Measurement And Analysis Of Peroxyacetyl Nitrate (PAN) In The North China Region And The Tibetan Plateau

As an environmental pollution issue, photochemical pollution has raised concern.Peroxyacetyl nitrate （PAN） is produced photochemically in boundary layer and couldcause damages to plants and human health. And it also acts as NOxreservoir due to itslong lifetime in cold condition. To know the background level and pollution pattern ofPAN in the North China region and the Tibetan Plateau, we used a PAN-GC analyzerand some other equipment to measure PAN and related species in several sites, whichwere deployed at an urban site in Beijing, at Shangdianzi, Wuqing and Namco. As theperformance of PAN-GC varied in different environment conditions, two methodswere used to correct the collected data and final data with fine quality have beenacquired. With the observational results we intend to get more imformation aboutPAN and photochemical somg in the above regions and understand more about thephysical and chemical factors influcing PAN’s variation.Measurement results in North China present average PAN concentrations of0.70ppb and0.20ppb in winter urban Beijing and in Wuqing District, Tianjin, respectively.And monthly average PAN concentrations at Shangdianzi were0.22,0.33and0.59ppb for January, February and March2011, respectively. Abundant PAN indicates thatphotochemistry is able to produce significant PAN even in winter in the region. Suchpollution is partly attributed to the stable boundry layer commonly occurring in winter,which also causes the accumulation of other primary pollutants, including NOxandCO. In such condition, O₃is effectively consumed by chemical reaction with NO andhence does not correlate well with PAN. At Shangdianzi, the PAN level in spring andearly summer is higher than the winter PAN level, indicates that the North Chinaregion experiences heavier photochemical pollution in warmer seasons. With lowdecomposition rate under low temperature, PAN plumes could be transportedregionally. It is shown that when southwest air stream is dominating the Shangdianzisite, there is a second acsending process in the PAN concentration, indicating theimportance of transport. However, the diurnal course of PAN also has differentpatterns in different seasons, caused probably by different temperature and transport conditions.Measurement results at Namco in the Tibetan Plateau shows different averagePAN levels ranging from0.23to0.90ppb. And positive correlation between PAN andO₃are shown from all periods. The large-scale air circulation seems to exert greatinfluence on the PAN and O₃concentrations. In summer, the troposphere over theTibetan Plateau is dominated by Asian Summer Monsoon, which transports air massesfrom South Asia to the Tibetan Plateau. PAN sourced from low altitude is transportedtogether with climbing air masses and eventually reachs our site. In addition to thissource, some elevated PAN values may also be caused by the downward transport ofair masses，which also cause the simultaneously increasing of O₃. We find increasingPAN occured from8:00LT to10:00LT on everyday, which we believe is caused bythe vertical mixing of low boundary layer, when the increasing wind speed and thedecresing vertical gradient of wind speed clearly exhibit the process. The southernAsia monsoon is believed to influence this process effectively by maintaining highsurface wind speed and transporting PAN from surface India Peninsula, leading tohigher PAN/O₃ratio.