The categorical tree model derived using the above parameters, indicates that daily minimum RH had the strongest association with MDA8 O3 (Fig. 2). Seventy-four percent of cases when MDA8 O3 was > 60 ppbv occurred when daily minimum RH was < 14.7% (Fig. 2 — Nodes 1 and 2). Further, categorically high MDA8 O3 was predicted when, in addition to low daily minimum RH, daily maximum T was low (≤ 25.6 °C and ≤ 24 °C for Nodes 4 and 6 in Fig. 1, respectively). Observations indicate that 49% of the cases when MDA8 O3 > 60 ppbv (i.e. categorically high or very high) during the AZD1080 considered were characterized by conditions of low min RH and low maximum T. It is also interesting to note that while the vast majority of cases with elevated MDA8 O3 are represented in Nodes 1 and 2 (Fig. 2), 50% of the cases with categorically medium MDA8 O3 occurred under these conditions. This indicates RuBP surface conditions during periods of elevated MDA8 O3 were not consistently, markedly different than periods without elevated MDA8 O3, and suggests that local surface conditions alone are not driving the production of O3 measured at surface sites in rural Nevada. The fact that the nodes with low daily minimum RH and low maximum daily T included the majority of days with high and very high MDA8 O3 is particularly interesting because as a secondary photochemical pollutant, low RH and high T would be expected to promote production of O3. The association with relatively lower maximum T is consistent with interception of air parcels from aloft, and thus, represents a line of evidence that transport of O3 from the free troposphere influences high O3 measured in rural Nevada.