Airborne　particulate　matter　(including　TSP,　PM10,　PM2.5,　and　PM1)　were　collected　at　an　urban　site　in　Xi＇an　during　winter　2010.　Individual　particles　were　analyzed　using　scanning　electron　microscopy　and　energy　dispersive　X-ray　spectrometer　(SEM-EDX).　The　morphologies,　size　distributions,　and　relative　abundance　of　aerosol　particles　in　each　size　were　summarized.　The　monomodal　particle　size　distribution　was　found　in　all　the　samples　under　different　weather　conditions,　with　the　peaks　located　at　less　than　1.0　mu　m.　The　majority　of　particles　were　composed　of　soot,　mineral　dust,　and　tar　balls,　with　minor　fly　ash　particles.　Soot　aggregates　were　the　predominant　species　(in　numbers),　ranging　from　56.6%　in　TSP　on　a　sunny　day　to　86.3%　in　PM1　on　a　cloudy　day,　with　an　average　of　73.2%　in　all　the　samples.　The　particle　mass　concentration　and　chemical　composition,　including　water-soluble　inorganic　ions,　elemental　compositions,　organic　carbon　(OC)　and　elemental　carbon　(EC)　contents　of　24-hr　integrated　PM2.5,　were　also　subject　to　chemical　bulk　analysis.　Soot　was　predominantly　observed　in　the　PM2.5　samples　(from　74.7%　to　82.7%　in　numbers),　whereas　EC　accounted　for　only　a　small　amount　(＜　8%)　of　the　PM2.5　mass.　Corresponding　to　the　mass　concentrations　of　geological　materials　(29.2%,　44.5%,　and　37.3%　on　sunny,　cloudy,　and　hazy　days),　the　number　concentrations　of　mineral　dust　and　fly　ash　particles　on　the　sunny,　cloudy,　and　hazy　days　were　14.6%,　7.1%,　and　7.7%,　respectively.