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Hailun is a county-level city in Heilongjiang Province, situated in the heart of Northeast China's Songnen Plain, approximately 200 kilometres north of Harbin. This strategic position places it within one of China's most important agricultural regions, known as the 'breadbasket of the north,' where vast expanses of cropland—primarily soybean, corn, and rice fields—dominate the landscape. The city's urban core, with a population of around 480,000, is characterised by low-rise buildings and a grid-like layout typical of many Chinese county seats, blending into a rural periphery that supports intensive farming. Terrain here is predominantly flat, with an average elevation of about 200 metres above sea level, which facilitates agricultural mechanisation but also influences air quality by limiting natural ventilation. Hailun lies near the Hulan River, a tributary of the Songhua River, though it is not directly on a major water body, reducing any moderating effects on pollution dispersion. The region is part of the broader Northeast China industrial belt, with nearby cities like Qiqihar contributing to regional air pollution through industrial emissions, which can drift into Hailun under prevailing winds. This urban–rural gradient means that while local pollution sources include general urban activities like vehicle exhaust and residential heating, agricultural practices such as crop residue burning in surrounding fields also play a significant role, especially during harvest seasons. The flat terrain and lack of significant topographic barriers allow pollutants to accumulate more easily, particularly during periods of atmospheric stability, making geographic context a key factor in understanding Hailun's air quality challenges.

In Hailun, air quality follows a distinct seasonal pattern shaped by the region's harsh continental climate, with cold, dry winters and warm, humid summers. Winter, from December to February, is the peak pollution season due to heavy reliance on coal-fired heating systems, which emit particulate matter and sulfur dioxide. Temperature inversions are common during these months, trapping pollutants near the ground under a layer of cold air, leading to persistent haze and reduced visibility. Sensitive groups, such as children and the elderly, should limit outdoor activities during this time and consider using air purifiers indoors. Spring, from March to May, sees some improvement as heating demand decreases, but pollution can spike in late spring due to agricultural burning of crop residues, combined with occasional dust storms from the arid regions to the west. Summer, from June to August, offers the best air quality, with higher temperatures and increased rainfall from the East Asian monsoon helping to wash pollutants from the air, making it an ideal season for outdoor recreation. Autumn, from September to November, begins with relatively clean air but deteriorates as temperatures drop and heating resumes, with October often marked by harvest-related burning. Throughout the year, wind patterns play a crucial role: westerly winds can bring industrial pollution from nearby cities, while calm conditions exacerbate local accumulation. For health guidance, sensitive individuals should monitor air quality forecasts closely, avoid strenuous activities during pollution peaks, and plan outdoor visits preferably in summer or early autumn.