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In recent years, air pollution has become a serious threat, causing adverse health effects and millions of premature deaths in China. This study examines the spatial-temporal characteristics of ambient air quality in five provinces (Shaanxi (SN), Xinjiang (XJ), Gansu (GS), Ningxia (NX), and Qinghai (QH)) of northwest China (NWC) from January 2015 to December 2018. For this purpose, surface-level aerosol pollutants, including particulate matter (PMx, x = 2.5 and 10) and gaseous pollutants (sulfur dioxide (SO₂), nitrogen dioxide (NO₂), carbon monoxide (CO), and ozone (O₃)) were obtained from China National Environmental Monitoring Center (CNEMC). The results showed that fine particulate matter (PM_2.5), coarse particulate matter (PM₁₀), SO₂, NO₂, and CO decreased by 28.2%, 32.7%, 41.9%, 6.2%, and 27.3%, respectively, while O₃ increased by 3.96% in NWC during 2018 as compared with 2015. The particulate matter (PM_2.5 and PM₁₀) levels exceeded the Chinese Ambient Air Quality Standards (CAAQS) Grade II standards as well as the WHO recommended Air Quality Guidelines, while SO₂ and NO₂ complied with the CAAQS Grade II standards in NWC. In addition, the average air quality index (AQI), calculated from ground-based data, improved by 21.3%, the proportion of air quality Class I (0–50) improved by 114.1%, and the number of pollution days decreased by 61.8% in NWC. All the pollutants’ (except ozone) AQI and PM_2.5/PM₁₀ ratios showed the highest pollution levels in winter and lowest in summer. AQI was strongly positively correlated with PM_2.5, PM₁₀, SO₂, NO₂, and CO, while negatively correlated with O₃. PM₁₀ was the primary pollutant, followed by O₃, PM_2.5, NO₂, CO, and SO₂, with different spatial and temporal variations. The proportion of days with PM_2.5, PM₁₀, SO₂, and CO as the primary pollutants decreased but increased for NO₂ and O₃. This study provides useful information and a valuable reference for future research on air quality in northwest China.