| The nitrate radical(NO3)is an important oxidant found in the troposphere during the night.The reactions of NO3 with VOCs and organic sulfur species can affect the abundance and lifetimes of these species.The reactions of NO3 with biologic volatile organic compounds(BVOC)have an important contribution to the production of nocturnal organic nitrates and secondary organic aerosols.NO3 can quickly form a thermal equilibrium with dinitrogen pentoxide(N2O5).The N2O5 heterogeneous process can produce HNO3.Nocturnal chemical processes involving NO3 and N2O5 can remove reactive nitrogen oxides(NOx)from the atmosphere and subsequently affect the ozone production.It is of great significance to carry out large-scale N03 radicals and N2O5 measurements in different regions and seasons in China to analysis the complex nocturnal atmospheric chemical processes.Based on the previous work in our research group,this paper carried out the optimization of the cavity ring down spectroscopy(CRDS)instrument and the observations and analysis of chemical processes involving NO3 and N2O5 in different regions and seasons in China.The main results are as follows:1)An inexpensive,compact CRDS instrument for sensitive measurement of nocturnal nitrogen oxides NO3 and N2O5 in ambient air was developed.The system parameters were configured reasonably,such as reflectivity of the mirrors.The temperature control device for the heating zone was optimized,which can ensure a stable heating temperature for the pyrolysis reaction.The absorption cross sections of NO3 were retrieved.Software program for data acquisition and processing was optimized.The minimum detection limits for the NO3 radical and N2O5 are estimated to be 1.1 ppt and 1.9 ppt(l?,2.5s).2)The N2O5 intercomparison by the self-made CRDS and another cavity enhanced absorption spectrometer(CEAS)was carried out.The two data sets showed a good agreement within their uncertainties,with an intercept of 15.6 ppt,slope of 0.94 and a correlation coefficient R2=0.97.The excellent agreement between the measurement by the CRDS and CEAS instruments demonstrates the capability of the two instruments for accurately measuring N2O5 with high sensitivity.However,under several special environmental conditions,such as high RH(RH>60%)and high PM2.5 concentrations(PM2.5>200?g/m3),large discrepancies also existed.The discrepancy between the measurements of the two instruments indicates that filters should be removed frequently during the heavily polluted days and increasing the flow rate in the cavity is an alternative approach to reduce the loss.3)The CRDS instrument was deployed to measure NO3 and N2O5 during two field campaigns in the urban and suburban areas of Beijing in winter.The observations provide nocturnal N03 and N205 variations.The results show that in the cold winter time,the concentrations of NO3 were below detection limit,the concentrations of N2O5 were higher in the suburban areas than that in the urban areas and the maximum value was larger than 1400ppt.Calculated NO3 production rates were 0.36±0.41 ppb/h and 0.94±0.83 ppb/h in the suburban and urban areas,respectively.Budgets for NO3 show that it was lost primarily to N2O5 hydrolysis in winter.In urban areas,NO3 reactions with BVOC,AVOC,NO and N2O5 hydrolysis contributed to 0.5%-5%,0.1%-2%,9.8%-45%and 28%-89.6%of NO3 loss.4)Field observations of NO3 and N2O5 were carried out in Taizhou Site and Beijing site,respectively.The formation and loss mechanism of NO3 and N2O5 in these regions were compared.The concentrations of NO3 and N2O5 were higher in Beijing than that in Taizhou.The lifetimes of NO3 were 0.23±0.25 min and 0.20±0.36 min while the lifetimes of N2O5 were 2.59±3.25 min and 0.93±1.13 min for Beijing and Taizhou regions,respectively.NO3 reactivities by the reactions with VOC were 2.33×10-2 s-1 and 1×10-2 s-1 and the fitted uptake coefficients of N2O5 were 0.066-0.109 and 0.027-0.107 in Beijing and Taizhou,respectively.For loss mechanism,the loss of NO3 at Beijing site is complicated due to variations in the aerosol surface area.Budgets for NO3 in Taizhou site show that direct loss contributed to 3.15%-42.86%of overall NO3 loss.5)The oxidation of BVOC by N03 is an important pathway for the generations of organic nitrates(ON)and secondary organic aerosols(SOA)at night.The ON productions were 0.057 ppb/h and 0.048 ppb/h in Beijing and Taizhou,respectively.The heterogeneous reaction of N2O5 plays an important role in the formation of nitrate.The nitrate formations during the whole night were 24.05±20.56?g/m3 and 14.83±6.01?g/m3 in Beijing and Taizhou,respectively.The NOx removal rates at these two sites were 0.29±0.13 h-1 and 0.14±0.02 h-1,respectively. |
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