Characteristics of intercontinental transport of tropospheric ozone from Africa to Asia

Abstract

Abstract. In this study, we characterize the transport of ozone from Africa to Asia through the analysis of the simulations of a global chemical transport model, GEOS-Chem, from 1987 to 2006. The receptor region Asia is defined within 5–60∘ N and 60–145∘ E, while the source region Africa is within 35∘ S–15∘ N and 20∘ W–55∘ E and within 15–35∘ N and 20∘ W–30∘ E. The ozone generated in the African troposphere from both natural and anthropogenic sources is tracked through tagged ozone simulation. Combining this with analysis of trajectory simulations using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, we find that the upper branch of the Hadley cell connects with the subtropical westerlies in the Northern Hemisphere (NH) to form a primary transport pathway from Africa to Asia in the middle and upper troposphere throughout the year. The Somali jet that runs from eastern Africa near the equator to the Indian subcontinent in the lower troposphere is the second pathway that appears only in NH summer. The influence of African ozone mainly appears over Asia south of 40∘ N. The influence shows strong seasonality, varying with latitude, longitude, and altitude. In the Asian upper troposphere, imported African ozone is largest from March to May around 30∘ N (12–16 ppbv) and lowest during July–October around 10∘ N (∼ 2 ppbv). In the Asian middle and lower troposphere, imported African ozone peaks in NH winter between 20 and 25∘ N. Over 5–40∘ N, the mean fractional contribution of imported African ozone to the overall ozone concentrations in Asia is largest during NH winter in the middle troposphere (∼ 18 %) and lowest in NH summer throughout the tropospheric column (∼ 6 %). This seasonality mainly results from the collective effects of the ozone precursor emissions in Africa and meteorology and chemistry in Africa, in Asia and along the transport pathways. The seasonal swing of the Hadley circulation and subtropical westerlies along the primary transport pathway plays a dominant role in modulating the seasonality. There is more imported African ozone in the Asian upper troposphere in NH spring than in winter. This is likely due to more ozone in the NH African upper troposphere generated from biogenic and lightning NOx emissions in NH spring. The influence of African ozone on Asia appears larger in NH spring than in autumn. This can be attributed to both higher altitudes of the elevated ozone in Africa and stronger subtropical westerlies in NH spring. In NH summer, African ozone hardly reaches Asia because of the blocking by the Saharan High, Arabian High, and Tibetan High on the transport pathway in the middle and upper troposphere, in addition to the northward swing of the subtropical westerlies. The seasonal swings of the intertropical convergence zone (ITCZ) in Africa, coinciding with the geographic variations of the ozone precursor emissions, can further modulate the seasonality of the transport of African ozone, owing to the functions of the ITCZ in enhancing lightning NOx generation and uplifting ozone and ozone precursors to upper layers. The strength of the ITCZ in Africa is also found to be positively correlated with the interannual variation of the transport of African ozone to Asia in NH winter. Ozone from NH Africa makes up over 80 % of the total imported African ozone over Asia in most altitudes and seasons. The interhemispheric transport of ozone from southern hemispheric Africa (SHAF) is most evident in NH winter over the Asian upper troposphere and in NH summer over the Asian lower troposphere. The former case is associated with the primary transport pathway in NH winter, while the latter case is associated with the second transport pathway. The intensities of the ITCZ in Africa and the Somali jet can each explain ∼ 30 % of the interannual variations in the transport of ozone from SHAF to Asia in the two cases.

Local and synoptic meteorological influences on daily variability in summertime surface ozone in eastern China

“…Weather classification is a well-established tool for characterizing atmospheric processes on multiple scales and further for studying the air-pollution-weather relationship (Han et al, 2018b). The methods for weather classification can be generally categorized into three groups: subjective, mixed, and objective, depending on the automatic degree during the classification process (Huth et al, 2008).…”

“…The FNL geopotential height fields (Sect. 2.1) at 850 hPa can capture the synoptic circulation variations over eastern China well (Han et al, 2018b). In this study, we used the geopotential height at 850 hPa in 2013-2018 as the input for the SOM.…”

“…These weather conditions can enhance stagnation and production of ozone (Camalier et al, 2007;. Variations in these local meteorological variables depend on the dominant weather systems (Davis et al, 1998;Han et al, 2018b;Leung et al, 2018).…”

“…Third, we further examine the sensitivity of daily surface ozone to the variation in synoptic weather systems. Considering the complexity of the synoptic meteorology in eastern China (Ding et al, 2017;Han et al, 2018b), we employ an objective clustering technique, the self-organizing map (SOM), to identify the predominant synoptic weather patterns (SWPs). In the following sections, we introduce the data and methods in Sect.…”

An important mechanism of regional O<sub>3</sub> transport for summer smog over the Yangtze River Delta in eastern China

“…1) with 32 vertical layers extending from the surface to 100 hPa. The simulation period spans from 21 to 25 August 2016 with 1-hourly model outputs, and the spinup time is the first 24 h. The physical parameterizations include the Noah land surface model (Tewari et al, 2004), the Mesoscale Model (MM5) similarity surface layer, the Yonsei University (YSU) boundary layer scheme (Hong et al, 2006), the Rapid Radiative Transfer Model (RRTM) longwave scheme (Mlawer et al, 1997), the Goddard shortwave scheme (Chou et al, 1998), the Morrison double-moment microphysics scheme (Morrison et al, 2009) and the Kain-Fritsch cumulus parameterization (Kain, 2004). The gasphase chemical mechanism is selected using the Regional Acid Deposition Model, version 2 (RADM2) (Chang et al, 1990;Stockwell et al, 1984) and includes 158 chemical reactions among 36 species.…”