Based　on　long-term　systematic　sampling,　information　is　currently　limited　regarding　the　impacts　of　different　air　pollution　levels　on　variations　of　bacteria,　fungi　and　ammonia-oxidizing　microorganisms　(AOMs)　in　fine　particulate　matter　(PM2.5),　especially　their　interactions.　Here,　PM2.5　samples　were　weekly　collected　at　different　air　pollution　levels　in　Beijing,　China　during　one-year　period.　Microbial　composition　was　profiled　using　Illumina　sequencing,　and　their　interactions　were　further　investigated　to　reveal　the　hub　genera　with　network　analysis.　Diversity　of　bacteria　and　fungi　showed　obvious　seasonal　variations,　and　the　heavy-　or　severe-pollution　levels　mainly　affected　the　diversity　and　composition　of　bacteria,　but　not　fungi.　While,　the　community　structure　of　both　bacteria　and　fungi　was　influenced　by　the　combination　of　air　pollution　levels　and　seasons.　The　most　abundant　bacterial　genera　and　some　genera　with　highest　abundance　in　heavy-　or　severe-pollution　days　were　the　hub　bacteria　in　PM2.5.　Whereas,　only　the　dominant　fungi　in　light-pollution　days　in　winter　were　the　hub　fungi　in　PM2.5.　The　complex　positive　correlations　of　bacterial　or　fungal　pathogens　would　aggravate　the　air　pollution　effects　on　human　health,　despite　of　their　low　relative　abundances.　Moreover,　the　strong　co-occurrence　and　co-exclusion　patterns　of　bacteria　and　fungi　in　PM2.5　were　identified.　Furthermore,　the　hub　environmental　factors　(e.g.,　relative　humidity　and　atmospheric　pressure)　may　play　central　roles　in　the　distributions　of　bacteria　and　fungi,　including　pathogens.　Importantly,　AOMs　showed　significant　co-occurrence　patterns　with　the　main　bacterial　and　fungal　genera　and　potential　pathogens,　providing　possible　microbiological　evidences　for　controlling　ammonia　emissions　to　effectively　reduce　PM2.5　pollution.　These　results　highlighted　the　more　obvious　air　pollution　impacts　on　bacteria　than　fungi,　and　the　complex　bacterial-fungal　interactions,　as　well　as　the　important　roles　of　AOMs　in　airborne　microbial　interactions　webs,　improving　our　understanding　of　bioaerosols　in　PM2.5.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.