–124% for CO and C2H6, respectively. However, 61–98% of the CO and C2H6 partial columns in the European model are shown to arise from the boundary conditions, making the global-scale model a more suitable alternative when modeling these two species. In the evaluation of the global model the average partial columns for 2006 are shown to be within 1–9% and 37–50% of the measurements for CO and C2H6, respectively. The global model sensitivity for assumptions made in this paper is also analyzed.
Highlights
During the last 30 years the trend in tropospheric carbon monoxide (CO) has turned from positive in the 1980s, to negative, in the 1990s and 2000s
The comparison of the European-scale model with the CO and C2H6 FTIR measurements is presented in Figs. 1 and 2, respectively
The boundary and initial conditions (BICs) influence is quantified by tracers and those are presented in Table 5, where it can be seen that columns at the modeled stations close to the model horizontal boundary are dominated by the BICs, while columns at the stations further away from the boundary include more information generated within the model
Summary
During the last 30 years the trend in tropospheric carbon monoxide (CO) has turned from positive in the 1980s, to negative, in the 1990s and 2000s. Ethane (C2H6) has shown a constant negative trend in both the 1980s and 1990s of roughly 1–3 % per year (Rinsland et al, 1998; Mahieu et al, 1997). These changes are important as tropospheric chemistry to a large extent is controlled by the hydroxyl (OH) radical, often referred to as the detergent of the atmosphere. One of the main OH sinks is the reaction with CO (Crutzen et al, 1999) In this oxidation, the greenhouse gas carbon dioxide is formed but the reaction is related to the formation or destruction of tropospheric ozone (O3), depending on the NOx concentrations in the ambient air. C2H6 has shown to be a major route for the formation of peroxyacetyl nitrate (PAN) which acts as a NOx reservoir and thereby accelerates O3 formation in the troposphere (Blake and Rowland, 1986; Finlayson-Pitts and Pitts, 2000)
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