Peroxyacetyl nitrate and peroxypropionyl nitrate in Porto Alegre, Brazil

Abstract

For 41 days between 25 May 1996 and 27 March 1997, peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) have been measured by electron capture gas chromatography at Santa Rita near Porto Alegre, RS, Brazil, where light-duty vehicles used either ethanol or a gasoline–MTBE blend. Daily maximum concentrations ranged from 0.19 to 6.67 ppb for PAN and 0.06 to 0.72 ppb for PPN. Linear regression of maximum PPN vs. maximum PAN yielded a slope of 0.105±0.004 ( R 2=0.974). Diurnal variations of ambient PAN often followed those of ozone with respect to time of day but not with respect to amplitude. This was reflected in the large relative standard deviations associated with the study-averaged PAN/ozone concentration ratio, 0.037±0.105 (ppb/ppb, n=789) and the maximum PAN/maximum ozone concentration ratio, 0.028±0.015 (ppb/ppb, range 0.005–0.078, n=41). On several days PAN accounted for large fractions of the total ambient NO x in the late morning and afternoon hours, e.g., PAN/NO x ⩽0.58 and PAN/(NO x –NO) ⩽0.76 on 27 March 1997. The amount of PAN lost by thermal decomposition (TPAN) was comparable in magnitude to that present in ambient air. The ratios TPAN/(PAN+TPAN) were up to 0.53, 0.67 and 0.64 during the warm afternoons of 25, 26 and 27 March 1997, respectively. The highest calculated value of TPAN was 5.6 ppb on 27 March 1997. On that day the 24 h-averaged value of TPAN (1.01 ppb) was nearly the same as that of PAN (1.09 ppb). Using computer kinetic modeling (SAPRC 97 chemical mechanism) and sensitivity analysis of VOC incremental reactivity, we ranked VOC present in Porto Alegre ambient air for their importance as precursors to PAN and to PPN. Using as input data the averages of VOC concentrations measured in downtown Porto Alegre during the ca. 1 yr period March 1996–April 1997, we calculated that the most important precursors to PAN and PPN were the SAPRC 97 model species ARO2 (which includes the aromatics xylenes, trimethylbenzenes, ethyltoluenes, etc.), which accounted for ca. 17% of the total PAN and total PPN formation potentials. Overall, the results indicate a major role for aromatics and alkenes and a minor role for acetaldehyde and ethanol as precursors to peroxyacyl nitrates in the Porto Alegre urban area.