In situ measurements of HNO3, NOy, NO, and O3 in the lower stratosphere and upper troposphere

In situ measurements of nitric acid (HNO3), reactive nitrogen (NOy), nitric oxide (NO), and ozone (O3) made in the upper troposphere (UT) and lower stratosphere (LS) between 29° and 33°N latitudes during September 1999 are used to examine NOy partitioning and correlations between the measured species in these regions. The fast-response (1 s) HNO3 measurements are acquired with a new autonomous CIMS instrument. In the LS, HNO3 accounts for the majority of NOy, and the sum of HNO3 and NOx accounts for approximately 90% of NOy. In the UT, the sum of HNO3 and NOx varies between 40% and 100% of NOy. Both HNO3 and NOy are strongly positively correlated with O3, with larger correlation slopes in the UT than in the LS. In the UT at low values of the quantity (NOy–NOx–HNO3), it is uncorrelated with O3, while at higher values, a positive correlation with O3 is found. Of these two air mass types, those with higher (NOy–NOx–HNO3) mixing ratios are likely associated with the presence of peroxyacetyl nitrate (PAN) that is produced by NOx-hydrocarbon chemistry.