Model simulation of NO 3 , N 2 O 5 and ClNO 2 at a rural site in Beijing during CAREBeijing-2006

A chemical box model was used to study nitrate radical (NO 3 ), dinitrogen pentoxide (N 2 O 5 ) and nitryl chloride (ClNO 2 ) in a rural site during the Campaign of Air Quality Research in Beijing 2006 (CAREBeijing-2006). The model was based on regional atmospheric chemistry mechanism version 2 (RACM2) with the heterogeneous uptake of N 2 O 5 and the simplified chloride radical (Cl) chemistry mechanism. A high production rate of NO 3 with a mean value of 0.8 ppbv/h and low mixing ratios of NO 3 and N 2 O 5 (peak values of 17 pptv and 480 pptv, respectively) existed in this site. Budget analysis showed that NO emission suppressed the NO 3 chemistry at the surface layer, the reaction of NO 3 with VOCs made a similar contribution to NO 3 loss as N 2 O 5 heterogeneous uptake. The NO 3 chemistry was predominantly controlled by isoprene, and NO 3 oxidation produced organic nitrate with a mean value of 0.06 ppbv/h during nighttime. The organic nitrate production initiated by NO 3 was equal to that initiated by OH, implying the importance of nighttime chemistry for secondary organic aerosol (SOA) formation. We confirmed that the N 2 O 5 heterogeneous reaction accounted for nighttime particle NO 3 − enhancement, with a large day to day variability, and made less of a contribution to NOx loss compared to that of OH reacting with NO 2 . Additionally, abundant ClNO 2 , up to 5.0 ppbv, was formed by N 2 O 5 heterogeneous uptake. ClNO 2 was sustained at a high level until noon in spite of the gradually increasing photolysis of ClNO 2 after sunrise. Chlorine activation caused by N 2 O 5 heterogeneous uptake increased primary ROx formation by 5% and accounted for 8% of the net ozone production enhancement in the morning.