NO₂

As the nadir-derived NO₂ VCDs were unrealistically large and VCDs above the ER-2 could not be retrieved, the retrievals were constrained to $2\times 10^{15}$ cm^-2 below and $2\times 10^{15}$ cm^-2 above the ER-2. For the transit flight to Hawaii on 21 September, the constraints used are $1\times 10^{15}$ cm^-2 below and $2\times 10^{15}$ cm^-2 above. These were estimated based on results from the Chemical Transport Model (CTM) at York University.

 

Table 6.9: Summary of NO₂ limb retrievals and integrated ER-2 in-situ NO₂ measurements (Gao et al., 1994) from 6 May and 21 September 1997 (see text for details).

Flight	Time	Location	$z_{\rm ER2}$	Retrieved NO2 VCD ( $\times10^{14}$ cm-2)
Date	(UTC)	($^{\circ }$N,$^{\circ }$W)	(km)	0-12 km	12-16 km	16- $z_{\rm ER2}$ km
6 May	2048	76.2,114.5	19.5	1.57	7.53	10.9
	2248	70.1,132.8	20.1	5.91	5.53	8.56
26 April	1932	87.0,148.0	19.1	3.91	6.21	9.88
	2106	83.1,148.0	19.4	5.51	6.55	7.94
	2130	80.1,148.0	19.6	9.43	5.39	5.19
21 September	2017	53.1,153,1	20.5	0.53	3.43	6.04
	2150	42.1,154.5	20.1	2.17	2.44	5.43
	2327	31.4,155.4	20.7	3.43	3.23	3.33
6 May	1817	66.6,147.3	Aa	-	-	4.07
	2007	78.5,133.8	B	-	-	8.68
	2026	77.6,123.9	C	-	-	7.93
	2128	73.3,104.1	D	-	-	9.57
	2147	72.3,111.8	E	-	-	8.49
	0011b	65.9,149.5	F	-	4.86	5.47
21 September	2212	39.9,154.7	H	-	-	2.76
	2215	37.7,154.9	I	-	-	1.90
	0051c	23.2,157.4	J	-	0.90	1.82

^a Columns derived from integrating in-situ measurements over segments of the flight: A Ascent from Fairbanks; B descent during first dive; C ascent after first dive; D descent during second dive; E ascent after second dive; F descent into Fairbanks; H decent during dive; I ascent after dive; J descent into Hawaii.
^b 7 May 1997.
^c 22 September 1997.

The retrievals from 6 May are discussed first. Overall, they appear to be reasonable and are roughly comparable with the in-situ VCDs from the two mid-flight dives. A VCD of 10¹⁵ cm^-2 well mixed between 16 and 20 km gives a mixing ratio of 1 ppbv, which seems reasonable for the location and season (de Grandpré et al., 1997). The values in the lowest layer, C₁, were quite variable indicating that the constant VCD of $2\times 10^{15}$ cm^-2 below the ER-2 is not always appropriate. The integrated in-situ amounts from 16-20 km increased by about 50% between the ascent and descent, the southern most portion of this flight, and the dives, which were further north. This suggests there is a substantial vertical gradient. The results from 26 April were more variable. The last scan in particular has smaller ACDs which were largely responsible for the decreased amount of NO₂ retrieved in C₃. As similar anomalies were observed in the ozone retrievals, it appears that some of this air may have originated inside the polar vortex. The decreases in C₃ forced unrealistically large amounts on NO₂ into the lowest layer, C₁. The values retrieved in C₂ from the three scans were quite consistent but slightly larger in comparison with the in-situ C₂, although the latter was made further south.

A general decrease in the NO₂ abundance was observed in C₂ and C₃ as the ER-2 flew south during the 21 September flight, similar to that observed for the ozone. This is expected as to a first approximation, the distribution of NO₂ follows that of ozone as they are closely coupled chemically. The estimate of $1\times 10^{15}$ cm^-2 below the ER-2 appears to be approximately correct for the first scan and too large for the second and third. This appears to be especially true for the third scan as the retrieval algorithm is pushing more NO₂ into C₂. The $0.53\times 10^{15}$ cm^-2 from 0-12 km from the first scan seems reasonable as this translates into 30 pptv for a well mixed layer (not an unreasonable approximation based on the US standard atmosphere NO₂ profile) which is consistent with that expected in a marine environment. Also similar to the ozone, this decrease with latitude was not as pronounced as was observed in the integrated in-situ VCDs.