Sanhe, nestled within the Ningxia Hui Autonomous Region of China, occupies a geographically complex position along coordinates 36.5639° N, 105.6439° E. The city’s terrain is characterised by the loess plateau, a landscape sculpted by centuries of wind and water erosion, resulting in undulating hills and relatively flat plains. Situated at an elevation of approximately 1400 meters, Sanhe experiences a continental climate with significant temperature variations. The Yellow River flows relatively close to Sanhe, though not directly adjacent, influencing local humidity and potentially contributing to dust transport. The surrounding region is a mix of agricultural land, primarily focused on grain production, and increasingly, industrial development linked to Ningxia’s resource extraction industries. This proximity to both agricultural practices (leading to ammonia emissions from fertilizers) and industrial zones (emitting particulate matter and sulfur dioxide) significantly impacts air quality. The urban-rural gradient is relatively sharp, with a rapid transition from concentrated residential and commercial areas to open farmland. This creates localized pollution hotspots within the city centre, exacerbated by limited wind dispersal due to the surrounding topography. The loess soil itself is a significant source of PM10 during dust storms, a frequent occurrence in the spring.

Sanhe’s air quality follows a distinct seasonal pattern. Spring (March-May) is typically the worst period, dominated by dust storms originating from the Gobi Desert and exacerbated by strong winds sweeping across the loess plateau. These winds lift significant quantities of particulate matter, particularly PM10, into the atmosphere. Summers (June-August) bring some relief, with increased rainfall suppressing dust and monsoon winds offering temporary dispersal. However, high temperatures can promote the formation of ground-level ozone, a secondary pollutant. Autumn (September-November) sees a return of drier conditions and increased coal burning for heating, leading to a rise in particulate matter and sulfur dioxide concentrations. Winter (December-February) is characterized by temperature inversions, trapping pollutants near the ground and creating stagnant air conditions. This, combined with continued coal use, results in consistently poor air quality. Sensitive groups – children, the elderly, and those with respiratory conditions – should avoid strenuous outdoor activity during spring and winter. October and November, before the full onset of winter heating, generally offer the most favourable conditions. Monitoring wind direction is crucial; winds from the northwest are likely to carry desert dust.