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An advanced approach of conditional non-linear optimal perturbation (CNOP) was adopted to identify the sensitive area for targeted observations of meteorological fields associated with PM.sub.2.5 concentration forecasts of a heavy haze event that occurred in the Beijing-Tianjin-Hebei (BTH) region, China, from 30 November to 4 December 2017. The results show that a few specific regions in the southern and northwestern directions close to the BTH region represent the sensitive areas. Numerically, when predetermined artificial observing arrays (i.e. possible \"targeted observations\") in the sensitive areas were assimilated, the forecast errors of PM.sub.2.5 during the accumulation and dissipation processes were aggressively reduced; specifically, these assimilations, compared with those in other areas that have been thought of as being important for the PM.sub.2.5 forecasts in the BTH region in previous studies, exhibited a more obvious decrease in the forecast errors of PM.sub.2.5 . Physically, the reason why these possible targeted observations can significantly improve the forecasting skill of PM.sub.2.5 was interpreted by comparing relevant meteorological fields before and after assimilation. Therefore, we conclude that preferentially deploying additional observations in the sensitive areas identified by the CNOP approach can greatly improve the forecasting skill of PM.sub.2.5, which provides, beyond all doubt, theoretical guidance for practical field observations of meteorological fields associated with PM.sub.2.5 forecasts.

Severe surface ozone (O.sub.3) pollution frequently occurred in North China and obviously damages human health and ecosystems. The meteorological conditions effectively modulate the variations in O.sub.3 pollution. In this study, the interannual relationship between O.sub.3 -related meteorology and late-spring snow cover in West Siberia was explored, and the reasons for its decadal change were also physically explained. Before mid-1990s, less snow cover could enhance net heat flux and stimulate positive phase of the Eurasian (EU) teleconnection in summer. The positive EU pattern resulted in hot, dry air and intense solar radiation in North China, which could enhance the natural emissions of O.sub.3 precursors and photochemical reactions in the atmosphere closely related to high O.sub.3 concentrations. However, after the mid-1990s, the southern edge of the dense snow cover area in West Siberia shifted northward by approximately 2.sup.\" in latitude and accompanied radiation and heat flux also retreated toward the polar region. The connections among snow anomalies, EU pattern and surface O.sub.3 became insignificant and thus influenced the stability of the predictability.

The key process in understanding carbon dynamics under different ecosystems is quantifying soil CO[sub.2] efflux. However, this process can change annually as it depends on environmental variables. The results of this paper present the effects of root network, soil temperature, and volumetric water content on soil CO[sub.2] efflux, which were investigated on Retisol of two types of land uses in Western Lithuania in 2017-2019: forest and grassland. It was determined that the average soil CO[sub.2] efflux in the grassland was 32% higher than in the forest land. The CO[sub.2] efflux, average across land uses, tended to increase in the following order: 2017 < 2018 < 2019. Dry weather conditions with high temperatures during the vegetation period governed the soil CO[sub.2] efflux increase by 14%. Soil temperature (up to 20 °C) and volumetric water content (up to 23-25%) had a positive effect on the soil CO[sub.2] efflux increase on Retisol. We established that the root's activity plays one of the main roles in the CO[sub.2] production rate-in both land uses, the soil CO[sub.2] efflux was influenced by the root length density and the root volume.