今晚空气一级优，北京下周一还有雪

Group 1 - The core viewpoint of the article highlights the current air pollution situation in Beijing, indicating a transition to improved air quality conditions due to changing weather patterns [1] - As of November 19, PM2.5 levels in Beijing exceeded standards for 44 hours, with severe pollution recorded for 18 hours and a peak concentration of 171 micrograms per cubic meter [1] - Forecasts predict that from the afternoon of November 19, air quality will improve to a level classified as "excellent" due to the influence of cold air and increased wind speeds [1] Group 2 - From November 20 to 25, frequent cold air activities are expected, leading to overall favorable dispersion conditions and maintaining air quality at good levels [1] - The weather during the upcoming weekend is expected to be mainly sunny, with daytime temperatures ranging from 3 to 4 degrees Celsius and nighttime lows dropping to between -8 and -6 degrees Celsius [1][3] - Light snow or flurries are anticipated in the western and northern regions on the evening of November 22, with a recommendation to monitor local weather forecasts [1]