Observations of total alkyl nitrates during Texas Air Quality Study 2000: Implications for O3 and alkyl nitrate photochemistry

Abstract

Observations of total alkyl nitrates (∑ANs) were obtained using thermal dissociationlaser-induced fluorescence at La Porte, Texas, from 15 August to 15 September 2000, along with an extensive suite of other nitrogen oxides, hydrocarbons, and O3. The ∑AN mixing ratios ranged as high as 5.2 ppbv. The median midday mixing ratio was 1.2 ppbv, and the median nighttime mixing ratio was 0.26 ppbv. These are higher mixing ratios than the sum of individual nitrates in virtually every prior study. The diurnal variation of ∑ANs was similar to that of HNO3 and of total peroxy nitrates, with a peak near 1300 local time (LT) indicating a photochemical source. Mixing ratios decreased rapidly in the afternoon, suggesting that ∑AN deposition is nearly as fast as HNO3 deposition. The observed correlation between O3 and ∑ANs has a slope that increases from 29 (R2 = 0.73) △O'x/△∑ANs at 0900-1200 LT to 41 (R2 = 0.74) △Ox/△∑ANs at 1400-1800 LT. We present calculations constrained by the observed hydrocarbons showing that both the mixing ratio of ∑ANs and the correlation of ∑ANs with O3 are to be expected on the basis of the branching ratios for alkyl nitrate formation in the RO2 + NO reaction. Copyright 2004 by the American Geophysical Union.